Consequences of Continental Glaciation

Consequences of Continental Glaciation

kacimo samy   2:22 م

Consequences of Continental Glaciation

Ice Loading and Glacial Rebound 

The concept of subsidence and rebound, due to continental glaciation and deglaciation. (Not to scale.)

When a large ice sheet (more than 50 km in diameter) grows on a continent, its weight causes the surface of the lithosphere to sink. In other words, ice loading causes glacial subsidence. Lithosphere, the relatively rigid outer shell of the Earth, can sink because the underlying asthenosphere is soft enough to flow slowly out of the way (figure above). Because of ice loading, much of Antarctica and Greenland now lie below sea level, so if their ice were instantly to melt away, these continents would be flooded by a shallow sea.

What happens when continental ice sheets do melt away? Gradually, the surface of the underlying continent rises back up, by a process called glacial rebound, and the asthenosphere flows back underneath to fill the space. This process doesn't take place instantly, the asthenosphere flows so slowly (at rates of a few millimetres per year) that it takes thousands of years for ice-depressed continents to rebound. Thus, glacial rebound is still taking place in some regions that were covered by ice during the Pleistocene Ice Age.

Sea-Level Changes: The Glacial Reservoir 

The link between sea level and global glaciation: glaciers store water so when glaciers grow, sea level falls, and when glaciers melt, sea level rises.

More of the Earth’s surface and near-surface freshwater resides in glacial ice than in any other reservoir. During the Pleistocene Ice Age, glaciers covered almost three times as much land area so they held significantly more water than they do today. In effect, water from the ocean reservoir transferred to the glacial reservoir and remained trapped on land. As a consequence, sea level dropped by as much as 100 m, and extensive areas of continental shelves became exposed as the coastline migrated seaward (figure above a–c). People and animals populated the exposed coastal plains. The drop in sea level also created land bridges across the Bering Strait between North America and northeastern Asia, providing convenient migration routes for prehistoric humans. 

Ice Dams, Drainage Reversals, and Lakes 

When ice freezes over a sewer opening in a street, neither meltwater nor rain can enter the drain, and the street floods. Ice sheets play a similar role in glaciated regions. The ice may block the course of a river, leading to the formation of a lake. In addition, the weight of a glacier changes the tilt of the land surface and therefore the gradients of streams, and glacial sediment may fill pre-existing valleys. In sum, continental glaciation modifies or destroys pre-existing drainage networks. While the glacier exists, streams find different routes and carve out new valleys; by the time the glacier melts away, these new streams have become so well established that old river courses may remain abandoned. 

Meltwater Floods 

Subsidence of the land surface at the toe of a glacier locally led to the growth of large ice-margin lakes. Inevitably, the ice dams that held back these lakes melted and broke. In a matter of hours to days, the contents of the lakes drained, creating immense flood-waters that stripped the land of soil and left behind huge ripple marks. For example, glacial Lake Missoula, in Montana, filled when glaciers advanced and blocked the outlet of a large valley. When the glaciers retreated, the ice dam broke, releasing immense torrents the Great Missoula Flood that scoured eastern Washington, creating a barren, soil-free landscape called the channelled scablands.

Ice-age lakes in North America.

The largest known ice-margin lake covered portions of Manitoba and Ontario, in south-central Canada, and North Dakota and Minnesota in the United States (figure above a). This body of water, Glacial Lake Agassiz, existed between 11,700 and 9,000 years ago, a time during which the most recent phase of the last ice age came to a close and the continental glacier retreated north. At its largest, the lake covered over 250,000 square km (100,000 square miles), an area greater than that of all the present Great Lakes combined. The sudden release of water from Lake Agassiz may have led to a sea-level rise of 1 to 3 m during a single year. 

Pluvial Features 

During the Pleistocene Ice Age, the climate in regions to the south of continental glaciers was wetter than it is today. Fed by enhanced rainfall, lakes accumulated in low-lying land at a great distance from the ice front. Many such pluvial lakes (from the Latin pluvia, rain) flooded interior basins of the Basin and Range Province in Utah and Nevada (figure above b). The largest pluvial lake, Lake Bonneville, covered almost a third of western Utah. When this lake suddenly drained after a natural dam holding it back broke, it left a bathtub ring of shoreline rimming the mountains near Salt Lake City. Today’s Great Salt Lake is but a small remnant of Lake Bonneville. 

Periglacial Environments 

Periglacial regions are not ice covered but do include substantial areas of permafrost.

In polar latitudes today, and in regions adjacent to the fronts of continental glaciers during the last ice age, the mean annual temperature stays low enough (below 5C) that soil moisture and groundwater freeze and, except in the upper few meters, stay solid all year. Such permanently frozen ground is called  permafrost. Regimes with widespread permafrost that do not have a cover of snow or ice are called periglacial environments (the Greek peri means around, or encircling; periglacial environments appear around the edges of glacial environments; figure above a). 

The upper few meters of permafrost may melt during the summer months, only to refreeze again when winter comes. As a consequence of the freeze-thaw process, the ground of some permafrost areas splits into pentagonal or hexagonal shapes, creating a landscape called patterned ground (figure above b). 

Permafrost presents a unique challenge to people who live in polar regions or who work to extract resources from these regions. For example, heat from a building may warm and melt underlying permafrost, creating a mire into which the building settles. For this reason, buildings in permafrost regions must be placed on stilts, so that cold air can circulate beneath them to keep the ground frozen. 

Credits: Stephen Marshak (Essentials of Geology)

Deposition Associated with Glaciation

Deposition Associated with Glaciation

kacimo samy   10:32 ص

Deposition Associated with Glaciation 

The Glacial Conveyor 

The glacial conveyor and the formation of lateral and medial moraines on glaciers.

Glaciers can carry sediment of any size and, like a conveyor belt, transport it in the direction of flow (that is, toward the toe;  figure above a). The sediment load either falls onto the surface of the glacier from bordering cliffs or gets plucked and lifted from the substrate and incorporated into the moving ice. Geologists refer to a pile of debris carried by or left by glaciers as a moraine. Sediment dropped on the glacier’s surface moves with the ice and becomes a stripe of debris. Stripes formed along the side edges of the glacier are lateral moraines. When a glacier melts, lateral moraines lie stranded along the side of the glacially carved valley, like bathtub rings. Where two valley glaciers merge, the debris constituting two lateral moraines merges to become a medial moraine, running as a stripe down the interior of the composite glacier (figure above b). Trunk glaciers created by the merging of many tributary glaciers contain several medial moraines. Sediment transported to a glacier’s toe by the glacial conveyor accumulates in a pile at the toe and builds up to form an end moraine.

Types of Glacial Sedimentary Deposits 

Several different types of sediment can be deposited in  glacial environments; all of these types together constitute  glacial drift. The term dates from pre-Agassiz studies of glacial deposits, when geologists thought that the sediment had “drifted” into place during an immense flood. Specifically,  glacial drift includes the following: 

Sedimentation processes and products associated with glaciation. Glacial sediment is distinctive.

• Till: Sediment transported by ice and deposited beneath, at the side, or at the toe of a glacier is called glacial till. Glacial till is unsorted because the solid ice of glaciers can carry clasts of all sizes (figure above a). 
• Erratics: Glacial erratics (figure above b) are cobbles and boulders that have been dropped by a glacier. Some lie within or on till piles, and others rest on glacially polished surfaces. 
• Glacial marine: Where a sediment-laden glacier flows into the sea, icebergs calve off the toe and raft clasts out to sea. As the icebergs melt, they drop the clasts. Sediment consisting of ice-rafted clasts mixed with marine sediment makes up glacial marine. 
• Glacial outwash: Till deposited by a glacier at its toe may be picked up and transported by meltwater streams that sort the sediment. The clasts are deposited by a braided stream network to form a broad area of gravel and sandbars called an outwash plain. This sediment is known as glacial outwash (figure above c). 
• Loess: When the warmer air above ice-free land beyond the toe of a glacier rises, the cold, denser air from above the glacier rushes in to take its place. A strong wind, called katabatic wind, therefore blows at the margin of a glacier. This wind picks up fine clay and silt and transports it away from the glacier’s toe. Where the winds die down, the sediment settles and forms a thick layer. This sediment, called loess, sticks together because of the electrical charges on clay flakes. Thus, steep escarpments can develop by erosion of loess deposits (figure above d).
• Glacial lake-bed sediment: Streams transport fine clasts, including rock flour, away from the glacial front. This sediment eventually settles in meltwater lakes, forming a layer of glacial lake-bed sediment that commonly contains varves. A varve is a pair of thin layers deposited during a single year. One layer consists of silt brought in during spring floods and the other of clay deposited in winter when the lake’s surface freezes over and the water is still (figure above e).

Depositional Landforms of Glacial Environments 

The formation of depositional landforms associated with continental glaciers.

Picture a group of hunters, dressed in reindeer skin, gazing southward from the crest of an ice cliff at the toe of a continental glacier in what is now southern Canada. It’s about 12,000 years ago, and the glacier has been receding for at least a millennium. The hunters would have been able to see a variety of landscape features, some formed by glacial erosion and some by deposition, due to moving ice and meltwater. We've already described erosional features, so now let’s focus on the depositional features of the landscape (figure above a, b).

From their vantage point, the hunters would probably have seen a few curving, hummocky ridges of sediment in the region between the glacier’s toe and the horizon. Each of these ridges is an end moraine, formed when the position of the glacier’s toe remained in the same location for a while. Ice keeps flowing to the toe, and like a giant conveyor belt, transports sediment to the toe. As the ice melts, this sediment accumulates to form a pile of till, and this pile comprises the end moraine. Geologists refer to the end moraine at the farthest limit of glaciation as the terminal moraine. In the northeastern United States, a large terminal moraine built up during the Pleistocene Ice Age this ridge of sediment now underlies Long Island, New York, and Cape Cod, Massachusetts (figure above c). When a glacier starts receding, it may stall several times the end moraines that form when a glacier stalls while receding are known as recessional moraines. The hummocky layer of till between end moraines is known as lodgment till or ground moraine. Since this till was deposited by moving ice, clasts within it may be aligned and scratched. 

Knob-and-kettle topography and drumlins characterize some areas that were once glaciated.

The hummocky surface of a moraine reflects both variations in the amount of sediment supplied by the ice and the development of kettle holes. A kettle hole is a roughly circular depression made when a block of ice that calved off the toe of a glacier became buried by till. When the block eventually melts, it leaves behind a depression (figure above a, b). Geologists refer to a land surface spotted with many kettle holes separated by rounded hills or ridges of sediment as “knob-and-kettle topography” (figure above c). 

In some locations, glacial ice flow molds underlying till into an elongate hill known as a drumlin (from the Gaelic word for small hill or ridge). Drumlins commonly occur in swarms, and tend to be about 50 m high. Their long axis trends parallel to the flow direction of the glacier. Notably, drumlins taper in the direction of flow a drumlin’s upstream end is steeper than its downstream end (figure above d, e).

Eskers are snake-like ridges of sand and gravel that form when sediment fills meltwater tunnels at the base of a glacier.

As we've noted, not all of the sediment or “drift” associated with glacial landscapes was deposited directly by ice, for meltwater also carries and deposits sediment. Water transported sediment, in contrast to till, tends to be sorted and stratified. Sediment deposited in meltwater tunnels beneath a glacier itself may remain as a sinuous ridge, known as an esker, when the glacier melts away (figure above a, b). Braided meltwater streams that flow beyond the end of a glacier deposit layers of sand and gravel that underlie glacial outwash plains. Meltwater may collect in a lake adjacent to the glacier’s toe, to form an ice-margin lake. Additional lakes and swamps may form in low areas on the ground moraine. Sediments deposited in eskers and glacial outwash plains serve as important sources of sand and gravel for construction, and the fine sediment of former glacial lake beds evolves into fertile soil for agriculture.

Credits: Stephen Marshak (Essentials of Geology)

Carving and Carrying by Ice

Carving and Carrying by Ice

kacimo samy   7:29 ص

Carving and Carrying by Ice

Glacial Erosion and Its Products 

Products of glacial erosion. Ice is a very aggressive agent of erosion.

During the last ice age, valley glaciers cut deep, steep-sided valleys into the Sierra Nevada mountains of California. In the process, some granite domes were cut in half, leaving a rounded surface on one side and a steep cliff on the other. Half Dome, in Yosemite National Park, formed in this way (figure above a); its steep cliff has challenged many rock climbers. Such glacial erosion also produces the knife-edge ridges and pointed spires of high mountains (figure above b) and broad expanses where rock outcrops have been stripped of overlying sediment and polished smooth (figure above c). In many localities, the rock surface visible today is the same rock surface once in contact with ice. In some places, subsequent rockfalls and river erosion have substantially modified the surface.

As glaciers flow, clasts embedded in the ice act like the teeth of a giant rasp and grind away the substrate. This process, glacial abrasion, produces long gouges, grooves, or scratches called glacial striations (figure above d). Striations range from 1 cm to 1 m across and may be tens of centimeters to tens of meters long. As you might expect, striations run parallel to the flow direction of the ice. Rasping by embedded sand yields shiny glacially polished surfaces. 

Glaciers pick up fragments of their substrate in several ways. During glacial incorporation, ice surrounds debris so the debris starts to move with the ice. During glacial plucking (or glacial quarrying), a glacier breaks off fragments of bedrock. Plucking occurs when ice freezes around rock that has just started to separate from its substrate, so that movement of the ice can lift off pieces of the rock. At the toe of a glacier, ice may actually bulldoze sediment and trees slightly before flowing over them. 

Landscape features formed by the glacial erosion of a mountains landscape.

Let’s now look more closely at the erosional features associated with a mountain glacier (figure above a). Freezing and thawing during the fall and spring help fracture the rock bordering the head of the glacier (the ice edge high in the mountains). This rock falls on the ice or gets picked up at the base of the ice, and moves downslope with the glacier. As a consequence, a bowl-shaped depression, or cirque, develops on the side of the mountain. If the ice later melts, a lake called a tarn may form at the base of the cirque. The shape of a cirque may be maintained or even amplified by rockfalls after the glacier is gone. An arête (French for ridge), a residual knife-edge ridge of rock, separates two adjacent cirques. A pointed mountain peak surrounded by at least three cirques is called a horn. The Matterhorn, a peak in Switzerland, is a particularly beautiful example of a horn; each of its four faces is a cirque (figure above b).

Glacial erosion severely modifies the shape of a valley. To see how, compare a river-eroded valley with a glacially eroded valley. If you look along the length of a river in unglaciated mountains, you’ll see that it typically flows down a V-shaped valley, with the river channel forming the point of the V. The V develops because river erosion occurs only in the channel, and mass wasting causes the valley slopes to approach the angle of repose. But if you look down the length of a glacially eroded valley, you’ll see that it resembles a U, with steep walls. A U-shaped valley (figure above c) forms because the combined processes of glacial abrasion and plucking not only lower the floor of the valley but also bevel its sides.

Glacial erosion in mountains also modifies the intersections between tributaries and the trunk valley. In a river system, the trunk stream serves as the local base level for tributaries, so the mouths of the tributary valleys lie at the same elevation as the trunk valley. The ridges (spurs) between valleys taper to a point when they join the trunk valley floor. During glaciation, tributary glaciers flow down side valleys into a trunk glacier. But the trunk glacier cuts the floor of its valley down to a depth that far exceeds the depth cut by the tributary glaciers. Thus, when the glaciers melt away, the mouths of the tributary valleys perch at a higher elevation than the floor of the trunk valley. Such side valleys are called hanging valleys. The water in a post-glacial stream that flows down a hanging valley  cascades over a spectacular waterfall to reach the post-glacial trunk stream (figure above d). As they erode, trunk glaciers also chop off the ends of spurs (ridges) between valleys, to produce truncated spurs.

A roche moutonnée is an asymmetric bedrock hill shaped by the flow of glacial ice.

Now let’s look at the erosional features produced by continental ice sheets. To a large extent, these depend on the nature of the pre-glacial landscape. Where an ice sheet spreads over a region of low relief, such as the Canadian Shield, glacial erosion creates a vast region of polished, flat, striated surfaces. Where an ice sheet  spreads over a hilly area, it deepens valleys and smooths hills. Glacially eroded hills may end up being elongate in the direction of flow and may be asymmetric, for glacial rasping smoothes and bevels the upstream part of the hill, creating a gentle slope, whereas glacial plucking eats away at the downstream part, making a steep slope. Ultimately, the hill’s profile may resemble that of a sheep lying in a meadow such a hill is called a roche moutonnée, from the French for sheep rock (figure above a, b).

Fjords: Submerged Glacial Valleys 

One of the many spectacular fjords of Norway. The water is an arm of the sea that fills a glacially carved valley. Tourists are standing on Pulpit Rock (Prekestolen).

As noted earlier, where a valley glacier meets the sea, the glacier’s base remains in contact with the ground until the water depth exceeds about four-fifths of the glacier’s thickness, at which point the glacier floats. Thus, glaciers can carve U-shaped valleys even below sea level. In addition, during an ice age, water extracted from the sea becomes locked in the ice sheets on land, so sea level drops significantly. Therefore, the floors of valleys cut by coastal glaciers during the Pleistocene Ice Age were cut much deeper than present sea level. Today, the sea has flooded these deep valleys, producing fjords. In the spectacular fjord-land regions along the coasts of Norway, New Zealand, Chile, and Alaska, the walls of submerged U-shaped valleys rise straight from the sea as vertical cliffs up to 1,000 m high (figure above). Fjords also develop where an inland glacial valley fills to become a lake.

Credits: Stephen Marshak (Essentials of Geology)

Ice and the Nature of Glaciers

Ice and the Nature of Glaciers

kacimo samy   6:34 ص

Ice and the Nature of Glaciers 

What Is Ice? 

The nature of ice and the formation of glaciers. Snow falls like sediment and metamorphoses to ice when buried.

Ice consists of solid water, formed when liquid water cools below its freezing point. We can apply concepts introduced in our earlier discussions of rocks and minerals to distinguish among various occurrences of ice. For example, we can think of a single ice crystal as a mineral specimen, for it is a naturally occurring, inorganic solid, with a definite chemical composition (H2O) and a regular crystal structure. Ice crystals have a hexagonal form, so snowflakes grow into six-pointed stars (figure above a). We can picture a layer of fresh snow as a layer of sediment, and a layer of snow that has been compacted so that the grains stick together as a layer of sedimentary rock (figure above b). We can also think of the ice that appears on the surface of a pond as an igneous rock, for it forms when molten ice (liquid water) solidifies. Glacial ice, in effect, is a metamorphic rock. It develops when pre-existing ice recrystallizes in the solid state, meaning that the molecules in solid water rearrange to form new crystals (figure above c).

How a Glacier Forms 

In order for a glacier to form, three conditions must be met. First, the local climate must be cold enough that winter snow does not melt entirely away during the summer. Second, there must be sufficient snowfall for a large amount of snow to accumulate. And third, the slope of the surface on which the snow accumulates must be gentle enough that the snow does not slide away in avalanches, and must be protected enough that the snow doesn't blow away. 

Glaciers develop in polar regions because, even though relatively little snow falls today, temperatures remain so cold that most ice and snow survive all year. Glaciers develop in mountains, even at low latitudes, because temperature decreases with elevation; at high elevations, the mean temperature stays cold enough for ice and snow to survive all year. Since the temperature of a region depends on latitude, the specific elevation at which mountain glaciers form also depends on latitude. In Earth’s present-day climate, glaciers form only at elevations above 5 km at the equator, but can flow down to sea level at  latitudes of between 60 and 90 degrees. 

The transformation of snow to glacier ice takes place as younger snow progressively buries older snow. Freshly fallen snow consists of delicate hexagonal crystals with sharp points. The crystals do not fit together tightly, so fresh snow contains about 90% air. With time, the points of the snowflakes become blunt because they either sublimate (evaporate directly into vapour) or melt, and the snow packs more tightly. As snow becomes buried, the weight of the overlying snow increases pressure, which causes remaining points of contact between snowflakes to melt. Gradually, the snow transforms into a packed granular material called firn, which contains only about 25% air (figure above d). Melting of firn grains at contact points produces water that crystallizes in the spaces between grains until eventually the firn transforms into a solid mass of glacial ice composed of interlocking ice crystals. Such glacial ice, which may still contain up to 20% air trapped in bubbles, tends to absorb red light and thus has a bluish colour. The transformation of fresh snow to glacier ice can take as little as tens of years in regions with abundant snowfall, or as long as thousands of years in regions with little snowfall. 

Categories of Glaciers 

Glaciers are streams or sheets of recrystallized ice that stay frozen all year long and flow under the influence of gravity. Today, they highlight coastal and mountain scenery in Alaska, the Cordillera of western North America, the Alps of Europe, the Southern Alps of New Zealand, the Himalayas of Asia, and the Andes of South America, and they cover most of Greenland and Antarctica. Geologists distinguish between two main categories: mountain glaciers and continental glaciers. 

A great variety of glaciers form in mountainous areas.

Mountain glaciers (also called alpine glaciers) exist in or adjacent to mountainous regions (figure above a). Topographical features of the mountains control their shape; overall, mountain glaciers flow from higher elevations to lower elevations. Mountain glaciers include cirque glaciers, which fill bowl-shaped depressions, or cirques, on the flank of a mountain; valley glaciers, rivers of ice that flow down valleys; mountain ice caps, mounds of ice that submerge peaks and ridges at the crest of a mountain range; and piedmont glaciers, fans or lobes of ice that form where a valley glacier emerges from a valley and spreads out into the adjacent plain (figure above b–d). Mountain glaciers range in size from a few hundred meters to a few hundred kilometres long.

Antarctica is an ice-covered continent.

Continental glaciers are vast ice sheets that spread over thousands of square kilometres of continental crust. Continental glaciers now exist only on Antarctica and Greenland (figure above a, b). Antarctica is a continent, so the ice beneath the South Pole rests mostly on solid ground. Locally, however, lakes of liquid water exist at the base of continental glaciers. In 2012, Russian geologists drilled into one of these lakes, Lake Vostock, 3.7 km below the surface of the Antarctic ice sheet. Continental glaciers flow outward from their thickest point (up to 3.5 km thick) and thin toward their margins, where they may be only a few hundred meters thick. 

Geologists also find it valuable to distinguish between types of glaciers on the basis of the thermal conditions in which the glaciers exist. Temperate glaciers occur in regions where atmospheric temperatures become warm enough for the glacial ice to be at or near its melting temperature during part or all of the year. Polar glaciers occur in regions where atmospheric temperatures stay so cold all year long that the glacial ice remains below melting temperature throughout the year. Of note, Earth is not alone in hosting polar glaciers Mars has them too (read below). 

Polar Ice Caps on Mars

The ice caps of Mars.

Mars has white polar ice caps that change in area with the season, suggesting that they partially melt and then refreeze (figure above a, b). The question of what the ice caps consist of remained a puzzle until fairly recently. It now appears that the Martian ice caps consist mostly of water (H2O) ice mixed with a small amount of dust. The ice caps attain a maximum thickness of 3 km. During the winter, atmospheric carbon dioxide freezes and covers the north polar cap with a 1-m-thick layer of frozen CO2 (dry ice). During the summer, this layer melts away. The south polar cap has a dry-ice blanket that is 8 m thick and doesn't melt away entirely in the summer. The difference between the north and south poles may reflect elevation, for the south pole is 6 km higher and therefore remains colder. 

High-resolution photographs reveal that distinctive canyons, up to 10 km wide and 1 km deep, spiral outward from the centre of the north polar ice cap. Why did this  pattern form? Recent calculations suggest that if the ice sublimates (transforms into gas) on the sunny side of a crack and refreezes on the shady side, the crack will migrate sideways over time. If the cracks migrate more slowly closer to the pole, where it’s colder, than they do farther away, they will naturally evolve into spirals.

The Movement of Glacial Ice 

How do glaciers move? Let’s consider the two mechanisms that allow glaciers to move plastic deformation and basal sliding. At conditions found below depths of about 60 m in a glacier, ice deforms by plastic deformation, meaning that the grains within it change shape very slowly, and new grains grow while old ones disappear. We can picture such changes to be a consequence of the rearrangement of water molecules within ice grains. If ice is warm enough for thin water films to form along grain boundaries, plastic deformation may also involve the microscopic slip of ice grains past their neighbours along the water films. In cases where significant quantities of meltwater accumulate at the base of a glacier, forming a layer either of liquid or of slurry like wet sediment, glaciers can move by basal sliding. During this process, the liquid water or water-saturated slurry layer holds the glacial ice above bedrock and thereby decreases friction; effectively, the glacier glides along on a wet cushion. 

Crevasses form in the upper layer of a glacier, in which the ice is brittle. Commonly, cracking takes place where the glacier bends while flowing over steps or ridges in its substrate.

As we noted earlier, plastic deformation takes place only at depths of greater than about 60 m in a glacier above this depth, known as the brittle–plastic transition, ice is too brittle to flow. As a glacier overall undergoes movement, its upper 60 m of ice deforms predominantly by cracking. A crack that develops by brittle deformation of a glacier is called a  crevasse (figure above). In large glaciers, crevasses can be hundreds of meters long, and they can open up to form open gashes up to 15 m across. 

Forces that drive the movement of glaciers.

Why do glaciers move? Ultimately, because the pull of gravity is strong enough to make ice flow (figure above a, b). A glacier flows in the direction in which its top surface slopes. Thus, valley glaciers flow down their valleys, and continental ice sheets spread outward from their thickest point. To picture the movement of a continental ice sheet, imagine pouring honey on a tabletop. The honey spreads out until the puddle reaches an even thickness. In the case of a continental ice sheet, a thick pile of ice builds up, and gravity causes the top of the pile to push down on the ice at the base. Eventually, the basal ice can no longer support the weight of the overlying ice and begins to deform plastically. When this happens, the basal ice starts squeezing out to the side, carrying the overlying ice with it. The greater the volume of ice that builds up, the wider the ice sheet can become. 

Flow velocities vary with location in a glacier. Overall, ice flows from the zone of accumulation to the zone of  toe.

Glaciers generally flow at rates of between 10 and 300 m per year. Not all parts of a glacier move at the same rate. For example, friction between rock and ice slows a glacier, so the centre of a valley glacier moves faster than its margins, and the top of a glacier moves faster than its base (figure above a, b). If water builds up beneath a valley glacier to the point where it lifts the glacier off its substrate, basal sliding starts and the glacier undergoes a glacial surge. During surges, glaciers have been clocked at speeds of 10 to 110 m per day! Sudden surges may generate ice quakes, whose seismic vibrations travel through the glacier and through the rock below.

Glacial Advance and Retreat 

Glaciers resemble bank accounts: snowfall accumulates and adds to the account, while ablation the removal of ice by sublimation (the evaporation of ice into water vapour), melting (the transformation of ice into liquid water, which flows away), and calving (the breaking off of chunks of ice) subtracts from the account. Snowfall adds to the glacier in the zone of accumulation, whereas ablation subtracts in the zone of ablation; the boundary between these two zones is the equilibrium line. 

Glacial advancement and retreat.

The leading edge or margin of a glacier is called its toe, or terminus (figure above a). If the rate at which ice builds up in the zone of accumulation exceeds the rate at which ablation occurs below the equilibrium line, then the toe moves forward into previously unglaciated regions. Such a change is called a glacial advance (figure above b). In mountain glaciers, the position of a toe moves downslope during an advance, and in continental glaciers, the toe moves outward, away from the glacier’s origin. If the rate of ablation below the equilibrium line equals the rate of accumulation, then the position of the toe remains fixed. But if the rate of ablation exceeds the rate of accumulation, then the position of the toe moves back toward the origin of the glacier; such a change is called a glacial retreat (figure above c). During a mountain glacier’s retreat, the position of the toe moves upslope. It’s important to realize that when a glacier retreats, it’s only the position of the toe that moves back toward the origin, for ice continues to flow toward the toe. Glacial ice cannot flow back toward the glacier’s origin. 

One final point before we leave the subject of glacial flow: beneath the zone of accumulation, a volume of ice gradually moves down toward the base of the glacier as new ice accumulates above it, whereas beneath the zone of ablation, a volume of ice gradually moves up toward the surface of the glacier, as overlying ice ablates. Thus, as a glacier flows, ice volumes follow curved trajectories (figure above a–c). For this reason, rocks picked up by ice at the base of the glacier may slowly move to the surface. 

Ice in the Sea 

On the moonless night of April 14, 1912, the great ocean liner Titanic struck a large iceberg in the frigid North Atlantic. Lookouts had seen the ghostly mass of frozen water only minutes earlier and had alerted the ship’s pilot, but the ship had been unable to turn fast enough to avoid disaster. The force of the blow split the steel hull, allowing water to gush in. Less than 3 hours later, the ship disappeared beneath the surface, and 1,500 people perished. 

Ice shelves, tidewater glaciers, and sea ice, the nature of coastal areas in glacial regions.

Where do icebergs, such as the one responsible for the Titanic’s demise, originate? In high latitudes, mountain glaciers and continental ice sheets flow down to the sea, and they either stop at the shore or flow into the sea. Glaciers whose terminus lies in the water are called tidewater glaciers. Valley glaciers may protrude farther into the ocean to become ice tongues. Continental glaciers entering the sea become broad, flat sheets known as ice shelves. In shallow water, glacial ice remains grounded (figure above a). But where the water becomes deep enough, the ice floats with four-fifths of the ice below the water’s surface. At the boundary between glacier and ocean, blocks of ice calve off and tumble into the water with an impressive splash. If a free-floating chunk rises 6 m above the water and is at least 15 m long, it is formally called an iceberg. Since four-fifths of the ice lies below the surface of the sea, the base of a large iceberg may actually be a few hundred meters below the surface (figure above b, c). 

Not all ice floating in the sea originates as  glaciers on land. In polar climates, the surface of the sea itself freezes, forming sea ice (figure above d). The north polar ice cap of the Earth consists of sea ice, formed on the surface of the Arctic Ocean. Some sea ice, such as that covering the interior of the Arctic Ocean, floats freely; but some protrudes outward from the shore. Vast areas of ice shelves and of sea ice have been disintegrating in recent years. For example, ice-free openings develop in the Arctic Ocean sea ice during the summers, and the area of the ice shelf in Antarctica has been decreasing rapidly. 

Credits: Stephen Marshak (Essentials of Geology)

Vanishing Rivers

Vanishing Rivers

kacimo samy   9:21 ص

Vanishing Rivers 

As Homo sapiens evolved from hunter-gatherers into farmers, areas along rivers became attractive places to settle. Rivers serve as avenues for transportation and are sources of food, irrigation water, drinking water, power, recreation, and (unfortunately) waste disposal. Further, their floodplains provide particularly fertile soil for fields, replenished annually by seasonal floods. Considering the multitudinous resources that rivers provide, it’s no coincidence that ancient cultures developed in river valleys and on floodplains. Nevertheless, over time, humans have increasingly tended to abuse or overuse the Earth’s rivers. Here we note four pressing environmental issues pertaining to rivers.

• Pollution: The capacity of some rivers to carry pollutants has long been exceeded, transforming them into deadly cesspools. Pollutants include raw sewage and storm drainage from urban areas, spilled oil, toxic chemicals from industrial sites, floating garbage, excess fertilizer, and animal waste. Some pollutants directly poison aquatic life, some feed algae blooms that strip water of its oxygen, and some settle out to be buried along with sediments. 
• Dam Construction: In 1950, there were about 5,000 large (over 15 m high) dams worldwide, but today there are over 38,000. Damming rivers has both positive and negative results. Reservoirs provide irrigation water and hydroelectric power, and they trap some floodwaters and create popular recreation areas. But in some locations their construction destroys “wild rivers” (the whitewater streams of hilly and mountainous areas) and alters the ecosystem of a drainage network by forming barriers to migrating fish, by decreasing the nutrient supply to organisms downstream, by removing the source of sediment for the delta, and by eliminating seasonal floods that replenish nutrients in the landscape.

The Central Arizona Project canal shunts water from the Colorado River to Phoenix.

• Overuse of Water: Because of growing populations, our thirst for river water continues to increase, but the supply of water does not. The use of water has grown especially in response to the “green revolution” of the 1960s, during which huge new tracts of land came under irrigation. Today, 65% of the water taken out of rivers is used for agriculture, 25% for industry, and 9% for drinking.  As a result, in some places human activity consumes the entire volume of a river’s water, so that the channel contains little more than a saline trickle, if that, at its mouth. For example, except during unusually wet years, the Colorado River’s channel contains almost no water where it crosses the Mexican border, for huge pipes and canals carry the water instead to Phoenix and Los Angeles (figure above). 
• Effects of Urbanization and Agriculture on Streams: When it rains in a naturally vegetated region, or in an agricultural region, much of the water that falls from the sky either soaks into the ground or gets absorbed by plants. Some of the soil moisture or groundwater eventually seeps into a nearby stream, but the remainder flows elsewhere underground. As a result, the amount of water that reaches nearby streams after a storm is less than the total amount of precipitation, and a significant lag occurs between the time when the water falls and when the stream’s discharge increases. Urbanization changes both the volume of water reaching the stream and the length of the time lag, because when developers transform fields and forests into parking lots, roads, and buildings, a layer of impermeable concrete and asphalt prevents rainfall from infiltrating, and the amount of living biomass is smaller. Storm sewers and streets divert water directly to streams, so not only does the volume of water entering the streams increase, but also the rate at which the volume changes increases. 
• Although we tend to think of farmland as “vegetated land,” it actually has less plant cover than does natural grassland or forest. That’s because the land surface between the crop rows remains bare during the growing season, and after harvest, entire fields become a broad expanse of exposed soil. Sheetwash flowing across the unprotected land surface erodes and carries with it significant volumes of sediment. Thus, a river’s sediment load increases significantly when farms replace  forests nearby.

Credits: Stephen Marshak (Essentials of Geology)

Energy Choices, Energy Problems

Energy Choices, Energy Problems

kacimo samy   7:23 ص

Energy Choices, Energy Problems 

The Age of Oil and the Oil Crunch 

World energy use, cost and reserves.

Energy usage in industrialized countries grew with dizzying speed through the mid-20th century, and during this time people came to rely increasingly on oil. Eventually, oil supplies within the borders of industrialized countries could no longer match the demand, and these countries began to import more oil than they produced themselves. Through the 1960s, oil prices remained low (about $1.80 a barrel), so this was not a problem. In 1973, however, a complex tangle of politics and war led the Organization of Petroleum Exporting Countries (OPEC) to limit its oil exports. In the United States, fear of an oil shortage turned to panic, and motorists began lining up at gas stations, in many cases waiting for hours to fill their tanks. The price of oil rose to $18 a barrel, and newspaper headlines proclaimed, “Energy Crisis!” Governments in industrialized countries instituted new rules to encourage oil conservation. During the last two decades of the twentieth century, the oil market stabilized. Since 2004, oil prices rose overall, passing the $147/bbl mark in 2008; but the price collapsed in late 2008 when the Great  Recession hit. More recently, the price has hovered around $100/bbl (figure above a). Will a day come when shortages arise not because of politics or limitations on refining capacity, but because there is no more oil to produce? As highly populous countries such as China and India industrialize, the use of fuels accelerates. To understand the issues involved in predicting the future of energy supplies, we must first classify energy resources. As noted earlier, we call a particular resource renewable if nature can replace it within a short time relative to a human life span (in months or, at most, decades). A resource is non-renewable if nature takes a very long time (hundreds to perhaps millions of years) to replenish it. Oil is a non-renewable resource, in that the rate at which humans consume it far exceeds the rate at which nature replenishes it, so we will inevitably run out of oil. The question is, when?

Historians in the future may refer to our time as the Oil Age because so much of our economy depends on oil. How long will the Oil Age last? A reliable answer to this question is hard to come by, because there is not total agreement on the numbers that go into the calculation, especially as the use of unconventional reserves increases, so estimates vary widely. Geologists estimate that we’ve already used a substantial proportion of our conventional reserves, but that there are still about 850 to 1,350 billion barrels of proven conventional oil reserves (figure above b), meaning reserves that have been documented and are still in the ground. Optimistically, there may be an additional 2,000 billion barrels of unproven conventional reserves, meaning oil that has not yet been found but might exist. Thus, the world possibly holds between 850 and 3,350 billion barrels of conventional oil. Presently, humanity guzzles oil at a rate of about 31 billion barrels per year. At this rate, conventional oil supplies will last until some time between 2050 and 2150, not too far in the future. 

Some geologists argue that the beginning of the end of the Oil Age has begun, because the rate of consumption now exceeds the rate of discovery and in many regions, the rate of production has already started to decrease. The peak of production for a given reserve is called Hubbert’s Peak, after the geologist who first emphasized that the production of reserves must decline because oil is a non-renewable resource. Hubbert’s Peak for the United States appears to have been passed in the 1970s. Some researchers argue that the global peak may occur between 2012 and 2014, but this number remains uncertain, and only time will tell. Conservation approaches, such as increasing the gas mileage of cars and increasing the amount of insulation in buildings, could stretch out supplies and make them last decades longer. 

Of course, the picture of oil reserves changes significantly if unconventional reserves are included in estimates. All told, perhaps 1.5 trillion barrels of oil may be trapped in tar sands, and 3 trillion barrels trapped in oil shale. But wide disagreement remains concerning whether it’s fair to include all of these reserves, because a significant proportion would be so difficult and expensive to access that they may never really be an economical energy source.

Even in the most optimistic scenario, including 3,000 billion bbls of conventional oil and perhaps 3,000 billion bbls of accessible unconventional oil, at current rates of consumption, supplies can last for only another 200 years, so the Oil Age will last a total of about 350 years (figure above c). On a timeline representing the 4,000 years since the construction of the Egyptian pyramids, this looks like a very short blip. We may indeed be living during a unique interval of human history.

Can Other Fossil Fuels Replace Oil? 

As true limits to the oil supply approach, societies are looking first at relatively abundant supplies of other conventional fossil fuels, namely natural gas and coal, as sources of energy (figure above d). Rough estimates suggest that world natural gas reserves may exceed 180 trillion cubic meters, which would provide approximately the same amount of energy as 1.2 trillion barrels of oil. But tapping into this gas supply requires expensive technologies for extraction and transport. Similarly, worldwide coal reserves are estimated to be about 850 trillion tons, which contains approximately the same amount of energy as 11 trillion barrels of oil. But the stated number for coal reserves does not distinguish clearly between accessible (mineable) coal and inaccessible coal, which is too deep to mine. And, as is the case for oil, there are political and environmental consequences to relying on gas or coal.

Environmental Issues of Fossil Fuel Use 

Marine oil spills. These can come from drilling rigs, or from tankers.

Environmental concerns about energy resources begin right at the source. Oil drilling requires substantial equipment, the use of which can damage the land. And as demonstrated by the 2010 Gulf of Mexico offshore well blowout, oil drilling can lead to tragic loss of life and disastrous marine oil spills (Offshore Drilling and the Deepwater  Horizon Disaster). Oil spills from pipelines or trucks sink into the subsurface and contaminate groundwater, and oil spills from ships and tankers create slicks that spread over the sea surface and foul the shoreline (figure above a, b). Coal and uranium mining also scar the land and can lead to the production of acid mine runoff, a dilute solution of sulphuric acid that forms when sulphur-bearing minerals such as pyrite (FeS2) in mines react with rainwater. The runoff enters streams and kills fish and plants. Collapse of underground coal mines may cause the ground surface to sink. 

Numerous air-pollution issues also arise from the burning of fossil fuels, which sends soot, carbon monoxide, sulphur dioxide, nitrous oxide, and unburned hydrocarbons into the air. Coal, for example, commonly contains sulphur, primarily in the form of pyrite, which enters the air as sulphur dioxide (SO2) when coal is burned. This gas combines with rainwater to form dilute sulphuric acid (H2SO4), or acid rain. For this reason, many countries now regulate the amount of sulphur that coal can contain when it is burned. But even if pollutants can be decreased, burning fossil fuels still releases carbon dioxide (CO2) into the atmosphere. As we discuss in Chapter 19, CO2 is a greenhouse gas, so a change in the amount of CO2 in the atmosphere can affect climate. Because of concern about CO2 production, research efforts are under way to develop techniques to capture CO2 at power plants, liquefy it, and pump it into reservoir rocks deep underground. This process is called carbon sequestration. 

Offshore Drilling and the Deepwater  Horizon Disaster

The Deepwater Horizon.  

A substantial proportion of the world’s oil reserves reside in the sedimentary basins that underlie the continental shelves of passive continental margins. To access such reserves, oil companies must build offshore drilling platforms. In water less than 600 m (2,000 ft) deep, companies position fixed platforms on towers resting on the sea floor. In deeper water, semi-submersible platforms float on huge submerged pontoons (figure above a). With these, oil companies can now access fields lying beneath 3 km (10,000 ft) of water. 

North America’s largest offshore fields occur in the passive-margin basin that fringes the coast of the Gulf of Mexico. More than 3,500 platforms operate in the Gulf at present, together yielding up to 1.7 million bbl/day. 

During both onshore and offshore exploration, drillers worry about the possibility of a blowout. A blowout happens when the pressure within a hydrocarbon reserve penetrated by a well exceeds the pressure that drillers had planned for, causing the hydrocarbons (oil and/ or gas) to rush up the well in an uncontrolled manner and burst out of the well at the surface in an oil gusher or gas plume. Blowouts are rare because, although fluids below the ground are under great pressure due to the weight of overlying material, engineers fill the hole with drilling mud with a density greater than that of clear water. The weight of drilling mud can counter the pressure of underground hydrocarbons and hold the fluids underground. But if drillers encounter a bed in which pressures are unexpectedly high, or if they remove the mud before the walls of the well have been sealed with a casing (a pipe, cemented in place by concrete) a blowout may happen. 

A catastrophic blowout occurred on April 20, 2010, when drillers on the Deepwater Horizon, a huge semi-submersible platform leased by BP, were finishing a 5.5-km-long (18,000 ft) hole in 1.5-kmdeep (5,000 ft) water south of Louisiana. Due to a series of errors, the casing was not sufficiently strong when workers began to replace the drilling mud with clear water. Thus the high-pressure, gassy oil in the reservoir that the well had punctured rushed up the drillhole. A backup safety device called a blowout preventer failed, so the gassy oil reached the  platform and sprayed 100 m (328 ft) into the sky. Sparks from electronic gear triggered an explosion, and the platform became a fountain of flame and smoke that killed 11 workers. An armada of fireboats could not douse the conflagration (figure above b), and after 36 hours, the still-burning platform tipped over and sank. 

Robot submersibles sent to the sea floor to investigate found that the twisted mess of bent and ruptured pipes at the well head was billowing oil and gas. On the order of 50,000 to 62,000 bbl/day of hydrocarbons entered the Gulf’s water from the well. Stopping this underwater gusher proved to be an immense challenge, and initial efforts to block the well, or to put a containment dome over the well head, failed. It was not until July 15 that the flow was finally stopped, and not until September 19 that a new relief well intersected the blown well and provided a conduit to pump concrete down to block the original well permanently. All told, about 4.2 million bbl of hydrocarbons contaminated the Gulf from the Deepwater Horizon blowout. The spill was devastating to wetlands, wildlife, and the fishing and tourism industries.

Alternative Energy 

Can nuclear power or hydroelectric power replace oil? Vast supplies of uranium, the fuel of traditional nuclear plants, remain untapped. Further, nuclear engineers have designed alternative plants, powered by breeder reactors, that essentially produce new fuel. But many people view nuclear plants with concern because of issues pertaining to radiation, accidents, terrorism, and waste storage, and these concerns have slowed the industry. A substantial increase in hydroelectric power production is not likely, as most major rivers have already been dammed, and industrialized countries have little appetite for taming any more. Similarly, the growth of geothermal-energy output seems limited. 

Because of the potential problems that might result from relying more on coal, hydroelectric, and nuclear energy, researchers have been increasingly exploring clean energy options (see figure 1 d). One possibility is solar power, and the cost of solar power is steadily decreasing. Unfortunately, technologies for large-scale solar energy do not yet exist.  Similarly, we can turn to wind power for relatively small-scale energy production, but covering the landscape with windmills is not appealing, and since wind production varies with wind speed, so it can’t provide a steady supply that the present-day energy grid requires. Fusion power may be possible some day, but physicists and engineers have not yet figured out a way to harness it. 

Clearly, society will be facing difficult choices in the not-so-distant future about where to obtain energy, and we will need to invest in the research required to discover new alternatives. By 2050, 40% of energy may be from renewable sources. In the near term, conservation can play an important role by diminishing demand for fossil fuels. 

Credits: Stephen Marshak (Essentials of Geology)

Oil and Gas

Oil and Gas

kacimo samy   7:43 ص

Oil and Gas

What Are Oil and Gas? 

For reasons of economics and convenience, industrialized societies today rely primarily on oil (petroleum) and natural gas for their energy needs. Oil and natural gas, both fossil fuels, consist of hydrocarbons, chain-like or ring-like molecules made of carbon and hydrogen atoms. Chemists consider hydrocarbons to be a type of organic chemical.

Some hydrocarbons are gaseous and invisible, some resemble a watery liquid, some appear syrupy, and some are solid. The viscosity (ability to flow) and the volatility (ability to evaporate) of a hydrocarbon product depend on the size of its molecules. Hydrocarbon products composed of short chains of molecules tend to be less viscous (meaning they can flow more easily) and more volatile (meaning they evaporate more easily) than products composed of long chains, because the long chains tend to tangle up with each other. Thus, short-chain molecules occur in gaseous form (natural gas) at room temperature, moderate-length-chain molecules occur in liquid form (gasoline and oil), and long-chain molecules occur in solid form (tar).

Hydrocarbon Systems 

Oil and gas do not occur in all rocks at all locations. That’s why the goal of controlling oil fields, regions that contain significant amounts of accessible oil underground, has sparked bitter wars. A known supply of oil and gas held underground is a hydrocarbon reserve; if the reserve consists dominantly of oil, it is usually called an oil reserve and if it consists dominantly of gas, it’s a gas reserve. The development of a reserve requires a specific association of materials, conditions, and time. Geologists refer to this association as a hydrocarbon system. We’ll now look at the components of a hydrocarbon system, namely the source rock, the thermal conditions of oil formation, the migratory pathway, and the trap. 

Source Rocks and Hydrocarbon Generation 

News stories often incorrectly imply that oil and gas are derived from buried trees or the carcasses of dinosaurs. In fact, the hydrocarbon molecules of oil and gas are derived from organic chemicals, such as fatty molecules called lipids, that were once in plankton. Plankton is made up of very tiny floating organisms including single-celled and very small multicellular plants (algae) as well as protists and microscopic animals. Typically, most planktonic organisms range in size from 0.02 to 2.0 mm in diameter. When the organisms die, they sink to the floor of the lake or sea that they lived in, and if the water is relatively “quiet” (nonflowing), accumulate.

If the sea-floor or lake-floor environment is rich with oxygen, dead plankton may be eaten or oxidized and transformed into CO2 and CH4 gas, which bubbles away. But in  oxygen-poor waters, the organic material can survive long enough to mix with clay and form an organic-rich, muddy ooze, that can then become buried by still more sediment so that it becomes preserved. Eventually, pressure due to the weight of overlying sediment squeezes out the water, and the ooze becomes compacted and, eventually, lithified to become black, organic shale. (Shale that does not contain organic matter tends to be gray, tan, or red.) Organic shale contains the raw materials from which hydrocarbons form, so we refer to it as a source rock.

The formation of oil. The process begins when organic debris settles with sediment. As burial depth increases, heat and pressure transform the sediment into black shale in which organic matter becomes kerogen. At appropriate temperatures, kerogen becomes oil, which then seeps upward.

If organic shale becomes buried deeply enough (2 to 4 km), it gets warmer, since temperature increases with depth in the Earth. Chemical reactions take place in warm source rocks and slowly transform the organic material in the shale into a mass of waxy molecules called kerogen (figure above). Shale containing 15% to 30% kerogen is called oil shale. If oil shale warms to temperatures of greater than about 90C, kerogen molecules break into smaller oil and natural gas molecules, a process known as hydrocarbon generation. At temperatures over about 160C, any remaining oil breaks down to form natural gas; and at temperatures over 225–250C, organic matter loses all its hydrogen and transforms into graphite (pure carbon). Thus, oil itself forms only in a relatively narrow range of temperatures, called the oil window.

Reservoir Rocks and Hydrocarbon Migration 

The clay flakes that comprise most of an oil shale fit together very tightly and thus prevent kerogen, and any liquid or gaseous hydrocarbons forming within the kerogen, from moving through the rock. Therefore, you can’t simply drill a hole into a source rock and pump out oil the oil won’t flow into the well fast enough to make the process cost efficient. Instead, to obtain oil or gas, companies drill into reservoir rocks, rocks that contain, or could contain, accessible oil or gas, meaning oil or gas that can flow through rock and be sucked into a well fairly easily. 

To be a reservoir rock, a body of rock must have space in which the oil or gas can reside and must have channels through which the oil or gas can move. The space can be in the form of openings, or pores, between clastic grains (which exist because the grains didn't fit together tightly and because cement didn't fill all the spaces during cementation) or in the form of cracks and fractures that developed after the rock formed. In some cases, groundwater passing through rock dissolves minerals to produce new pore space. Porosity refers to the proportion of pore space in a rock. Not all rocks have the same porosity for example, shale has low porosity (10%), whereas poorly cemented sandstone has high porosity (35%). By saying that sandstone has a porosity of 35%, we mean that about a third of a block of the sandstone consists of open space. The oil or gas in a reservoir rock occurs in the pores, and thus is distributed through the rock it does not occur in open pools underground. Permeability refers to the degree to which pore spaces are connected to one another. In a rock with high permeability, there are many tiny channel ways linking pores, and/or many interconnected cracks cutting through the rock, so that fluids are not trapped in pores but rather can flow  through the rock. The greater the porosity, the greater the capacity of a reservoir rock to hold oil; and the greater the rock’s permeability, the easier it is for the oil to be extracted. 

Initially, oil resides in the source rock. Because it is buoyant relative to groundwater, the oil migrates into the overlying reservoir rock. The oil accumulates beneath a seal rock in a trap.

To fill the pores of a reservoir rock, oil and gas must first migrate (move) from the source rock into a reservoir rock, a process that can take thousands to millions of years to happen (figure above). Why do hydrocarbons migrate? Oil and gas are less dense than water, so they try to rise toward the Earth’s surface to get above groundwater, just as salad oil rises above the vinegar in a bottle of salad dressing. Natural gas, being less dense, ends up rising above oil. In other words, buoyancy drives oil and gas upward. Typically, a hydrocarbon system must have a good migration pathway, such as a set of permeable fractures, in order for large volumes of hydrocarbons to move.

Traps and Seals 

If oil or gas escapes from the reservoir rock and ultimately reaches the Earth’s surface, where it leaks away at an oil seep, there will be none left underground to extract. Thus, for an oil reserve to exist, oil and gas must be held underground in the reservoir rock by means of a geologic configuration called a trap.

There are two components to an oil or gas trap. First, a seal rock, a relatively impermeable rock such as shale, salt, or unfractured limestone, must lie above the reservoir rock and stop the hydrocarbons from rising further. Second, the seal and reservoir rock bodies must be arranged in a geometry that localizes the hydrocarbons in a restricted area. Geologists recognize several types of hydrocarbon trap geometries, four of which are described below.

Types of Oil and Gas Traps

Geologists who work for oil companies spend much of their time trying to identify underground traps. No two traps are exactly alike, but we can classify most into the following four categories.

Examples of oil traps.  A trap is a configuration of a seal rock over a reservoir rock, in a geometry that keeps the oil underground.

• Anticline trap: In some places, sedimentary beds are not horizontal, as they are when originally deposited, but have been bent by the forces involved in mountain building. These bends, as we have seen, are called folds. An anticline is a type of fold with an arch-like shape (figure above a). If the layers in the anticline include a source rock overlain in turn by a reservoir rock that is overlain by a seal rock, then we have the recipe for an oil reserve. The oil and gas rise from the source rock, enter the reservoir rock, and rise to the crest of the anticline, where they are trapped by a seal.
• Fault trap: A fault is a fracture on which there has been sliding. If the slip on the fault crushes and grinds the adjacent rock to make an impermeable layer along the fault, then oil and gas may migrate upward along bedding in the reservoir rock until they stop at the fault surface (figure above b). Alternatively, a fault trap develops if the slip on the fault juxtaposes an impermeable rock layer against the reservoir rock. 
• Salt-dome trap: In some sedimentary basins, the sequence of strata contains a thick layer of salt, deposited when the basin was first formed and seawater covering the basin was shallow and very salty. Sandstone, shale, and limestone overlie the salt. The salt layer is not as dense as sandstone or shale, so it is buoyant and tends to rise up slowly through the overlying strata. Once the salt starts to rise, the weight of surrounding strata squeezes the salt out of the layer and up into a growing, bulbous salt dome. As the dome rises, it bends up the adjacent layers of sedimentary rock. Oil and gas in reservoir rock layers migrate upward until they are trapped against the boundary of the salt dome, for salt is not permeable (figure above c). 
• Stratigraphic trap: In a stratigraphic trap, a tilted reservoir rock bed “pinches out” (thins and disappears) up its dip between two impermeable layers. Oil and gas migrating upward along the bed accumulate at the pinchout (figure above d).

Credits: Stephen Marshak (Essentials of Geology)

Stratigraphic Formations and Their Correlation

Stratigraphic Formations and Their Correlation

kacimo samy   6:40 ص

Stratigraphic Formations and Their Correlation

We can summarize information about the sequence of sedimentary strata at a location by drawing a stratigraphic column. Typically, we draw columns to scale, so that the relative thicknesses of layers portrayed on the column reflect the thicknesses of layers in the outcrop. Then, we divide the sequence of strata represented on a column into stratigraphic formations (“formations,” for short), a sequence of beds of a specific rock type or group of rock types that can be traced over a fairly broad region. The boundary surface between two formations is a type of geologic contact. (Fault surfaces and the boundary between an igneous intrusion and its wall-rock are also types of contacts.) Typically, a formation has a specific geologic age.

The stratigraphic formations and stratigraphic column for the Grand Canyon in Arizona.

Lets see how the concept of a stratigraphic formation applies to the Grand Canyon. The walls of the canyon look striped, because they expose a variety of rock types that differ in color and in resistance to erosion. Geologists identify major contrasts distinguishing one interval of strata from another, and use them as a basis for dividing the strata into formations, each of which may consist of many beds (figure above a–c). Some formations include a single rock type, whereas others include interlayered beds of two or more rock types. Not all formations have the same thickness, and the thickness of a single formation can vary with location. Commonly, geologists name a formation after a locality where it was first identified or first studied. For example, the “Schoharie Formation” was first defined based on exposures in Schoharie Creek, in New York.

If a formation consists of only one rock type, we may incorporate that rock type in the name (for example, Kaibab Limestone), but if a formation contains more than one rock type, we use the word “formation” in the name (such as Toroweap Formation). Note that in the formal name of a formation, all words are capitalized. Several adjacent formations in a succession may be lumped together as a stratigraphic group. 

Where did the concept of a stratigraphic formation come from? While excavating canals in England, William Smith discovered that formations cropping out at one locality resembled formations cropping out at another, in that their beds looked similar and contained similar fossil assemblages. In other words, Smith was able to define the stratigraphic  relationship between the strata at one locality and the strata at another, a process now called correlation.

The principles of correlation.

How does correlation work? Typically, geologists correlate formations between nearby regions based on similarities in rock type. We call this method lithologic correlation (figure above a, b). For example, the sequence of strata on the southern rim of the Grand Canyon clearly correlates with the sequence on the northern rim, because they contain the same rock types in the same order. In some cases, a sequence contains a key bed, or marker bed, which is a particularly unique layer that provides a definitive basis for correlation. 

To correlate rock units over broad areas, we must rely on fossils to define the relative ages of sedimentary units. We call this method fossil correlation. Geologists use fossil correlation for studies of broad areas because sources of sediments and depositional environments may change from one location to another. The beds deposited at one location during a given time interval may look quite different from the beds deposited at another location during the same time interval. But if fossils of the same relative age occur at both locations, we can say that the strata at the two locations correlate. (Note that the fossils are not necessarily of the same species they won’t be if the depositional environments are different but they are of the same age.) Fossil correlation may also come in handy when rock types are not distinctive enough to allow correlation. For example, imagine that the Santuit Sandstone and Oswaldo Sandstone of figure above a look the same. Only fossils may distinguish one layer from the other, if the intervening Milo Limestone is absent.

A geologic map depicts the distribution of rock units and structures.

Once William Smith succeeded in correlating stratigraphic formations throughout central England, he faced the challenge of communicating his ideas to others. One way would be to create a table that compared stratigraphic columns from different locations. But since Smith was a surveyor, and worked with maps, it occurred to him that he could outline and colour in areas on a map to represent areas in which strata of a given relative age occurred. He did this using the data he had collected, and in 1815, produced the first modern geologic map. In general, a geologic map portrays the spatial distribution of rock units at the Earth’s surface. Significantly, the pattern displayed on a geologic map provides insight into the presence and orientation of geologic structures in the map area (figure above a, b). The inside of this book’s back cover provides a geologic map of North America.

Credits: Stephen Marshak (Essentials of Geology)