The Cenozoic Era: The Modern World Comes to Be

The Cenozoic Era: The Modern World Comes to Be

kacimo samy   4:58 ص

The Cenozoic Era

The Cenozoic Era is focused which we know today of the world, the modern world.

Paleogeography

 The two main active continental orogenic systems on the Earth today. The Alpine-Himalayan system formed when Africa, India, and Australia collided with Asia (inset). The Cordilleran and Andean systems reflect the consequences of convergent-boundary tectonism along the eastern Pacific Ocean.

During the last 65 million years, the map of the Earth has continued to change, gradually producing the configuration of continents and plate boundaries we see today. The final stages of the Pangaea breakup separated Australia from Antarctica and Greenland from North America, and formed the North Sea between Britain and continental Europe. The Atlantic Ocean continued to grow because of sea-floor spreading on the Mid- Atlantic Ridge, and thus the Americas have moved westward, away from Europe and Africa. Meanwhile, the continents that once constituted Gondwana drifted northward as the intervening Tethys Ocean was consumed by subduction. Collisions of the former Gondwana continents with the southern margins of Europe and Asia resulted in the formation of the largest orogenic belt on Earth today, the Alpine-Himalayan chain (figure above). India and a series of intervening volcanic island arcs and micro-continents  collided with Asia to form the Himalayas and the Tibetan Plateau to the north, while Africa along with some volcanic island arcs and micro-continents collided with Europe to  produce the Alps. 

As the Americas moved westward, convergent plate boundaries evolved along their western margins. In South America, convergent-boundary activity built the Andes, which remains an active orogen to the present day. In North America, convergent-boundary activity continued without interruption until about 40 Ma (the Eocene Epoch), yielding, as we have seen, the Laramide orogeny. Then, because of the rearrangement of plates off the western shore of North America, a transform boundary replaced the convergent boundary in the western part of the continent by 25 Ma. When this happened, volcanism and compression ceased in western North America, the San Andreas Fault system formed along the coast of the United States, and the Queen Charlotte Fault system developed off the coast of Canada. Along the San Andreas and Queen Charlotte faults today, the Pacific Plate moves northward with respect to North America at a rate of about 6 cm per year. In the western United States, convergent-boundary tectonics continues only in Washington, Oregon, and northern California, where subduction of the Juan de Fuca Plate generates the volcanism of the Cascade volcanic chain.

The Basin and Range Province is a rift. The inset shows a cross section along the red line.

As convergent tectonics ceased in the western United States south of the Cascades, the region began to undergo rifting (extension) in roughly an east-west direction. The result was the formation of the Basin and Range Province, a broad continental rift whose development has stretched the region to twice its original width (figure above). The Basin and Range gained its name from its topography the province contains long, narrow mountain ranges separated from each other by flat, sediment filled basins. This topography formed when the crust of the region was broken up by normal faults. Blocks of crust above these faults slipped down and tilted. Crests of the tilted blocks form the ranges, and the depressions between them, which rapidly filled with sediment eroded from the ranges, became basins. 

The Basin and Range Province terminates just north of the Snake River Plain, the track of the hot spot that now lies beneath Yellowstone National Park. As North America drifts westward, volcanic calderas formed along the Snake River Plain; Yellowstone National Park straddles the most recent caldera.

Recall that in the Cretaceous Period, the world was relatively warm and sea level rose so that extensive areas of continents were submerged. During the Cenozoic Era, however, the global climate rapidly became cooler, and by the early Oligocene Epoch (34 Ma), Antarctic glaciers reappeared for the first time since the Triassic. The climate continued to grow colder through the Late Miocene Epoch, leading to the formation of grasslands in temperate climates. About 2.5 Ma, the Isthmus of Panama formed, separating the Atlantic completely from the Pacific, changing the configuration of oceanic currents, perhaps leading the Arctic Ocean to freeze over. 

The maximum advance of the Pleistocene ice sheet in North America.

During the overall cold climate of the past 2 million years, continental glaciers have expanded and retreated across northern continents at least 20 times, resulting in the  Pleistocene Ice Age (figure above). Each time the glaciers grew, sea level fell so much that the continental shelf became exposed to air. At times, a land bridge formed across the Bering Strait, west of Alaska, providing migration routes for animals and people from Asia into North America. A partial land bridge also formed from southeast Asia to Australia, making human migration to Australia easier. Erosion and deposition by the glaciers created much of the landscape we see today in northern temperate regions. About 11,000 years ago, the climate warmed, and we entered the interglacial time interval we are still experiencing today.

The present-day Bahamas serve as an example of what the interior of the United States might have looked like during intervals of the Paleozoic. Shallow land areas were submerged and became the site of shallow-marine sedimentation.

Life evolution

When the skies finally cleared in the wake of the K-T boundary catastrophe, plant life recovered, and soon forests of both angiosperms and gymnosperms grew. The grasses, which first appeared in the Cretaceous, spread across the plains in temperate and subtropical climates by the middle of the Cenozoic Era, transforming them into vast grasslands. The dinosaurs, except for their descendants the birds, were gone for good. Mammals rapidly diversified into a variety of forms to take their place. In fact, most of the modern groups of mammals that exist today originated at the beginning of the Cenozoic Era, giving this time the nickname Age of Mammals. During the latter part of the era, huge mammals appeared (such as mammoths, giant beavers, giant bears, and giant sloths), but these became extinct during the past 10,000 years, probably because of hunting by humans.

It was during the Cenozoic that our own ancestors first appeared. Ape-like primates diversified in the Miocene Epoch (about 20 Ma), and the first human-like primate appeared at about 4 Ma, followed by the first members of the human genus, Homo, at about 2.4 Ma. Fossil evidence, primarily from Africa, indicates that Homo erectus, capable of making stone axes, appeared about 1.6 Ma, and the line leading to Homo sapiens (our species) diverged from Homo neanderthalensis (Neanderthal man) about 500,000 years ago. According to the fossil record, modern people appeared about 200,000 years ago, and initially shared the planet with two other species of the genus Homo, the Neanderthals and the Denisovans. The last Neanderthals and Denisovans died off over 25,000 years ago, leaving Homo sapiens as the only human species on Earth. 

Earth’s history reflects the complex consequences of plate  interactions, sea-level changes,  atmospheric changes, life evolution, and even meteorite impact. In the past few millennia, humans have had a huge effect on the planet, causing changes significant enough to be obvious in the geologic record of the future.
Credits: Stephen Marshak (Essentials of Geology)

The Mesozoic Era: When Dinosaurs Ruled

The Mesozoic Era: When Dinosaurs Ruled

kacimo samy   5:25 ص

The Mesozoic Era

The Mesozoic Era where Dinosaurs were dominant spanned from 251-65 Ma.

The Early and Middle Mesozoic Era  (Triassic–Jurassic Periods, 251–145 Ma) 

Paleogeography

Pangaea began to break up in the Triassic, and by Jurassic time, a narrow North Atlantic Ocean existed.

Pangaea, the super-continent formed at the end of the Paleozoic Era, existed for about 100 million years, until rifting commenced during the Late Triassic and Early Jurassic Periods and the super-continent began to break up. By the end of the Jurassic Period, rifting had succeeded in splitting North America from Europe and Africa. The Mid-Atlantic Ridge formed, and the North Atlantic Ocean started to grow (figure above). According to the record of sedimentary rocks, Earth overall had a warm climate during the Triassic and Early Jurassic. But during the Late Jurassic and Early Cretaceous, the climate cooled. Pangaea’s interior remained a non-marine environment in which red sandstone and shale, now exposed in the spectacular cliffs of Zion National Park, were deposited. By the Middle Jurassic Period, sea level began to rise, and a shallow sea submerged much of the Rocky Mountain region. On the western margin of North America, convergent margin tectonics became the order of the day. Beginning with Late Permian and continuing through Mesozoic time, subduction generated volcanic island arcs and caused them, along with micro-continents and hot-spot volcanoes, to collide with North America. Thus, North America grew in land area by the accretion of crustal fragments on its western margin. Because these fragments consist of crust that formed elsewhere, not originally on or adjacent to the continent, geologists call them exotic terranes. From the end of the Jurassic through the Cretaceous Period, a major continental volcanic arc, the Sierran arc, grew on the western margin of North America itself; you’ll learn more about this arc later.

Life evolution

During the Jurassic, giant dinosaurs roamed the land. This painting shows several species.

During the early Mesozoic Era, a variety of new plant and animal species appeared, filling the ecological niches left vacant by the Late Permian mass extinction. Reptiles swam in the oceans, and new kinds of corals became the predominant reef builders. On land, gymnosperms and reptiles diversified, and the Earth saw its first turtles and flying reptiles. And at the end of the Triassic Period, the first true dinosaurs evolved. Dinosaurs differed from other reptiles in that their legs were positioned under their bodies rather than off to the sides, and they were possibly warm blooded. By the end of the Jurassic Period, gigantic sauropod dinosaurs (weighing up to 100 tons), along with other familiar examples such as stegosaurus, thundered across the landscape, and the first feathered birds, such as Archaepteryx, took to the skies (figure above). The earliest ancestors of mammals appeared at the end of the Triassic Period, in the form of small, rat-like creatures.

The Late Mesozoic Era  (Cretaceous Period, 145–65 Ma) 

Paleogeography

During the Late Cretaceous, a continental volcanic arc formed. A fold thrust belt formed to the east, as did
a transcontinental seaway.

During the Cretaceous Period, the Earth’s climate continued to shift to warmer conditions, and sea level rose significantly, reaching levels that had not been attained  for the previous 200 million years. Great seaways flooded most of the continents (figure above). In fact, during the latter part of the Cretaceous Period, a shark could have swum from the Gulf of Mexico to the Arctic Ocean, or across much of western Europe. 

Paleogoegraphy is Late Cretaceous through Eocene time.

The breakup of Pangaea continued through the Cretaceous Period, with the opening of the South Atlantic Ocean and the separation of South America and Africa from Antarctica and Australia. India broke away from Gondwana and headed rapidly northward toward Asia (figure above a, b). Along the continental margins of the newly formed Mesozoic oceans, passive-margin basins developed that filled with great thicknesses of sediments. In western North America, the Sierran arc, a large continental volcanic arc that initiated at the end of the Jurassic Period, continued to be active. This arc resembled the present day Andean arc of western South America. Though the volcanoes of the Sierran arc have long since eroded away, we can see their roots in the form of the plutons that now constitute the granitic batholith of the Sierra Nevada Mountains. Compressional stresses along the western North American convergent boundary activated large thrust faults east of the arc, an event geologists refer to as the Sevier orogeny. This orogeny produced a fold-thrust belt whose remnants you can see today in the Canadian Rockies and in western Wyoming (figure above c). At the end of the Cretaceous Period, continued compression along the convergent boundary of western North America caused slip on large faults in the region of Wyoming, Colorado, eastern Utah, and northern Arizona. In contrast to the faults of fold-thrust belts, these faults penetrated deep into the Precambrian rocks of the continent, and thus movement on them generated basement uplifts (figure above d). Overlying layers of Paleozoic strata warped into large monoclines, folds whose shape resembles the drape of a carpet over a step. This event, which geologists call the Laramide orogeny, formed the structure of the present Rocky Mountains in the United States. Geologists have determined that sea-floor spreading rates may have been as much as three times faster during the Cretaceous than they are today. As a result, more of the oceanic crust was younger and warmer than it is today, and since young sea floor lies at a shallower depth than does older sea floor (due to isostasy; see Interlude D), Cretaceous mid-ocean ridges occupied more volume than they do today. The extra volume of the ridges displaced sea water, causing sea level to rise. Also during the Cretaceous, huge submarine plateaus formed from basalts erupted at hot-spot volcanoes. The existence of these plateaus implies that particularly active mantle plumes, or super plumes, reached the base of the lithosphere. Growth of submarine plateaus displaced sea water and thus also contributed to sea-level rise. Volcanism associated with extra-rapid sea-floor spreading, as well as with submarine plateau growth, likely released CO2 into the atmosphere. Geologists hypothesise that this increased atmospheric CO2 concentration led to a global rise in atmospheric temperature. Rising temperatures would cause sea water to expand and polar ice sheets to melt, both phenomena that would make sea level go up even more. Considering all the phenomena that caused sea level to rise during the Cretaceous, it’s no surprise that the continents flooded and that large epicontinental seas formed during this era.

Life evolution

In the seas of the late Mesozoic world, modern fish appeared and became dominant. In contrast with earlier fish, the new fish had short jaws, rounded scales, symmetrical tails, and specialised fins. Huge swimming reptiles and gigantic turtles (with shells up to 4 m across) preyed on the fish. On land, cycads largely vanished, and angiosperms (flowering plants), including hardwood trees, began to compete successfully with conifers for dominance of the forest. Dinosaurs reached their peak of success at this time, inhabiting almost all environments on Earth. Social herds of grazing dinosaurs roamed the plains, preyed on by the fearsome Tyrannosaurus rex (a Cretaceous, not a Jurassic, dinosaur, despite what Hollywood says). Pterosaurs, with wingspans of up to 11 m, soared overhead, and birds began to diversify. Mammals also diversified and developed larger brains and more specialised teeth, but for the most part, they remained small and rat-like.

The “K-T boundary event” 

The Cretaceous tertiary impact. The aftermath probably caused extinction of the dinosaurs and other species.

Geologists first recognised the K-T boundary (K stands for Cretaceous and T for Tertiary) from 18th-century studies that identified an abrupt global change in fossil assemblages. Until the 1980s, most geologists assumed the faunal turnover took millions of years. But modern dating techniques indicate that this change happened almost instantaneously and that it represents a sudden mass extinction of most species on Earth. The dinosaurs, which had ruled the planet for over 150 million years, simply vanished, along with 90% of plankton species in the ocean and up to 75% of plant  species. What kind of catastrophe could cause such a sudden and extensive mass extinction? The cause of the K-T mass extinction remained a mystery until the late 1970s, when Walter Alvarez, an American geologist, and his colleagues examined a shale layer deposited exactly at the K-T boundary. They found that this shale contained relatively high concentrations of iridium, an element that comes primarily from meteorites. Further study showed that the clays of this age contained other unusual materials, such as tiny glass spheres formed when a spray of molten rock freezes, grains of coesite (a mineral that forms when intense shock waves pass through quartz), and even carbon from burned vegetation. All these features pointed to the occurrence of a huge meteorite impact at the time of the K-T boundary. Subsequently, geologists found a 100-km-diameter and 16-km-deep meteorite crater buried beneath younger strata of the Yucatan  Peninsula in Mexico (figure above a, b). Isotopic dating indicates that formation of the crater occurred at 65 o 0.4 Ma, the time of the K-T boundary event. Because of its age and size, this crater, known as the Chicxulub crater, may be the grave of the deadly object whose impact with Earth eliminated so much life.

The impact caused so much destruction because it not only formed a crater, blasting huge quantities of debris into the sky, but probably also generated 2-km-high tsunamis that inundated the shores of continents and generated a blast of hot air that set forests on fire. The blast and the blaze together could have ejected so much debris into the atmosphere that for months there would have been perpetual night and winter like cold. In addition, chemicals ejected into the air could have combined with water to produce acid rain. These conditions would cause photosynthesis to all but cease, and thus would break the food chain and trigger extinctions.
Credits: Stephen Marshak (Essentials of Geology) 

The Paleozoic Era: Continents Reassemble and Life Gets Complex

The Paleozoic Era: Continents Reassemble and Life Gets Complex

kacimo samy   12:34 م

The Paleozoic Era

The Paleozoic Era comprises of Cambrian to Ordovician Periods which time span is 542-251 Ma.

The Early Paleozoic Era  (Cambrian–Ordovician Periods, 542–444 Ma) 

Paleogeography

Land and sea in the early Paleozoic Era.

At the beginning of the Paleozoic Era, Pannotia broke up, yielding smaller continents including Laurentia (composed of North America and Greenland), Gondwana (South America, Africa, Antarctica, India, and Australia), Baltica (Europe), and Siberia (figure above a). New passivemargin basins formed along the edges of these new continents. In addition, sea level rose, so that vast areas of continental interiors were flooded with shallow seas called epicontinental seas (figure above b). These regions are now cratonic platforms. In many places, water depths in epicontinental seas reached only a few meters, creating a well-lit marine environment in which life abounded. Deposition in these seas, therefore, yielded layers of fossiliferous sediment. Sea level, however, did not stay high for the entire early Paleozoic Era; regressions and transgressions took place, the former marked by unconformities and the latter by accumulations of sediment. The layer cake of strata in the Grand Canyon is rock formed from such sediment. The geologically peaceful world of the early Paleozoic Era in Laurentia abruptly came to a close in the Middle Ordovician Period, for at this time its eastern margin rammed into a volcanic island arc and other crustal fragments. The resulting collision, called the Taconic orogeny, deformed and metamorphosed strata of the continent’s margin and produced a mountain range in what is now the eastern part of the Appalachians (figure above c). 

Life evolution

A museum diorama illustrates what early Paleozoic marine organisms may have looked like.

The fossil record indicates that soon after the Cambrian began, life underwent remarkable diversification. This event, which paleontologists refer to as the Cambrian explosion, took several million years. What caused this event? No one can say for sure, but considering that it occurred roughly at the time a supercontinent broke up, it may have had something to do with the production of new ecological niches and the isolation of populations that resulted when small continents formed and drifted apart. The first animals to appear in the Cambrian Period had simple tube- or cone-shaped shells, but soon thereafter, the shells became more complex. Shells on other organisms may have evolved as a means of protection against predation by organisms such as conodonts, small, eel-like organisms with hard parts that resemble teeth. By the end of the Cambrian, 

trilobites were grazing the sea floor. Trilobites shared the environment with mollusks, brachiopods, nautiloids, gastropods, graptolites, and echinoderms (figure above). Thus, a complex food chain arose, which included plankton, bottom feeders, and at the top, predators. Many of the organisms crawled over or swam around reefs composed of mounds of sponges with mineral skeletons. The Ordovician Period saw the first crinoids and the first vertebrate animals, jawless fish. At the end of the Ordovician, mass extinction took place, perhaps because of the brief glaciation and associated sea-level lowering of the time.

The Middle Paleozoic Era  (Silurian–Devonian Periods, 444–359 Ma) 

Paleogeography

Paleogeography and fossils of Silurian and Devonian time

 .

As the world entered the Silurian Period, global climate warmed, sea level rose, and the continents flooded once again. In some places, where water in the epicontinental seas was clear and could exchange with water from  the oceans, huge reef complexes grew, forming a layer of  fossiliferous limestone on the continents. Also, distinct orogenies took place, yielding new mountain belts during the middle Paleozoic Era. For example, collisions on the eastern side of  Laurentia during Silurian and Devonian time produced the Caledonian orogen (affecting eastern Greenland, western Scandinavia, and Scotland) and the Acadian orogen in the region that is now the Appalachians (figure above a). Throughout much of the middle Paleozoic, the western margin of North America continued to be a passive-margin basin. Finally, in the Late Devonian, the quiet environment of the west-coast passive margin ceased, possibly because of a collision with an island arc. This event, known as the Antler orogeny, was the first of many orogenies to affect the western margin of the continent. The Caledonian, Acadian, and Antler orogenies all shed deltas of sediment onto the continents; these deposits formed thick successions of red beds, such as those visible today in the Catskill Mountains (figure above b). 

Life evolution

Life on Earth underwent radical changes in the middle Paleozoic Era. In the sea, new species of trilobites,  gastropods, crinoids, and bivalves replaced species that had disappeared during the mass extinction at the end of the Ordovician Period. On land, vascular plants with woody tissues, seeds, and veins (for transporting water and food) rooted for the first time. With the evolution of veins and wood, plants could grow much larger, and by the Late Devonian Period the land surface hosted swampy forests with tree-sized relatives of club mosses and ferns. Also at this time, spiders, scorpions, insects, and crustaceans began to exploit both dry-land and freshwater habitats, and jawed fish such as sharks and bony fish began to cruise the oceans. Finally, at the very end of the Devonian Period, the first amphibians crawled out onto land and inhaled air with lungs (figure above c).

The Late Paleozoic Era  (Carboniferous–Permian Periods, 359–251 Ma)

Paleogeography

Paleogeography at the end of the Paleozoic Era.

The climate cooled significantly in the late Paleozoic. Seas gradually retreated from the continents, so that during the Carboniferous Period, regions that had hosted the limestone-forming reefs of epicontinental seas now became coastal areas and river deltas in which sand, shale, and organic debris accumulated. In fact, during the Carboniferous Period, Laurentia lay near the equator, so it enjoyed tropical and semitropical conditions that favored lush growth in swamps. This growth left thick piles of plant debris that transformed into coal after burial. Much of Gondwana and Siberia, in contrast, lay at high latitudes, and by the Permian Period became covered by ice sheets. The late Paleozoic Era also saw a succession of continental collisions, culminating in the formation of a single supercontinent, Pangaea (figure above a). The largest collision occurred during Carboniferous and Permian time, when Gondwana rammed into Laurentia and Baltica, causing the Alleghanian orogeny of North America (figure above b). 

 Features of the Appalachian Mountains in the eastern United States.

During this event, the final stage in the development of the Appalachians, eastern North America rammed against northwestern Africa, and what is now the Gulf Coast region of North America squashed against the northern margin of South America. A vast mountain belt grew, in which deformation generated huge faults and folds. We now see the eroded remnants of rocks deformed during this event in the Appalachian and Ouachita Mountains. Along the  continental side of the range, a wide band of deformation called the Appalachian fold-thrust belt formed (figure above). Movement on the faults displaced strata and resulted in the formation of large folds. At depth, the thrust faults merged with a near-horizontal sliding surface, called a detachment, just above the Precambrian basement. Stresses generated during the Alleghanian orogeny were so strong that preexisting faults in the continental crust clear across North America became active again. The movement produced uplifts and sediment-filled basins in the Midwest and in the region of the present-day Rocky Mountains. Geologists refer to the late Paleozoic uplifts of the Rocky Mountain region as the Ancestral Rockies.

The assembly of Pangaea involved a number of other collisions around the world as well. Notably, Africa collided with southern Europe to form the Hercynian orogen. Also, a rift or small ocean in Russia closed, leading to the uplift of the Ural Mountains, and parts of China along with other fragments of Asia attached to southern Siberia. 

Life evolution

A museum diorama of a Carboniferous coal swamps includes a giant dragonfly, with a wingspan of about 1 m.
The insect photo gives series of its size relative to human.

The fossil record indicates that during the late Paleozoic Era, plants and animals continued to evolve toward more familiar forms. In coal swamps, fixed-wing insects including huge dragonflies flew through a tangle of ferns, club mosses, and scouring rushes, and by the end of the Carboniferous Period insects such as the cockroach, with foldable wings, appeared (figure above). Forests containing gymnosperms (“naked seed” plants such as conifers) and cycads (trees with a palm-like stalk and fern-like fronds) became widespread in the Permian Period. Amphibians and, later, reptiles populated the land. The appearance of reptiles marked the evolution of a radically new component in animal reproduction: eggs with a protective covering. Such eggs permitted reptiles to reproduce without returning to the water. The late Paleozoic Era came to a close with a major mass extinction event, during which over 95% of marine species disappeared. Why this event occurred remains a subject of debate. According to one hypothesis, the terminal Permian mass extinction occurred as a result of an episode of extraordinary volcanic activity in the region that is now Siberia; basalt sheets extruded during the event are known as the “Siberian traps.” Eruptions could have clouded the atmosphere, acidified the oceans, and disrupted the food chain.

Credits: Stephen Marshak (Essentials of Geology)

The Proterozoic: The Earth in Transition

The Proterozoic: The Earth in Transition

kacimo samy   10:54 ص

Growth of Continents 

The Proterozoic Eon spans roughly 2 billion years, from about 2.5 Ga to the beginning of the Cambrian Period at 542 Ma thus, it encompasses almost half of Earth’s history. During Proterozoic time, Earth’s surface environment changed from being an unfamiliar world of fast-moving plates, small continents, and an oxygen-poor atmosphere, to the more familiar world of slower plates, large continents, and an oxygen-rich atmosphere. First, let’s look at changes to the continents. New continental crust continued to form during the Proterozoic Eon, but at progressively slower rates in fact, by the middle of the eon, over 90% of the Earth’s continental crust had formed. As Archean proto-continents collided with each other and with volcanic island arcs and hot-spot volcanoes, still larger continents gradually assembled. Significantly, the interiors of these larger continents became isolated from heating by subduction-related igneous activity that happened along its margins. Interior regions, therefore, slowly cool and strengthen until they become very rigid and durable. Such a region of cold, relatively stable continental crust is called a craton. All cratons that exist today had formed by about 1 Ga (figure above); therefore, crust in cratons (the old parts of continents) ranges from about 3.85 Ga to about 1 Ga. 

To understand the character of a craton, let’s examine North America’s craton a bit more closely. We see that it consists of two regions (figure above). Throughout the shield, outcrops expose Precambrian “basement,” which consists of igneous and metamorphic rocks older than about 1 Ga. The landscape of the shield tends to have fairly low relief there are small hills and valleys, but no dramatic mountain ranges. Most of North America’s shield lies in Canada, so geologists refer to it as the Canadian Shield. Throughout the cratonic platform, which surrounds the shield and also underlies Hudson Bay, a blanket of Paleozoic or Mesozoic cover strata overlies the Precambrian basement. 

By using isotopic dating on samples from both outcrops and drill holes, geologists have been able to subdivide the basement of North America’s craton into distinct blocks or provinces, each of which has been given a name (figure above). It appears that the Canadian Shield consists of several Archean crust blocks sutured together by Proterozoic orogens. The basement of the cratonic platform in the United States, in contrast, grew when a series of volcanic island arcs and continental slivers “accreted” (attached) to the margin of the Canadian Shield between  1.8 and 1.6 Ga. In the Midwest, granite plutons intruded much of this accreted region, and rhyolite ash flows covered it, between 1.5 and 1.3 Ga. Successive collisions ultimately brought together most continental crust on Earth into a single supercontinent, named Rodinia, by around 1 Ga. The last major collision during the formation of Rodinia produced a large orogen called the Grenville orogen. 

If you look at a popular (though not universally accepted) reconstruction of Rodinia, you can identify the crustal provinces that would eventually become the familiar continents of today (figure above a). Several studies suggest that sometime between 800 and 600 Ma, Rodinia “turned inside out,” in that Antarctica, India, and Australia broke away from western North America and swung around and collided with the future South America, possibly forming a short-lived supercontinent that some geologists refer to as Pannotia (figure above b). The map of the Earth clearly changed radically during the Proterozoic. But that’s not all that changed fossil evidence suggests that this eon also saw important steps in the evolution of life. When the Proterozoic began, most life was prokaryotic, meaning that it consisted of single-celled organisms (archaea and bacteria) without a nucleus. Though studies of chemical fossils hint that eukaryotic life, consisting of cells that have nuclei, originated as early as 2.7 Ga, the first possible body fossil of a eukaryotic organism occurs in 2.1-Ga rocks, and abundant body fossils of eukaryotic organisms can be found only in rocks younger than about 1.2 Ga. Thus the proliferation of eukaryotic life, the foundation from which complex organisms eventually evolved, took place during the Proterozoic. The last half-billion years of the Proterozoic Eon saw the remarkable transition from simple organisms into complex ones. Ciliate protozoans (single-celled organisms coated with fibers that give them mobility) appear at about 750 Ma. 

A great leap forward in complexity of organisms occurred during the next 150 million years of the eon, for sediments deposited perhaps as early as 620 Ma and certainly by 565 Ma contain several types of multicellular organisms that together constitute the Ediacaran fauna, named for a region in southern Australia where fossils of these organisms were first found. Ediacaran species survived into the beginning of the Cambrian before becoming extinct. Their fossil forms suggest that some of these invertebrate organisms resembled jellyfish, while others resembled worms (figure above a). The evolution of life played a key role in the evolution of Earth’s atmosphere. Before life appeared, there was hardly any free oxygen (O2) in the atmosphere. With the appearance of photosynthetic organisms, oxygen began to enter the  atmosphere. But it was not until about 2.4 Ga that the concentration of oxygen in the atmosphere increased dramatically. This event, called the great oxygenation event, happened when other environments were no longer able to absorb or dissolve the oxygen produced by organisms, so the oxygen began to accumulate as a gas in air. As a result, the oceans became oxidizing environments that, for reasons described in chemistry books, could no longer contain large quantities of dissolved iron. Between 2.4 Ga and 1.8 Ga, huge amounts of iron settled out of the ocean to form colorful sedimentary beds known as banded iron formation (BIF). BIF consists of alternating layers of iron oxide minerals (hematite or magnetite) and jasper (red chert) as illustrated by the chapter opening photo. Radical climate shifts occurred on Earth at the end of the Proterozoic Eon. Specifically, accumulations of glacial sediments occur worldwide in late Proterozoic stratigraphic sequences. What’s strange about the occurrence of these sediments is that they can be found even in regions that were located at the equator during these times (figure above b). This observation implies that the entire planet was cold enough for glaciers to form at the end of the Proterozoic. Geologists still are debating the history of these global ice ages; in one model, glaciers covered the land and perhaps the entire ocean surface froze, resulting in snowball Earth (figure above c). The shell of ice cut off the oceans from the atmosphere, causing oxygen levels in the sea to drop drastically, so many life forms died off. What brought an end to snowball Earth conditions? According to one model, the icy sheath also prevented atmospheric CO2 from dissolving in seawater, but it did not prevent volcanic activity from continuing to add CO2 to the atmosphere. Earth would have remained a snowball forever, were it not for the addition of volcanic CO2, a greenhouse gas that traps heat in the atmosphere much as glass panes trap heat in a greenhouse. As the CO2 concentration increased, Earth warmed up and eventually the glaciers melted. 

Introducing the Phanerozoic Eon 

As the Proterozoic came to a close, Earth’s climate warmed and continents drifted apart life evolved and diversified to occupy the new environments that formed. Over a relatively short period of time, shells appeared and the fossil record became much more complete. This event defines the end of the Proterozoic Eon and therefore, of the Precambrian, and the start of the Phanerozoic Eon. Of note, geologists recognized the siginificance of this event long before they could assign it a numerical age (currently 542 Ma). The Phanerozoic Eon consists of three eras the Paleozoic (Greek for ancient life), the Mesozoic (middle life), and the Cenozoic (recent life). Geologists have divided the Mesozoic and Cenozoic each into three periods and the Paleozoic into six periods. In the sections that follow, we consider changes in the map of our planet’s surface (its paleogeography), as manifested by the distribution of continents, seas, and mountain belts, as well as life evolution that happened during the three eras.

Credits: Stephen Marshak (Essentials of Geology)

The Archean Eon: Birth of the Continents and Life

The Archean Eon: Birth of the Continents and Life

kacimo samy   10:52 ص

The Archean Eon

The Archean Eon has given us where we lives now, land.

Land Appears 

The boundary between the Archean (from the Greek word for beginning) Eon and the Hadean Eon occurs at about 3.85 Ga. Effectively, this date marks the time at which substantial quantities of crustal rocks, including rocks that originated as marine sediments, formed. With the advent of the Archean, crust was locally cool and stable enough for rocks to survive and for isotopic clocks to start ticking. Geologists still debate about whether plate tectonics in the form that occurs today operated in the early part of the Archean Eon. Most researchers picture an early Archean Earth with rapidly moving small plates, numerous volcanic island arcs, and abundant hot-spot volcanoes. Others propose that early Archean lithosphere was too warm and buoyant to subduct, and that plate tectonics could not have operated until the later part of the Archean or later; these authors argue that plume-related volcanism or some other process was the main source of new crust until the late Archean. Regardless of which model ultimately proves more accurate, it is clear that the Archean was a time during which significant volumes of new continental crust were generated. What processes produced continental crust? According to one model, early crust formed from mafic igneous rocks that originally extruded or intruded at convergent plate boundaries and/or hot-spot volcanoes. Once formed, these rocks were too buoyant to be subducted, so when the arcs and plateaus collided with one another, they sutured together to form larger blocks that remained at the Earth’s surface. The development of convergent plate boundaries along the margins of these blocks, and of rifts and hot spots within the blocks, led to production of flood basalts. Partial melting of basaltic crust yielded felsic and intermediate rocks. As collisions continued, the blocks coalesced into still larger proto-continents (figure above a, b), which slowly cooled and became stronger. As a result of these processes, the first long-lived blocks of durable continental crust came into existence between 3.2 and 2.7 Ga, and by the end of the Archean Eon, about 80% of the Earth’s continental area had formed (figure above c). A clear stratigraphic record of marine sediment deposition appears in remnants of Archean crust, indicating that oceans filled in the Archean and have existed ever since. Presumably, permanent oceans could survive only after the Earth’s surface had cooled below the boiling point of water. Prior to that time, gaseous H2O saturated the atmosphere in fact, prior to ocean formation, H2O and CO2 were the dominant gases of the atmosphere. Once the oceans formed, however, the atmosphere lost most of its H2O. And once liquid water existed, most atmospheric CO2 dissolved into it, so CO2 also went from being a major component of the atmosphere to being a trace component. Thus, the Archean saw the atmosphere change from being a foggy mixture of H2O and CO2 into being a transparent gas dominantly composed of nitrogen (N2) gas. Since nitrogen is inert (it doesn't chemically react with or dissolve in other materials), it was left behind.

The First Life 

Clearly, the Archean Eon saw many firsts in Earth history. Not only did the first continents appear during the Archean, but probably also the first life. The search for the earliest evidence of life continues to make headlines in the popular media. Most geologists currently conclude that life has existed on Earth since at least  3.5 Ga, and perhaps since 3.8 Ga, for rocks of this age contain chemical signatures of organisms. The oldest undisputed fossil forms of bacteria and archaea occur in 3.2-Ga rocks  (figure above a). (Shapes resembling such organisms occur in rocks as old as 3.5 Ga, but their identity remains less certain.) Archean strata at some localities contain stromatolites, distinctive layered mounds of sediment. Stromatolites that developed after about 3.2 Ga form because cyanobacteria secrete a mucuslike substance to which sediment settling from water sticks. As the mat gets buried, new cyanobacteria colonize the top of the sediment, building a mound upward (figure above b); modern examples locally occur in shallow, tropical waters. What specific environment on the Archean Earth served as the cradle of life? Laboratory experiments conducted in the 1950s led many researchers to think that life began in warm pools of surface water, beneath a methane- and ammoniarich atmosphere streaked by bolts of lightning. More recent researchers suggest instead that submarine hot-water vents, so-called black smokers, served as the hosts of the first organisms. These vents emit clouds of ion-charged solutions from which sulphide minerals precipitate and build chimneys. The earliest life in the Archean Eon may well have been thermophilic (heat-loving) bacteria or archaea that dined on pyrite at dark depths in the ocean alongside these vents. Later in the Archean, organisms evolved the ability to carry out photosynthesis, and moved into shallower, well lit water. As the Archean Eon came to a close, the first continents had formed, and life had colonized not only the depths of the sea but also the shallow marine realm. Plate tectonics had commenced, continental drift was taking place, collisional mountain belts were forming, and erosion was occurring. The atmosphere was gradually accumulating oxygen, although probably this gas still accounted for only a very small percentage of the air; Archean air was unbreathable. The stage was set for another major change in the Earth System.

Credits: Stephen Marshak (Essentials of Geology)

The Hadean Eon: Before the Rock Record

The Hadean Eon: Before the Rock Record

kacimo samy   9:45 ص

Hadean Eon

James Hutton, the 18th-century Scottish geologist who was the first to provide convincing evidence that the Earth was very old, could not measure Earth’s age directly, and indeed speculated that there may be “no vestige of a beginning.” But isotopic dating studies conducted in recent decades have shown that it is possible, in fact, to assign a numerical age to our planet’s formation. Specifically, dates obtained for a class of meteorites thought to be remnants of the planetesimal cloud out of which the Earth formed yield an age of 4.57 Ga. Geologists currently take this age to be the Earth’s birth date. But the oldest whole rock yet found is only 4.03 Ga, and a clear record of Earth history, as recorded in continental crustal rocks, does not begin until about 3.85 Ga. Geologists refer to the mysterious time interval between the birth of Earth and 3.85 Ga as the Hadean Eon (from Hades, the Greek god of the underworld) because during this interval the Earth’s surface was, at times, like an inferno. Many major events happened during the Hadean. By about 4.5 Ga, the Earth underwent internal  differentiation gravity pulled molten iron down to the center of the Earth, where it accumulated to form the core, leaving a mantle composed of ultramafic rock. Researchers suggest that soon after or perhaps during differentiation, a Mars-sized protoplanet collided with the Earth. The energy of this collision blasted away a significant fraction of Earth’s mantle; the resulting debris formed a ring of silicate-rock debris orbiting the Earth. This ring then coalesced to form the Moon, which, when first formed, was less than 20,000 km away. (By comparison, the Moon is 384,000 km from Earth today.)

This speculative painting depicts the Hadean Earth's surface as a magma ocean pummeled by meteorites. The Moon was much closer then, but might not have been visible through the dense atmosphere.

In the wake of differentiation and Moon formation, the Earth was so hot that much of its surface was an ocean of seething magma (figure above). Rafts of solid rock formed temporarily on the surface of the magma ocean, but these eventually sank and remelted. This stage lasted at least until about 4.4 Ga. After that time, the amount of radioactive heat generation decreased (because elements with short half-lives had decayed), so the Earth might have become cool enough for solid rocks to form at its surface. The evidence for this statement comes from western Australia, where geologists have found 4.4-Ga grains of a durable mineral called zircon. During the Hadean Eon, outgassing of the Earth’s mantle began to take place. This means that volatile (gassy) elements or compounds originally incorporated in mantle minerals were released and bubbled out of volcanoes, along with lava. The gases accumulated into a toxic atmosphere of water (H2O), methane (CH4), ammonia (NH3), hydrogen (H2), nitrogen (N2), carbon dioxide (CO2), sulfur dioxide (SO2), and other gases. Some researchers speculate that gases from comets colliding with Earth may have contributed additional gases to the early atmosphere. If the Hadean Earth’s surface was sufficiently cool for an extensive solid crustal rock to form beginning at 4.4 Ga, then the first oceans may have accumulated soon thereafter, when water in the atmosphere condensed and fell as rain. Though mineral grains as old as 4.4 Ga exist, the oldest whole rock yet found on Earth has an age of only 4.03 Ga, and most rocks are younger than 3.85 Ga. What destroyed most, if not all, of the pre-3.85-Ga rock (and oceans, if they existed) of the Earth? The answer comes from studies of cratering on the Moon. These studies suggest that the Moon and, therefore, all inner planets of the Solar System underwent intense meteor bombardment between 4.0 and 3.85 Ga. Researchers speculate that this bombardment would have pulverized and/ or melted almost all crust that had existed on Earth at the time, and would have destroyed the existing atmosphere and ocean. Only after the bombardment ceased could long-lasting crust, atmosphere, and oceans begin to form. The discovery of 3.85-Ga marine sedimentary rocks in Greenland suggests that the appearance of land and sea happened quite soon after bombardment ceased. What did the Earth’s surface look like at 3.85 Ga? An observer probably would have found small, barren landmasses, spotted with volcanoes, poking up above an acidic sea. But both land and sea would have been obscured by murky, dense (CO2- and SO2-rich) air.

Credits: Stephen Marshak (Essentials of Geology)