What Do We Mean by Plate Tectonics?

What Do We Mean by Plate Tectonics?

kacimo samy   2:32 ص

What Do We Mean by Plate Tectonics?

The paleomagnetic proof of continental drift (plate tectonics) and the discovery of sea-floor spreading set off a scientific revolution in geology in the 1960s and 1970s. Geologists realised that many of their existing interpretations of global geology, based on the premise that the positions of continents and oceans remain fixed in position through time, were simply wrong! Researchers dropped what they were doing and turned their attention to studying the broader implications of continental drift and sea-floor spreading. It became clear that these phenomena required that the outer shell of the Earth was divided into rigid plates that moved relative to each other. New studies clarified the meaning of a plate, defined the types of plate boundaries, constrained plate motions, related plate motions to earthquakes and volcanoes, showed how plate interactions can explain mountain belts and seamount chains, and outlined the history of past plate motions. From these, the modern theory of plate tectonics evolved. Below, we first describe lithosphere plates and their boundaries, and then outline the basic principles of plate tectonics theory.

The Concept of a  Lithosphere Plate 

 Nature of the lithosphere and its behaviour.

We learned earlier that geoscientists divide the outer part of the Earth into two layers. The lithosphere consists of the crust plus the top (cooler) part of the upper mantle. It behaves relatively rigidly, meaning that when a force pushes or pulls on  it, it does not flow but rather bends or breaks (figure above a). The lithosphere floats on a relatively soft, or “plastic,” layer called the asthenosphere, composed of warmer ( 1280°C) mantle that can flow slowly when acted on by a force. As a result, the asthenosphere convects, like water in a pot, though much more slowly.

Continental lithosphere and oceanic lithosphere differ markedly in their thicknesses. On average, continental lithosphere has a thickness of 150 km, whereas old oceanic lithosphere has a thickness of about 100 km (figure above b). (For reasons discussed later in this chapter, new oceanic lithosphere at a mid-ocean ridge is much thinner.) Recall that the crustal part of continental lithosphere ranges from 25 to 70 km thick and consists largely of low-density felsic and intermediate rock. In contrast, the crustal part of oceanic lithosphere is only 7 to 10 km thick and consists largely of relatively high-density mafic rock (basalt and gabbro). The mantle part of both continental and oceanic lithosphere consists of very high-density ultramafic rock (peridotite). Because of these  differences, the continental lithosphere “floats” at a higher level than does the oceanic lithosphere. 

The location of plate boundaries and the distribution of earthquakes.

The lithosphere forms the Earth’s relatively rigid shell. But unlike the shell of a hen’s egg, the lithospheric shell contains a number of major breaks, which separate it into distinct pieces. As noted earlier, we call the pieces lithosphere plates, or simply plates. The breaks between plates are known as plate boundaries (figure above a). Geoscientists distinguish twelve major plates and several microplates. 

The Basic Principles of Plate Tectonics 

With the background provided above, we can restate plate tectonics theory concisely as follows. The Earth’s lithosphere is divided into plates that move relative to each other. As a plate moves, its internal area remains mostly, but not perfectly, rigid and intact. But rock along plate boundaries undergoes intense deformation (cracking, sliding, bending, stretching, and squashing) as the plate grinds or scrapes against its neighbours or pulls away from its neighbours. As plates move, so do the continents that form part of the plates. Because of plate tectonics, the map of Earth’s surface constantly changes.

Identifying Plate Boundaries 

How do we recognize the location of a plate boundary? The answer becomes clear from looking at a map showing the locations of earthquakes (figure above b). Recall from Chapter 1 that earthquakes are vibrations caused by shock waves that are generated where rock breaks and suddenly slips along a fault. The epicentre marks the point on the Earth’s surface directly above the earthquake. Earthquake epicentres do not speckle the globe randomly, like buckshot on a target. Rather, the majority occur in relatively narrow, distinct belts. These earthquake belts define the position of plate boundaries because the fracturing and slipping that occurs along plate boundaries generates earthquakes. Plate interiors, regions away from the plate boundaries, remain relatively earthquake-free because they do not accommodate as much movement. While earthquakes serve as the most definitive indicator of a plate boundary, other prominent geologic features also develop along plate boundaries.

Note that some plates consist entirely of oceanic lithosphere, whereas some plates consist of both oceanic and continental lithosphere. Also, note that not all plates are the same size (figure above c). Some plate boundaries follow continental margins, the boundary between a continent and an ocean, but others do not. For this reason, we distinguish between active margins, which are plate boundaries, and passive margins, which are not plate boundaries. Earthquakes are common at active margins, but not at passive margins. Along passive margins, continental crust is thinner than in  continental interiors. Thick (10 to 15 km) accumulations of sediment cover this thinned crust. The surface of this sediment layer is a broad, shallow (less than 500 m deep) region called the continental shelf, home to the major fisheries of the world. 

The three types of plate boundaries differ based on the nature of relative movement.

Geologists define three types of plate boundaries, based simply on the relative motions of the plates on either side of the boundary (figure above a–c). A boundary at which two plates move apart from each other is a divergent boundary. A boundary at which two plates move toward each other so that one plate sinks beneath the other is a convergent boundary. And a boundary at which two plates slide sideways past each other is a transform boundary.

Credits: Stephen Marshak (Essentials of Geology)

The biggest issues facing Africa largest mining gathering – Indaba 2016

The biggest issues facing Africa largest mining gathering – Indaba 2016

kacimo samy   8:50 م

The biggest issues facing Africa& largest mining gathering – Indaba 2016

On Monday more than 7,000 delegates from 100 countries and territories across six continents will descend on Cape Town for the biggest mining gathering on the planet in one of arguably the toughest years in the history of African mining.

The continent is facing economic uncertainty, doubts over mining legislation and regulation, power cuts and looming strikes. Even the optimists, that is the government of South Africa, admit there could be as many as 32,000 jobs under threat in the country’s mines.

This would be a severe blow to an industry, that despite several setbacks in the last five years, is still top dog in Africa employing 400,000 people. Pay talks are expected in the platinum industry later this year, with the dominant union AMCU, that could lead to a strike costing billions and thousands of jobs. The union led a five-month strike in platinum two years ago.

AMCU will make its debut at this year’s indaba and is likely to arrive with characteristic uncompromising views on mining that could unsettle one or two foreign investors. 

“We feel it is not ok for corporations to come into our country make decisions about what is happening in our country and then leave,” says Manzini Zungu, the spokesman for AMCU.  

One of the highlights of this year’s gathering is that South Africa’s new mining minister Mosebenzi Zwane will open proceedings and give the mining world a hint of how his government plans to stem the flow of capital out.

For years a pall of gloom has formed over the Mining Indaba in Cape Town with analysts and industry insiders tutting about how pessimistic foreign investors are and how there are more supply companies and fewer mining companies every year.

“The problem is South Africa is sending out messages that it is closed for business,” one delegate and former mine boss said to me in Cape Town last year.

Infrastructure is also a worry. From  Zambia to Mpumalanga mines are suffering power cuts and a lack of roads, water and rails.

On the other side of the coin, infrastructure, along with the support for new mining projects, is the focus of the Mining Indaba with the aim of prising billions out of foreign investors.

The organizers are upbeat even if the mining industry is not. They claim billions of dollars in investment have been channelled in to African mining, through the indaba, creating 5,000 jobs since 2007.It claims 2,300 international companies will have delegates at this year’s gathering.

The Johannesburg Stock Exchange, clearly caught up in the sustained enthusiasm of the organizers, will be there in force in the hope of recruiting new listings. Of the 391 companies listed on the JSE, 68 are from metals mining and resources. The fact that the JSE wants more, at the very least, is a vote of confidence for a very battered industry that has seen billions in value of resources companies wiped off the board in the last few years.

When the talking starts in Cape Town there are likely to be more than a few sparks. One of the first speakers on Monday will be the former Nigerian Reserve Bank governor Lamido Sanusi, the man whose unflinching stance against corruption and his own government in 2011 earned him Forbes Africa Person of the Year award. The Emir of Kano, who rarely minces his words, will speak as chairman of Black Rhino Group , an energy infrastructure company with investments across Africa.

Other vociferous mining bigwigs expected as speakers will be Ben Magara, the CEO of Lonmin, Mark Cutifani, the CEO of Anglo American, Tom Albanese, the head of Vendanta Group, Robert Friedland, the founder of Ivanhoe Resources and Mark Bristow, the CEO of Randgold Resources.

There will be plenty of talk at the Mining Indaba, in Cape Town’s vast International Trade Centre, but talk can be cheap. If mining is to remain a major industry on this continent serious investment and hard work is needed this year.

More than 20 years ago the Mining Indaba was launched in one room at a Cape Town hotel. The industry must be careful not to allow the industry to shrink back into one hotel room in 20 years’ time.

Credits: Chris Bishop, CNBCAfrica

New technique to discover copper deposits

New technique to discover copper deposits

kacimo samy   7:46 ص

Magmatic rock which formed large porphyry deposit in Chile.

A geologist at the University of Exeter has developed a new and relatively inexpensive way to establish whether certain types of magmatic rocks are more likely to contain valuable metal deposits.

In a study published in Nature Geoscience, Dr Ben Williamson, of the University’s Camborne School of Mines, together with Dr Richard Herrington from the Natural History Museum, London, have proposed a new method to explore for porphyry-type copper deposits. These deposits provide around 75 per cent of the world’s copper and a significant amount of molybdenum and gold which makes them extremely important to the world economy. The deposits, which originally form at several kilometres depth below the Earth’s surface, above large magma chambers, are relatively rare, particularly the largest deposits which are most economic to mine. In addition, most near-surface deposits have already been discovered. Any new method to locate deeper deposits is therefore of great interest to the mining industry.

The project, funded by Anglo American, a major global mining company, compared the chemical compositions of minerals from magmatic rocks that host porphyry deposits against those which are barren. A case study was then undertaken of a major new porphyry discovery in Chile, to test their theory. Minerals from magmatic rocks which host porphyry deposits have distinctive chemical characteristics which can be used as one of a suite of indicators to home-in on porphyry deposits. Unravelling the causes of the distinctive chemical signatures has also brought new insights into the formation of porphyry copper deposits, and more generally the generation of the magmatic rocks from which they form, which are an important component of the Earth’s crust. The main finding in this regard is that the magma chamber below the porphyry undergoes discrete injections of water-rich melts or watery fluids which enhance the magma’s ability to transfer copper and other metals upwards to form a porphyry copper deposit.

Dr Ben Williamson, of the University of Exeter’s Camborne School of Mines, said: “This new method will add to the range of tools available to exploration companies to discover new porphyry copper deposits. Our findings also provide important insights into why some magmas are more likely to produce porphyry copper deposits than others, and add to our understanding of how their parent magmatic rocks evolve.”

Source: University of Exeter

Evidence for Sea-Floor Spreading

Evidence for Sea-Floor Spreading

kacimo samy   8:48 ص

Evidence for Sea-Floor Spreading 

For a hypothesis to become a theory, researchers must demonstrate that the idea really works. During the 1960s, geologists found that the sea-floor spreading hypothesis successfully explains several previously baffling observations. Here we discuss two: (1) the existence of orderly variations in the strength of the measured magnetic field over the sea floor, producing a pattern of stripes called marine magnetic anomalies; and (2) the variation in sediment thickness on the ocean crust, as measured by drilling.

Marine Magnetic Anomalies

Recognizing anomalies 

Geologists can measure the strength of Earth’s magnetic field with an instrument called a magnetometer. At any given location on the surface of the Earth, the magnetic field that you measure includes two parts: one produced by the main dipole of the Earth generated by circulation of molten iron in the outer core, and another produced by the magnetism of near-surface rock. A magnetic anomaly is the difference between the expected strength of the Earth’s main dipole field at a certain location and the actual measured strength of the magnetic field at that location. Places where the field strength is stronger than expected are positive anomalies, and places where the field strength is weaker than expected are negative anomalies.

The discovery of marine magnetic anomalies.

Geologists towed magnetometers back and forth across the ocean to map variations in magnetic field strength (figure above a). As a ship cruised along its course, the magnetometer’s gauge might first detect an interval of strong signal (a positive anomaly) and then an interval of weak signal (a negative anomaly). A graph of signal strength versus distance along the traverse, therefore, has a sawtooth shape (figure above b). When geologists compiled data from many cruises on a map, these marine magnetic anomalies defined distinctive, alternating bands. If we color positive anomalies dark and negative anomalies light, the pattern made by the anomalies resembles the stripes on a candy cane (figure above c). The mystery of this marine magnetic anomaly pattern, however, remained unsolved until geologists recognized the existence of magnetic reversals.

Magnetic reversals

Magnetic polarity reversals and the chronology of reversals. 

Recall that Earth’s magnetic field can be represented by an arrow, representing the dipole, that presently points from the north magnetic pole to the south magnetic pole. When researchers measured the paleomagnetism of a succession of rock layers that had accumulated over a long period of time, they found that the polarity (which end of a magnet points north and which end points south) of the paleomagnetic field preserved in some layers was the same as that of Earth’s present magnetic field, whereas in other layers it was the opposite  (figure above a, b). 

At first, observations of reversed polarity were largely ignored, thought to be the result of lightning strikes or of local chemical reactions between rock and water. But when repeated measurements from around the world revealed a systematic pattern of alternating normal and reversed polarity in rock layers, geologists realized that reversals were a worldwide, not a local, phenomenon. They reached the unavoidable conclusion that, at various times during Earth history, the polarity of Earth’s magnetic field has suddenly reversed! In other words, sometimes the Earth has normal polarity, as it does today, and sometimes it has reversed polarity (figure above c). A time when the Earth’s field flips from normal to reversed polarity, or vice versa, is called a magnetic reversal. When the Earth has reversed polarity, the south magnetic pole lies near the north geographic pole, and the north magnetic pole lies near the south geographic pole. Thus, if you were to use a compass during periods when the Earth’s magnetic field was reversed, the north-seeking end of the needle would point to the south geographic pole. Note that the Earth itself doesn't turn upside down it is just the magnetic field that reverses.

In the 1950s, about the same time researchers discovered polarity reversals, they developed a technique that permitted them to measure the age of a rock in years. Geologists applied the technique to determine the ages of rock layers in which they obtained their paleomagnetic measurements, and thus determined when the magnetic field of the Earth reversed. With this information, they constructed a history of magnetic reversals for the past 4.5 million years; this history is now called the magnetic-reversal chronology. The time interval between successive reversals is called a chron. 

A diagram representing the Earth’s magnetic-reversal chronology (figure above d) shows that reversals do not occur regularly, so the lengths of different polarity chrons are different. For example, we have had a normal-polarity chron for about the last 700,000 years. Before that, a reversed-polarity chron occurred. The youngest four polarity chrons (Brunhes, Matuyama, Gauss, and Gilbert) were named after scientists who had made important contributions to the study of magnetism. As more measurements became available, investigators realized that some short-duration reversals (less than 200,000 years long) took place within the chrons, and they called these shorter durations “polarity subchrons.” Using isotopic dating, it was possible to determine the age of chrons back to 4.5 Ma.

Interpreting marine magnetic anomalies

The progressive development of magnetic anomalies and the long-term reversals chronology.

Why do marine magnetic anomalies exist? In 1963, researchers in Britain and Canada proposed a solution to this riddle. Simply put, a positive anomaly occurs over areas of the sea floor where underlying basalt has normal polarity. In these areas, the magnetic force produced by the magnetite grains in basalt adds to the force produced by the Earth’s dipole the sum of these forces yields a stronger magnetic signal than expected due to the dipole alone (figure above a). A negative anomaly occurs over regions of the sea floor where the underlying basalt has a reversed polarity. In these regions, the magnetic force of the basalt subtracts from the force produced by the Earth’s dipole, so the measured magnetic signal is weaker than expected. 

The sea-floor-spreading model easily explains not only why positive and negative magnetic anomalies exist over the sea floor, but also why they define stripes that trend parallel to the mid-ocean ridge and why the pattern of stripes on one side of the ridge is the mirror image of the pattern on the other side  (figure above b). To see why, let’s examine stages in the process of sea-floor spreading (figure above c). Imagine that at Time 1 in the past, the Earth’s magnetic field has normal polarity. As the basalt rising at the mid-ocean ridge during this time interval cools and solidifies, the tiny magnetic grains in basalt align with the Earth’s field, and thus the rock as a whole has a normal polarity. Sea floor formed during Time 1 will therefore generate a positive anomaly and appear as a dark stripe on an anomaly map. As it forms, the rock of this stripe moves away from the ridge axis, so half goes to the right and half to the left. Now imagine that later, at Time 2, Earth’s field has reversed polarity. Sea-floor basalt formed during Time 2, therefore, has reversed polarity and will appear as a light stripe on an anomaly map. As it forms, this reversed-polarity stripe moves away from the ridge axis, and even younger crust forms along the axis. The basalt in each new stripe of crust preserves the polarity that was present at the time it formed, so as the Earth’s magnetic field flips back and forth, alternating positive and negative anomaly stripes form. A positive anomaly exists over the ridge axis today because sea floor is forming during the present chron of normal polarity. 

Closer examination of a sea-floor magnetic anomaly map reveals that anomalies are not all the same width. Geologists found that the relative widths of anomaly stripes near the Mid-Atlantic Ridge are the same as the relative durations of paleomagnetic chrons (figure above d). This relationship between anomaly-stripe width and polarity-chron duration indicates that the rate of sea-floor spreading has been constant along the Mid-Atlantic Ridge for at least the last 4.5 million years. If you assume that the spreading rate was constant for tens to hundreds of millions of years, then it is possible to estimate the age of stripes right up to the edge of the ocean.

Evidence from Deep-Sea Drilling 

In the late 1960s, a research drilling ship called the Glomar Challenger set out to sail around the ocean drilling holes into the sea floor. This amazing ship could lower enough drill pipe to drill in 5-km-deep water and could continue to drill until the hole reached a depth of about 1.7 km (1.1 miles) below the sea floor. Drillers brought up cores of rock and sediment that geoscientists then studied on board. 

On one of its early cruises, the Glomar Challenger drilled a series of holes through sea-floor sediment to the basalt layer. These holes were spaced at progressively greater distances from the axis of the Mid-Atlantic Ridge. If the model of sea-floor spreading was correct, then not only should the sediment layer be progressively thicker away from the axis, but the age of the oldest sediment just above the basalt should be progressively older away from the axis. When the drilling and the analyses were complete, the prediction was confirmed. Thus, studies of both marine magnetic anomalies and the age of the sea floor proved the sea-floor-spreading model.


Credits: Stephen Marshak ( Essentials of Geology)

The Discovery of Sea-Floor Spreading

The Discovery of Sea-Floor Spreading

kacimo samy   7:43 ص

The Discovery of Sea-Floor Spreading

New Images of Sea-Floor Bathymetry 

Bathymetry of mid-ocean ridges and abyssal plains.

Military needs during World War II gave a boost to sea-floor exploration, for as submarine fleets grew, navies required detailed information about bathymetry, or depth variations. The invention of echo sounding (sonar) permitted such information to be gathered quickly. Echo sounding works on the same principle that a bat uses to navigate and find insects. A sound pulse emitted from a ship travels down through the water, bounces off the sea floor, and returns up as an echo through the water to a receiver on the ship. Since sound waves travel at a known velocity, the time between the sound emission and the echo detection indicates the distance between the ship and the sea floor. (Recall that  velocity distance/time, so distance velocity s time.) As the ship travels, observers can obtain a continuous record of the depth of the sea floor. The resulting cross section showing depth plotted against location is called a bathymetric profile (figure above a, b). By cruising back and forth across the ocean many times, investigators obtained a series of bathymetric profiles and from these constructed maps of the sea floor. (Geologists can now produce such maps much more rapidly using satellite data.) Bathymetric maps reveal several important features.

Other bathymetric features of the ocean floor.

• Mid-ocean ridges: The floor beneath all major oceans includes abyssal plains, which are broad, relatively flat regions of the ocean that lie at a depth of about 4 to 5 km below sea level; and mid-ocean ridges, submarine mountain ranges whose peaks lie only about 2 to 2.5 km below sea level (figure above a). Geologists call the crest of the mid-ocean ridge the ridge axis. All mid-ocean ridges are roughly symmetrical bathymetry on one side of the axis is nearly a mirror image of bathymetry on the other side. 
• Deep-ocean trenches: Along much of the perimeter of the Pacific Ocean, and in a few other localities as well, the ocean floor reaches depths of 8 to 12 km deep enough to swallow Mt. Everest. These deep areas occur in elongate troughs that are now referred to as trenches (figure above b). Trenches border volcanic arcs, curving chains of active volcanoes. 
• Seamount chains: Numerous volcanic islands poke up from the ocean floor: for example, the Hawaiian Islands lie in the middle of the Pacific. In addition to islands that rise above sea level, sonar has detected many seamounts (isolated submarine mountains), which were once volcanoes but no longer erupt. Volcanic islands and seamounts typically occur in chains, but in contrast to the volcanic arcs that border deep ocean trenches, only one island at the end of a seamount and island chain remains capable of erupting volcanically today. 
• Fracture zones: Surveys reveal that the ocean floor is diced up by narrow bands of vertical cracks and broken-up rock. These fracture zones lie roughly at right angles to mid-ocean ridges. The ridge axis typically steps sideways when it intersects with a fracture zone.

New Observations on the Nature of Oceanic Crust 

By the mid-20th century, geologists had discovered many important characteristics of the sea-floor crust. These discoveries led them to realize that oceanic crust differs from continental crust, and that bathymetric features of the ocean floor provide clues to the origin of the crust. Specifically: 

• A layer of sediment composed of clay and the tiny shells of dead plankton covers much of the ocean floor. This layer becomes progressively thicker away from the mid-ocean ridge axis. But even at its thickest, the sediment layer is too thin to have been accumulating for the entirety of Earth history. 
• By dredging up samples, geologists learned that oceanic crust is fundamentally different in composition from continental crust. Beneath its sediment cover, oceanic crust bedrock consists primarily of basalt it does not display the great variety of rock types found on continents. 
• Heat flow, the rate at which heat rises from the Earth’s interior up through the crust, is not the same everywhere in the oceans. Rather, more heat rises beneath mid-ocean ridges than elsewhere. This observation led researchers to speculate that hot magma might be rising into the crust just below the mid-ocean ridge axis. 
• When maps showing the distribution of earthquakes in oceanic regions became available in the years after World War II, it became clear that earthquakes do not occur randomly, but rather define distinct belts (figure below). Some belts follow trenches, some follow mid-ocean ridge axes, and others lie along portions of fracture zones. Since earthquakes define locations where rocks break and move, geologists realized that these bathymetric features are places where motion is taking place.

A 1953 map showing the distribution of earthquake locations in the ocean basins. Note that earthquakes occur in belts.

Harry Hess and His “Essay in Geopoetry” 

Harry Hess's basic concept of sea-floor spreading. Hess implied, incorrectly, that only the crust moved. We will see that this sketch is an oversimplified.

In the late 1950s, Harry Hess, after studying the observations described above, realized that because the sediment layer on the ocean floor was thin overall, the ocean floor might be much younger than the continents. Also, because the sediment thickened progressively away from mid-ocean ridges, the ridges themselves likely were younger than the deeper parts of the ocean floor. If this was so, then somehow new ocean floor must be forming at the ridges, and thus an ocean basin could be getting wider with time. But how? The association of earthquakes with mid-ocean ridges suggested to him that the sea floor was cracking and splitting apart at the ridge. The discovery of high heat flow along mid-ocean ridge axes provided the final piece of the puzzle, for it suggested the presence of very hot molten rock beneath the ridges. In 1960, Hess suggested that indeed molten rock (basaltic magma) rose upward beneath mid-ocean ridges and that this material solidified to form oceanic crust basalt (figure above). The new sea floor then moved away from the ridge, a process we now call sea-floor spreading. Hess realized that old ocean floor must be consumed somewhere, or the Earth would have to be expanding, so he suggested that deep ocean trenches might be places where the sea floor sank back into the mantle. Hess suggested that earthquakes at trenches were  evidence of this movement, but he didn’t understand how the movement took place. Other geologists, such as Robert Dietz, were coming to similar conclusions at about the same time.

Hess and his contemporaries realized that the sea-floor-spreading hypothesis instantly provided the long sought explanation of how continental “drift” occurs. Continents passively move apart as the sea floor between them spreads at mid-ocean ridges, and they passively move together as the sea floor between them sinks back into the mantle at trenches. (As we will see later, geologists now realize that it is the lithosphere that moves, not just the crust.) Thus, sea-floor spreading proved to be an important step on the route to plate tectonics the idea seemed so good that Hess referred to his description of it as “an essay in geopoetry.” But first, the idea needed to be tested, and other key discoveries would have to take place before the whole theory of plate tectonics could come together. 

Paleomagnetism and the Proof of Continental Drift

Paleomagnetism and the Proof of Continental Drift

kacimo samy   7:20 ص

Paleomagnetism and the Proof of Continental Drift

More than 1,500 years ago, Chinese sailors discovered that a piece of lodestone, when suspended from a thread, points in a northerly direction and can help guide a voyage. Lodestone exhibits this behaviour because it consists of magnetite, an iron rich mineral that, like a compass needle, aligns with Earth’s magnetic field lines. While not as magnetic as lodestone, several other rock types contain tiny crystals of magnetite, or other magnetic minerals, and thus behave overall like weak magnets. In this section, we explain how the study of such magnetic behaviour led to the realization that rocks preserve paleomagnetism, a record of Earth’s magnetic field in the past. An understanding of paleomagnetism provided proof of continental drift and, contributed to the development of plate tectonics theory. As a foundation for introducing paleomagnetism, we first provide additional detail about the basic nature of the Earth’s magnetic field.

Earth’s Magnetic Field 

Features of Earth’s magnetic field.

Circulation of liquid iron alloy in the outer core of the Earth generates a magnetic field. (A similar phenomenon happens in an electrical dynamo at a power plant.) Earth’s magnetic field resembles the field produced by a bar magnet, in that it has two ends of opposite polarity. Thus, we can represent Earth’s field by a magnetic dipole, an imaginary arrow (figure above a). Earth’s dipole intersects the surface of the planet at two points, known as the magnetic poles. By convention, the north magnetic pole is at the end of the Earth nearest the north geographic pole (the point where the northern end of the spin axis intersects the surface). The north-seeking (red) end of a compass needle points to the north magnetic pole.

Earth’s magnetic poles move constantly, but don’t seem to stray further than about 1,500 km from the geographic poles, and averaged over thousands of years, they roughly coincide with Earth’s geographic poles (figure above b). That’s because the rotation of the Earth causes the flow to organize into patterns resembling spring-like spirals, and these are roughly aligned with the spin axis. At present, the magnetic poles lie hundreds of kilometres away from the geographic poles, so the magnetic dipole tilts at about 11° relative to the Earth’s spin axis. Because of this difference, a compass today does not point exactly to geographic north. The angle between the direction that a compass needle points and a line of longitude at a given location is the magnetic declination (figure above c).

Invisible field lines curve through space between the magnetic poles. In a cross-sectional view, these lines lie parallel to the surface of the Earth (that is, are horizontal) at the equator, tilt at an angle to the surface in mid-latitudes, and plunge perpendicular to the surface at the magnetic poles (figure above d). The angle between a magnetic field line and the surface of the Earth, at a given location, is called the magnetic inclination. If you place a magnetic needle on a horizontal axis so that it can pivot up and down, and then carry it from the magnetic equator to the magnetic pole, you’ll see that the inclination varies with latitude it is 0° at the magnetic equator and 90° at the magnetic poles. (Note that the compass you may carry with you on a hike does not show inclination because it has been balanced to remain horizontal.)

What Is Paleomagnetism? 

Paleomagnetism and how it can form during the solidification and cooling of lava.

In the early 20th century, researchers developed instruments that could measure the weak magnetic field produced by rocks and made a surprising discovery. In a rock that formed millions of years ago, the orientation of the dipole representing the magnetic field of the rock is not the same as that of present day Earth (figure above a). To understand this statement, consider an example. Imagine travelling to a location near the coast on the equator in South America where the inclination and declination are presently 0°. If you measure the weak magnetic field produced by, say, a 90-million-year-old rock, and represent the orientation of this field by an imaginary bar magnet, you’ll find that this imaginary bar magnet does not point to the present day north magnetic pole, and you’ll find that its inclination is not 0°. The reason for this difference is that the magnetic fields of ancient rocks indicate the orientation of the magnetic field, relative to the rock, at the time the rock formed. This record, preserved in rock, is paleomagnetism.

Paleomagnetism can develop in many different ways. For example, when lava, molten rock containing no crystals, starts to cool and solidify into rock, tiny magnetite crystals begin to grow (figure above b). At first, thermal energy causes the tiny magnetic dipole associated with each crystal to wobble and tumble chaotically. Thus, at any given instant, the dipoles of the magnetite specks are randomly oriented and the magnetic forces they produce cancel each other out. Eventually, however, the rock cools sufficiently that the dipoles slow down and, like tiny compass needles, align with the Earth’s magnetic field. As the rock cools still more, these tiny compass needles lock into permanent parallelism with the Earth’s magnetic field at the time the cooling takes place. Since the magnetic dipoles of all the grains point in the same direction, they add together and produce a measurable field.

Apparent Polar Wander A Proof That Continents Move

Apparent Polar wander paths and their interpretation.

Why doesn't the paleomagnetic dipole in ancient rocks point to the present-day magnetic field? When geologists first attempted to answer this question, they assumed that continents were fixed in position and thus concluded that the positions of Earth’s magnetic poles in the past were different than they are today. They introduced the term paleopole to refer to the  supposed position of the Earth’s magnetic north pole in the past. With this concept in mind, they set out to track what they thought was the change in position of the paleopole over time. To do this, they measured the paleomagnetism in a succession of rocks of different ages from the same general location on a continent, and they plotted the position of the associated  succession of paleopole positions on a map (figure above a). The successive positions of dated paleopoles trace out a curving line that came to be known as an apparent polar-wander path. 

At first, geologists assumed that the apparent polarwander path actually represented how the position of Earth’s magnetic pole migrated through time. But were they in for a surprise! When they obtained polar-wander paths from many different continents, they found that each continent has a different apparent polar-wander path. The hypothesis that continents are fixed in position cannot explain this observation, for if the magnetic pole moved while all the continents stayed fixed, measurements from all continents should produce the same apparent polar-wander paths.

Geologists suddenly realized that they were looking at apparent polar-wander paths in the wrong way. It’s not the pole that moves relative to fixed continents, but rather the continents that move relative to a fixed pole (figure above b). Since each continent has its own unique polar-wander path (figure above c), the continents must move with respect to each other. The discovery proved that Wegener was essentially right all along, continents do move!

Wegener’s Evidence for Continental Drift

Wegener’s Evidence for Continental Drift

kacimo samy   7:00 ص

Wegener’s Evidence for Continental Drift 

Wegener suggested that a vast supercontinent, Pangaea, existed until near the end of the Mesozoic Era (the interval of geologic time that lasted from 251 to 65 million years ago). He suggested that Pangaea then broke apart, and the landmasses moved away from each other to form the continents we see today. Let’s look at some of Wegener’s arguments and see what led him to formulate this hypothesis of continental drift.

The Fit of the Continents 

Almost as soon as maps of the Atlantic coastlines became available in the 1500s, scholars noticed the fit of the continents. The northwestern coast of Africa could tuck in against the eastern coast of North America, and the bulge of eastern South America could nestle cozily into the indentation of southwestern Africa. Australia, Antarctica, and India could all connect to the southeast of Africa, while Greenland, Europe, and Asia could pack against the northeastern margin of North America. In fact, all the continents could be joined, with remarkably few overlaps or gaps, to create Pangaea. Wegener concluded that the fit was too good to be coincidence and thus that the continents once did fit together.

Locations of Past Glaciations 

Glaciers are rivers or sheets of ice that flow across the land surface. As a glacier flows, it carries sediment grains of all sizes (clay, silt, sand, pebbles, and boulders). Grains protruding from the base of the moving ice carve scratches, called striations, into the substrate. When the ice melts, it leaves the sediment in a deposit called till, that buries striations. Thus, the occurrence of till and striations at a location serve as evidence that the region was covered by a glacier in the past (see chapter opening photo). By studying the age of glacial till deposits, geologists have determined that large areas of the land were covered by glaciers during time intervals of Earth history called ice ages. One of these ice ages occurred from about 326 to 267 Ma, near the end of the Paleozoic Era. 

Wegner's evidence for continental drift came from analysing the geologic record.

Wegener was an Arctic climate scientist by training, so it’s no surprise that he had a strong interest in glaciers. He knew that glaciers form mostly at high latitudes today. So he suspected that if he plotted a map of the locations of late Paleozoic glacial till and striations, he might gain insight into the locations of continents during the Paleozoic. When he plotted these locations, he found that glaciers of this time interval occurred in southern South America, southern Africa, southern India, Antarctica, and southern Australia. These places are now widely separated and, with the exception of Antarctica, do not currently lie in cold polar regions (figure above a). To Wegener’s amazement, all late Paleozoic glaciated areas lie adjacent to each other on his map of Pangaea. Furthermore, when he plotted the orientation of glacial striations, they all pointed roughly outward from a location in southeastern Africa. In other words, Wegener determined that the distribution of glaciations at the end of the Paleozoic Era could easily be explained if the continents had been united in Pangaea, with the southern part of Pangaea lying beneath the center of a huge ice cap. This distribution of glaciation could not be explained if the continents had always been in their present positions.

The Distribution of Climatic Belts 

If the southern part of Pangaea had straddled the South Pole at the end of the Paleozoic Era, then during this same time interval, southern North America, southern Europe, and northwestern Africa would have straddled the equator and would have had tropical or subtropical climates. Wegener searched for evidence that this was so by studying sedimentary rocks that were formed at this time, for the material making up these rocks can reveal clues to the past climate. For example, in the swamps and jungles of tropical regions, thick deposits of plant material accumulate, and when deeply buried, this material transforms into coal. And in the clear, shallow seas of tropical regions, large reefs develop. Finally, subtropical regions, on either side of the tropical belt, contain deserts, an environment in which sand dunes form and salt from evaporating seawater or salt lakes accumulates. Wegener speculated that the distribution of late Paleozoic coal, reef, sand-dune, and salt deposits could define climate belts on Pangaea. 

Sure enough, in the belt of Pangaea that Wegener expected to be equatorial, late Paleozoic sedimentary rock layers include abundant coal and the relicts of reefs. And in the portions of Pangaea that Wegener predicted would be subtropical, late Paleozoic sedimentary rock layers include relicts of desert dunes and deposits of salt (figure above b). On a present-day map of our planet, exposures of these ancient rock layers scatter around the globe at a variety of latitudes. On Wegener’s Pangaea, the exposures align in continuous bands that occupy appropriate latitudes.

The Distribution of Fossils 

Today, different continents provide homes for different species. Kangaroos, for example, live only in Australia. Similarly, many kinds of plants grow only on one continent and not on others. Why? Because land-dwelling species of animals and plants cannot swim across vast oceans, and thus evolved independently on different continents. During a period of Earth history when all continents were in  contact, however, land animals and plants could have migrated among many continents. 

With this concept in mind, Wegener plotted fossil occurrences of land-dwelling species that existed during the late Paleozoic and early Mesozoic Eras (between about 300 and 210 million years ago) and found that these species had indeed existed on several continents (figure above c). Wegener argued that the distribution of fossil species required the continents to have been adjacent to one another in the late Paleozoic and early Mesozoic Eras.

Matching Geologic Units 

 Further evidence of drift: rocks on different sides of the ocean match.

An art historian can recognize a Picasso painting, an architect knows what makes a building look “Victorian,” and a geoscientist can identify a distinctive assemblage of rocks. Wegener found that the same distinctive Precambrian rock assemblages occurred on the eastern coast of South America and the western coast of Africa, regions now separated by an ocean  (figure above a). If the continents had been joined to create Pangaea in the past, then these matching rock groups would have been adjacent to each other, and thus could have composed continuous blocks or belts. Wegener also noted that features of the Appalachian Mountains of the United States and Canada closely resemble mountain belts in southern Greenland, Great Britain, Scandinavia, and northwestern Africa (figure above b, c), regions that would have lain adjacent to each other in Pangaea. Wegener thus demonstrated that not only did the coastlines of continents match, so too did the rocks adjacent to the coastlines.

Criticism of Wegener’s Ideas 

Wegener’s model of a supercontinent that later broke apart explained the distribution of ancient glaciers, coal, sand dunes, rock assemblages, and fossils. Clearly, he had compiled a strong circumstantial case for continental drift. But as noted earlier, he could not adequately explain how or why continents drifted. He left on his final expedition to Greenland having failed to convince his peers, and he died without knowing that his ideas, after lying dormant for decades, would be reborn as the basis of the broader theory of plate tectonics. 

In effect, Wegener was ahead of his time. It would take three more decades of research before geologists obtained sufficient data to test his hypotheses properly. Collecting this data required instruments and techniques that did not exist in Wegener’s day. Of the many geologic discoveries that ultimately opened the door to plate tectonics, perhaps the most important came from the discovery of a phenomenon called paleomagnetism, so we discuss it next.

Credit: Stephen Marshak (Essentials of Geology)