Theories of Geography Part 3 – Plate tectonics

Plate Tectonics

Evidence of Continental Drift: Wegener’s Continental Drift Hypothesis

The Lithosphere is always in a state of work in progress. In 1912, a German geologist named Alfred Wegener came up with an outlandish theory known as continental drift. His theory was based upon the following clues.

1. Continental Fit

One of the first clues he had was that the continents were once joined once, by noting the jigsaw puzzle–like geometry of Africa’s west coast and South America’s east coast. This was called “Continental Fit”.

2. Fossils

Fossils of Mesosaurus, a freshwater reptile, have only been found in Africa and South America. The fossil remains of Cynognathus, a land reptile, are found in South America and Africa. A fern called Glossopteris was found fossilized on all of the southern continents. Since these continents all have different climates now, Wegener proposed that they once all shared a similar climate as one landmass. The evidence of another land reptile, Lystrosaurus, was found in Africa, India, and Australia.

3. Coal Fields

He noticed the presence of coal fields in the temperate regions, while they could only be formed in the Tropical regions.

4. Glacial Flow

Wegner noticed that all over the southern hemisphere there are glacier deposits left over from millions of years ago. India, which is now located above the equator, shows signs of glaciers moving across it from the south. Since, it cannot be explained without continental drift why would glaciers move toward India from the equator? The clue Wegner had was of a single giant ice sheet that moved outward from Antarctica.

5. Similarity in Rocks

The similarity in the rock structure on opposite sides of the Atlantic was another clue. So, Wegener proposed that the present continents were once joined in a super continent named Pangaea and later the drifted apart. Wegener proposed that the Pangaea broke into continents and the new continents drove away themselves in two directions viz. Equatorward and Westward movements

He said that the movements towards the equators were because of the gravitational differential forces and force of buoyancy. The Westward movement occurred because of the tidal force of sun and moon. He proposed that the Pangaea began to separate into the Gondwanaland and Angaraland in the Carboniferous period and the space between the two was filled with water that was called Tethys Sea. Later the Gondwanaland disrupted during the Cretaceous period and with this, the Indian subcontinent (peninsula), Madagaskar, Australia and Antarctica broke away from the Gondwanaland. Similarly the North America broke away from the Angaraland and drifted westward due to Tidal forces. He went on further proposing that South America broke way from Africa and moved westwards due to Tidal forces. This theory was interesting and thrilling but Wegener was unable to explain what the forces behind this drift were. So, the result was that Alfred Wegener was derided by the scientific community; his proposal was called “geopoetry”.  However, the later discoveries in deep-sea science led Wegener’s basic proposition to be accepted as fact, and today a good deal is known about how the continental drift occurs.

6. Paleomagnetism

Earth has a magnetic field that causes a compass needle to always point toward the North magnetic pole. When the magnetic minerals cool down, the domains within the magnetic mineral take on an orientation parallel to any external magnetic field present at the time they cooled below this temperature. Using this, it can be determined what the orientation of the magnetic field present was at the time the rock containing the mineral cooled, and thus it is able to determine the position of the magnetic pole at that time. Magnetite is the most common magnetic mineral in the Earth’s crust. The studies showed that the magnetic pole had apparently moved through time. When similar measurements were made on rocks of various ages in North America, however, a different path of the magnetic pole was found. This would first imply that either the Earth has had more than one magnetic pole at various times in the past, which cannot happen. The second implication is that the different continents have moved relative to each other over time. This led to the confirmation of the theory of continental drift.

Plate Tectonics and Seafloor Spreading

Lithosphere is made up of about a dozen giant and several smaller sections called plates, and these move in various directions in processes known collectively as plate tectonics. The below graphics shows the plates and their general direction of Movement.

Earthquakes, volcanoes, and other geologic events are concentrated where plates separate, collide, or slide past one another. Where they separate, rifting produces very low land elevations (e.g. well below sea level at the Dead Sea of Israel and Jordan) or the emergence of new crust on the ocean floor (e.g. in the middle of the Atlantic Ocean). The central item in the Plate Tectonics is the Mid-Oceanic Ridge. The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid- oceanic ridge system the longest mountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long.

When the ocean floors such as Mid-Atlantic Ridge and the East Pacific Rise, new lithosphere is “born” as molten material rises from the earth’s mantle and cools into solid rock. Plate tectonics are often explained by the useful analogy of a “conveyor belt” in constant motion. On either side of the long, roughly continuous ridges, the two young plates move away from one another, carrying islands with them; this process is called seafloor spreading. Seafloor spreading has few impacts on us, but when the earth’s plates collide, there is cause for great concern: tectonic forces are among the planet’s greatest natural hazards. The seismic activity (seismic refers to earth vibrations, mainly earthquakes) that causes earthquakes and tsunamis (tidal waves) and the volcanism (movement of molten earth material) of volcanoes and related features are the most dangerous tectonic forces.


We have already studied that the Oceanic lithosphere is thinner and denser, whereas continental lithosphere is thicker and lighter. Both of these crustal plates float on the plastic asthenosphere. We can visualize this as two blocks of wood floating in water, where a thicker block rides higher above the water surface than a thinner block. This implies that the thicker continental surfaces rise higher above the ocean floors. In the below graphics, there are four plates viz. A, B, C and D.  Plate A and B are pulling apart along their common boundary, which lies along the axis of a midoceanic ridge. When they pull apart, it creates a gap in the crust that is filled by magma rising from the mantle beneath. At greater depth under the rift, magma solidifies into plutonic rocks. The boundary between the plates A and B is called a spreading boundary.

In the right, we see that the oceanic lithosphere of plate B is moving toward the continental lithosphere of plate C. Where these two plates collide, they form a converging boundary. Here, since the oceanic plate is comparatively thin and dense, in contrast to the thick, buoyant continental plate, the oceanic lithosphere bends down and plunges into the asthenosphere. The process in which one plate is carried beneath another is called subduction.

The descending lithosphere is melted again as it dives into the earth’s mantle along a deep linear feature called trench (such as the Mariana Trench off Japan). Subduction is another stage along the “conveyor belt” process that will eventually see this material recycled as newborn lithospheric crust. This subduction process releases enormous amounts of energy. The great stress of one plate pushing beneath another is released in the form of an earthquake. The world’s largest recorded earthquakes—registering 9.5 (Chile, 1960), 9.2 (United States, 1964), and 9.1 (Indonesia, 2004), respectively, on the Richter scale, which measures the strength of the earthquake at its source—struck along these subduction zones. This sudden displacement of a section of oceanic lithosphere is also what triggers a tsunami and the attendant loss of life and property such a powerful wave can cause. Further, the Volcanism generally occurs at places near the subduction zones.

Movement of Plates – Faulting

In some other places, the lithospheric plates grind and slide along one another. The processes of rock crowding together or pulling apart along these fracture lines is known as faulting. The movement along various kinds of faults causes earthquakes, the emergence of new landforms, and other consequences. They are of the following types:

  • Normal – tension in the crust (a ‘pulling apart”)
  • Reverse – Compression in crust (a ‘pushing in’)
  • Reverse Thrust Fault
  • Transform

Subduction is responsible for high rates of volcanism, earthquakes, and mountain building. When the large pieces of material on the subducting plate are pressed into the overriding plate, it results in the Orogeny or Mountain formation. These areas are subject to many earthquakes.

Faulting Versus Folding

Please note that both faulting of the Rocks and folding of the Rocksplay role in creation of the Earthquake, however, the role of later is also dependent upon the former. Earthquakes usually occur where Earth’s crust has cracks and is weak. The cracks through which these vibrations pass are called Faults. The movement of rocks along these faults cause earthquakes. As a result of the earthquake, the rocks on the surface of earth change from their earlier position. Their up and down bending into elevations and hollows is called folding of rocks. When the folding continues for a long time, the beds of the rocks can no longer bear the pressure of the force. They break and the rocks may be thrown up on one side and down on the other, thus resulting in Faulting.

The Lithospheric Plates System and Plate Boundaries

The Earth’s surface is composed of six major lithospheric plates’ viz. Pacific, American, Eurasian, African, Austral-Indian, and Antarctic. Apart from those, there are some lesser plates and sub plates also. The below graphics shows these Lithospheric Plates.

Some important notable observations about these plates are as follows:

  • American plate includes most of the continental lithosphere of North and South America.
  • Most part of the Eurasian plate is continental lithosphere, but it is fringed on the west and north by a belt of oceanic lithosphere.
  • African Plate is also known as the Nubia Plate. It is a mix of continental and oceanic lithosphere.
  • The great Pacific plate occupies much of the Pacific Ocean basin and consists almost entirely of oceanic lithosphere.
  • The Antarctic plate is almost completely enclosed by a spreading plate boundary. This means that the other plates are moving away from the pole.
  • The continent of Antarctica forms a central core of continental lithosphere completely surrounded by oceanic lithosphere.
  • The Austral-Indian plate is mostly oceanic lithosphere but contains two cores of continental lithosphere– Australia and peninsular India. The recent studies show that they may be different parts of two different plates.

Plate Boundaries

The above discussed Lithospheric Plates are composed of lithosphere, about 100 km thick, that “float” on the plastic asthenosphere. While the continents do indeed appear to drift, they do so only because they are part of larger plates that float and move horizontally on the upper mantle asthenosphere. The plate boundaries can be identified because they are zones along which maximum earthquakes occur.  Plate interiors have much fewer earthquakes. There are three types of plate boundaries:

  • Convergent Plate Boundaries: where plates move toward each other.
  • Divergent Plate boundaries: where plates move away from each other.
  • Transform Plate Boundaries: where plates slide past one another.

Convergent Plate Boundaries

The convergent plate boundaries are also responsible for nearly 75% of Earth’s volcanoes.  There are following types of Convergent Boundaries:

Ocean-Ocean Convergent Plate Boundary

When two oceanic plates meet and collide against each other, the denser of the two plates is pulled under the other and is subducted. It descends into the asthenosphere, or upper mantle, where it will lead to the generation of new magma. Such boundary would be called an Ocean-ocean convergent plate boundary.

Please note that when one oceanic plate is subducted under the other, the resulting new magma is less dense than the surrounding rock. Therefore it easily rises and erupts on the seafloor, ultimately building a volcano or a volcanic island in the sea. Areas of ocean-ocean convergence are characterized by ocean trenches, seafloor volcanoes, and volcanic islands.

Island Volcanic Arc

At ocean-ocean convergent boundaries, the resulting body of many volcanoes is called an island volcanic arc. An island volcanic arc may include islands that develop in the sea from the build-up of volcanic rocks. Thus, Island volcanic arcs are a chain of islands and mountains that form on the overriding or non-subducting oceanic plate. Examples of such arcs are Japan, the Philippines, the Tonga Islands, the Aleutian Islands, and the West Indies Islands etc. All of them have developed parallel to the direction of subduction.



Ocean-Continental Convergent Boundary

Convergence of an oceanic plate with a continental plate is similar to ocean-ocean convergence and often results in the volcanic. When an oceanic plate collides with a continental plate, the oceanic plate is always pulled under and subducted because it is denser than the continental plate. When the oceanic plate is subducted under the continental plate, it leads to the generation of new magma, which upwells and forms volcanoes on the non-subducting plate, or the continental plate. Thus Volcanoes are common on Ocean-Continent Boundary also. At ocean-continent boundaries, the resulting body of volcanoes is called a continental volcanic arc. Continental volcanic arcs are chains of volcanoes found on the margin of the continent above a subduction zone at ocean-continent boundaries. The most visible example is Andes Mountains off the west coast of the U.S. Hefre we should also note that Pacific Ring of Fire, where subduction is taking place at numerous trenches that border the continental shores, has  450 volcanoes, more than 75% of all the volcanoes on Earth. This makes plate convergence responsible for nearly all volcanic activity on Earth.

Continent-Continent Boundary

When the continent and continent converge, the crust at both the sides is too light and buoyant to be subducted, so neither plate is subducted in continent-continent convergent boundary. Both continental masses press against the other, and both become compressed and ultimately fused into a single block with a folded mountain belt forming between them.

This is the type of activity is responsible for forming the Himalayas, and is still going on. The Himalayas are still growing, as we all know. Please note that due to intense pressure between the colliding plates, metamorphic rocks formation is common at such boundaries. Please also note that Volcanoes are not common at Continent-continent convergent boundaries because there is no subduction of plates. Subduction is prerequisite for formation of the new magma.

Divergent Plate Boundaries

The Continental Drift Theory says that all the continents were once joined together in one giant supercontinent called Pangaea. Because of plate tectonics, Pangaea broke apart and the continents began their slow migration to their present locations. The Atlantic Ocean opened up in between North America and the west coasts of Europe and Africa. The agent for causing this is the Mid- Atlantic Ridge, a divergent plate boundary, where two plates are rifting and moving away from each other.  Thus, divergent plate boundaries are places of extension stress, where the crust is being extended, thinned, and rifted.

In the convergent plate boundaries are the destructive plate boundaries where the crustal material is consumed at the subduction zones. However, the divergent plate boundaries are constructive boundaries because it leads to formation of new Lithosphere. The creation of the new crustal material takes place at mid-ocean ridges, where the oceanic crust is rifted open and magma wells up to fill the opening. The magma then hardens to form the igneous rocks that make up the oceanic crust. This is the mechanism which forms maximum amount of rock material on earth.

Comparison: Divergent and Convergent Plate Boundaries

Important Points to Remember

Convergent Boundaries Divergent Boundaries
Explosive Volcanoes Quiet, Non explosive volcanoes
High Silicic Magma: The magma comes from the subduction of lithospheric crust so it has more of silicate. High Basaltic Magma: Oceanic crust is created at the mid-oceanic ridges; it forms from up welling magma that cools and solidifies to igneous rock. Most of this is Basaltic.
Stratovolcanoes Shield Volcanoes
Consumption of the Ocean Floor Creation of Ocean Floor
Shallow, Intermediate as well as Deep Focus Earthquakes


Shallow Focus earthquakes only


Continental Rift Zones

Please note that the divergent plate boundaries can also develop on the continents, and here, we name them as Continental Rift Zones. Most of the features of Oceanic Divergent boundaries are valid for them also such as thinned crust; normal faults; shallow earthquakes; basaltic volcanoes etc. While the Continental Rift Zones develop, the earth is stretched and thinned, leading to development of a small body of water. When the rifting keeps continuing, the body of water grows bigger to form a juvenile ocean. After millions of years of rifting, the body of water becomes a mature ocean with two separate continents on each side. Red Sea and Gulf of Aden is the best example of this phenomenon.

Transform Plate Boundary

Transform plate boundaries are places where two plates are sliding past each other. At these boundaries, the plates are neither compression nor extension stress, but are under shear stress. Then there is neither creation nor consumption of the lithospheric material. So, the transform plate boundaries are basically faults and nothing else. The transform plate boundaries can cause horizontal displacement of hundreds of kilometers of land on the continents which results in several types of landscapes such as ridges and troughs. In oceans, transform plate boundaries are part of fracture zones. Earthquakes are most common at transform plate boundaries. Volcanoes rarely develop at transform plate boundaries because transform boundaries do not allow for the upwelling or new creation of magma.

January 11, 2018

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