Chapter 4 Distribution Of Oceans And Continents

Continental Drift

Looking at a world map, one might notice the striking similarity in the shapes of the coastlines on opposite sides of the Atlantic Ocean, particularly between South America and Africa. This apparent fit has long fascinated observers. As early as 1596, Dutch cartographer Abraham Ortelius suggested that these continents might have once been connected. Later, Antonio Pellegrini even created a map depicting this idea.

However, it was the German meteorologist Alfred Wegener who, in 1912, proposed a comprehensive theory known as the Continental Drift Theory. This theory aimed to explain the distribution of landmasses and oceans across the globe.

Wegener hypothesized that, in the distant past, all the Earth's continents were joined together into a single, giant landmass called Pangaea (meaning "all Earth"). This supercontinent was surrounded by a vast global ocean, which he named Panthalassa (meaning "all water").

According to Wegener's theory, around 200 million years ago, Pangaea began to break apart. It first split into two large continents: Laurasia in the Northern Hemisphere and Gondwanaland in the Southern Hemisphere. Over millions of years, Laurasia and Gondwanaland continued to fragment, eventually forming the continents we see today, which then slowly drifted to their current positions.

Evidence In Support Of The Continental Drift

Wegener and others compiled various lines of evidence from geology, paleontology, and climatology to support the idea that continents had moved.

The Matching Of Continents (Jig-Saw-Fit)

One of the most compelling pieces of evidence is the remarkable geometrical fit between the coastlines of continents, especially the eastern coast of South America and the western coast of Africa. In 1964, Sir Edward Bullard used computer modeling to fit the continents together along the 1,000-fathom ($~1,800 \, m$) depth contour (which represents the edge of the continental shelf, arguably a better fit than the present shoreline) and found a very precise match, supporting the idea that they were once joined.

Rocks Of Same Age Across The Oceans

Geological studies and radiometric dating techniques have revealed that rock formations and geological structures found on widely separated continents have striking similarities in age and composition. For instance, very old rocks (around 2 billion years old) found along the coast of Brazil are virtually identical to those found in western Africa. Furthermore, the oldest marine sedimentary rocks found along the coasts of South America and Africa date back only to the Jurassic period, suggesting that the ocean separating them formed around that time or later.

Tillite

Tillite is a type of sedimentary rock formed from glacial deposits (moraine). Evidence of extensive glaciation from the Permo-Carboniferous period (around 300-250 million years ago) is found in several continents now located in the Southern Hemisphere, including India, Africa, the Falkland Islands, Madagascar, Antarctica, and Australia. These areas, which are now far apart and in varied climatic zones, exhibit deposits (the "Gondwana system" of sediments) with a basal layer of thick tillite. The presence of such widespread glacial deposits, particularly in areas now near the equator (like parts of India), strongly suggests that these landmasses were once connected and located near the South Pole, experiencing a common ice age. The similarity in the sedimentary sequences above the tillite also indicates a shared geological history.

Placer Deposits

The discovery of rich placer deposits of gold along the Ghana coast in West Africa is another piece of evidence. Placer deposits are concentrations of heavy minerals accumulated by gravity, typically in stream beds. The source rocks for this gold are found in Brazil, across the Atlantic, but not in Ghana. This supports the idea that the Ghana coast was once adjacent to the gold-bearing regions of Brazil before the Atlantic Ocean opened up, allowing gold to be eroded from Brazil and deposited in the area that would become Ghana.

Distribution Of Fossils

The finding of identical plant and animal fossils on continents now separated by vast oceans provides biological evidence for continental drift. For example, fossils of the small freshwater reptile Mesosaurus have been found only in the Irati formation of Brazil and the Whitehill formation of South Africa. These two locations are currently separated by 4,800 km of the Atlantic Ocean. Since Mesosaurus was adapted to fresh or brackish water and could not have crossed a vast ocean, its presence on both continents suggests they were once connected. Similarly, the distribution of specific land-dwelling creatures like Lemurs across India, Madagascar, and Africa led some to propose a now-submerged land bridge called 'Lemuria' to explain their spread, an idea consistent with these landmasses being part of a larger continent.

Force For Drifting

While Wegener provided strong evidence that continents had moved, his explanation for the driving forces behind this movement was less convincing to the scientific community at the time. He proposed two main forces:

• Pole-Fleeing Force (or Centrifugal Force): He suggested this force, related to the Earth's rotation, caused continents to drift away from the poles towards the equator. This force contributes to the Earth's equatorial bulge.
• Tidal Force: He also invoked the gravitational attraction of the Moon and the Sun, which causes tides, as a potential force. Wegener believed these forces, acting over immense periods, could cause continents to move.

However, calculations by physicists showed that these forces were far too weak to overcome the resistance of the continental crust and mantle, leading many scientists to reject Wegener's theory due to the lack of a plausible driving mechanism.

Post-Drift Studies

Although Wegener's theory faced initial skepticism, particularly regarding the driving forces, significant scientific progress after World War II brought new information, especially from studying the ocean floor. These discoveries revitalized interest in the idea of mobile continents and led to the development of modern plate tectonic theory.

Convectional Current Theory

In the 1930s, well before the widespread acceptance of continental drift, geologist Arthur Holmes proposed a mechanism that could potentially drive continental movement. He suggested the existence of convection currents within the Earth's mantle. These currents, driven by heat generated from the radioactive decay of elements and residual heat from Earth's formation, cause the mantle material to slowly rise (where it is hotter and less dense) and sink (where it is cooler and denser) in a continuous cycle. Holmes theorized that these slow but powerful currents within the mantle could exert drag on the overlying crust (or lithosphere), causing it to move. This concept provided a potential plausible driving force that was missing from Wegener's original theory.

Mapping Of The Ocean Floor

Extensive oceanographic research, particularly funded after World War II, revealed that the ocean floor was not flat and featureless as previously thought, but had significant relief and complex geological structures. This included the discovery of vast submerged mountain ranges (mid-oceanic ridges), deep trenches, abyssal plains, and submarine volcanoes. Importantly, these studies showed that volcanic activity was concentrated along the mid-oceanic ridges. Techniques for dating ocean floor rocks also revealed that they were significantly younger than rocks found on continents. Furthermore, rocks sampled symmetrically on either side of mid-oceanic ridges showed remarkably similar ages and magnetic properties.

Ocean Floor Configuration

Detailed mapping of the ocean floor has revealed key features that provide crucial insights into the processes driving continental movement. The ocean floor can be broadly divided into three main relief divisions based on depth and form.

Diagram showing key features of the ocean floor including continental shelf, slope, rise, abyssal plain, and mid-ocean ridge.

Continental Margins

These represent the submerged edges of the continents, forming the transition zone between the continents and the deep ocean basins. They include the continental shelf (a gently sloping extension of the continent), the continental slope (a steeper descent), the continental rise (a more gradual slope leading to the deep ocean), and sometimes deep-oceanic trenches. Deep-oceanic trenches are particularly significant features, as they are associated with areas where the ocean floor is being recycled back into the mantle.

Abyssal Plains

These are vast, flat, and largely featureless plains found in the deep ocean basins, situated between the continental margins and the mid-oceanic ridges. Abyssal plains are areas where fine-grained sediments, transported from the continents and accumulated over millions of years, cover the underlying oceanic crust, creating smooth topography.

Mid-Oceanic Ridges

This is a massive, interconnected underwater mountain range system that spans all the world's ocean basins, making it the longest mountain chain on Earth (though mostly submerged). A key characteristic of mid-oceanic ridges is a central rift valley running along the crest. This rift is a zone of intense volcanic activity and shallow earthquakes. The volcanoes along these ridges were previously described as mid-oceanic volcanoes and are sites where new oceanic crust is formed.

Distribution Of Earthquakes And Volcanoes

When maps showing the global distribution of earthquakes and volcanoes are examined, they reveal striking patterns that align closely with certain geographical features. Earthquakes and volcanoes are not randomly scattered across the globe but are concentrated in specific belts.

Map illustrating the primary belts of earthquake and volcanic activity around the world.

One prominent belt follows the line of the mid-oceanic ridges, running down the center of the Atlantic Ocean and extending into the Indian Ocean. Earthquakes along this belt typically have shallow focal depths.

Another major belt of activity coincides with the Alpine-Himalayan mountain system and, most significantly, the Circum-Pacific belt, which rims the Pacific Ocean (often called the "Ring of Fire" due to its numerous active volcanoes). Earthquakes in these zones are often deep-seated, originating at significant depths within the Earth.

The close correlation between the locations of earthquakes, volcanoes, deep-ocean trenches, and mid-ocean ridges strongly suggests a relationship between these features and underlying geological processes.

Concept Of Sea Floor Spreading

Building upon the new data gathered from post-World War II ocean floor studies, particularly concerning mid-oceanic ridges and the magnetic properties of oceanic rocks, geologist Harry Hess proposed the concept of Sea Floor Spreading in 1961.

Key observations supporting this hypothesis included:

• Frequent volcanic eruptions are concentrated along the crests of mid-oceanic ridges, bringing molten rock (lava) to the surface.
• Rocks on the ocean floor exhibit alternating stripes of normal and reversed magnetic polarity, mirroring the Earth's magnetic field reversals over time. These magnetic stripes are symmetrical on either side of the mid-oceanic ridge crest. Crucially, rocks closest to the ridge crest are youngest and have the current (normal) magnetic polarity, while rocks get progressively older and show alternating polarities as one moves away from the ridge.
• The age of the oceanic crust is surprisingly young compared to continental crust. No oceanic crust found anywhere on Earth is older than about 200 million years, whereas continental rocks can be over 3.2 billion years old.
• Sediments on the ocean floor are relatively thin, especially near the mid-oceanic ridges. If the oceans were ancient and static, thick layers of sediment would have accumulated over billions of years. The thin sedimentary cover supports a younger age for the ocean floor.
• Earthquakes along mid-oceanic ridges are shallow, while deep earthquakes are found near oceanic trenches.

Hess synthesized these observations into his sea floor spreading hypothesis. He proposed that magma from the mantle rises at the mid-oceanic ridges, erupts, and solidifies to form new oceanic crust. As more magma rises, it pushes the newly formed crust away from the ridge crest in both directions. This process effectively causes the ocean floor to spread apart.

Recognizing that the Earth is not expanding, Hess also suggested that old oceanic crust must be consumed elsewhere. He proposed that the ocean floor moves across the Earth's surface and eventually sinks back into the mantle at deep-oceanic trenches. This concept explained why oceanic crust is relatively young and why trenches are associated with deep earthquakes.

Diagram showing magma rising at a mid-oceanic ridge, forming new crust that spreads outwards, and old crust subducting at a trench.

Plate Tectonics

The concepts of continental drift and sea floor spreading were unified in the late 1960s into the overarching theory of Plate Tectonics. This theory, largely developed by scientists like McKenzie and Parker, and Morgan in 1967, revolutionized geology.

The theory states that the Earth's rigid outer layer, the lithosphere (composed of the crust and the uppermost, rigid part of the mantle), is broken into a number of large and small pieces called tectonic plates or lithospheric plates. These plates are not static; they move slowly and continuously across the Earth's surface, riding on the weaker, ductile layer of the upper mantle called the asthenosphere.

A plate can consist of both continental and oceanic lithosphere. The thickness of the lithosphere varies, being generally thinner under oceans (5-100 km) and thicker under continents (up to 200 km or more). Plates are classified as primarily continental or oceanic based on whether continental or oceanic lithosphere makes up the larger portion of the plate (e.g., the Pacific Plate is mostly oceanic, while the Eurasian Plate is largely continental).

Map illustrating the boundaries and names of the Earth's major and some minor tectonic plates.

Plate boundaries, where plates interact, are zones of intense geological activity, characterized by earthquakes, volcanoes, mountain building, and the formation of trenches and ridges. The Earth's lithosphere is divided into seven major plates and several minor ones.

Major Plates:

1. Antarctica Plate (includes surrounding oceanic crust)
2. North American Plate (includes western Atlantic)
3. South American Plate (includes western Atlantic)
4. Pacific Plate
5. Indo-Australian Plate (includes Indian, Australian, and New Zealand landmasses and oceanic crust)
6. Africa Plate (includes eastern Atlantic)
7. Eurasian Plate (includes adjacent oceanic crust)

Some important Minor Plates:

• Cocos Plate (East Pacific, west of Central America)
• Nazca Plate (East Pacific, west of South America)
• Arabian Plate (Middle East)
• Philippine Sea Plate (Western Pacific)
• Caroline Plate (Western Pacific, north of New Guinea)
• Fuji Plate (Southwest Pacific, northeast of Australia)
• Juan de Fuca Plate (Eastern Pacific, off US/Canada coast)

Plate tectonic theory clarifies that it is not just the continents that move, but the entire lithospheric plates on which they rest. This movement has been ongoing throughout Earth's history and will continue in the future. Pangaea is now understood as a configuration that occurred when several plates carrying continental masses converged during the late Paleozoic and early Mesozoic eras.

Divergent Boundaries

These are plate boundaries where two plates are moving away from each other. As the plates separate, magma rises from the mantle to fill the gap, creating new lithosphere. These sites are known as spreading sites.

• Example: The Mid-Atlantic Ridge is a classic example, where the North American and South American plates are pulling away from the Eurasian and African plates, respectively. This continuous process forms new oceanic crust along the ridge.

Convergent Boundaries

These are plate boundaries where two plates are moving towards each other and collide. At convergent boundaries, one plate is typically forced beneath the other and sinks into the mantle. This process is called subduction, and the area where it occurs is a subduction zone. Crust is consumed or destroyed at convergent boundaries.

Convergence can happen in three main scenarios:

• Oceanic-Continental Convergence: An oceanic plate collides with a continental plate. The denser oceanic plate subducts beneath the less dense continental plate. This often results in the formation of a continental volcanic arc (like the Andes) and a deep oceanic trench (like the Peru-Chile Trench) offshore.
• Oceanic-Oceanic Convergence: Two oceanic plates collide. The older, denser oceanic plate subducts beneath the younger, less dense one. This leads to the formation of an island volcanic arc (like the Mariana Islands) and an associated deep oceanic trench (like the Mariana Trench).
• Continental-Continental Convergence: Two continental plates collide. Since both are relatively low density, neither plate readily subducts deep into the mantle. Instead, the continental crust is compressed, thickened, faulted, and folded, resulting in the formation of large mountain ranges (like the Himalayas or the Alps).

Transform Boundaries

These are plate boundaries where two plates slide horizontally past each other, with little or no creation or destruction of lithosphere. The movement along these boundaries occurs along transform faults. These faults are often found linking segments of mid-oceanic ridges, where the spreading is offset. Because plates are not perfectly rigid spheres and spreading rates can vary, transform faults accommodate the differential movement between adjacent spreading segments. Activity along transform faults is characterized by frequent, strong earthquakes but generally not volcanic activity or mountain building.

Rates Of Plate Movement

Scientists can measure the rates at which plates are moving using various techniques, including analyzing the magnetic stripes on the ocean floor, using satellite-based GPS measurements, and dating rocks from plate boundaries. The pattern of magnetic reversals recorded in the oceanic crust provides a historical record of spreading rates. By matching the pattern of magnetic stripes on either side of a mid-ocean ridge with the known history of Earth's magnetic field reversals, geologists can determine how much new crust has formed over a given time period, and thus calculate the spreading rate.

Plate movement rates vary significantly across the globe. Some plates move very slowly, perhaps less than 2.5 cm per year (e.g., the Arctic Ridge), while others move much faster, exceeding 15 cm per year (e.g., the East Pacific Rise near Easter Island).

Force For The Plate Movement

Unlike Wegener's inadequate forces, the driving mechanism for plate movement in modern plate tectonics is primarily attributed to forces originating within the Earth's mantle. While the exact combination of forces is still debated, the leading hypothesis involves mantle convection.

• Heat within the Earth, primarily from radioactive decay of isotopes (like Uranium, Thorium, and Potassium) and residual heat from its formation, creates temperature differences within the mantle.
• Warmer, less dense mantle material rises, while cooler, denser material sinks, forming slow-moving circulation cells (convection cells) within the asthenosphere.
• This convective flow within the mantle is thought to exert drag on the rigid overlying lithospheric plates, causing them to move.
• Additional forces contributing to plate motion include 'ridge push' (the force exerted by new, hot material pushing plates away from mid-ocean ridges) and 'slab pull' (the force exerted by a dense, subducting oceanic plate pulling the rest of the plate down into the mantle).

The concept of mantle convection, first proposed by Arthur Holmes, was crucial in providing a plausible driving force for plate tectonics, overcoming the main objection to Wegener's original drift theory.

Movement Of The Indian Plate

The Indo-Australian Plate is one of the major tectonic plates, encompassing the Indian subcontinent, Australia, and parts of the surrounding oceanic crust. Its movement and collision with the Eurasian plate have played a key role in shaping the geography of South Asia.

Sequence of maps illustrating the journey of the Indian subcontinent from its position south of the equator around 140 million years ago to its collision with Asia and subsequent northward movement.

• The northern boundary of the Indian plate is a prominent example of continent-continent convergence, where it collides with the Eurasian plate along the Himalayas. This collision is responsible for the formation and ongoing uplift of the Himalayan mountain range.
• To the east, the plate boundary extends through the Rakinyoma Mountains of Myanmar and along the island arc of the Java Trench, where oceanic lithosphere is subducting.
• The eastern margin of the Australian part of the plate is a divergent boundary (spreading site) located in the southwest Pacific Ocean.
• The western margin follows the Kirthar Mountains in Pakistan, continues along the Makrana coast, and connects to the spreading ridge in the Red Sea rift, extending southeastwards along the Chagos Archipelago.
• The boundary between the Indian plate and the Antarctic plate to the south is also a divergent boundary (oceanic ridge).

Geological evidence indicates that around 225 million years ago, the Indian subcontinent was a large island situated off the coast of Australia, separated from the Asian continent by the ancient Tethys Sea. India began its northward journey around 200 million years ago, after Pangaea started breaking up.

A significant event during India's rapid northward drift (starting around 60 million years ago) was the massive outpouring of basaltic lava that formed the Deccan Traps, covering a large part of peninsular India. At this time, India was still relatively close to the equator.

Around 40-50 million years ago, the Indian plate collided with the Eurasian plate. This ongoing collision has caused immense compressional forces, resulting in the folding, faulting, and uplift of the Tethys Sea sediments and the edges of both continental plates, leading to the formation of the Himalayas. Scientists believe this process continues today, contributing to the rising height of the Himalayas.

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