Chapter 4 Distribution Of Oceans And Continents

In the previous chapter, we explored the Earth's internal structure. Looking at a world map, we see continents covering about 29% of the surface and oceans covering the rest. However, the positions of these large landmasses and water bodies haven't always been the same throughout Earth's history, and they continue to change.

This raises intriguing questions: Where were the continents and oceans located in the distant past? What processes cause them to move and change their positions? How do scientists uncover information about their past arrangements?

This chapter will explore the concepts and evidence behind the dynamic nature of Earth's surface, addressing how continents and oceans have shifted over geological time.

Continental Drift

Observe the coastlines bordering the Atlantic Ocean – the eastern edges of North and South America seem to fit remarkably well with the western edges of Europe and Africa, like pieces of a jigsaw puzzle. This striking symmetry led many early thinkers to speculate that these landmasses might have once been connected.

As early as 1596, Dutch mapmaker Abraham Ortelius proposed this possibility. Later, Antonio Pellegrini drew maps depicting the continents joined. However, it was the German meteorologist Alfred Wegener who, in 1912, presented a comprehensive explanation in his **"Continental Drift Theory"**.

Wegener proposed that around 200 million years ago, all the continents were merged into a single giant landmass he named **Pangaea** (meaning "all Earth"). This supercontinent was surrounded by a vast single ocean called **Panthalassa** (meaning "all water").

According to Wegener's theory, Pangaea began to break apart. Initially, it split into two major landmasses: **Laurasia** in the north (including present-day North America, Europe, and Asia) and **Gondwanaland** in the south (including present-day South America, Africa, India, Australia, and Antarctica). These two large masses then further fragmented over millions of years to form the continents we see today.

Wegener supported his theory with various lines of evidence gathered from different continents. These included observations related to the shape of coastlines, geological similarities, fossil distribution, and ancient climates.

Evidence In Support Of The Continental Drift

Wegener and subsequent scientists presented several pieces of evidence supporting the idea that continents were once joined:

The Matching Of Continents (Jig-Saw-Fit)

The most obvious evidence is the visual fit between the coastlines of continents across the Atlantic, particularly the eastern coast of South America and the western coast of Africa. Computer-assisted analysis by Bullard in 1964, attempting to match the continents at the 1,000-fathom (approx. 1,800m) depth contour rather than the current eroding shoreline, demonstrated an almost perfect fit, providing strong visual support for them once being connected.

Rocks Of Same Age Across The Oceans

Modern radiometric dating techniques allow scientists to determine the absolute age of rocks. Studies have revealed belts of rocks with similar ages and characteristics that cross from one continent to another, now separated by oceans. For example, ancient rock formations in Brazil's coast match those in western Africa, both being around 2,000 million years old. Furthermore, the oldest marine sediments found along the coasts of South America and Africa date back only to the Jurassic period, suggesting that the ocean basin between them formed after that time.

Tillite

**Tillite** is a specific type of sedimentary rock formed from deposits left by glaciers. Evidence of extensive ancient glaciation (indicated by thick tillite layers) dating to the same period has been found in geographically distant landmasses currently located in the Southern Hemisphere: India, Africa, the Falkland Islands, Madagascar, Antarctica, and Australia. These landmasses share a common sequence of sedimentary rocks known as the Gondwana system. The widespread distribution of these specific glacial deposits strongly suggests that these regions were once joined together in a cold climate zone, far from their current tropical or temperate locations, providing evidence for both past climate changes and continental movement.

Placer Deposits

The presence of rich **placer deposits** (concentrations of valuable minerals eroded from their source rock and transported by water) of gold along the coast of Ghana in West Africa is noteworthy because there is no known source rock for gold in Ghana itself. The source rocks for these gold deposits are found in Brazil. This suggests that when Africa and South America were joined, the rivers carrying gold eroded from the Brazilian highlands deposited it in the area that is now the Ghana coast.

Distribution Of Fossils

The discovery of identical plant and animal fossils on continents now separated by vast oceans poses a challenge unless the continents were previously connected. For example, fossils of land-dwelling or freshwater species that could not have crossed large marine barriers are found on multiple continents. The presence of **Lemurs** in India, Madagascar, and Africa led some early geographers to hypothesize a now-submerged land bridge called 'Lemuria'. Even more compelling is the fossil evidence of **Mesosaurus**, a small freshwater reptile. Its skeletons are found *only* in the Irati Formation of Brazil and the Southern Cape Province of South Africa – two regions now separated by the Atlantic Ocean, approximately 4,800 km apart. Such limited and specific distribution strongly supports the idea that these landmasses were once connected.

Force For Drifting

Wegener proposed that the continents drifted due to two main forces: the **pole-fleeing force** and **tidal forces**. The pole-fleeing force was linked to the Earth's rotation, which causes a bulge at the equator and might push landmasses away from the poles. Tidal forces, caused by the gravitational pull of the Moon and the Sun, influence oceanic tides. Wegener believed these forces, acting over millions of years, could be strong enough to move continents. However, the scientific community at the time largely rejected Wegener's proposed forces, deeming them too weak to account for the movement of entire continents.

Post-Drift Studies

Although Wegener's theory faced criticism regarding the proposed forces, subsequent research, particularly after World War II, provided crucial new information that revitalized interest in the idea of moving continents. Much of this new data came from extensive studies of the ocean floor.

Convectional Current Theory

In the 1930s, well after Wegener's initial proposal, British geologist **Arthur Holmes** discussed the possibility of **convection currents** operating within the Earth's mantle. He suggested that heat generated by radioactive decay within the mantle creates thermal differences, driving slow, circular movements of partially molten rock. Holmes proposed that these convection currents could provide the necessary force to move continental landmasses. This theory offered a plausible mechanism that had been missing from Wegener's original hypothesis and would later become a key component of plate tectonics.

Mapping Of The Ocean Floor

Post-World War II advancements in sonar and other technologies allowed for detailed mapping of the ocean floor. This revealed that the seafloor is not flat but possesses complex topography, including vast mountain ranges (mid-oceanic ridges) and deep valleys (trenches). These features, particularly the mid-oceanic ridges, were found to be volcanically very active.

Crucially, dating of rocks from the oceanic crust showed them to be significantly **younger** than continental rocks. Furthermore, rocks located symmetrically on either side of the crest of mid-oceanic ridges showed remarkable similarities in age, composition, and magnetic properties, with the youngest rocks found directly at the ridge crest and becoming progressively older with distance from the crest. These findings were inconsistent with the idea of ancient, static ocean basins and provided vital clues about the dynamic nature of the seafloor.

Ocean Floor Configuration

Understanding the relief features of the ocean floor is essential for comprehending the distribution and movement of continents and oceans. The ocean floor can be broadly divided into three main zones based on depth and landforms. (This refers to Figure 4.1).

Continental Margins

These are the transitional areas between the continents and the deep ocean basins. They include the **continental shelf** (gently sloping submerged edge of a continent), the **continental slope** (steeper descent), the **continental rise** (accumulated sediment at the base of the slope), and deep-oceanic **trenches**. Trenches are particularly important in understanding plate interactions.

Abyssal Plains

**Abyssal plains** are vast, flat, or gently sloping areas that make up the deep ocean floor, situated between the continental margins and the mid-oceanic ridges. They are covered by fine sediments, including clay and silt, that have accumulated over millions of years, originating from the continents and the ocean itself.

Mid-Oceanic Ridges

The **mid-oceanic ridge** system is an enormous, interconnected underwater mountain range that stretches across the globe, making it the longest mountain chain on Earth, though mostly submerged. Its central feature is a **rift valley** running along the crest, which is a zone of intense volcanic activity where new oceanic crust is formed. The ridge system includes a fractured plateau and flank zones extending outwards from the central rift. As mentioned in the previous chapter, these are sites of mid-oceanic volcanoes.

Distribution Of Earthquakes And Volcanoes

Examining the global distribution patterns of earthquakes and volcanoes reveals a striking correlation with specific geological features, particularly the mid-oceanic ridges, mountain belts, and oceanic trenches. (This refers to Figure 4.2).

A prominent line of seismic activity (earthquakes) is observed running along the center of the Atlantic Ocean, roughly parallel to the coasts, coinciding precisely with the **Mid-Atlantic Ridge**. This pattern continues into the Indian Ocean and branches towards East Africa and along a belt extending towards New Guinea.

Other significant concentrations of earthquakes occur along the **Alpine-Himalayan mountain belt** and especially around the margins of the **Pacific Ocean**. This zone around the Pacific is famously known as the **"Ring of Fire"** due to the high frequency of both earthquakes and active volcanoes.

A distinction exists in the depth of earthquake origins (foci): earthquakes along mid-oceanic ridges typically have **shallow foci** (closer to the surface), while those in the Alpine-Himalayan belt and the Pacific Ring of Fire are often **deep-seated** (originating at greater depths).

The distribution of **volcanoes** also closely follows these same patterns, with major volcanic belts coinciding with the areas of intense earthquake activity, particularly along the mid-oceanic ridges and the Pacific margins.

Concept Of Sea Floor Spreading

The discoveries from post-World War II ocean floor research, combined with palaeomagnetic studies (studying the history of Earth's magnetic field recorded in rocks), provided critical evidence that significantly expanded upon Wegener's drift theory. These key findings included:

• High levels of **volcanic activity** are concentrated along the entire length of the mid-oceanic ridges, with vast quantities of lava erupting onto the seafloor.
• Rocks sampled from the oceanic crust show a pattern related to their distance from the ridge crest. Rocks equidistant from the crest on either side have nearly identical age, chemical composition, and magnetic properties (recording the direction of Earth's magnetic field at the time of their formation). Crucially, the rocks closest to the ridge crest are the youngest and have the current magnetic polarity, while rocks get progressively older and exhibit alternating stripes of normal and reversed magnetic polarity as one moves away from the ridge.
• Oceanic crustal rocks are remarkably **younger** than continental rocks. The oldest known oceanic crust is around 200 million years old, whereas continental rock formations can be as ancient as 3,200 million years.
• Contrary to expectations if oceans were as old as continents, the layer of **sediments** on the ocean floor is relatively thin, and sediment cores drilled from the deepest parts show sediment layers are no older than about 200 million years.
• Earthquakes along mid-oceanic ridges are shallow, while those in deep oceanic trenches are deep-seated.

Based on these observations, particularly the magnetic striping and the age pattern of oceanic crust, **Harry Hess** proposed the hypothesis of **"sea floor spreading"** in 1961. Hess theorized that magma rises from the mantle at the crest of mid-oceanic ridges, cools to form new oceanic crust, and this new crust continuously pushes the older crust away from the ridge in both directions. This process causes the ocean floor to effectively "spread" outwards.

Recognizing that the Earth is not expanding (i.e., the spreading of one ocean doesn't cause others to shrink), Hess further proposed that old oceanic crust is simultaneously being destroyed or consumed elsewhere. He suggested that this occurs at deep oceanic trenches, where the aging oceanic crust sinks back down into the mantle, a process called **subduction**. (This refers to Figure 4.3 visualizing the process).

The concept of seafloor spreading provided a compelling mechanism for continental movement and laid the groundwork for the development of the unifying theory of Plate Tectonics.

Plate Tectonics

The concept of seafloor spreading, combined with other geological and geophysical data gathered after Wegener's time, led to the development of the theory of **Plate Tectonics** in 1967 by scientists like **McKenzie and Parker**, and **Morgan**, working independently. This theory revolutionized our understanding of how the Earth's surface operates.

Plate tectonics posits that the Earth's rigid outer layer, the **lithosphere**, is broken into several large and small pieces called **tectonic plates** or **lithospheric plates**. These plates are massive, irregular-shaped slabs composed of both the crust and the uppermost, rigid part of the mantle. They float and move horizontally over the weaker, partially molten layer beneath, the asthenosphere.

The thickness of the lithosphere varies; it is typically thinner under oceans (5-100 km) and thicker under continents (around 200 km). A plate is often named either a **continental plate** or an **oceanic plate** based on which type of lithosphere constitutes the larger portion of the plate (e.g., the Pacific plate is mostly oceanic, while the Eurasian plate is largely continental, although most plates contain both). (This refers to Figure 4.5 showing the major and minor plates).

The theory identifies **seven major tectonic plates** and several important minor plates:

Major Plates:

1. Antarctica Plate (including surrounding oceanic crust)
2. North American Plate (includes western Atlantic floor)
3. South American Plate (includes western Atlantic floor)
4. Pacific Plate
5. India-Australia-New Zealand Plate
6. Africa Plate (includes eastern Atlantic floor)
7. Eurasia Plate (including adjacent oceanic crust)

Minor Plates:

• Cocos Plate (located between Central America and the Pacific Plate)
• Nazca Plate (located between South America and the Pacific Plate)
• Arabian Plate (primarily the landmass of Saudi Arabia)
• Philippine Plate (situated between the Asiatic/Eurasian Plate and the Pacific Plate)
• Caroline Plate (located north of New Guinea, between the Philippine and Indian Plates)
• Fuji Plate (a smaller plate in the South Pacific)
• Juan de Fuca Plate (off the coast of North America)

A key difference from Wegener's concept is that it is the entire **plate** that moves, carrying continents (or parts of them) and oceanic crust along with it, rather than continents somehow plowing through stationary oceanic crust. Plates have been in constant motion throughout geological history and will continue to move in the future. Pangaea, the supercontinent, is understood in plate tectonics as a configuration that resulted from the convergence of previously separated continental fragments carried on different plates.

Palaeomagnetic data has been vital in reconstructing the past positions of continents. By analyzing the magnetic orientation in rocks of different ages from a continent, scientists can determine the latitude at which those rocks formed relative to Earth's magnetic poles, thus tracing the continent's movement over time. (This refers to Figure 4.4 showing historical plate positions).

Types Of Plate Boundaries

Plate interactions primarily occur at their boundaries, leading to distinct geological activity and features. There are three main types of plate boundaries:

Divergent Boundaries

These are areas where tectonic plates are moving **away from each other**. As the plates pull apart, magma rises from the mantle to fill the gap, creating new lithosphere (specifically, new oceanic crust). These zones are called **spreading sites**. Mid-oceanic ridges are the most prominent examples of divergent boundaries, such as the Mid-Atlantic Ridge, where the North American and Eurasian plates, and the South American and African plates, are separating.

Convergent Boundaries

These are zones where tectonic plates are moving **towards each other**. When plates collide, one plate is typically forced beneath the other and descends into the mantle, where it is recycled. This process is called **subduction**, and the area where it occurs is a **subduction zone**. Crust is destroyed at convergent boundaries. Convergence can happen in three ways:

• Oceanic Plate - Continental Plate Convergence: Denser oceanic lithosphere subducts beneath the lighter continental lithosphere, often forming oceanic trenches and coastal mountain ranges with volcanoes (e.g., the Andes mountains and the Peru-Chile Trench).
• Oceanic Plate - Oceanic Plate Convergence: One oceanic plate subducts beneath another, creating oceanic trenches and chains of volcanic islands (island arcs) parallel to the trench (e.g., the Mariana Trench and the Mariana Islands).
• Continental Plate - Continental Plate Convergence: When two continental plates collide, neither is dense enough to subduct significantly. Instead, the crust is compressed, folded, and thickened, resulting in the formation of large mountain ranges (e.g., the Himalayas, formed by the collision of the Indian and Eurasian plates).

Transform Boundaries

These boundaries occur where tectonic plates slide **horizontally past each other**. At transform boundaries, lithosphere is neither created (as at divergent boundaries) nor destroyed (as at convergent boundaries). The movement along these boundaries occurs along large fractures called **transform faults**. These faults are often found connecting segments of mid-oceanic ridges. Differential spreading rates along the ridge or the Earth's curvature can cause plates to slide laterally relative to adjacent segments along these faults.

Rates Of Plate Movement

The speed at which tectonic plates move can be measured using various techniques, including studying the magnetic stripes on the ocean floor (which act like a geological tape recorder of spreading) and using precise satellite-based GPS measurements. Plate movement rates vary significantly across the globe.

The slowest spreading rates are found along ridges like the Arctic Ridge, where plates move at less than 2.5 cm per year. The fastest rates are observed at parts of the East Pacific Rise, near Easter Island, where plates are separating at over 15 cm per year.

Force For The Plate Movement

While Wegener's proposed forces were inadequate, the theory of plate tectonics identifies a more powerful driving mechanism. It recognizes that the Earth's interior is not static but dynamic.

The primary driving force behind plate movement is believed to be **convection currents in the mantle**. As envisioned by Arthur Holmes, residual heat from Earth's formation and heat generated by the decay of radioactive elements deep within the Earth cause the lower mantle material to heat up, become less dense, and slowly rise. Near the surface, this material spreads out horizontally beneath the lithospheric plates, dragging them along. As the material cools, it becomes denser and sinks back down into the deeper mantle, completing a convective cycle or "convection cell". This slow circulation of the ductile mantle material provides the fundamental energy to move the rigid lithospheric plates across the Earth's surface.

Movement Of The Indian Plate

The present-day **Indian Plate** is a large tectonic plate that includes the Indian subcontinent, the Australian continent, and a significant portion of the surrounding oceanic crust.

Its boundaries are diverse:

• To the **north**, the boundary is a **convergent boundary** with the Eurasian Plate, specifically involving the collision of two continental landmasses, which is responsible for the uplift of the Himalayas. This is a continent-continent convergence zone (or subduction zone from the perspective of the oceanic part of the plate).
• To the **east**, the boundary extends through the Rakinyoma Mountains in Myanmar and curves southward along an island arc system near the Java Trench.
• The **eastern margin** in the southwest Pacific (east of Australia) is a **spreading site** (divergent boundary) forming an oceanic ridge.
• The **western margin** generally follows the Kirthar Mountains in Pakistan, extends along the Makrana coast, and connects to the spreading ridge system originating from the Red Sea rift and continuing southeastward along the Chagos Archipelago.
• The boundary between the Indian Plate and the Antarctic Plate to the **south** is primarily a **divergent boundary** marked by an oceanic ridge trending roughly east-west.

Tracing the history of the Indian subcontinent's movement using palaeomagnetic data reveals a remarkable journey. Around **225 million years ago**, India was a large island located far south off the Australian coast, separated from the Asian continent by the ancient **Tethys Sea**. (This refers to Figure 4.6 tracing India's movement).

India began its rapid northward drift approximately **200 million years ago**, following the breakup of Pangaea. Around **140 million years ago**, the subcontinent was situated as far south as 50°S latitude, with the Tethys Sea and the Tibetan block separating it from the main Asiatic landmass.

During this northward movement, a significant geological event occurred around **60 million years ago**: massive outpouring of basaltic lava that formed the extensive **Deccan Traps** in peninsular India. At this time, India was still relatively close to the equator.

The climactic event of India's journey was its **collision with Asia**, which began around **40-50 million years ago**. This collision crumpled the crust along the boundary, initiating the dramatic uplift of the **Himalayan mountain range**. Geologists believe that this collision and the ongoing uplift of the Himalayas are still occurring today, albeit at a slow geological pace.

Exercises

Multiple Choice Questions

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Answer The Following Questions In About 30 Words

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Answer The Following Questions In About 150 Words

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