Distribution of Oceans and Continents

Learning Outcomes

1. Understand the concept of continental drift and plate tectonics.
2. Analyze the evidence supporting the movement of continents.
3. Recognize the role of sea floor spreading in oceanic and continental distribution.
4. Differentiate between divergent, convergent, and transform boundaries.
5. Explore how plate movements influence geographic and seismic features.

Continental Drift Theory

The notion that continents were once part of a supercontinent and have since drifted apart has been explored by numerous scientists. Alfred Wegener, a German meteorologist, presented a comprehensive theory regarding continental drift in 1912. He proposed that the continents formed a single large mass, Pangaea, surrounded by a mega-ocean called Panthalassa. About 200 million years ago, Pangaea began to split into two large masses, Laurasia in the north and Gondwanaland in the south, which then broke into smaller continents over time.

Key Evidences for Continental Drift

1. Jig-Saw Fit of Continents: The coastlines of Africa and South America exhibit an extraordinary match, similar to pieces of a puzzle. In 1964, Bullard’s computerized map confirmed the accuracy of this fit, especially at a depth of 1,000 fathoms.
2. Rocks of Same Age Across Oceans: Radiometric dating helped correlate rock formations on different continents, such as ancient rocks from Brazil and Western Africa, showing a close match. Early marine deposits along the South American and African coastlines are of Jurassic age, indicating that oceans did not exist before that time.
3. Tillite Evidence: Tillite is a glacial deposit found in the Gondwana system of sediments. These deposits occur across six landmasses in the Southern Hemisphere, including India, Africa, Falkland Islands, Antarctica, and Australia, proving similar geologic histories and supporting the idea of continental drift.
4. Placer Deposits: Rich placer deposits of gold along the Ghana coast lack a nearby source rock. The veins containing gold exist in Brazil, suggesting that these two landmasses were once joined.
5. Distribution of Fossils: Identical fossil species found across marine barriers raised questions. For instance, Mesosaurus, a small reptile adapted to shallow waters, was found only in South Africa and Brazil, now separated by an ocean. This supports the notion that these continents were once connected.

Important Note:

• Continental drift theory explains the movement of landmasses but doesn’t account for the mechanism behind their movement, which later discoveries, like plate tectonics, clarified.

Post-Drift Discoveries and Plate Tectonics

Following World War II, new insights from ocean floor mapping revitalized the study of continents and oceans. The key finding was that the ocean floor is not flat but includes features like submerged mountain ranges and deep trenches.

1. Convectional Current Theory: Arthur Holmes in the 1930s discussed the idea of convection currents in the mantle caused by radioactive heating. This theory provided an explanation for the force driving the movement of continents, addressing a gap in Wegener’s theory.
2. Ocean Floor Configuration: Mid-oceanic ridges are sites of intense volcanic activity, where new crust forms. The study of magnetic properties in rocks near ridges revealed symmetrical patterns on either side, indicating the spreading of the ocean floor.
3. Sea Floor Spreading Hypothesis: Proposed by Harry Hess in 1961, this hypothesis suggested that the ocean floor spreads at mid-oceanic ridges due to constant volcanic eruptions. As new lava emerges, it pushes the crust outward, resulting in the expansion of oceans. Hess theorized that old oceanic crust is consumed at oceanic trenches, where it sinks back into the earth’s mantle.

Ocean Floor Features and Earthquake Distribution
The ocean floor is divided into several sections that provide crucial insights into the distribution of oceans and continents:

1. Continental Margins: These include continental shelves, slopes, and deep-ocean trenches, the latter being significant in understanding continental distribution.
2. Abyssal Plains: These flat, sediment-rich plains lie between continental margins and mid-ocean ridges.
3. Mid-Ocean Ridges: Forming the longest mountain chains, these ridges are characterized by volcanic activity at the crest.

The distribution of earthquakes and volcanoes closely follows these ocean floor features. Seismic activity is concentrated along mid-oceanic ridges, while deep-seated earthquakes occur near trenches. The Pacific Rim, also known as the Ring of Fire, is notable for its high concentration of active volcanoes.

Concept of Plate Tectonics

In 1967, McKenzie, Parker, and Morgan integrated earlier ideas to develop the plate tectonics theory, which explains the movement of lithospheric plates over the asthenosphere. These plates consist of continental and oceanic lithosphere and are rigid, moving as units over the earth’s surface.

Types of Plates

• Major Plates: Include Pacific, Eurasian, African, North American, South American, Indian-Australian, and Antarctic plates.
• Minor Plates: Examples are the Cocos plate between Central America and the Pacific and the Nazca plate between South America and the Pacific.

Plate Boundaries

1. Divergent Boundaries: Where plates move apart, creating new crust. The Mid-Atlantic Ridge is an example.
2. Convergent Boundaries: Where plates collide, and one plate is subducted beneath another. This can occur between oceanic and continental plates or continental plates themselves.
3. Transform Boundaries: Where plates slide past each other without creating or destroying crust. These boundaries often result in earthquakes along fault lines.

Important Note:

• Convergent boundaries lead to subduction, where one plate is forced beneath another, often causing significant seismic and volcanic activity.

Rate and Force of Plate Movements

The rate of plate movement varies, with the Arctic Ridge being the slowest (less than 2.5 cm/year) and the East Pacific Rise near Easter Island being the fastest (over 15 cm/year). The movement is driven by convection currents in the mantle. These currents are fueled by heat from radioactive decay and residual heat within the earth. The continuous cycle of heating and cooling creates convective flow, which moves the rigid plates above.

Movement of the Indian Plate

The Indian plate includes Peninsular India and parts of Australia. The northern boundary is defined by the Himalayas, where continent-continent convergence occurs. To the east, the plate extends into Myanmar and further into the Java Trench.

India’s geological journey began about 200 million years ago when it broke away from Pangaea and started moving northward. By 40-50 million years ago, India collided with Asia, leading to the uplift of the Himalayas. The process continues to this day, with the Himalayas still rising.

Multiple-Choice Question
Who proposed the concept of sea floor spreading?
Answer: Harry Hess