Distribution of Oceans and Continents

Introduction

The surface of the Earth is divided into land (29%) and water (71%). This unequal distribution, and the arrangement of continents and ocean basins, is not random — it is the result of billions of years of geological activity. The theory that best explains this distribution is Plate Tectonics, which emerged from earlier ideas about Continental Drift and Sea Floor Spreading.

Continental Drift Theory — Alfred Wegener (1912)

German meteorologist Alfred Wegener proposed the Continental Drift Theory in 1912. He argued that all continents were once joined together as a single supercontinent called Pangaea (meaning "All Land"), surrounded by a single ocean called Panthalassa (meaning "All Seas").

• Around 200 million years ago, Pangaea began to break apart. It first split into:
• Laurasia (northern supercontinent: North America, Europe, Asia)
• Gondwanaland (southern supercontinent: South America, Africa, India, Australia, Antarctica)

These then broke apart further to form today's continents.

1. 1.Evidence for Continental Drift:
2. 2.Jigsaw Fit: The coastlines of continents, especially Africa and South America, fit together like puzzle pieces.
3. 3.Rock similarities: Identical rock types and geological structures found on opposite sides of the Atlantic Ocean (e.g., matching mountain ranges in Brazil and West Africa).
4. 4.Fossil evidence: Same fossils ( · Mesosaurus · , · Glossopteris · flora, · Lystrosaurus · ) found on continents now separated by oceans, suggesting they were once connected.
5. 5.Palaeoclimatic evidence: Evidence of glaciation in tropical Africa, and tropical coal deposits in Antarctica, suggest continents were once at different latitudes.
6. 6.Placer deposits: Gold in Ghana matches geological veins in Brazil across the Atlantic.

Sea Floor Spreading — Harry Hess (1960s)

• American geologist Harry Hess proposed the theory of Sea Floor Spreading in the 1960s. He suggested:
• Molten material from the mantle rises at mid-ocean ridges (underwater mountain chains), pushes the sea floor apart, and creates new oceanic crust.
• Old oceanic crust sinks back into the mantle at ocean trenches (subduction zones).
• This conveyor-belt-like movement provided the mechanism that Wegener's theory lacked.

1. 1.Evidence for Sea Floor Spreading:
2. 2.Magnetic anomalies: Rocks on either side of mid-ocean ridges show symmetric magnetic reversal patterns (paleomagnetism), like a tape recording of Earth's magnetic reversals.
3. 3.Age of ocean floor rocks: Rocks are youngest at mid-ocean ridges and get progressively older away from the ridge — exactly as expected if new floor is being created at the ridge.
4. 4.Heat flow: Highest heat flow is measured at mid-ocean ridges, consistent with rising hot magma.
5. 5.Sediment thickness: Ocean sediments are thin near ridges (young crust) and thicker far from ridges (older crust).

Plate Tectonics Theory

• Plate Tectonics, developed in the 1960s-70s, combined Continental Drift and Sea Floor Spreading into a unified theory. The key ideas are:
• Earth's lithosphere is broken into 7 major plates (and several minor plates).
• These plates move on the semi-fluid asthenosphere driven by convection currents in the mantle.

| Boundary Type | What Happens | Example |
|---|---|---|
| Divergent | Plates move apart; new crust formed | Mid-Atlantic Ridge |
| Convergent | Plates collide; one may subduct | Himalayas, Mariana Trench |
| Transform | Plates slide past each other | San Andreas Fault (California) |

• Types of convergence:
• Oceanic-Oceanic: One plate subducts; volcanic island arcs form (e.g., Japan)
• Oceanic-Continental: Oceanic plate subducts (denser); coastal mountain ranges and trenches form (e.g., Andes)
• Continental-Continental: Neither subducts; both fold upward to form fold mountains (e.g., Himalayas)

Formation of Major Ocean Basins and Continents

• The breakup of Pangaea explains:
• The Atlantic Ocean opened as South America and Africa separated.
• The Indian Ocean formed as India moved northward from Gondwanaland.
• The Himalayas formed when India collided with the Eurasian plate.
• Australia separated from Antarctica and moved northward.

Isostasy

• Students often say Wegener proved continental drift — he only proposed it. The mechanism was missing until Sea Floor Spreading filled the gap.
• Do not confuse Pangaea (single landmass) with Panthalassa (single ocean).
• Laurasia = northern part; Gondwanaland = southern part — after Pangaea split.
• At a transform boundary, crust is neither created nor destroyed — plates only slide past each other.
• The Himalayas result from continental-continental convergence (neither plate subducts), NOT from oceanic-continental convergence.

Wegener's Continental Drift Theory proposed a single supercontinent Pangaea that broke apart. Hess's Sea Floor Spreading provided the mechanism. Plate Tectonics unified these into a comprehensive theory explaining the distribution of continents and oceans, mountain formation, earthquakes, and volcanoes through the movement of lithospheric plates on the asthenosphere.