Continental Drift

The theory of Continental Drift was first proposed by Alfred Wegener in 1912. He suggested that the continents were once joined together in a supercontinent called Pangaea and over time drifted to their current positions.

• About 200 million years ago, all the continents were part of a single massive landmass called Pangaea.
• Surrounding this supercontinent was the vast ocean called Panthalassa.
• Around 180 million years ago, Pangaea started to break apart:
  • Laurasia in the north, which later became North America, Europe, and Asia.
  • Gondwanaland in the south, which included South America, Africa, India, Australia, and Antarctica.
• These continents slowly drifted apart, leading to the formation of the present-day continents.

Evidence for Continental Drift

Wegener used different types of evidence to support his theory that continents had drifted apart.

1. Matching Coastlines (Jigsaw Fit):
   • The coastlines of continents like South America and Africa appear to fit together like pieces of a puzzle. This suggested that these continents were once connected.
2. Similar Rocks Across Oceans:
   • Similar types of rocks and mountains were found on continents now separated by oceans. For example, rocks in Brazil match those in West Africa, indicating that these continents were once connected.
3. Tillite Deposits:
   • Tillite rocks, which are formed by glaciers, have been found in places like India, Africa, South America, and Antarctica. This suggests these areas were once part of the same landmass located near the South Pole.
4. Fossil Evidence:

   Fossils of the same species of plants and animals have been found on continents that are now far apart. For example:

   • Fossils of the freshwater reptile Mesosaurus have been found in both South America and Africa, which suggests these continents were once connected because this reptile couldn’t have crossed the ocean.

Forces for Continental Drift

Wegener suggested that two forces might be responsible for moving the continents:

• Pole-Fleeing Force: This force comes from the Earth’s rotation and could have pushed continents away from the poles.
• Tidal Forces: Wegener also thought that the gravitational pull of the Sun and the Moon might have helped move the continents.

However, these forces were not strong enough to explain the movement of the continents, so the theory wasn’t fully accepted until later studies provided better explanations.

Post-Drift Studies

Later studies helped provide more evidence and explanations for how continents moved.

1. Convectional Current Theory:
   • In the 1930s, Arthur Holmes proposed that convection currents in the Earth’s mantle could move the continents. These currents work like boiling water.
   • Hot mantle material rises, moves sideways under the Earth’s crust, cools, and sinks back down. This motion causes the tectonic plates to slowly move across the Earth’s surface.
2. Ocean Floor Mapping

   After World War II, scientists mapped the ocean floor and made key discoveries:

   
   
   • Mid-Ocean Ridges: Long underwater mountain chains where new ocean floor is formed.
   • Ocean Trenches: Deep underwater valleys where old ocean floor sinks back into the Earth’s mantle. This process is called subduction.

Ocean Floor Configuration

The ocean floor is not flat. It has three main regions:

1. Continental Margins

   Continental margins are the underwater edges of continents and include:

   
   
   • Continental shelf: The shallow underwater part near the coast.
   • Continental slope: A steep slope leading from the edge of the continental shelf down to the ocean floor.
   • Continental rise: The area where sediment collects at the bottom of the slope.
2. Deep-Sea Basins
   • Deep-sea basins are flat, deep areas of the ocean floor that are covered by thick layers of sediment. These sediments come from eroded material from the land.
3. Mid-Ocean Ridges
   • Mid-ocean ridges are underwater mountain ranges where new oceanic crust is created. Magma from the mantle rises through the ridge and cools, forming new crust that spreads outwards.

Distribution of Earthquakes and Volcanoes

Earthquakes and volcanoes are often found where tectonic plates meet.

• Earthquakes: These occur when plates move against each other. For example, earthquakes happen where plates collide, pull apart, or slide past each other.
• Volcanoes: Magma from the Earth’s mantle rises to the surface through cracks in the Earth’s crust. Volcanoes are especially common at mid-ocean ridges and subduction zones.

Sea Floor Spreading

In the early 1960s, Harry Hess proposed the theory of sea floor spreading, which explained how the ocean floor is constantly being created and destroyed.

At mid-ocean ridges, magma from the mantle rises, creating new oceanic crust. As the new crust forms, it pushes the older crust away from the ridge, causing the sea floor to spread.

• Magnetic Stripes: Rocks on the ocean floor record the Earth’s magnetic field. The pattern of magnetic stripes on either side of a mid-ocean ridge shows that new oceanic crust is being created and pushed away.
• Rock Age: Rocks near mid-ocean ridges are younger, and rocks farther from the ridge are older, which proves that the ocean floor is spreading.
• Sediment Thickness: Sediment on the ocean floor is thinner near the mid-ocean ridges because the crust there is newer.

Plate Tectonics

The theory of plate tectonics explains that the Earth’s outer shell is divided into tectonic plates. These plates move over the semi-fluid layer beneath them, called the asthenosphere.

• Divergent Boundaries: Plates move apart from each other. This happens at mid-ocean ridges, where new crust is created (e.g., the Mid-Atlantic Ridge).
• Convergent Boundaries: Plates move towards each other. One plate may be forced beneath another (subduction), or the plates may collide, causing mountain formation (e.g., the Himalayas, where the Indian Plate collided with the Eurasian Plate).
• Transform Boundaries: Plates slide past each other, causing earthquakes. A well-known example is the San Andreas Fault in California.

List of Major and Minor Tectonic Plates

The Earth’s surface is divided into several major and minor tectonic plates:

• Pacific Plate: The largest tectonic plate, mostly under the Pacific Ocean.
• North American Plate: Covers North America, parts of the Atlantic Ocean, and parts of Siberia.
• South American Plate: Includes South America and part of the Atlantic Ocean.
• Eurasian Plate: Covers Europe, Asia (except India), and parts of the Atlantic Ocean.
• African Plate: Includes the continent of Africa and parts of the surrounding oceans.
• Indo-Australian Plate: A combination of the Indian Plate (which includes India) and the Australian Plate.
• Antarctic Plate: Covers Antarctica and the surrounding Southern Ocean.

• Nazca Plate: Located off the west coast of South America.
• Cocos Plate: Found near Central America.
• Caribbean Plate: Located in the Caribbean Sea.
• Philippine Sea Plate: Found near the Philippines.
• Arabian Plate: Covers the Arabian Peninsula.
• Scotia Plate: Found in the southern Atlantic near Antarctica.

Movement of the Indian Plate

The Indian Plate is an important example of tectonic plate movement.

• Northward Drift: Around 150 million years ago, the Indian Plate broke away from Gondwanaland and began drifting northward.
• Collision with the Eurasian Plate: About 40-50 million years ago, the Indian Plate collided with the Eurasian Plate, forming the Himalayas.
• Ongoing Movement: The Indian Plate continues to move north at about 5 cm per year, which causes the Himalayas to rise and triggers earthquakes in the region.

Forces for Plate Movement

Tectonic plates move because of convection currents in the Earth’s mantle. These currents are caused by heat from the Earth’s core. The hot material rises, cools as it moves across, and then sinks back down, creating a cycle that pushes the plates.

Conclusion

• The continents were once joined in Pangaea and have drifted apart over millions of years.
• The plate tectonics theory explains how tectonic plates move, interact, and cause earthquakes, volcanoes, and mountain-building processes.
• The Indian Plate’s movement created the Himalayas, and its ongoing movement continues to shape the Earth’s surface.