Chapter Notes

Continental Drift

Have you ever looked at a world map and noticed how the coast of South America seems like it could fit snugly against the coast of Africa? You're not the first! This observation is the starting point for the theory of Continental Drift.

While a Dutch map maker named Abraham Ortelius first suggested this possibility in 1596, it was a German meteorologist, Alfred Wegener, who developed a complete theory in 1912.

Wegener's theory proposed that about 200 million years ago, all the continents were joined together into a single, massive supercontinent. He named this supercontinent PANGAEA, which means "all earth." This landmass was surrounded by a single, vast ocean called PANTHALASSA, meaning "all water."

According to Wegener, Pangaea began to break apart. It first split into two large landmasses:

• Laurasia in the north.
• Gondwanaland in the south.

These two landmasses then continued to break apart and drift over millions of years into the continents we know today.

Evidence in Support of the Continental Drift

Wegener didn't just base his theory on the shape of the continents; he gathered several pieces of evidence to support his claims.

The Matching of Continents (Jig-Saw-Fit)

The most obvious evidence is the remarkable match between the coastlines of South America and Africa. This fit is not just approximate. In 1964, a scientist named Bullard used a computer program to match the continents not at their current coastlines, but at the edge of their continental shelves (the 1,000-fathom line), and found the fit to be nearly perfect.

Rocks of Same Age Across the Oceans

Using modern radiometric dating, scientists have found that the belt of ancient rocks from the Brazil coast, dated at 2,000 million years old, perfectly matches a similar belt of rocks in western Africa. This suggests these rock formations were created at the same time and in the same place, before the continents split.

Tillite

Tillite is a type of sedimentary rock formed from the deposits left by glaciers. Geologists have found ancient tillite from the same glacial period in several different parts of the Southern Hemisphere, including India, Africa, Falkland Island, Madagascar, Antarctica, and Australia. The presence of the same glacial deposits on these now-separate continents strongly suggests they were once a single landmass covered by the same ice sheet.

Placer Deposits

An interesting puzzle exists on the coast of Ghana in Africa, which has rich deposits of gold. However, the source rocks for this gold are nowhere to be found in Ghana. The source, gold-bearing veins, is actually found in Brazil. This makes perfect sense if the two continents were once joined—the gold would have eroded from the Brazilian plateau and been deposited in what is now Ghana.

Distribution of Fossils

Scientists have found fossils of identical plants and animals, which lived on land or in fresh water, on continents that are now separated by vast oceans.

• The fossils of Mesosaurus, a small reptile that lived in shallow brackish water, are only found in two places: South Africa and Brazil. It's highly unlikely this small creature could have swum across the entire Atlantic Ocean.
• The presence of Lemurs in India, Madagascar, and Africa led some scientists to theorize about a former land bridge or contiguous landmass called 'Lemuria' connecting them.

Force for Drifting

While Wegener provided strong evidence that the continents drifted, his explanation for why they drifted was less convincing to other scientists. He proposed two main forces:

1. Pole-fleeing force: Related to the Earth's rotation, which creates a bulge at the equator. Wegener thought this force caused continents to drift towards the equator.
2. Tidal force: The gravitational pull of the sun and moon, which causes ocean tides. He believed this force, over millions of years, could pull the continents westward.

Most scientists at the time considered these forces far too weak to move entire continents, and his theory was largely dismissed for this reason.

Post-drift Studies

For many years, the idea of continental drift was on the fringe of science. However, after World War II, new technologies allowed for detailed studies of the ocean floor, which revealed surprising information and revived interest in the idea that the Earth's surface is dynamic.

Convectional Current Theory

In the 1930s, long before the ocean floor was mapped, a scientist named Arthur Holmes proposed a possible mechanism for continental drift. He suggested that the Earth's mantle contains convection currents.

• These currents are generated by heat from radioactive elements in the mantle, which creates thermal differences.
• Hotter, less dense material rises, spreads out, cools, and then sinks back down, creating a continuous circular flow.
• Holmes argued that these powerful currents could be the force strong enough to move continents.

Mapping of the Ocean Floor

Detailed mapping revealed that the ocean floor is not a flat, featureless plain. It has immense underwater mountain ranges, called mid-oceanic ridges, and extremely deep trenches. Key findings from this research were:

• The mid-oceanic ridges are volcanically very active.
• Rocks of the oceanic crust are much younger than continental rocks.
• Rocks on either side of a mid-oceanic ridge showed a symmetrical pattern: rocks equidistant from the ridge crest were the same age and had similar compositions. The youngest rocks were at the crest itself, and they got progressively older farther away from it.

Ocean Floor Configuration

The ocean floor can be divided into three main parts: continental margins, abyssal plains, and mid-oceanic ridges.

Continental Margins

This is the transition zone between the continents and the deep ocean floor. It includes the continental shelf, slope, rise, and deep-oceanic trenches. The trenches are particularly important as they are the deepest parts of the oceans.

Abyssal Plains

These are vast, flat plains that lie between the continental margins and the mid-oceanic ridges. They are covered with sediments that have washed off the continents.

Mid-Oceanic Ridges

This is a massive, interconnected chain of underwater mountains, forming the longest mountain range on Earth. The central part of the ridge, called the crest, has a rift system and is a zone of intense volcanic activity.

Distribution of Earthquakes and Volcanoes

When maps of earthquake and volcano locations are studied, a clear pattern emerges.

• There is a line of seismic and volcanic activity that runs right through the middle of the Atlantic Ocean, following the mid-oceanic ridge.
• Another major area of activity is the "rim of fire" around the Pacific Ocean.
• Earthquakes at the mid-oceanic ridges are shallow, while earthquakes near the trenches and continental margins are often very deep.

Concept of Sea Floor Spreading

The new information from ocean floor mapping led a scientist named Hess to propose the theory of sea floor spreading in 1961. This theory explained the observations that Wegener's theory could not.

The key points of sea floor spreading are:

1. Constant volcanic eruptions occur at the crest of mid-oceanic ridges.
2. This new lava pushes the oceanic crust apart, causing the sea floor to "spread" away from the ridge on both sides.
3. As the floor spreads, older rock is pushed farther away from the ridge. This explains why rocks are youngest at the ridge and older as you move away.
4. To prevent the Earth from getting bigger, the older oceanic crust eventually sinks back into the mantle at deep oceanic trenches. This process is called consumption.

Plate Tectonics

The ideas of continental drift and sea floor spreading were combined in 1967 into the modern, comprehensive theory of Plate Tectonics, developed by McKenzie, Parker, and Morgan.

This theory states that the Earth's outer layer, the lithosphere (which includes the crust and the top part of the mantle), is not one solid piece. Instead, it is broken into massive, irregularly-shaped slabs of rock called tectonic plates. These plates "float" and move horizontally on the hotter, more fluid layer of the mantle below, known as the asthenosphere.

There are seven major plates and several minor ones.

Major Plates

• Antarctica and the surrounding oceanic plate
• North American plate
• South American plate
• Pacific plate
• India-Australia-New Zealand plate
• Africa with the eastern Atlantic floor plate
• Eurasia and the adjacent oceanic plate

Minor Plates

Some important minor plates include the Cocos plate, Nazca plate, Arabian plate, Philippine plate, and Caroline plate.

Plate Boundaries

The most geologically active areas on Earth are the boundaries where these plates meet. There are three types of plate boundaries.

Divergent Boundaries

This is where two plates pull away from each other. Magma from the mantle rises to fill the gap, creating new crust. These are also known as spreading sites. [!example] The Mid-Atlantic Ridge is a classic example of a divergent boundary, where the North American and Eurasian plates are moving apart.

Convergent Boundaries

This is where two plates collide. The result depends on the types of plates colliding, but crust is always destroyed as one plate dives under the other in a process called subduction.

• Oceanic-Continental: The denser oceanic plate sinks under the continental plate.
• Oceanic-Oceanic: One oceanic plate sinks under the other.
• Continental-Continental: The two continents collide and crumple, forming massive mountain ranges.

Transform Boundaries

This is where two plates slide horizontally past each other. Crust is neither created nor destroyed. These boundaries are often marked by large faults.

Rates of Plate Movement

Scientists can determine the speed of plate movement by studying the magnetic stripes on the ocean floor. The rates vary widely:

• Slowest: The Arctic Ridge moves at less than 2.5 cm/yr.
• Fastest: The East Pacific Rise moves at more than 15 cm/yr.

Force for the Plate Movement

The driving force behind plate movement is the convection cells in the mantle, as first proposed by Arthur Holmes. The slow, circular movement of hot, softened rock in the mantle drags the rigid plates on the surface along with it. The heat for this process comes from two main sources: radioactive decay within the Earth and residual heat left over from the planet's formation.

Movement of the Indian Plate

The theory of plate tectonics beautifully explains the geography of India and the formation of the Himalayas.

• The Indian plate includes Peninsular India and Australia.
• About 200 million years ago, India began its long northward journey after breaking away from Gondwanaland.
• During its journey, around 60 million years ago, a massive outpouring of lava created the Deccan Traps in what is now western India. At this time, the subcontinent was still near the equator.
• Finally, about 40-50 million years ago, the Indian plate collided with the Eurasian plate.
• Since both were continental plates, neither would subduct. Instead, the crust buckled, folded, and was pushed upwards, creating the Himalayan mountains.