LR247.2.5: Continental Drift

This image shows Earth's continents. What do you notice about their shapes?

Observe the map of the Earth's continents. Do you think they ever could have fit together like a puzzle?

In 1912, Alfred Wegener first developed the theory of continental drift. As a geologist, he spent a great deal of time learning as much as he could about Earth's history and structure. What he found was remarkable: the continents looked like puzzle pieces! Wegener hypothesized that millions of years ago, Earth's continents formed a single supercontinent, which he named Pangaea. While scientists were initially skeptical of Wegener's theory, over time, evidence was discovered that clearly supported his hypothesis.

This image shows how the super continent Pangaea has broken up over time due to continental drift.

One piece of evidence that supported Wegener's hypothesis came when geologists discovered that rocks on the east coast of South America matched rocks on the west coast of Africa in terms of age and composition. According to the theory of continental drift, Africa and South America were connected originally, and this discovery provided proof. Geologists found matching plant and animal fossils in both South America and Africa as well. For example, fossils of Mesosaurus, a reptile, were found on both continents. Since it would have been impossible for Mesosaurus to travel across an entire ocean, it was concluded that the continents must have previously been joined to allow for Mesosaurus to live on both.

One of the animals whose fossil record supports the theory of continental drift is a reptile called Mesosaurus. Mesosaurus lived in freshwater. Fossils of Mesosaurus were found in both South America and Africa.

Some scientists were still not convinced. How can huge continents move? In the 1960s, new evidence was found. During a routine mapping of the ocean floor of the Atlantic Ocean, an underwater mountain chain was found. On both sides of the mountain chain, the ocean floor was moving. This was due to a process called sea-floor spreading. Sea-floor spreading provided evidence for the theory of how the continents moved.

During seafloor spreading, magma rises up from the mantle. It cools and hardens to form new crust.

Check for Understanding:

1. The theory of continental drift states that the continents once formed a single landmass and have drifted apart over time.

2. Pangaea is the name of the supercontinent that all of Earth's continents at one time formed.

3. Fossil evidence supports the theory of continental drift.

Using evidence from different scientific fields, scientists have constructed maps showing the movement of the continents over time. Over millions of years, the single landmass of Pangaea became the seven separate continents shown on maps today.

Check for Understanding:

1.Alfred Wegner, a German geologist, proposed that the “puzzle pieces” of the different continents had once fit together as a single landmass.

2. Around 300 million years ago, the continents were joined together in a supercontinent called Pangaea.

3. Continental drift is the idea that continents have moved across Earth’s surface over time.

At first, scientists were skeptical of Wegener’s theory of continental drift. Over time, evidence was discovered that supported his hypothesis that the continents were moving. One piece of evidence that supported Wegener’s theory of continental drift was the existence of landforms of a similar age on different continents. Scientists noticed that mountain ranges in South America and Africa were formed during a similar time period. Additionally, coal fields in Europe and North America contained coal of a similar age.

Scientists also found evidence in fossils that supported Wegener's theory of continental drift. For example, fossils of Glossopteris, a type of fern plant, have been found in Africa, South America, Australia, India, and Antarctica. In order for the seeds from this plant to be distributed across several continents, scientists believe the continents must have been joined together at some point.

Scientists have found fossils of land bearing animals and plants on multiple continents. This suggests the continents were once joined together.

A continent’s climate gets warmer the closer it moves toward the Equator. A continent’s climate gets cooler the closer it moves toward the poles. Fossils of tropical organisms have been found near the poles. This indicates that the continent has drifted toward the poles over time.

Check for Understanding:

There are two main features of evidence that support the theory of continental drift: land features and fossils.

1. Land features that appear in different continents are of a similar age, suggesting that the continents have shifted over time.

2. Fossils of the same species have been found on different continents around the world, suggesting that those continents were once closer together.

3. Remains of tropical organisms have been found in cold regions, suggesting that the continents may have been in different parts of the world a long time ago.

Even though there were many pieces of evidence to support Wegener’s ideas, they were still pushed aside because he could not explain how the continents were able to move. However, in the mid-1900s scientists discovered new information that helped them understand how the continents had drifted. At this time, scientists began mapping the ocean floor. They found that not all of the ocean floor was flat and smooth. In parts of the sea floor, there were under-water mountains called mid-ocean ridges. Mid-ocean ridges are giant mountain ranges that run along the ocean floor. Some of these mountain ranges are longer than any mountain range found on the Earth’s surface.

Mid-ocean ridges form along cracks in the oceanic crust. Rock samples from the sea floor showed that the youngest rock is closest to the ridge, and the oldest rock is the farthest away from the ridge. Scientists realized parts of the sea floor must be moving if older rocks are constantly pushed outward. Scientists wondered if the sea floor movement could explain Wegener’s theory of continental drift.

The sea floor is moving, similar to a conveyor belt, as new rocks are pushed away from the ridge.

In order to explain why the sea floor was moving, scientists proposed a process called sea-floor spreading. Sea-floor spreading occurs when magma rises through the cracks in a mid-ocean ridge, cools, and hardens to form new oceanic crust. During sea-floor spreading, the midpoint of the ridge breaks, and two pieces of oceanic crust move in opposite directions as new crust is created. The older oceanic crust moves farther away from the mid-ocean ridge as new oceanic crust is formed.

Oceanic crusts are moving away from the mid-ocean ridge on both sides as new sea floor is created near the midpoint of the ridge.

Checking for Understanding:

1. Deep-ocean trenches are canyons that make up the deepest part of the ocean floor.

2. Subduction is the process in which oceanic crust becomes dense enough to sink beneath a deep-ocean trench and return to the mantle.

• Oceanic crust cools as it moves away from the mid-ocean ridge.

•.As the oceanic crust cools, the oceanic crust becomes denser and sinks deeper.

• The oceanic crust will become denser, and with the help of gravity, will sink deep enough to go beneath the deep-ocean trench and into the mantle.

Check for Understanding:

1. In the late 1800s, Alfred Wegener proposed that continents formed a single landmass called Pangaea over 300 million years ago.

2. Continental drift is the movement of continents across Earth's surface over time. There is evidence that continental drift has occurred:

• Similar land features such as coal fields have been found in North America and Europe.

• Fossils of same species have been found on multiple continents.

• Remains of tropical organisms have been found in cold regions, and glacier striations have been found in warm regions.

3. During the process of sea-floor spreading magma rises through the cracks in the mid-ocean ridges, cools, and hardens to form new oceanic crust.

• Deep-ocean trenches are deep canyons where oceanic crust sinks back into the mantle.

• The process of creating new oceanic crust and the crust going back into the mantle takes around 200 million years.