Continental Drift and Plate Tectonics

Continental Drift

Today, most people know that the landmasses on Earth move around. But people haven’t always believed this. It wasn’t until the early 20th century that German scientist Alfred Wegener suggested that the Earth’s continents were drifting. He called this movement Continental Drift. He was not the first or only person to think this, but he was the first to talk about the idea publicly.

Wegener came up with this idea because he noticed that the coasts of western Africa and eastern South America looked like puzzle pieces. He wondered if they might have once fit together and then drifted apart. Looking at the continents he theorized that they had once been joined together as a supercontinent around 225 million years ago. This continent was later named Pangaea.

Did you know?

The name Pangaea comes from the Ancient Greek words “pan,” meaning entire, and “Gaia,” meaning Earth. 

People now think that other supercontinents may also have existed. These supercontinents are thought to have come before Pangaea.

The idea of moving landmasses seems obvious now, but Wegener’s Theory of Continental Drift was not accepted for many years. Why? Well, for one thing, Wegener did not have a convincing explanation for the cause of the drifting. First, he suggested that the continents were moving around due to the Earth’s rotation. This later turned out to be wrong. Secondly, he was a meteorologist, not a geologist. This meant that geologists didn’t think he knew what he was talking about and did not take him seriously.

Fossil Evidence

One type of evidence that strongly supported the Theory of Continental Drift is the fossil record. Scientists have found fossils of similar types of plants and animals in rocks of similar age. These rocks were on the shores of different continents. This suggests that the continents were once joined.

For example, fossils of Mesosaurus, a freshwater reptile, have been found both in Brazil and western Africa. Also, fossils of the land reptile Lystrosaurus have been found in Africa, India and Antarctica.

Plate Tectonics

The Theory of Plate Tectonics builds on Wegener’s Theory of Continental Drift. In the Theory of Plate Tectonics, it is tectonic plates, rather than continents, which are moving.

Tectonic plates are pieces of the lithosphere. The lithosphere is made up of the crust and the upper mantle. These pieces float on a layer of partly liquid rock called the asthenosphere. Tectonic plates are able to move because the lithosphere is stronger and more dense than the rock below it.

There are currently seven plates that make up most of the continents and the Pacific Ocean. They are:

1. African Plate
2. Antarctic Plate
3. Eurasian Plate
4. Australian Plate
5. North American Plate
6. Pacific Plate
7. South American Plate

There are eight other smaller secondary plates. There are also other microplates, which are very small.

Did you know?

Tectonic plates not only move land masses, they also move oceans! This is because both continents and oceans are on the Earth’s crust.

You may imagine that these plates are zipping along, but in fact, they are moving VERY SLOWLY! Some move as slowly as 10–40 mm/year. This is about as fast as your fingernails grow. Others move as fast as 160 mm/year. This is about as fast as hair grows.

Moving Plates

Geologists came to accept the Theory of Plate Tectonics in the late 1950s and early 1960s after coming to understand the concept of seafloor spreading. Seafloor spreading happens on the seafloor where oceanic plates are moving away from each other. We say that these plates are diverging. When this happens, cracks occur in the lithosphere. This allows magma to come up and cool, forming a new seafloor.

The opposite of divergence is convergence. This happens when plates are moving towards each other. Material may push upwards forming mountains or downwards into the mantle. When the material from one plate is pushed on top of another, we call that obduction. When material is pushed downwards, we call that subduction. The material lost through subduction is roughly balanced by the formation of new ocean crust by seafloor spreading.

When tectonic plates meet, some dramatic things can happen. Let’s look at some of these in more detail.

Mountains and Volcanoes

What do mountains and volcanoes have in common? If you said that they are both large, steep landforms made of rock, you would be right. If you said that both are formed when tectonic plates are pushed and pulled, you would also be right!

Whether mountains or volcanoes form depends on the type of tectonic plates and where the plates collide.

To predict which will happen, you need to know two things.

1. There are two major types of tectonic plates - oceanic and continental.
2. Oceanic plates are more dense than continental plates.

Let’s look at how tectonic plates form mountains and volcanoes.

When two oceanic plates diverge it forms, undersea volcanoes. Volcanoes are caused by cracks in the Earth’s crust.

Image - Text Version

Shown is a colour illustration of oceanic plates creating an undersea volcano.

The illustration is a rectangular cube made up of three layers. The top layer is pale blue and translucent. The middle layer is brown. The bottom layer is orange. 

The bottom layer has three brown arrows. The arrows on the right and left point up, out, and back down in an arch. The arrow in the centre points straight up into the layer above. 

Here, a triangle of orange points up through the brown layer and the blue layer, to the surface. From this orange triangle, larger brown arrows point up and out to both sides. into the brown layer.

An example of this is the Mid-Atlantic Ridge, It extends from the Arctic Ocean to beyond the southern tip of Africa.

There are so many volcanoes in the Mid-Atlantic Ridge that it is considered the longest mountain range in the world!

Iceland is located on this ridge. The red triangles on the picture show where there are active volcanoes.

Image - Text Version

Shown is a colour map of Iceland divided into two tectonic plates.

Iceland is illustrated in beige. It is an oval-shaped island with many jagged peninsulas. It is surrounded by blue, labelled "Atlantic Ocean."

A wide grey stripe curves through the middle of the island. It starts at the top right edge of the illustration, and extends down to the bottom left corner. A second stripe branches off from the middle, extending below and right, just past the southern edge of the island. This whole stripe is labelled "Mid-Atlantic Ridge." 

There are eight red triangles along the ridge, at different points across the island. These represent active volcanos. 


The area of the map to the left of the ridge is labelled "North American Plate," with a large arrow pointing to the left. The area to the right of the ridge is labelled "Eurasian Plate," with a large arrow pointing to the right.

When two continental plates converge on land, one plate will slide beneath the other. This causes rock to get all folded and bunched up. These folded rocks form mountains.

The crust in the region of a mountain is thicker than the surrounding crust. The Himalayan Mountains are an example of this.

Image - Text Version

Shown is a colour illustration of oceanic plates creating an undersea volcano.

The illustration is a rectangular cube made up of two layers. The top surface of the cube is green. The layer below is brown. The bottom layer is orange.

The brown layer is divided into two overlapping sections.

The left section curves down under the right one, into the orange layer below. This is illustrated with dark brown arrows showing its direction of movement. 

The left section curves up and over the right one. It forms one of several brown, pointed mountains on the green surface. This is also illustrated with dark brown arrows.

there is a large red bullseye at a spot where the two layers push against each other.

Let’s see what happens when an oceanic plate converges with a continental plate.