Continental Drift and Plate Tectonics
As you work through the following slides of information, make sure you use the information to complete the tasks on your worksheet. By the end of the PDF, you should be able to describe the theory of continental drift, and explain how it is similar and different from the theory of plate tectonics. You should also be able to identify several pieces of evidence that support the theory of plate tectonics and continental drift. The tasks you will need to complete are highlighted in green text boxes. You may work with your group members to complete the tasks.
The Earth s rocky crust solidified billions of years ago, soon after the Earth formed. The crust is not a solid shell; it is broken up into huge, thick plates that drift atop the soft, underlying mantle. The plates are made of rock and drift all over the globe. They move both horizontally and vertically. Over long periods of time, the plates also change in size as their margins are added to, crushed together or pushed back into the Earth s mantle. The plates are between 80 to 400km thick.
Earth s Major Plates Now complete the following tasks on your worksheet: - TASK 1
Continental Drift Look at the picture below. Have you ever noticed that South America and Africa look like they might fit together like a jigsaw puzzle? This was first observed by the English writer Sir Frances Bacon in the 17 th Century. In 1910, German meteorologist Alfred Wegner developed a theory to explain how this might have been possible. He suggested that between 200 and 300 million years ago, all seven continents were one giant landmass. He called this landmass Pangaea, which means all lands. He also hypothesised that Pangaea has split into pieces and had, over time, drifted apart to form the continents as they exist today. He believed that the plates could drift slowly atop the liquid asthenosphere.
Wegner believed that there was a supercontinent around 290 million years ago called Pangaea. It was made up of all of the Earth s land masses, and existed from the Permian (290mya) through Jurassic (145mya) periods. It began breaking apart in the Jurassic Period, forming the continents Gondwanaland and Laurasia.
The Earth as it was 300 million years ago (left) and as it is today (right). The arrows indicate motion of the plates, and show that the continents are still moving apart or even colliding with each other. By the end of the Cretaceous period (65mya), Laurasia and Gondwanaland has begun to break apart, separating into land masses that look like our modernday continents. Now complete the following tasks on your worksheet: - TASK 2
The Evidence Wegner needed evidence to support his theory. Just because the continents looked like they fitted together was not proof that they did. after conducting some research, he discovered several plant and animal fossils that were identical even though they were found on opposite sites of the ocean. To gather more evidence, Wegner then looked at earth s mountain ranges. After studying the Appalachian Mountains along the eastern seaboard of North America and the Scottish Highlands in Europe, he discovered that the types of rocks on both sides of the atlantic ocean matched. Wegner believed this would only be possible if America and Europe were once together and has later drifted apart. However, many scientists did not accept Wegner s theory, as he could not explain how the continents had drifted apart. The theory of continental drift states that crustal plates move, but does not explain what causes this movement.
The Evidence Edward Suess, an Austrian geologist, found fossil evidence of the Glossopteris tree in India, South America, Southern Africa, Australia and Antarctica, and hypothesised that all these continents must have once been joined together - forming Gondwanaland. The continents of Gondwanaland were joined by a land bridge - a strip of land that connects adjacent continental land masses.
Other fossil evidence found included: Now complete the following tasks on your worksheet: - TASK 3 - TASK 4 - TASK 5
Convection Currents In 1929, a geologist named Arthur Holmes, suggested that the Earth s thick second layer, the mantle, has currents powerful enough to carry the continents sitting on top of them. He compared the process to a giant conveyor belt. These currents are known as convection currents. The idea that continents move due to convection currents in the Earth s mantle forms the basis of the theory of Plate tectonics.
Convection currents - hot air and liquids rise in the mantle and so does hot molten rock. Likewise, cooler rock drops. Heat from deep within the Earth causes the cooler molten rock to heat up again, and it rises up in the mantle again. When hot mantle rock come into contact with relatively cold crust, it cools and sinks. Look at the diagram opposite. Notice how the direction of the convection current (movement in a clockwise and anticlockwise direction) is responsible for moving plates together or apart.
Evidence for Convection Currents Erta Ale is a volcano in located in the Danakil Depression, in North Eastern Ethiopia. It is a shield volcano known for its persistent lava lake activity. The lava lakes seen at Erta Ale can show the convection currents of magma columns. The magma inside a volcano must be continually circulated within the magma chamber, so heat loss does not lead to solidification. The crust of the lava lake has been observed at moving between 10cm to 40cm per second atop the convection currents. During the movement of the magma, the crust at the surface cracks and the denser magma at the lava lake surface (which includes the crust) sinks whilst the hotter gas-rich magma rises to the surface. Erta Ale lava lake
At diverging plate boundaries, convection currents produce mid- ocean ridges and are responsible for the formation of new crust. Convection currents also produce subduction zones, where one plate slides under another and melts into the top of the mantle. Now complete the following tasks on your worksheet: - TASK 6 - TASK 7
Satellite Evidence Proof of drifting continents can be obtained from satellites orbiting the Earth. Using GPS technology, even minute shifts in the Earth s surface can be detected. In 2006, after 10 years of measuring, these satellites proved Wagner s theory of Continental drift - showing that Europe and North America area moving apart about 1.5cm every year. Now complete the following tasks on your worksheet. - TASK 8