Plate Tectonics (WJEC GCSE Chemistry: Combined Science)

How the surface of the Earth has changed

• About 230 million years ago, all of the land was joined together into one supercontinent called Pangea
• Over time, the surface of the Earth has changed
  • Pangea has broken up as the land masses moved
  • Eventually, forming the Earth as we know it today
• We now know that this change is due to tectonic plates moving a few centimetres per year
  • This movement is known as continental drift

Theory of continental drift

•  In 1912, Alfred Wegener proposed his theory of continental drift
• He noticed that the Earth's continents, particularly Africa and South America, fit together
• This led him to suggest that the Earth's continents had once been part of a single landmass
  • He believed that this landmass had drifted apart over millions of years

Overview of continental drift theory

Over millions of years, one supercontinent has drifted apart to form the present day Earth

Evidence for Wegener's theory

• To support his theory, Wegener looked for, and found evidence:
• Rock layers and mountain ranges
  • Wegener noticed similarities in rock layers and mountain ranges on different continents
    • For example, the Appalachian Mountains in North America seemed to continue into Scotland's Caledonian Mountains

• Fossils

  • Wegener found fossils of the same plants and animals on different continents:
    • The fern Glossopteris found in rocks in South America, Africa, Asia, Australasia and Antarctica
    • The reptiles Mesosaurus and Cynognathus found in Africa and South America
    • The reptile Lystrosaurus found in Africa, Asia and Antarctica

Distributions of fossils

The same fossils on different continents supported Wegener's theory of continental drift

• There was other evidence such as glacial features in tropical regions and coal deposits in Antarctica
• All of these pieces of evidence supported the theory that the continents had once been joined

The weakness of Wegener's theory

• Wegener had several pieces of evidence to support his theory of continental drift
  • But, these were all observations
• The weakness of his theory was the lack of evidence to explain how continental drift happened
  • This led to his theory not being accepted  

Evidence of continental drift

• As technology advanced, evidence was found to explain how continental drift happens
  • Ocean floor studies found large mountain ranges and canyons that would not exist without movement
  • Samples from the Atlantic Ocean floor found that the rock in the middle of the ocean was younger than the rock at the edges 
  • No Atlantic Ocean rock samples were older than around 175 million years, while rocks from the mainland were found that were billions of years old
  • Magnetic data from rocks showed that Britain had moved and had patterns matching North America 
• This evidence suggested that new ocean floor was being created and moving outwards while rock near the edges of the continents was sinking back into the crust
• The evidence proved that the Earth's crust was moving and provided some evidence for tectonic plates

Earth's tectonic plates

Earth's crust consists of seven major and several minor tectonic plates

How do tectonic plates move?

• Radioactive atoms within the Earth's core decay, releasing large amounts of thermal energy
• The thermal energy heats the mantle, which expands
• This means that the material in the mantle nearest the core becomes less dense and rises towards the lithosphere
  • The lithosphere consists of the crust and upper mantle
• Cooler material, which is more dense, sinks to replace the hot, less dense material 
• This is a continual process that causes convection currents to occur in the mantle
• These convection currents can slowly move the tectonic plates above them

Convection currents in the mantle

The heat released by the core causes convection currents in the mantle which move the tectonic plates