Tectonic Plates
Section: Earth and Space | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Structure of the Earth
You may recall from Stage 7 that the outer layers of the Earth are the solid, rocky crust. The crust rests on the more fluid mantle.
Earth is made of four main layers, each with a different state and composition
Earth's Layers
| Layer | State | Key Features |
|---|---|---|
| Inner Core | Solid | Temperature over 5000°C; made of iron and nickel; extremely high pressure |
| Outer Core | Liquid | Made of molten iron and nickel; responsible for Earth's magnetic field |
| Mantle | Semi-solid (very thick fluid) | Heated by the core; convection currents occur here; does not flow easily |
| Crust | Solid | Thin, rocky outer layer; broken into tectonic plates |
Heat Inside the Earth
The mantle is heated from the innermost part of the Earth: the inner core. The inner core is estimated to be at a temperature of over 5000°C. The high temperature of the inner core is due to:
- Thermal energy left over from the formation of the Earth
- Friction inside the Earth
- The type of reactions that happen in the rocks
- Remember from Unit 3: thermal energy is transferred through fluids by convection
- Convection currents occur in fluids
Movement of Tectonic Plates
Convection currents in the mantle cause tectonic plates to move (Source: Cambridge Checkpoint Science Learner's Book)
How Convection Currents Move Plates
The inner part of the mantle gets thermal energy from the core. The fluid in the mantle then expands when heated and becomes less dense than the fluid surrounding it. This hotter, less dense fluid in the mantle rises towards the crust, cools and sinks again, resulting in a convection current.
Slow-moving convection currents in the mantle drag the tectonic plates above them along with the flow
- The mantle is a very thick fluid and does not flow easily like water
- The convection currents move very slowly
- As the convection currents in the mantle move across underneath the crust, the tectonic plates that make up the crust are pulled along
- Just as the convection currents are slow, the movement of the tectonic plates is also slow
- Tectonic plates move at varying rates between 0.6 and 10 cm per year
- Key process: the inner core heats the mantle
- hot mantle material rises (less dense)
- cool mantle material sinks (more dense)
- this creates convection currents
- convection currents drag tectonic plates along
Practical Investigation: Modelling Convection in the Mantle
Convection currents in the mantle can be modelled in the classroom - for example, by heating a container of syrup (or wax) with a candle placed underneath, with biscuits floating on top representing tectonic plates.
- Safety risk: the hot syrup/liquid could splash and cause burns, or splash into eyes
- How the risk is reduced: wear eye protection (safety goggles), stand back from the equipment, and use tongs / heatproof gloves rather than handling the hot container directly
What are Tectonic Plates?
Scientists have mapped the movement of the tectonic plates (Source: Cambridge Checkpoint Science Learner's Book)
- Tectonic plates: large sections of the Earth's crust that float on the mantle and move slowly due to convection currents beneath them
Major Tectonic Plates
- Pacific Plate - largest plate, mostly oceanic
- North American Plate
- South American Plate
- Eurasian Plate - covers Europe and most of Asia
- African Plate
- Indo-Australian Plate
- Antarctic Plate
- Many smaller plates including: Caribbean Plate, Arabian Plate, Iranian Plate, Philippine Plate, Nazca Plate, Cocos Plate
- The Earth's crust is broken into approximately 15 major plates and many smaller ones
- Plates move in different directions at different speeds
- Most geological activity (earthquakes, volcanoes) occurs at plate boundaries
Why Do Tectonic Plates Float on the Mantle?
Tectonic plates are less dense than the mantle beneath them.
- Tectonic plates (the crust) are less dense than the mantle/magma below them
- Less dense materials float on top of denser materials - in the same way a less dense object (e.g. wood) floats on a denser liquid (e.g. water)
- This is why the tectonic plates sit above the mantle, rather than sinking into it
Evidence for Tectonic Plates
Scientists have gathered several types of evidence that support the theory of plate tectonics.
1. Continental Jigsaw (Continental Coasts)
The continents fit together like pieces of a jigsaw puzzle (Source: Cambridge Checkpoint Science Learner's Book)
When you look at a world map, it looks like the continents could fit together like pieces of a giant jigsaw puzzle - a picture cut into small interlocking pieces that can be put back together to form the original image.
Scientists have done this with the continents on Earth. The continental coasts can fit together as shown in the diagram. Scientists say that this continental jigsaw appearance is evidence for tectonic plates. There is a hypothesis that there was once only one large continent that eventually separated. The separate parts became some of the tectonic plates, and the convection currents from the mantle drove their movement.
- The east coast of South America fits remarkably well with the west coast of Africa
- this suggests they were once joined together
2. Fossil Record
- Fossil record: the collection of thousands of fossils that provide information about the time before humans were on Earth. Fossils are the remains of dead animals and plants that have turned to stone over millions of years
Fossils of Mesosaurus have been found in both South America and Africa (Source: Cambridge Checkpoint Science Learner's Book)
The fossil record provides more evidence for tectonic plates and their movement.
Mesosaurus Fossils
Fossils of an extinct reptile called Mesosaurus have been found in the parts of Africa and South America that would fit together in the jigsaw model of the continents. Mesosaurus lived about 275 million years ago and was similar to a crocodile. These animals lived at the coast and in shallow water. It is not likely that they would have crossed the 5000 km-wide Atlantic Ocean that now separates these continents.
Glossopteris Fossils
300 million-year-old fossils of the plant Glossopteris have been found in Antarctica, India, Australia, Africa and South America. This gives more evidence for the hypothesis that these continents were once joined.
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