Unit 5-2: Plate Tectonics

Moving plates cover the Earth How did the Earth come to have the features it currently has? –Why don’t we see craters everywhere from the meteors that struck the earth eons ago? –Have features only been worn down by the sands of time? –Why then, do we have mountains that are not very weathered, and others that are?

Moving plates cover the Earth The answer to these questions lies in the theory of Plate Tectonics: –Plates are rigid pieces of the crust that ‘float’ on the mantle. –These plates move steadily but incredibly slowly. –The movement of these plates as they move past each other is responsible for the Earth’s current features.

Moving plates cover the Earth These plates are always moving, and in different directions. –There is no gap between these plates, so they are always in contact with one another. Even if they are moving apart! –This movement and contact results in different features.

Moving plates cover the Earth The African Plate and the South American Plate are moving away from each other. The Nazca Plate and the South American Plate are moving towards each other. –These two different movements result in different land features on the continents. –This is what caused the Andes Mountains in South America.

Properties of the Plates The crust and mantle were originally thought to be made of entirely different material. It is now known that the crust and the uppermost layer of the mantle are composed of mostly the same material. This sharing of material is what makes up the Earth’s lithosphere.

Properties of the Plates The lithosphere is rigid, but is broken up into the plates. The lithosphere is about 100km thick. The lithosphere is made almost entirely of basalt. –The continents however, are made of granite, which is why they are the highest layer of the lithosphere. (granite is less dense than basalt)

Why do the plates move? The plates that make up the lithosphere float on a second layer – the Astenosphere. –The rock in the astenosphere is heated to the point that it is able to flow. –This layer moves in a slow convection current. Heat travels up from the outer core, flows in the direction of plate movement, Sinks back down towards the core.

Why do the plates move? It has been found that the plates move apart from each other where the astenosphere’s convection current rises. It has also been found that the plates collide where the astenosphere’s convection current sinks.

Evidence for Plate Tectonics The most prominent evidence for plate tectonics is volcanic and seismic activity. –Scientists have tried to find the causes of earthquakes and volcanic eruptions. –They observed that earthquakes do not occur in random places, but in the same areas over and over. –Likewise, volcanoes seem to follow a pattern or path.

Evidence for Plate Tectonics Earthquakes and volcanoes occur at the boundaries between plates. –This is where immense stress is built up When this stress is released, causes earthquakes –This is also where we have either molten rock being pushed up to the surface, Where the astenosphere is rising. –Or this is where the plate is being pulled down into the earth, Where the astenosphere is sinking.