12.2 Features of Plate Tectonics

The earth is made up of several layers. There are 4 “main layers”. Crust Mantle Outer Core Inner Core

Crust: Outermost layer up to 70 km thick Crust: Outermost layer up to 70 km thick. Continental plates are made of granite, Oceanic plates are made of basalt. Mantle: 70% of the earth’s volume (up to 2900 km thick). It is mostly solid and can be divided into two main parts.

Upper mantle is mostly molten rock containing iron and magnesium Upper mantle is mostly molten rock containing iron and magnesium. It is thick like toothpaste.   Lower mantle is made of solid, dense material like magnesium and iron

Outer core: The layer below the mantle Outer core: The layer below the mantle. At 23000 km thick, it is the only all-liquid section. It is comprised mainly of liquid iron and nickel.   Inner core: This layer is a sphere with a radius of about 1200 km. The temperature is about 5000-6000 oC but the inner core is still a solid due to really high pressure.

The difference in spin speed of the outer and inner core may be what causes the earth to have a magnetic field.   But to understand how all of this functions, scientists have made two more divisions to the earth’s layers.

The lithosphere includes the crust and the upper part of the mantle and is mostly solid. The lithosphere can be 65-100 km thick.

The asthenosphere is just below the lithosphere The asthenosphere is just below the lithosphere. There is a partly molten layer in the upper mantle. This liquid rock allows for the tectonic plates above to “float” and move about. Remember, the convection currents in the mantle drive continental drift. This is called mantle convection.

Remember that the tectonic plates are always moving Remember that the tectonic plates are always moving. If two plates are moving away from each other (diverging) then we see a ridge (underwater: oceanic ridge, on land: rift valley).

The newer rock being formed pushes the older rock away The newer rock being formed pushes the older rock away. This is called ridge push.   When two plates collide, one is usually pushed underneath the other. This is called subduction. Areas where this happens (subduction zones) usually experience earthquakes and volcanoes.

As the plate starts to subside, it also experiences slab pull, which helps with subduction further.   The area where two plates are in contact is called a plate boundary. There are two main factors to consider: the composition of the plates (type of plate) and the direction the plates are moving relative to one another.

Diverging plates = spreading apart.   Converging plates = moving together. There are 3 main converging plate interactions we will study:

Oceanic-continental plate convergence.   -This is when an oceanic plate hits a continental plate. -Oceanic plate subducts -Causes the formation of an underwater trench -The freshly melted basalt collects underneath the incoming continental plate and can form cone volcanoes. -The crumpling and folding of the continental plate also forms mountains

Oceanic-Oceanic plate convergence.   -This is when an ocean plate hits an ocean plate. -Whichever ocean plate is denser will subduct. -May cause a volcanic island arc.

Continental-continental plate convergence.   Unlike with oceanic plates, continental plates do not subduct when they hit each other. As a result, both plates are forced upwards. This creates the largest type of mountain ranges (ex: Himalayas.)

Transform plate boundaries   All of the above descriptions involve “head on” collisions between plates. But sometimes the plates “rub shoulders” with each other (pass by and only rub together, not run into each other).

Because there is no “head on collisions”, there are no mountains or volcanoes formed. However, there are still faults in the ground (transform faults) and you still experience earthquakes. A famous example is the San Andreas fault.