Questions raised in class today

How thick is the earth's crust?

The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick^[1] and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick and is mostly composed of slightly less dense rocks than those of the oceanic crust.



What happens to the oceanic crust when it is subducted under the continental crust?

Life cycle
Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. The youngest oceanic crust is at the oceanic ridges, and it gets progressively older away from the ridges.

The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr^-1 half-rate) produce thinner crust (4-5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at a lower depth, thereby producing less melt and thinner crust.

The oceanic crust subducts at what are known as convergent boundaries. These boundaries can exist between 2 oceanic plates .

In the second situation, the oceanic plate always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic plate, so oceanic crust is seldom more than 200 million years old.^[5] The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson cycle.