Oceanic Crust

Objectives:

1. To be able to define what oceanic crust is;

2. To be able to explain how oceanic crust forms;

3. To be able to describe the composition of oceanic crust; and

4. To be able to draw a diagram of the structure of oceanic crust.

Definition

• The oceanic crust is the part of Earth's lithosphere which underlies the ocean basins. It is thinner (generally less than 10 km thick) but more dense than continental crust, about 3.3 g/cc (grams per cubic centimeter).

Formation

1. Oceanic crust is created at mid-ocean/oceanic ridges and destroyed by being pulled back into the mantle in subduction zones by the processes of plate tectonics.

2. Most of the present day oceanic crust is less than 200 million years old because it is continuously being recycled.

3. Seafloor spreading forms oceanic crust.

Composition

1. Oceanic crust is composed of mafic basaltic rocks.

2. See Ophiolite link for more on composition.

Structure• It is also more dense than continental crust and

considerably thinner averaging 5-10 km versus the average continental crust thickness of around 20 to 80 km.

• Oceanic crust is coarsest at its bottom and is finer grained at its top.

• About 60% of the Earth's surface is underlain by oceanic crust.

• Ophiolite Sequence = structure (see diagrams)

Structure (cont.)

Structure (cont.)

Watch the Seafloor Move

Subduction Zones

• Formed by seafloor spreading

• Composed of basaltic rock (higher in iron)

• Ophiolite structure

• Destroyed at Subduction Zones

• Cascadia Subduction Zone in California

Conclusion