Oceanic Crust
* Composition: Primarily composed of basalt, a dark-colored, fine-grained igneous rock rich in magnesium and iron. It also contains some gabbro, a coarser-grained intrusive rock with similar composition.
* Density: Typically around 2.9 g/cm³.
* Thickness: Relatively thin, ranging from 5 to 10 kilometers.
Continental Crust
* Composition: More diverse, but generally richer in silica and lighter elements like aluminum and sodium. Common rock types include granite, a light-colored, coarse-grained igneous rock, and various metamorphic rocks.
* Density: Generally around 2.7 g/cm³.
* Thickness: Much thicker than oceanic crust, ranging from 30 to 70 kilometers.
Why the difference in density?
* Mineral Composition: The heavier, denser minerals like magnesium and iron are more abundant in oceanic crust, contributing to its higher density. Conversely, continental crust has more abundant lighter elements like silica and aluminum, resulting in lower density.
* Age: Oceanic crust is constantly being created at mid-ocean ridges and is older further away from the ridges. As oceanic crust ages, it cools and becomes denser. This also contributes to the difference in density between older and younger oceanic crust.
Significance of Density Difference
The density difference between oceanic and continental crust is a key factor in plate tectonics. Oceanic crust, being denser, subducts beneath continental crust at convergent plate boundaries, driving processes like volcanic arcs and mountain building.
