The two primary types of Earth's crust are continental crust and oceanic crust.
These two crust types differ significantly in composition, thickness, and density, shaping the Earth's surface and influencing geological processes.
Continental Crust
- Composition: Predominantly composed of granitic rocks, which are relatively low in density.
- Thickness: Significantly thicker than oceanic crust, averaging around 30-50 kilometers (19-31 miles), but can reach up to 70 kilometers (43 miles) under mountain ranges.
- Density: Less dense than oceanic crust, around 2.7 g/cm³. This lower density contributes to its higher elevation relative to the ocean basins.
- Age: Generally older than oceanic crust; some continental rocks are billions of years old.
- Formation: Formed through complex processes involving plate tectonics, volcanism, and the differentiation of the Earth's mantle. The formation involves the accumulation of lighter materials over long periods.
Oceanic Crust
- Composition: Primarily composed of basaltic rocks, which are denser than granitic rocks.
- Thickness: Much thinner than continental crust, averaging around 5-10 kilometers (3-6 miles).
- Density: Denser than continental crust, around 3.0 g/cm³. This higher density causes it to sink lower into the Earth's mantle.
- Age: Relatively young, typically less than 200 million years old.
- Formation: Formed at mid-ocean ridges through volcanic activity, where magma from the Earth's mantle rises and cools to create new oceanic crust.
Summary Table
| Feature | Continental Crust | Oceanic Crust |
|---|---|---|
| Composition | Granitic | Basaltic |
| Thickness | 30-70 km | 5-10 km |
| Density | ~2.7 g/cm³ | ~3.0 g/cm³ |
| Age | Billions of years | < 200 million years |
| Formation | Complex tectonic processes | Mid-ocean ridges |
The difference in density explains why continental crust "floats" higher on the Earth's mantle than oceanic crust. The constant creation and destruction of oceanic crust at plate boundaries is an essential part of plate tectonics.
