The Earth layer that most of us are familiar with is the one that we are standing on, the crust. Crust, however, is further divided into two main types: oceanic crust and continental crust. Differences in chemical composition and density allow us to distinguish between these two types.

Comparing the two different kinds of crust.
• Oceanic Crust: Oceanic crust, as the name implies, is the crust that underlies most of the world’s oceans. Oceanic crust is typically about 5–7 kilometers (3–4 miles) thick and is composed primarily of basalt. The average density of oceanic crust is 3.0 g/cm³ (i.e., three times the density of liquid water). Oceanic crust is very young when compared to the rest of the Earth. It is about 180 million years old.

Did You Know?
Only four elements together make up 87.4% of the Earth’s crust. They are oxygen (46.6%), silicon (27.8%), aluminum (8.1%), and iron (5.0%). The remaining 12.6% is composed of small amounts of calcium, sodium, potassium, magnesium, and various other elements.
• Continental Crust: Continental crust underlies the continents. The thickness of continental crust typically varies between 30–40 kilometers (19–25 miles), but can extend up to 70 kilometers (43 miles) underneath mountains. Although continental crust is not as uniform in composition as oceanic crust, it generally contains a fairly high proportion of granite. Continental crust has an average density of 2.7 g/cm³. Since continental crust is less dense than oceanic crust, continental crust will always “ride over” oceanic crust wherever the two types of crust meet (see Unit B – Plate Tectonics for further details). Continental crust can be quite old with a maximum age of approximately 4 billion years.