Question

The average thickness of the earth's crust is about ________ miles.

Answer 1

The average thickness of the earth's crust is about 18 miles under the continent and 3 miles under the ocean.

Explanation:

Crust is defined as the outermost shell which is solid in every planet and natural satellite. Crust is formed above the mantle.

The crust is composed of different chemicals and generally formed by igneous process and modified later by environmental hazards like soil erosion, eruption of volcano, sedimentation caused during floods etc.  

Generally, the crust of in a planet tend to be uniform in composition, properties and also thickness.

But in Earth crust is of two types. Because the crust formed in the land or say below the continents have different compositions and physio-chemical properties compared to the crust formed below oceans.

So the crust in earth is classified in two types. They are continental crust and oceanic crust.

Even the formation of continental and oceanic crust differs from each other because the solid outermost shell of the continents are harder than the outermost solid shell of the oceans.

So the thickness of the continental crust is about 18 miles and oceanic crust is about 3 miles only. Thus the continental crust is thicker and harder than oceanic crust.

Answer 2

Average oceanic crust thickness = (3+6)/2 = 4.5 mi
Average continental crust thickness = (20+30)/2 = 25 mi

Average Earth's crust thickness = (4.5+25)/2 = 14.75 mi