Crust — Science definition

Earth's thin, rocky outer layer — oceanic crust (about 5–10 km thick, basalt) under the oceans, and thicker continental crust (about 30–50 km thick, granite) under land.

Where it is taught

In use

The thin outer skin left on top is the crust — so thin compared with the whole planet that, on a scale model of Earth, it would be thinner than the skin of an apple.

Earth's Internal Structure

Questions using crust

Problem 1

A student labels crust, mantle and lithosphere as three separate non-overlapping compositional layers and calls slow mantle convection proof of liquid rock. Correct both statements.
Answer and explanation

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 2

Independent chapter check: A student labels crust, mantle and lithosphere as three separate non-overlapping compositional layers and calls slow mantle convection proof of liquid rock. Correct both statements.
Answer and explanation

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 3

Earth's radius is about 6,370 km and the crust is about 35 km thick under continents. What percentage of the radius is the crust, to one decimal place?
Answer and explanation

0.5 %

35 / 6,370 × 100 ≈ 0.5 percent — the crust is a thin skin.

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