The Earth’s crust is the thinnest of the planet’s major layers. The Earth’s interior is arranged in concentric shells of differing compositions and thicknesses, with the crust forming the outermost layer. Even though it is the layer humans inhabit, its depth is insignificant compared to the vastness of the layers beneath it. This solid shell represents a tiny fraction of the planet’s overall structure by both thickness and volume.
Defining the Earth’s Crust
The Earth’s crust is not uniform in thickness, exhibiting a significant difference between its two main types. Oceanic crust, which underlies the oceans, is thin, typically ranging from 5 to 10 kilometers deep. This dense layer is primarily composed of basaltic rocks.
In contrast, the continental crust, which forms the landmasses, is substantially thicker. It varies from 20 kilometers to as much as 70 kilometers, particularly beneath major mountain ranges. This less dense layer is made up of mostly granitic rocks. Even the thickest continental crust is shallow compared to the planet’s radius of approximately 6,371 kilometers.
The crust ends at a boundary known as the Mohorovičić discontinuity, or Moho, where the rock density suddenly increases as it transitions into the mantle. The entire crust makes up only about 1% of the Earth’s total volume. This small proportion demonstrates that the deepest part of the continental crust is merely a superficial boundary layer.
The Immense Scale of the Mantle
Immediately beneath the thin crust lies the mantle, a layer of silicate rock that dwarfs the crust in scale. It extends downward from the Moho to the outer core, a distance of approximately 2,900 kilometers. This immense thickness means the mantle is hundreds of times deeper than even the thickest sections of the continental crust.
The mantle is the largest layer, making up about 84% of the Earth’s total volume. To put this size into perspective, if the Earth were the size of a peach, the crust would be thinner than the skin, while the mantle would represent nearly all of the fruit’s flesh. Although the mantle is predominantly solid, it behaves like an extremely viscous fluid over geological timescales, driving the movement of the tectonic plates on the surface. This massive, flowing layer is the engine of the planet’s internal dynamics.
Comparing Thicknesses of the Core
The Earth’s core is also vastly thicker than the crust. The core begins where the mantle ends and extends to the planet’s center, a combined radius of approximately 3,480 kilometers. The outer core alone is a liquid layer of iron and nickel that is about 2,200 to 2,300 kilometers thick.
Beneath the liquid outer core is the solid inner core, which has a radius of roughly 1,221 to 1,250 kilometers. The combined core layers represent about 15% of the Earth’s total volume. The total thickness of the core from its outer boundary to the center is thousands of times greater than the maximum depth of the crust.