Crust (geology)

In geology, the crust is the outermost solid shell of a planet, dwarf planet, or natural satellite. It is usually different from the layer underneath, called the mantle, because of its chemical makeup. For icy satellites, the crust might be defined by whether it is solid or liquid compared to the layer below.

The crusts of Earth, Mercury, Venus, Mars, Io, the Moon, and other planetary bodies formed through processes involving hot, molten rock. After forming, these crusts changed over time due to wind, water, impacts from space rocks, volcanic activity, and the settling of particles.

Most terrestrial planets have crusts that look fairly similar across their surfaces. Earth is special because it has two different kinds of crust: continental crust, which makes up the land we live on, and oceanic crust, which lies under the oceans. These two types have different chemicals and physical traits and formed through different geological processes.

Types of crust

Planetary geologists divide crust into three categories based on how and when it formed.

Primary crust / primordial crust

This is a planet's "original" crust. It forms when a hot, liquid surface called a magma ocean cools and hardens. Early in the solar system, planets likely had surfaces made of magma oceans. As these cooled, they turned solid and formed a crust. This original crust was often broken apart and remade many times by big impacts from space.

Scientists are still unsure about the exact nature of primary crust because none of Earth's original crust remains today. Earth's strong forces of erosion and recycling of the crust have worn away or changed all rocks older than about 4 billion years. However, scientists can learn about primary crust by studying other planets. For example, some parts of Mercury and the Moon might be examples of primary crust.

Secondary crust

Secondary crust forms when parts of the mantle (the layer below the crust) melt. This melted material is usually made of silicate minerals and forms a type of rock called basalt.

This is the most common type of crust in our solar system. It covers most of Mercury, Venus, Earth, and Mars, as well as dark areas on the Moon called maria. On Earth, secondary crust forms mainly at places where new ocean floor is created.

Tertiary crust

Tertiary crust is changed more by chemical processes than primary or secondary crust. It can form in a few different ways, such as when secondary crust melts again or when pieces of crust are broken down and moved by wind, water, or ice.

The only known example of tertiary crust is the crust that makes up the continents on Earth. It is not known if other planets have tertiary crust, since Earth is the only planet in our solar system where the crust moves and changes in this way.

Earth's crust

Main article: Earth's crust

Earth's crust is a thin layer that covers the outside of our planet. It makes up less than 1% of Earth's total size. This crust is part of something called the lithosphere, which includes the crust and the top part of Earth's mantle. The lithosphere is split into pieces called tectonic plates, and these plates move around, helping to let heat escape from inside Earth into space.

Moon's crust

Further information: Geology of the Moon

Scientists believe that a big object called "Theia" crashed into the early Earth, and pieces flew off to form the Moon. When the Moon was just forming, its outside was very hot, like a big pool of melted rock called a "lunar magma ocean". As it cooled, a kind of rock called plagioclase feldspar hardened and floated to the top, making most of the Moon's outer layer, or crust. This crust is made of lots of plagioclase, with some other minerals mixed in.

The Moon's crust is thick, ranging from about 20 to 120 kilometers. It is also thicker on the side of the Moon that is far from Earth. Most of this crust formed very early, not long after the Moon itself came into being. Later, some smaller amounts of rock added to the crust, but these are just a small part of it all. The Moon does not have moving pieces of its outer layer like Earth does.