Earth's crust

Earth's crust is the thick outer shell of rock that makes up our planet. It is the top part of Earth's layers and is very important because it is where we live, grow food, and find many of the resources we use every day. The crust makes up less than one percent of Earth's radius and volume, but it is where all life on Earth exists.

The crust is part of something called the lithosphere, which also includes the upper part of the mantle. The lithosphere is broken into pieces known as tectonic plates. These plates move slowly and help carry heat from inside Earth out into space.

Beneath the crust lies the mantle, which is made of a special kind of rock called peridotite. This rock is denser than the crust, which is why the crust floats on top of it. The place where the crust meets the mantle is called the Mohorovičić discontinuity. Scientists learn about this boundary by studying how seismic waves travel through Earth.

As you go deeper into the crust, it gets much hotter. The temperature can rise from about 700 °C to 1,600 °C as you reach the mantle. In some places, the temperature can increase by as much as 30 °C for every kilometer you go down. This heat helps shape Earth's surface over millions of years.

Composition

Main articles: Abundance of elements in Earth's crust and Goldschmidt classification

The Earth's crust is made of two main types of rock. The continental crust is thicker, ranging from 25 to 70 kilometers, and is made mostly of lighter rocks like granite. In some places, like the Tibetan Plateau, it can be even thicker. The oceanic crust is thinner, about 5 to 10 kilometers, and made of denser rocks like basalt.

Both types of crust float on the softer rock below them, called the mantle. Because the continental crust is thicker and lighter, it sits higher above sea level, forming the land we live on, while the oceanic crust sits lower, creating the deep ocean basins. The most common minerals in the continental crust are feldspars, quartz, and pyroxenes.

Formation and evolution

Further information: Earth § Formation

Earth formed about 4.6 billion years ago from a cloud of dust and gas around the young Sun. As pieces of rock and dust crashed together, they formed a planet and created a lot of heat, melting Earth completely. As Earth cooled, it began to form its first layer of rock, called the primary crust. This early crust was often broken apart by huge impacts and then remade from the melted rock left behind.

Over time, Earth developed new layers of rock, called secondary and tertiary crust. The secondary crust forms where new ocean floor is created, and the tertiary crust forms the older landmasses we see today. The oldest pieces of land rock are found in places like Western Australia and the Northwest Territories. These rocks are billions of years old. Earth's land layers are preserved because they are less likely to be destroyed by the movement of Earth's plates.