Craton

A craton ( /ˈkreɪtɒn/ KRAYT-on, /ˈkrætɒn/ KRAT-on, or /ˈkreɪtən/ KRAY-tən; from Ancient Greek: κράτος kratos "strength") is a very old and stable part of Earth's land. It is part of the lithosphere (the Earth's top layers, the crust and the lithospheric mantle).

Cratons are made of very old rocks called basement rocks covered by younger sedimentary rocks. They have a thick crust and go deep into the Earth. Cratons contain some of the oldest continental crust rocks we know of. They formed a very long time ago, in the Archaeen (4 to 2.5 billion years ago) and the Proterozoic (2.5 billion to 538.8 million years ago) geologic eons. Most formed in the Archaeen.

Terminology

The word craton is used to talk about the stable parts of Earth’s land that don’t change much. These areas stay steady even when other parts of the land move and shift.

Cratons have two main layers: a deep layer made of strong, changed rocks, and a top layer with younger rocks that sit gently on top. The stable parts of land called continental shields show these deep rocks at the surface. The words shields and platforms describe how land looks, not how it moves under the ground.

The idea of a craton was first suggested in 1921 by a scientist named Leopold Kober. He used a word that means “strong land.” Later, another scientist named Hans Stille made the word shorter to craton.

Examples

Some well-known cratons include the Dharwar Craton in India, North China Craton, the East European Craton, the Amazonian Craton in South America, the Kaapvaal craton in South Africa, the North American Craton (also called the Laurentia Craton), the Gawler craton in South Australia, the Archean Wyoming Craton, and the Superior Craton in Canada.

Structure

Cratons have thick roots deep in the Earth. The rock under cratons is colder and thicker than usual rock, which helps them stay strong and stable. Unlike other parts of the Earth’s surface, cratons float because they are less dense.

Pieces of rock carried up by molten rock from deep below the surface show us what cratons are made of. These rocks are very strong because they have less water and more light elements like magnesium compared to heavier ones. This helps scientists learn about the deep origins of cratons.

Formation

The process that forms cratons is called cratonization. Scientists do not know all the details, but the first cratons probably formed during the Archean eon. This is shown by the age of diamonds, which come from deep inside cratons and are usually more than 2 billion years old. Ancient rocks from the Archean make up only a small part of today’s cratons, which means most of today’s continents formed later.

Cratonization probably ended during the Proterozoic. Later, continents grew mostly by adding material at their edges through a process called accretion. Scientists have a few ideas about how the deep roots of cratons formed. One idea is that collisions between continents helped build these roots. Another idea is that hot, rising rock from deep in the Earth made thick layers under the cratons. A third idea is that old ocean floor sank under the early continents, adding special rock below. There is also a theory that big impacts from space helped form cratons long ago. Each of these ideas helps explain clues scientists have found.

Erosion

The long-term wearing down of cratons is called the "cratonic regime." This process makes flat surfaces through pediplanation and etchplanation, creating areas known as peneplains. These processes change with the climate—pediplanation happens in dry areas, while etchplanation occurs in wet areas. Over time, cratons can shift between having more ocean nearby and more land far away.

Many cratons have stayed fairly flat since very old times. For example, the Yilgarn craton in Western Australia was already flat by Middle Proterozoic times, and the Baltic Shield had become smooth land during the Late Mesoproterozoic when rapakivi granites formed.