Craton

A craton ( /ˈkreɪtɒn/ KRAYT-on, /ˈkrætɒn/ KRAT-on, or /ˈkreɪtən/ KRAY-tən; from Ancient Greek: κράτος kratos "strength") is an old and stable part of continental lithosphere (the Earth's two topmost layers, the crust and the lithospheric mantle). Cratons have often survived cycles of merging and rifting of continents, and are generally found in the interiors of tectonic plates.

Cratons are made of ancient crystalline basement rocks covered by younger sedimentary rocks. They have a thick crust and deep roots extending several hundred kilometres into Earth's mantle. Cratons contain the oldest continental crust rocks on Earth. They were formed in the Archaeen (4 to 2.5 billion years ago) and the Proterozoic (2.5 billion- 538.8 million year ago) geologic eons. Most were 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 as 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. Studies show that the rock under cratons is colder and much thicker than usual rock, helping 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 are still unsure about many details, but the first cratons likely formed during the Archean eon. This is shown by the age of diamonds, which come from deep within cratons and are usually over 2 billion years old. Ancient rocks from the Archean make up only a small part of today’s cratons, suggesting that most of today’s continents formed later.

Cratonization probably finished during the Proterozoic. Later growth of continents happened mostly by adding material at their edges through a process called accretion. Scientists have several ideas about how the deep roots of cratons formed. One idea is that repeated collisions between continents helped build these roots. Another idea is that hot, rising rock from deep in the Earth created 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 large impacts from space helped form cratons long ago. Each of these ideas helps explain different clues scientists have found.

Erosion

The long-term wearing down of cratons is called the "cratonic regime." This process includes making 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.