Craton

A craton is an ancient and stable part of the Earth's continental lithosphere. The lithosphere consists of the crust and the uppermost part of the mantle. While many parts of the Earth's surface are constantly shifting, cratons remain remarkably steady. They often sit in the middle of tectonic plates. This stability allows them to survive many cycles of continents merging or breaking apart. Scientists use the term craton to distinguish these stable regions from more active, unstable areas.

Cratons are built from two distinct layers. The first is the cratonic basement. This layer consists of metamorphosed crystalline and metamorphic rocks. When this basement rock is visible at the surface, the area is called a shield. The second layer is the platform. A platform is a layer of younger, weakly deformed sedimentary rock that sits on top of the basement.

Together, these layers form a massive, coherent domain of the continental crust.

One of the most important features of a craton is its deep lithospheric root. These roots extend several hundred kilometers down into the Earth's mantle. Cratons have a much thicker lithosphere than the oceanic lithosphere found under the oceans. For example, cratonic lithosphere can be up to 4 billion years old. In contrast, oceanic lithosphere is often only 180 million years old. These deep roots are unusually cold and have a low density. This low density prevents the craton from sinking into the deep, hot mantle.

The history of the word "craton" began in 1921. An Austrian geologist named Leopold Kober first proposed a similar term for stable platforms. Later, a geologist named Hans Stille shortened the term to "craton." Modern understanding of how these structures form changed significantly in 1978. That year, Thomas H. Jordan published a paper in the journal Nature. He proposed that cratons formed through a process called cratonization. This process involves the melting of the upper mantle to create stable continental cores.

Cratonization likely began during the Archaean eon, between 4 and 2.5 billion years ago. Most cratons were formed during this time. Some also formed during the Proterozoic eon, which lasted until 538.8 million years ago. Evidence for this ancient age comes from diamonds found in craton roots. These diamonds are often over 2 billion or even 3 billion years old. Currently, Archaean rock makes up about 7% of the world's cratons. However, all continents on Earth contain some crust from the Archaean eon.

Scientists have several theories about how these deep roots actually form. Jordan suggested that high temperatures in the Archaean caused massive melting. About 30 to 40 percent of the source rock turned into magma. This left behind a solid residue called peridotite. This residue was enriched in lightweight magnesium, making it less dense than the surrounding mantle. Other theories include the repeated continental collision model. This model suggests that colliding continents "knead" the crust to create deep roots.

There is also a molten plume model and an impact origin model involving large asteroids.

There are many famous examples of cratons across the globe. In South America, there is the Amazonian Craton. In Africa, you can find the Kaapvaal Craton. Other major examples include the North American Craton, also known as Laurentia. In India, there is the Dharwar Craton. Canada is home to the Superior Craton. Australia has the Gawler Craton. These massive structures serve as the ancient cores of our modern continents.

Because cratons are so old, they undergo a long process of erosion. This is sometimes called the "cratonic regime." Over vast amounts of time, wind and water wear the surfaces down. This can create very flat surfaces known as peneplains. Some cratons, like the Yilgarn Craton in Australia, were already very flat long ago. Studying these stable rocks helps geologists understand the deep composition of our planet. By looking at rock fragments called xenoliths, they can see what the deep mantle looks like.