Geologists study the earth’s crust to determine its composition, age, and tectonic history. The Precambrian shield is a high-grade igneous and metamorphic crystalline basement that underpins continents. A platform covers some parts of the shield, forming a craton. Continents and cratons are not the same. Shields are exposed areas of Precambrian rock covered by a platform. Different tectonic plates are made up of different shields, and submerged cratons exist. Analyzing rocks and ocean depths is necessary to identify them.
Geologists analyze the earth’s crust in detail to determine its composition, age and tectonic history. One important distinction they made is between the Precambrian (more than 542 million years ago) high-grade igneous and metamorphic crystalline basement that makes up the continents, called the shield, and a large sedimentary layer that covers some parts of this area , called platform . Together, the two layers form a craton, which is the foundation of a continent. An example of a shield is the Canadian Shield, which underpins modern Canada.
Continents and cratons are not exactly the same thing. For example, the island of New Guinea, just north of Australia, is part of the Australian craton, but is not part of the Australian continent because the strait between New Guinea and the mainland is currently flooded. During the last ice age, this strait was dry and both land bodies were part of the same land mass.
A shield is, specifically, an exposed area of Precambrian igneous or metamorphic rock. When it’s covered with a platform, it’s called a basement. A platform can be hundreds of feet (meters) thick, but must have only been created within the last few hundred million years. The shields themselves can be billions of years old, and the oldest continental ones, found in Canada and a small part of Australia, are up to 4 billion years old, just 570 million years after the formation of the Earth itself.
Different tectonic plates are made up of different shields. People might think it would be possible to tell them apart just by looking at a map of the world, but it’s not that easy. To truly see them you need to analyze rock from all over the world and know the depths of the world’s oceans. For example, Greenland and North America are part of the same craton, but not India, Europe and the rest of Asia. There are even submerged cratons, such as the Kerguelen Plateau in the southern Indian Ocean.
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