The Earth’s continents are constantly being shaped by plate tectonics, resulting in mountain building, rifting, and deformation. Within the continental lithosphere, which includes the crust and the uppermost part of the mantle, there are ancient, stable regions that have resisted these forces for billions of years. These geological anchors are known as cratons, forming the rigid, long-lasting cores of the continents. Their stability and survival are central to understanding the evolution of the planet’s surface and interior.
Defining Cratons and Their Structure
A craton is a large, cohesive part of the continental lithosphere that has remained largely undeformed since the Precambrian eon, typically between 500 million and over 4 billion years old. These stable blocks are usually found in the interior of modern continents, having survived multiple cycles of supercontinent assembly and breakup. Cratonic crust is characterized by ancient crystalline basement rocks, primarily composed of lightweight felsic igneous rock like granite.
The most distinguishing structural feature is its lithospheric mantle root, often called a keel, which extends deep into the Earth’s mantle. This root is far thicker than the lithosphere underlying younger regions, reaching depths of 200 to over 300 kilometers. These roots are anomalously cold and rigid compared to the surrounding mantle, acting as a deep foundation for the crust above.
The Mechanism of Cratonic Stability
The longevity of cratons is rooted in two primary factors: chemical depletion and thermal insulation. The cratonic mantle root is made of highly melt-depleted peridotite, stripped of dense elements like iron and aluminum through ancient melting events. This process leaves the residue less dense than the surrounding fertile mantle material.
This low-density composition makes the cratonic keel chemically buoyant, giving it a positive buoyancy that resists downward motion into the convecting asthenosphere. This buoyancy anchors the craton, preventing it from being consumed or significantly deformed by the churning mantle. The depleted material is also refractory, resisting further melting.
The great thickness and cold temperature of the cratonic root provide thermal insulation. The deep, cold root acts as a barrier, preventing heat from the underlying asthenosphere from easily transferring into the lithosphere. This reduced heat flow keeps the cratonic material stronger and more viscous than the surrounding lithosphere, further resisting deformation.
Shields, Platforms, and Global Distribution
Cratons manifest at the surface in two distinct forms: shields and platforms. A shield is the portion of a craton where the ancient, usually Precambrian, crystalline basement rock is widely exposed at the surface. These areas, like the Canadian Shield or the Baltic Shield, reveal the igneous and metamorphic core of the continent.
A platform, conversely, is the part of the craton where the ancient basement rock is covered by a layer of younger, relatively flat-lying sedimentary rocks. The basement rock is hidden beneath these horizontal layers, such as in the North American Interior Platform. Cratons are found on every continent, with major examples including the Kaapvaal Craton, the Siberian Craton, and the Amazonian Craton.
Economic and Historical Importance
Cratons hold significance for both mineral resources and the reconstruction of Earth’s past. Their stability has allowed them to act as fixed points around which supercontinents have repeatedly assembled and broken apart, providing a geological record of early Earth processes.
The ancient, cold, and deep roots of cratons are the only places where diamonds are found in primary deposits. Diamonds form under extreme pressure and temperature deep in the mantle and are brought to the surface rapidly by violent volcanic eruptions. These eruptions create carrot-shaped kimberlite pipes, which almost exclusively erupt through the thick, stable lithosphere of cratonic cores. The age and stability of cratons are therefore directly linked to the world’s most significant diamond deposits.