Kimberlite is a rare, dark-colored igneous rock that originates deep within the Earth’s mantle. Its chemical composition is classified as ultramafic, meaning it is rich in magnesium and iron but low in silica. When fresh, the unweathered rock often has a distinct slate-blue or greenish-blue color, which early miners termed “blue ground.” Kimberlite represents one of the deepest-sourced melts known to science, though it never forms a typical volcano.
The Volcanic Origins of Kimberlite Pipes
Kimberlite forms from the partial melting of the deep mantle, occurring at depths between 150 and 450 kilometers below the surface. The melt is uniquely enriched in volatile components, specifically carbon dioxide and water, dissolved within the magma. The presence of these gases drives the kimberlite’s rapid journey to the surface.
Once formed, the volatile-rich magma is propelled upward through the lithosphere in a rapid, explosive eruption. This ascent fractures and shatters the surrounding country rock as it rises toward the surface. The entire process occurs quickly, often in hours or days, which prevents the magma from cooling or chemically altering.
The resulting geological structure is a distinct, carrot-shaped column known as a kimberlite pipe or diatreme. Near the surface, the pipe widens into a funnel shape due to the intense explosion and subsequent collapse of surrounding material. These vertical conduits transport material from the deepest parts of the planet’s interior to the crust, where they cool and solidify into kimberlite rock.
Geographic Location: Finding Kimberlite on Ancient Cratons
Kimberlite is not found randomly across the globe but is instead almost exclusively located within areas of ancient, stable continental crust known as cratons. These vast, old blocks of rock, such as the Kaapvaal Craton in Southern Africa, the Siberian Craton in Russia, and the Slave Craton in Canada, have remained tectonically stable for billions of years. The kimberlite pipes are concentrated in these regions because of a specific geological requirement: a thick, cold lithospheric mantle.
This ancient continental lithosphere, often referred to as a “mantle keel,” extends to depths of 200 kilometers or more, much deeper than the lithosphere beneath oceans or younger continental regions. The low temperature and high pressure within these deep, cold keels are necessary for the long-term stability of certain high-pressure minerals. If the kimberlite magma were to erupt through a thinner, hotter lithosphere, any high-pressure minerals it carried would be destroyed before reaching the surface.
This unique combination of a deep mantle source and a thick, cold crustal pathway limits kimberlite occurrences to specific, well-defined areas on nearly every continent. For instance, major deposits are found in parts of Australia, like the Kimberley region, and in the extensive landmasses of Africa and North America. The presence of these cratons dictates where geologists look for kimberlite.
Kimberlite’s Primary Significance: Diamonds and Mantle Insights
The most widely known importance of kimberlite is its role as the primary host rock for natural diamonds. Diamonds are not formed by the kimberlite itself, but rather deep beneath the surface, approximately 150 to 700 kilometers down in the mantle, where extreme pressure and temperature allow carbon atoms to crystallize into this unique structure. The kimberlite magma acts as a rapid delivery system, capturing these pre-existing diamond crystals (xenocrysts) and carrying them intact to the Earth’s surface.
The swift ascent is what preserves the diamonds, preventing them from turning back into graphite under lower-pressure conditions near the crust. Although only a small fraction of discovered kimberlite pipes contain diamonds in economically recoverable quantities, the vast majority of the world’s mined diamonds originate from these formations. The distinct mineral assemblage found within kimberlite, such as pyrope garnets and chrome diopside, are often used as indicator minerals to help prospectors locate potential diamond-bearing pipes.
Beyond its economic value, kimberlite provides scientists with unparalleled insights into the inaccessible deep Earth. As the magma travels upward, it rips off and carries fragments of the surrounding rock, known as xenoliths, from various depths of the mantle and crust. These xenoliths are essentially pristine samples of the Earth’s interior, providing a direct window into the composition, temperature, and pressure conditions of the planet’s deepest layers.
Scientists study these rock fragments and the mineral inclusions within them to understand the chemical evolution and dynamic processes of the mantle. Inclusions found within diamonds can reveal information about the presence of water or other fluids deep within the Earth’s interior that are otherwise impossible to sample. Kimberlite is an invaluable tool for deep-Earth geophysics and geochemistry.