Formation of the Earth
Experiments and the high density of diamonds tell us that they crystallize at very high pressures. In nature, this means that diamonds are created by geologic processes at great depth within the Earth, generally more than 150 kilometers down, in a region beneath the crust known as the mantle. This diagram shows the interior structure of the Earth. The three concentric layers -- the core, mantle, and crust -- formed within a few hundred million years of Earth's coalescence 4.5 billion years ago. The core is primarily an iron-nickel alloy and makes up a large fraction of the mass of Earth. The vast mantle is sandwiched between the core and the thin crust and is composed predominantly of magnesium and iron silicate minerals. Our planet's crust is a thin, rocky skin. Diamonds can form in most of Earth's interior but not near its surface, where graphite is the stable form of carbon. Indeed, diamonds only survive at the Earth's surface because great heat is required to break down the diamond structure.
The interior structure of the Earth
How do Diamonds move to the Earth's surface?
Diamonds ascend to the Earth's surface in rare molten rock, or magma, that originates at great depths. Carrying diamonds and other samples from the Earth's mantle, this magma rises and erupts in small but violent volcanoes. Just beneath such volcanoes is a carrot-shaped "pipe" filled with volcanic rock, mantle fragments, and some embedded diamonds. The rock is called kimberlite after the city of Kimberley, South Africa, where the pipes were first discovered in the 1870s. Another rock that provides diamonds is lamproite.
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The volcano that carries diamonds to the surface emanates from deep cracks and fissures called dikes. It develops its carrot shape near the surface when gases separate from the magma, perhaps accompanied by the boiling of groundwater, and a violent supersonic eruption follows. The volcanic cone formed above the kimberlite pipe is very small in comparison with volcanoes like Mount St. Helens, but the magma originates at depths at least 3 times as great. These deep roots enable kimberlite to tap the source of diamonds. Magmas are the elevators that bring diamonds to Earth's surface.
The rough diamond in the kimberlite rock Photo by ChrisMago
Diamonds are found on cratons
The locations of diamond deposits are determined by the geologic fact that diamonds are found primarily in two rare types of rocks - kimberlite and lamproite. These rocks occur as "pipes" (cone structures pushed to the surface by volcanic activity) only in cratons, those portions of the Earth's crust that have been stable for long periods of time. Diamond-bearing kimberlites of economic significance are found in archons, which are those portions of cratons that are older than 2.5 billion years. Protons, which are those portions of cratons that are 1.6 - 2.5 billion years old, are less likely to have diamond-bearing pipes, and when they do, the pipes are likely to be lamproites, as at the Argyle mine in Australia. Tectons, which are those portions of cratons between 800 million and 1.6 billion years old, are unlikely to contain either kimberlites or lamproites.
World distribution of cratons