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. Other processes, explored later in this exhibition, bring diamonds to where people can find them. 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 continental cores

The search for diamonds has determined that most are derived from kimberlite pipes in the oldest, nuclear portions of the continents, where the basement rocks are older than 1.5 billion
years.

The oldest parts of continents are called cratons and can be divided into two terranes: Archeanage archons, which are older than 2,500 million years, and Proterozoic-age protons, which are 1,600 - 2,500 million years old. The distribution of these terranes is shown on the map. Kimberlite pipes occur in many parts of the continental crust, but most diamond-rich ones are found in archons. This fact suggests that most diamonds were formed and stored deep below the cratons, in the area shown in the lower figure, and were later transported to the surface by kimberlite and lamproite magmas that extracted them and other samples from the mantle.

Distribution of kimberlites worldwide

Where do diamonds come from?

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