HOW DO DIAMONDS MOVE TO EARTH’S SURFACE?
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 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.
Diamonds ascend to the Earth’s surface in rare molten rock, or magma, that originates at great depths. Carrying diamonds and other samples from 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.
The volcano that carries diamond 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 ground water, 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.
Kimberlites are generally much younger than the diamonds they bring to Earth’s surface. Kimberlites and lamproites have been dated between 50 and 1,600 million years old. Diamonds associated with harzburgites are about 3.3 billion years old — more than two thirds the age of Earth itself, and those from eclogites generally range from 3 billion to less than 1 billion years old. These age differences help clarify a picture of diamonds having crystallized and been stored beneath the ancient continental cratons and only later being lifted to Earth’s surface by kimberlites.