Kimberlite is an igneous rock, formed from the cooling of molten magma. Kimberlite is composed of at least 35% olivine, together with other minerals such as mica, serpentine, and calcite. Geologists call it an ultrabasic rock, which means it does not contain any quartz or feldspar, the two most common rock-forming minerals. Olivine, the main mineral constituent of the rock, is an olive-green, grayish green, or brown mineral made up of magnesium, iron, and silica. Kimberlite is unique because it originates over 100 miles (150 kilometers) deep in the earth and travels in a matter of hours to the earth's surface where it forms small volcanic features. In 1888, the name kimberlite was proposed for this particular rock, based upon the occurrence of these rocks in the vicinity of Kimberley, South Africa.
Diamonds occur in only two rock types on earth, kimberlites and lamproites.
Large volumes of an olivine-rich rock type called peridotite occur at great depths in the earth in a layer called the mantle. At these depths, the combined temperature and pressure is high enough to partially melt some of the peridotite. If volatile gases, such as carbon dioxide and water, are present, they may propel the molten peridotite upwards, forming a kimberlite magma. As the hot kimberlite magma rises slowly upward into regions of lower temperature and pressure in the upper mantle and overlying crust, minerals start to crystallize and the volatile gases expand and exert increasingly higher pressures on the surrounding rocks, eventually breaking some of the surrounding rock and incorporating it into the magma. Closer to the earth's surface the internal pressures of the magma and volatile gases become so great that the kimberlite becomes explosive. Kimberlite magma can rise toward the surface at speeds estimated at up to 400 meters (1,200 feet) per second, ripping up more and more pieces from the surrounding rock, which gives the kimberlite its characteristic texture.
Kimberlites have been intruded into the earth's crust at various times. The intrusions are clustered in about ten time periods ranging from 1,600 million years to about 55 million years ago.
Kansas kimberlites were intruded in the Cretaceous Period, about 90 million years ago.
Kimberlite Pipes, South Africa:
Most of the world's kimberlites occur in South Africa, where over 3,000 kimberlite pipes have been found. Over 200 are known in North America, of which about 40 occur near the Colorado-Wyoming state line. Important new discoveries have been made in northern Canada in the last decade. Of the thousands of known kimberlites, fewer than about 1,000 contain diamonds, and of those, only about 50 to 60 have contained enough diamonds to be mined economically.
-- Excerpt from:
Kansas Geological Survey Website, 2001, and Berendsen, P., Weiss, T., and Dobbs, K., Kansas Kimberlites: Kansas Geological Survey Public Information Circular 16