Geologically, Namibia forms part of an extremely old region, with Pre-Cambrian granitic and metamorphic rocks dating back over two billion years. These shield or 'basement' rocks are usually covered by more recent sedimentary rocks, mostly deposited during the Mesozoic era (65 to 235 million years ago). Tectonic activity or movement in the earth's crust over the last 100 million years or so created a number of rifts through which magma was able to reach the surface (see Kimberlite (diamond) pipes
) and resulted in the uplifting of most of the area above sea level.Kimberlite (diamond) pipes
Diamond is a crystalline form of ordinary carbon created under conditions of extreme pressure and temperature. In nature, such conditions are only found deep below the earth's surface in the lower crust or upper mantle. Under certain circumstances in the past (usually associated with tectonic activity) the rock matrix in which diamonds occurred was subjected to such great pressure that it became fluid and welled up to the earth's surface in a volcanic pipe of fluidised material. The situation is similar to a conventional volcanic eruption, except that instead of basaltic magma being erupted through fissures in the crust, the volcanic material is a peculiar rock called kimberlite. This contains a wide assortment of minerals (including diamond) in addition to large chunks of other rocks that have been caught up in the process.
The pipes are correctly termed kimberlite pipes, and occur throughout southern Africa from the Cape to Zaire. However, only a small proportion of those discovered have proved to contain diamonds in sufficient abundance to be profitably worked. Namibia's diamonds derive not from primary kimberlite pipes, but from secondary diamond deposits – areas where diamonds have been washed down and deposited by old rivers, which have eroded kimberlite pipes in the interior on their way.