Light micrograph of a sample of basalt from the Ulten Valley, South Tyrol, Italy. Basalt is formed when the earth's mantle melts. The thin, low-silica magma cools relatively quickly when released to the Earth's surface or ocean. The magma usually contains inclusions (xenocryst) of minerals or entire rocks (xenolite) with a higher melting point, which were carried along during the ascent. Basalt consists mainly of a mixture of iron and magnesium silicates with pyroxene and calcium-rich feldspar (plagioclase) and also olivine. Magnification: x30 when printed at 15 centimetres wide.