Polarised light micrograph of a specimen of amphibolite from a scree of the Messner-Brook, St Nikolaus, Ultental, South Tyrol, Italy. Amphibolite is composed mainly of amphibole and plagioclase feldspar (bright), with little or no quartz. It is dense with a weakly foliated or schistose (flaky) structure. This rock was formed by metamorphic transformation of basalt and gabbro at great depths and temperatures of 550-700 degrees Celsius and pressures of 200-1200 megapascals. These conditions are known as amphibolite facies. The metamorphic transformation occurred around 335 million years ago (Variscan orogeny). Magnification: x30 when printed 15 centimetres wide.