this is a piece of unmetamorphosed basalt, which formed at the earth's surface. this rock would plot here on our chart at relatively low temperature and pressure. as basalt is metamorphosed to different combinations of temperature and pressure, its mineral composition changes as the rock reequilibrates to its new condition. these zones on the chart are called metamorphic facies. each facies is defined by the formation during metamorphism of a particular mineral or mineral assemblage. for example, this is a metamorphosed basalt containing the mineral amphibole. amphibole forms at a temperature between 450 and 700 degrees centigrade, and at a pressure corresponding to a depth of at least 6 kilometers. this metamorphosed basalt would plot here on the chart in the amphibolite facies. this is a metamorphosed basalt that contains zeolites, another mineral group, and it plots here in the zeolite facies. the zeolite facies corresponds to less intense metamorphic conditions than does the amphibolite facies, so rocks containing zeolites are said to be of lower metamorphic