Geology majority of the geologic history of the Earth. There are three major types of rock: igneous, sedimentary, and metamorphic. The rock cycle illustrates the relationships among them (see diagram). When a rock crystallizes from melt (magma or lava), it is an igneous rock. This rock can be weathered and eroded, then redeposited and lithified into a sedimentary rock. It can then be turned into a metamorphic rock by heat and pressure that change its mineral content, resulting in a characteristic fabric. All three types may melt again, and when this happens, new magma is formed, from which an igneous rock
Pitchstone that is very resistant to erosion. The pitchstone ridge of An Sgùrr on the Isle of Eigg, Scotland, was possibly formed as a lava flow in a valley. Pitchstone from the Isle of Arran was used as the raw material for making various items from the Mesolithic through the Neolithic to the Early Bronze Age. Mesolithic use appears to have been limited to the Isle of Arran itself, while in later periods the material or items made from it were transported around Britain. Pitchstone Pitchstone is a dull black glassy volcanic rock formed when felsic lava or magma cools quickly.