How Rocks Are Formed

Briefly explain the main features of the rock cycle and the formation of the three types of rocks. [6]


The main features of the rock cycle are sedimentary rocks, metamorphic rocks and intrusive and extrusive igneous rocks.

Sedimentary rocks are formed when sediments that have been eroded from rocks due to weathering are gradually accumulated over time in layers. When they build up in layers, this weight presses the sediment particles together, compacting them. When water passes through the spaces in between the particles helps to cement them together even more. This process of compacting and cementing sediment forms sedimentary rock.

Due to movements in the crust, rocks such as sedimentary rocks are frequently pulled under the surface of the earth, where temperatures increase dramatically. Also, rocks experience pressure caused by tons of other rocks pressing down on it from above; heat and pressure work together to alter the rocks under the earth's surface. This kind of change, which results from both rising temperature and pressure, is called metamorphism, and the resulting rock is a metamorphic rock.

When magma rises from deep within the earth and explodes out of a volcano, it is called lava, and it cools quickly on the surface. Rock formed in this way is called extrusive igneous rock. However, if magma gets pushed slowly upward toward the earth's surface instead of exploding from a volcano, it will cool at a much slower rate than lava erupting from a volcano. The kind of rock formed in this way is called intrusive igneous rock; it intrudes, or pushes, into the earth's interior and cools beneath the surface.