Some may even be glassy, and show no grains at all. Rocks that are composed entirely of mineral grains are said to have crystalline texture . These are classified based on characteristic properties. During the next three weeks we will discuss the various properties of each rock type, and classify subgroups of rocks within each major type. Fragments found in sedimentary rocks may sometimes be the skeletal material known as fossils.
Metamorphic rocks can begin as sedimentary, igneous or metamorphic.
The rock cycle describes the events that lead to the formation of a rock. The sediments may then be cemented and compacted to form a sedimentary rock. The sedimentary rock is then subjected to heat and pressure becoming a metamorphic rock once again. Perhaps the metamorphic rock is then melted under the surface of the Earth to become magma. This magma is then spewed out of a volcano , cooled, and now is an igneous rock.