A typical rock is a mixture of solid chemicals, known as minerals, which are pressed tightly together. If you look closely you can see the separate minerals as grains or small crystals in a rock. Each different mineral is a naturally formed inorganic substance with a particular chemical composition. There are over 4,500
different minerals but only a small number of these are major constituents of rocks.
TYPES OF ROCK
Geologists can classify almost all rocks into one of three main types, depending on how the rocks form. The types are known as Igneous Rock, Sedimentary Rock and Metamorphic Rock. Over millions of years, each type can slowly change into one of the others in an endless process called the rock cycle. The rock in the Earth’s crust is continually being destroyed and recycled. Rock on the surface is worn down to fragments and eventually settles to become sedimentary rock. Rock underground is melted to form igneous rock, or compressed and cooked, by the heat of molten rock, to form metamorphic rock. Movements in the Earth’s crust force underground rock back to the surface.
About 90 per cent of the rock in the Earth’s crust is igneous. Igneous rocks form when molten rock cools and solidifies. When this happens underground, the molten rock (magma) solidifies slowly giving crystals time to form. The magma becomes a hard, crystalline rock with large grains such as granite. Igneous rock can also form on the Earth’s surface when lava escapes from a volcano. The lava solidifies quickly, especially if it flows into water. It forms with no or very small, visible crystals such as basalt.
Sand, mud, and even the remains of living organisms can all turn into rock. These sediments settle on the sea floor, building up in layers. Over time, deep layers are compressed by the weight of the sediment on top and water seeping through the layers depositing minerals that glue the sediment particles together. As a result the sediment becomes sedimentary rock. Travertine, limestone and sandstone all form this way. The layers are sometimes visible as horizontal bands called strata.
Deep underground, rock can be subjected to intense heat and pressure. These forces, while not melting the rock outright, can cause minerals to recrystallize in new forms. The result is a hard, crystalline type of rock called metamorphic rock, which frequently has wavy or stripy patterns. Metamorphic rock often forms in mountainous regions, where Earth’s crust is buckling and folding under tremendous pressure. Quartzite, marble and slate are examples of Metamorphic rock.
ROCKS & THE STONE TRADE
As previous explained geologists name a rock according to its mineral composition and the environment in which it formed. The size of the grain is often important too. The stone trade uses just a few of the same names, granite, marble, slate, quartzite, travertine, sandstone and quartzite, but they have very different meanings.
GRANITE & THE STONE TRADE
Geologists use the term ‘granite’ for igneous rocks that formed within the earth’s crust, which are mainly made up of the minerals quartz, feldspars and mica. But the stone trade defines most hard rocks composed mainly of these silicon minerals as ‘granite’ irrespective of whether the rock is igneous, metamorphic or sedimentary. This means that ‘granites’ in the trade can include metaconglomerates (metamorphosed sedimentary rocks), gneisses (metamorphosed igneous or sedimentary rocks) and basalts (volcanic igneous rocks) as well as true granites.
MARBLE & THE STONE TRADE
If a rock is composed mainly of calcite (carbonate mineral) geologists call it a limestone if it is unmetamorphosed and marble if it has been metamorphosed. The stone trade says a rock of this composition is ‘marble’ or ‘limestone’ depending on whether it will take a good polish or not.