If you have been to any of the places where rocks are exposed at the surface of the Earth, such as in cliffs and quarries, or in mountainous areas, you may have already noticed that rocks come in a variety of forms. Let’s find out how many types of rocks are there.
Even if you have only had the chance to examine the pebbles in your garden, you will probably have seen that the color, shape, and hardness of the pebbles can vary quite considerably. Remember that soft, white, crumbly chalk and hard, black basalt is both rocks. Even soft, sticky clays are considered to be rocks.
It is important for biologists to be able to pass on information about any particular rock in as brief and accurate a way as possible so that geological information can be accumulated.
In scientific language, a rock could be described as an acid, igneous rock containing more than 10 percent quartz, and in which the alkali feldspars predominate over the calcic feldspars. But this is very long-winded and not easy to understand it would be enough to say that the rock is granite.
If is possible to group all the hundreds of different rock types in the world into families to give an idea of the origin and/or the make-up of those rocks. Every rock can be given a name to convey more particular information. In fact, rocks are usually divided into three major groups on the basis of the way in which they were formed. Rocks may be igneous, sedimentary, or metamorphic in origin.
The word igneous comes from the Latin ignis meaning fire and immediately gives an idea of how these rocks were formed; that is, that the great heat inside the earth has been involved in their formation. Igneous rocks are formed by the cooling of magma or melted rock which has been made more fluid by gasses. Granite is a typical igneous rock.
As the name implies, sedimentary rocks are formed from compacted, hardened, and cemented sediments such as sands to form sandstones, clays to form compacted, hardened, and cemented sediments such as sands to form sandstones, clays to form shales and limey oozes to form chalk.
As you have seen the sediments arise from the wearing away of other rocks. Metamorphic rocks are rocks that have been changed by heat and pressure. A slate is the metamorphic version of shale.
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