What are Rocks?
The word "rock" has many different meanings to different people. For example, engineers think of rocks as materials which must be blasted to make dams, roads and so on; a builder thinks of rocks as hard resistant building material; a child thinks of the pebble on the beach that they skip over the water. However, a geologist has to be more specific and defines a rock as a collection or aggregate of mineral grains. Most rocks fall into this category; however, there are some exceptions, such as conglomerate, which is made up of pebbles (themselves mineral aggregates); obsidian, which is a volcanic glass that cooled so fast that no mineral crystals had time to form; and coal, which is an aggregate of plant material as opposed to mineral matter. The term rock generally refers to aggregates of mineral grains that are coherent, in other words, the grains stay together and don't disintegrate under normal conditions; thus sand and mud, which disaggregate, are not rocks.
More than one type of mineral may be present in a rock, for example quartz, feldspar and muscovite in a granite, or the entire rock may be made up of many grains of a single mineral, for example quartz in some sandstones or calcite in limestone. There are more than two thousand three hundred different types of mineral that have been identified, luckily many are rare and only a small number make up most of the common rocks.
|Rock made up of an aggregate of the same
mineral (calcite in marble)
|Rock made up of several different minerals|
(quartz, feldspar and mica in a granite)
The mineral grains can have various shapes and are held together in the rock in a number of ways. Minerals can be rounded or iregular in shape. The rounded grains are held together by cement, just as sand grains are cemented together to form sandstone. Irregular or angular shaped grains are interlocking, and this is how grains in granite or gabbro are held together.
|Rock consisting of cemented rounded grains
||Rock consisting of interlocking irregular grains|
These features are important tools in classifying rocks and assigning them to one of the three major rock groups. Sedimentary rocks are made up of fragments of pre-existing rocks which have been weathered and eroded. The rocks which result have rounded or angular grains held together by mineral cements. Igneous and metamorphic rocks on the other hand crystallized at high temperatures and the minerals are interlocking and irregular.