The lithosphere, or outer surface of the earth, is made up of rock. Rocks are natural substances that are made up of different materials. They are fused together through natural processes to form the hard substance that we call rock. Rock may be covered with soil, ice or snow. It lies beneath rivers and oceans. Rocks have been used as tools, weapons and building materials by human beings for millions of years. Rocks are made up of two or more minerals. For example, the minerals quartz and feldspar are both present in granite, which is a rock. They are mined in order to extract the minerals they contain. These rocks are called ores. Rocks also contain most of the gems used by us. Scientists glean valuable information from the study of rocks.
The rock cycle
Rocks go through a rock cycle. This is a continuous process which has been taking place over millions of years. Rocks are constantly being broken up by weathering. Smaller pieces of rock are shifted by gravity, wind, water and ice. These pieces are eventually deposited elsewhere, and other material is deposited on top of them, causing the formation of layers called sediment. As the layers increase, they get compacted and cemented together by materials like clay. If rocks underground are subjected to high temperature or pressure, the minerals contained in them undergo a change. This is called metamorphosis. All types of rocks can go through metamorphosis. When rock is subjected to very high temperatures, it melts and is known as magma. When large quantities of magma cool slowly beneath the earth’s crust, they crystallise to form intrusive igneous rocks. If the magma is forced above the crust in a volcanic eruption, rocks called extrusive igneous rocks are formed. Once rocks are on the earth’s surface, the whole process starts again.
Classification of rocks
Rocks are classified according to the way in which they were formed. The main types of rocks are:
- a) Igneous rocks- They are formed due to volcanic activity and are the oldest rocks. Granite, basalt and pumice are all igneous rocks.
- b) Sedimentary rocks- They are formed by deposits carried by water. They may contain fossils. Some examples of sedimentary rocks are flint, sandstone and chalk.
- c) Metamorphic rocks- These rocks are formed under extreme heat and pressure over a long period of time. Marble and slate are examples of metamorphic rocks.
The word ‘mineral’ is often taken to refer to substances in food, like calcium and phosphorous. It is also used in reference to materials like natural gas. In this instance, ‘ mineral’ is used according to the standard definition used by mineralogists.
Minerals are used to make many of our everyday objects from rock salt to pencils. There are about four thousand different minerals. They are made up of chemical elements. Most minerals are made up of two or more elements, but a few are made up of just one element. Diamonds and graphite are both made up of carbon. Yet, diamond is the hardest known substance, but graphite is one of the softest. Water is not a mineral, but ice is. It is the most common mineral found on earth. Quartz is also a commonly found mineral.
The definition of minerals
Materials which have the following characteristics may be defined as minerals:
- a) They occur naturally
- b) They are inorganic
- c) They are solid
- d) Their chemical composition is unique
- e) Their atomic or internal structure is ordered
Minerals are identified according to their colour, hardness, shape, lustre and streak.
The difference between rocks and minerals
Minerals are found in rock.
Minerals have a definite chemical composition. Rocks do not.
Rocks vary in shape and colour, but these characteristics are generally uniform in minerals.
Rocks may contain organic matter like fossils. Minerals are inorganic.
The study of rocks is called petrology and the study of minerals is called mineralogy.