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The Rock Cycle

Rock Cycle

When scientists talk about the rock cycle, they are describing a slow, continuing process of rocks transforming from one type to another in a repeating sequence. In order to understand the process fully in detail we first need to be clear about the terms 'rock', 'stone', 'minerals' and 'elements'.

In simple terms a rock can be described as a large natural mass of stone. It is a hard substance made of minerals in its natural setting, whereas stone is a detached piece of rock.

A mineral: There are five key characteristics that help define and describe a mineral:

  • They occur naturally which is to say they are not man-made.
  • They are inorganic which means they are not alive. They are not composed of animal or plant cells.
  • Minerals are solid. They cannot be in a liquid or gaseous state.
  • They have a definite structure and chemical composition made up of different elements
  • They are able to grow as crystals due to their ordered atomic arrangement

An element is a substance that cannot be broken down into a more basic substance by chemical means. Aristotle described an element more accurately as: 'one of those bodies into which other bodies can be decomposed and which itself is not capable of being divided into other'. There are 117 known elements (as of 2007), with 92 of these involved in the composition of minerals. Oxygen and silicone are the two most common making up about 76% of the total with 7 other main elements contributing to most of the rest. The chart below shows the distribution of elements in the world's minerals:

Oxygen= 46% - 50%
Silicon = 26% - 28%
Aluminium= 7% - 8%
Iron= 4% - 6%
Calcium= 2.4% - 3%
Potassium= 2.3% - 2.5%
Sodium= 2.1% - 2.5%
Magnesium= 2% - 4%
Hydrogen= 1%
83 Others= 2%

So minerals, made up of elements, are the building blocks of rock. Differences in the mix of mineral and how they are formed determine what type of rock is created. Now these rocks are themselves transforming from one type to another through a continuing recycled process which is governed by the changes our planet is constantly going through. These three main rock types are known as Igneous, Sedimentary and Metamorphic.

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