What is a rock?
They are very tiny grains of different minerals, compressed together in chemical reaction to form a bigger mass. Rocks make up non-water part of the earth’s crust.
A mineral and a rock do not mean the same thing. Minerals are made of individual or a combination of elements. A mineral is composed of the same substance throughout. A rock is made of a combination of two or more different minerals. They may also contain organic compounds.
They come in infinite sizes, shapes, colors, weights, textures and strengths. Rocks constantly change from one type to another, and from place to place, in a cycle known as the rock cycle. This change does not occur in short periods—they take thousands to millions of years to change.
Rocks may be hard or soft. It depends on the way the tiny grains interlock with each other. Rocks with rough grains fitting perfectly into each other tend to leave no room for moisture. As a result, they are harder and non-porous in nature. Granite is a good example.
Rocks with rounded or fine grains often have spaces that hold moisture, and tend to be softer. They crumble quicker than hard rocks. Rocks that have lots of spaces holding water, or through which water can pass are called porous rocks. Sandstone is a great example.
Soils, sands and rocks.
Sand is the fine debris resulting from broken down rocks (weathered). Sand therefore carries the same or similar properties of the rocks that they came from. Soils are sands that contain air, water and organic materials, in proportions that support the growth of plants and organisms that live in them. Many plants and animals rely on soils for survival.
How do rocks form?
Different rocks form from different processes. In the next few pages, we will see how the three main rock types (Igneous Rocks, Sedimentary Rocks and Metamorphic Rocks) form. Let us begin with igneous rocks...