The Three Great Kinds of Rocks

Wherever you may be as you read this, you are almost certain to be surrounded by rock. The soil in your garden is made up of tiny grains of rock and rests on a bed of solid rock. Bricks and concrete are also made from grains of rock. Even if you are reading this on board a ship or by the seaside, rock is still not far away. The water on which you may be sailing covers a huge basin of rock, and the sand on the beach is made up of tiny grains of rock.

Rock comes in all shapes and sizes, from soil and sand, through stones and pebbles to mountains, which are huge pieces of rock. In fact, the earth is a great ball of rock. If we could weigh our planet, it would tip the scales at 6,600,000,000,000,000,000,000 tons.


You will find that collecting rocks can be an interesting hobby, especially when you live near hills or mountains, where many different kinds can be found. Stones and pebbles have many different patterns and colors. You can make them look more than attractive by polishing them in a tumbler. This is a little machine that rubs them with a special powder. Some stones and pebbles may be so hard that you may have to polish them for weeks. Others may be so soft that they break up into powder in a tumbler.

As your collection grows, you will want to group, or classify, your finds in some way. You could do this by arranging the stones into groups of different colors, or perhaps into those with bands and those without bands. You could also classify them according to the places where you found the stones.

But a geologist (a scientist who studies the earth) would find that these ways of classifying rocks do not tell him very much about them. He would prefer to group the rocks according to the minerals they contain or the ways in which they form.


Minerals are the chemical substances of which rocks are made. There are many different minerals and some rocks are mixtures of minerals. You will need many small groups to classify rocks by their minerals. So it is better to classify rocks according to the way they form because there are only three kinds of rock formations. They are igneous, sedimentary, and metamorphic rocks. Every rock you handle whether it is a heavy stone, a pebble or a handful of sand or soil-belongs to one of these three groups.

Igneous rocks are those that form when hot molten rock rises from the earth’s interior, then cools and hardens. Igneous is a good name for this kind because it means ‘fiery’. Sedimentary rocks grow from layers of sediment– deposit of sand, mud, clay, minerals and even animal or plant remains. The layers build up and slowly harden into rock. Metamorphic rocks form when a great heat of pressure underground acts on existing rocks and changes them into new rocks. Metamorphic means ‘changed’. Although they form underground, these rocks may later come to the surface. Sedimentary rocks are the most common of these three great classes. They cover two-thirds of the earth’s solid surface, including the ocean floors. Most of the remaining rock is igneous, and igneous rock always lies beneath a sedimentary rock. It forms the bases of continents and cores of mountains and supports the sedimentary rock ocean floor.

Red sandstone: sandstone is a sedimentary rock. It is formed from small grains of rock which change, under great pressure, into hard layers. Sandstone is sometimes used in the construction of buildings.
Red sandstone: sandstone is a sedimentary rock. It is formed from small grains of rock which change, under great pressure, into hard layers. Sandstone is sometimes used in the construction of buildings.
Marble is a metamorphic rock that is formed from limestone. It has great beauty and strength.
Marble is a metamorphic rock that is formed from limestone. It has great beauty and strength.
This igneous rock basalt was once a molten rock. It is often crushed and used to make roads.
This igneous rock basalt was once a molten rock. It is often crushed and used to make roads.

Igneous Rock-Hot and Cold

Round the earth’s solid inner core is an outer core of rock which is molten. Other rocks inside the earth are hot enough to melt, but the great pressure of cold rock on the surface keeps it solid. Where the pressure is lessened, such as in a passage opening toward’s the earth surface, the rock can melt. This molten rock is called magma. It rises slowly towards the surface. If it reaches the surface, it spouts from volcanoes in fiery rivers of molten rock that is called lava. Eventually, the lava cools and hardens to produce the igneous rock. Sometimes the magma does not get as far as the surface but cools and hardens beneath. The surface rocks may then gradually weather away to show the mass of igneous rock beneath. Igneous rocks that form at the surface is called extrusive rocks, and those that form below are called intrusive rocks.

There are many different types of igneous rocks, depending on the minerals in the lava or magma and on how fast it cools and hardens. If the cooling is very rapid, the result is a natural black glass called obsidian. The broken edges of obsidian, like those of any glass, are very sharp. American Indians made arrowheads, spear points and knives out of obsidian chips.

Most igneous rocks are made of masses of small grains or crystals. The crystals form as a molten rock cools and hardens. The slower the cooling, the larger the crystals. Glassy rocks form when the cooling is so rapid that there is no time for crystals to grow. Lava cools more quickly than the magma because it is open to the air at the earth’s surface. It usually forms a fine- grained rock called basalt. Basalt may be crushed and used in a road building. Sometimes basalt produces long columns of rock with several straight, flat sides. The steps of the Giant’s Causeway in Northern Ireland are the tops of basalt columns. This outcrop of rock sticks out into the sea towards Scotland. According to the legend, it was the beginning of the giant’s roadway across the water.

The grains in basalt are so small that they cannot be seen without the unaided eye. Another common igneous rock is granite, which has larger grains or crystals. This is because granite is an intrusive rock. It forms when magma cools underground. Being underground, away from the cold air, a magma cools slowly thus allowing large crystals to grow.  Granite and basalt are made up of several minerals. Each mineral forms as a liquid crystals as the liquid rocks cools. In granite, those crystals are large enough to be seen. They make up grainy patterns of pink and grey in rock.

Whole mountains of granite, once the cores of bigger mountains, lie exposed the earth;s surface. Parts of the Highlands in Scotland are made up of granite. The rock is very hard, and when polished, the grains produce a beautiful shine. Granite is often used for making roads and in large public buildings.

Much lava contains bubbles of gas. When the lava cools, it hardens into a kind of solid froth called pumice. The bubbles, which are trapped inside, make the pumice so light that it floats on the water. Pumice is often ground into fine powder and used as a polisher.

Cliff made of long columns of basalt.
Cliff made of long columns of basalt.
Magma, or molten rock, slowly rises towards the earth's surface. The magma may cool before it reaches the surface, forming a rock called granite.
Magma, or molten rock, slowly rises towards the earth’s surface. The magma may cool before it reaches the surface, forming a rock called granite.
Limestone is made from the shells of millions of tiny sea creatures.
Limestone is made from the shells of millions of tiny sea creatures.
Granite is made up of different minerals which forms separate crystals.
Granite is made up of different minerals which form separate crystals.

Shaping Sedimentary Rocks

Everything in our world us continually being down. Rocks are worn away by wind and ice, minerals turn into liquid by water, and all living things die. But our world is also ever-challenging, and these materials and remains are not lost but are used again to form sedimentary rocks.

Grains of rocks produced by the erosion (wearing away) of rock masses eventually settle in layers on beds of seas, lakes and rivers. As these layers get thicker and heavier, the lower ones are pressed down. They may also be squeezed by rock movements, and minerals left in them by water may cement the grains together. In ways, over a period of hundreds of thousands of years, the sediments become layers of hard rock.

Sandstone is one example of this kind of sedimentary rock. The layers that make it up can often be seen in sandstone cliffs. Sandstone maybe strong enough to be used as building stone. Shale is a similar sedimentary rock formed from tiny bits of mud and clay. The rocks can easily be split into layers, and is crushed to make tiles, bricks and cement. Shale is the most common in sedimentary rocks.

Certain minerals can be dissolved by water. This happens when water flows over rocks containing those minerals. That is how the sea become salty. In some places, water containing mineral in its dissolved state may be trapped in a pool. If a water evaporates (changes into vapour or gas) with the heat of the sun, the mineral is left as a layer in the ground where the pool was. A rock slowly builds up if this occur many times. Rock salt is one kind of a sedimentary rock that forms this way.

Limestone is a well-known sedimentary rock that forms from the remains of animals. It begins as tiny shells of dead sea animals settle in layers on the ocean floor. Gradually it hardens into rocks. Some of the shells are large enough for their remains to be seen in pieces of limestone. Lime- a substance used in glass, and in mortar for joining bricks and stones-is produced from limestone.

Coal is sedimentary rock. It comes from peat, which is formed from layers of the dead plants. Sometimes the outline of a leaf can be seen in a piece of coal. In fact, fossils (preserved remains) can be found only in sedimentary rocks. In other rocks, the heat that made them destroys any remain of plants and animals. Sedimentary rocks, therefore, are like books in which scientists can read the story of life on earth.

Metamorphic Rocks: All Change

There are always movements going all over the earth’s surface and inside it. These movements sometimes put a great strain on the rocks. Masses of hot igneous rock may rise towards the surface and heat other rocks in their path. Great movements may squeeze, fold and twist the rocks. Because of this, rocks change. Sedimentary and igneous rocks became metamorphic rocks, and existing metamorphic rocks may change to new metamorphic rocks. Shale becomes slate; the thin layers of soft shale change into the hard layers of slate that are sometimes used for making roofs. Limestone become marble, another metamorphic rock.

Pure marble is white, but small amounts of other minerals and rocks. Marble is sometimes used to make buildings look more beautiful, and many of the world’s finest statues are made up of white marble. Quartzite is a metamorphic rock formed from sandstone. The sand grain in a sandstone joined together, producing a very hard rock. Schist and gneiss are general names for two kinds of metamorphic rocks. A schist is made up of thin layers of minerals, and a gneiss contains light and dark bands.

When you look at the rocks, remember that new rocks like them are being formed all the time. The rocks you see will eventually be destroyed, their remains new rocks will be formed. Rocks are hard and last for a long time, but even they are a part of nature’s ever changing pattern.


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