There are various celestial bodies in the universe. The sun, the moon and the earth are celestial bodies, and these bodies have been divided into stars, planets, satellites and so on. In the solar system there are the earth and other planets and satellites. How might these have been formed? How far might they be spread out in the sky? What might these bodies be made of? That curiosity is still found in people, and it is where this unit begins.
The group made up of the sun and the planets, satellites, asteroids and comets that go round the sun is called the solar system, or the solar family. The earth, which is in the solar system, is the clean home of us all. Most of its outer and inner part is made of soil and rock. And in exactly the same way, most parts of all the celestial bodies have been found to be made of rock.
It looks strange that a unit called The Earth and Universe opens with rocks rather than with stars, and the last sentence above is the reason. Mars is rock. The moon is rock. Mercury is rock. The asteroids between Mars and Jupiter are rock. The solid part of this planet, from the pebble in your hand down as far as anybody can drill, is rock. So when you learn to read a rock you are not learning something small and local; you are learning the alphabet that most of the solar system is written in. That is why the pictures sent back by a spacecraft that lands on Mars are pictures of stones, and why the first thing anybody does with them is compare them with stones on Earth.
Rock
You must have seen hard substances like the ones in the picture around you. These hard substances are rocks. Rocks are found in the inner as well as the outer part of the earth. Rocks are made of various minerals mixed together. The colour, the structure and the heaviness of different rocks are different from one another. Rocks break into pieces and soil is formed, and that is why small pieces of rock are found in soil.
Three words are used in these two paragraphs and it is worth putting them in order, because each one is made from the one before it. A mineral is a single natural substance with a definite composition, and you met a few in the last unit: bauxite, haematite, chalcopyrite. A rock is several minerals mixed together, which is exactly why two rocks can look completely different even though the earth only has so many minerals to work with; changing the recipe changes the rock. And soil is what you get when rock is broken into small enough pieces and mixed with the remains of living things. So the pebble in figure 11.2, the mud in a field and the ore in the last unit are three stages of the same material, and the only difference between them is how far it has been broken down.
Formation of rock
Think for a moment: how might rocks have been formed in nature, and is it possible or not possible to make rock in a factory?
The water that flows from rivers, streams and waterfalls carries away soil, sand, plants and wood. Those substances settle at the bottom of the river and of the sea. After they have been pressed down for a long time, extreme pressure and heat make them set hard and take the form of rock. Some rocks are hard and some are somewhat soft. The colours of different rocks are also found to be different from one another. Their particles can be seen, some fine and some large. The surface of some is rough and of some is smooth. Some split into layers when they are broken, while some are powdery like sand.
It is a good question and it deserves an answer. Yes, and you are surrounded by the results. Concrete is small stones and sand held together by cement, which is very nearly a hand made conglomerate; the only difference is that a river took thousands of years over it and a mixer takes an afternoon. Bricks are clay that has been hardened by heat instead of by burial. And glass is what you get when sand is melted and cooled quickly, which is exactly how obsidian forms in a volcano, and glass is in fact sometimes called man made obsidian. So the honest answer is that we can copy the recipes, and quite well. What we cannot copy is the one ingredient that costs nothing and cannot be hurried, which is time. A factory can imitate pressure and heat easily enough. It cannot imitate a million years.
Read the sedimentation paragraph once more and then take it somewhere astonishing. Rock made that way forms at the bottom of a sea. Now consider that the rock at the very summit of Sagarmatha, the highest point on the surface of this planet, is limestone, and limestone is made from the shells of animals that lived in shallow water. Those shells settled on a sea floor, were buried, were pressed into rock, and were then pushed up almost nine kilometres as India drove into Asia and the Himalaya rose. It is still rising. The same story explains the shaligram that people collect from the bed of the Kali Gandaki: those spiral patterns are the shells of ammonites, sea creatures that died long before there were any mountains here at all. So the two most famous stones in the country, one on the highest peak and one in a river gorge, are both evidence of the same thing: this land was once under water, and rocks remember.
Sorting rocks by what you can see and feel
Activity 11.2 asks you to collect rock samples from around your school or home, give each one a number, and sort them on five properties. Notice which five they are: hard or soft, colour, layered or not, rough or smooth, and the general structure. Not one of them needs a laboratory, and that is the point. Everything a geologist does begins with looking carefully at a stone in the hand, and the five columns in that table are exactly the questions a geologist asks first.
The column headed layered or not is the one that will tell you most, and it is worth knowing why before you fill it in. Layers are what settling makes. Sand and mud and shells arriving year after year build up in flat sheets, so a rock that splits into layers is announcing that it was made by settling. Rock that cooled from molten material has no reason to be layered at all, because nothing was arriving in stages; it simply set. So one glance at a broken edge often tells you which of the three families a rock belongs to, before you have asked a single other question. Keep that in mind for the next topic, where the three families are named.
Types of rock
On the basis of the hardness or softness, the colour, whether it is layered or not, whether it is rough or smooth, and the structure, rocks are of various types. On the basis of their origin and their method of formation, rocks can be classified into three groups: igneous rock, sedimentary rock and metamorphic rock. The next topic takes those three one at a time.
Read that sentence again and notice the words on the basis of origin and method of formation. The three groups are not colours, or hardnesses, or sizes. They are three answers to one question: how was this rock made? Cooled from molten material, settled in layers, or changed by heat and pressure from something it used to be. That is a classification by history, and it is a genuinely unusual thing to meet at this level. A rock does not carry a label saying where it came from, so somebody had to work backwards from what they could see to what must have happened, which is what the whole of geology is. And it explains why the same grey stone can belong to different groups depending on a story you cannot see: the appearance is a clue, but the history is the answer.
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