1. Igneous rock
Igneous rock has been forming ever since the time when the earth itself began. Magma, the mixture of liquid and gaseous substances in the inner part of the earth, comes out beyond the surface of the earth for geological reasons, cools and becomes hard, and igneous rock is then formed. Fossils are not found in these rocks.
The book states that fossils are not found in igneous rock and then moves on, and it is worth stopping, because that is not a random fact but something you could have worked out for yourself. A fossil is the preserved shape of a living thing. Igneous rock was molten before it set, at temperatures high enough to melt stone. Nothing that was ever alive survives being poured into that, so there is nothing left to preserve. Sedimentary rock is the exact opposite: it forms gently, by burial, at the bottom of quiet water, which is precisely the condition a shell or a bone needs in order to last. So the presence or absence of fossils is not a curiosity, it is a reliable clue about where a rock came from, and it explains why almost everything anybody knows about ancient life on this planet has been read out of one of the three families and not the other two.
Some important igneous rocks
| Rock | What it is |
|---|---|
| Granite | A black and brown coloured rock which is very hard and strong. Because its surface is flat, attractive and smooth, it is generally laid on floors. |
| Pumice | A light rock with many holes and pores all through it. |
| Obsidian | A glass like and weak kind of rock. It is also called volcanic glass. |
Pumice is the one rock in this whole unit that will float on water, and after a volcanic eruption near the sea, rafts of it can drift for months. Ask yourself why, and you will find you already have the answer. In Unit 9 you learned that whether a thing floats depends on the density of the whole object, air pockets included, and not on the material it is made of. That is why a solid lump of iron sinks and an iron box floats. Pumice is exactly the same trick played by nature: it forms when frothy magma full of gas bubbles cools so quickly that the bubbles are trapped in the rock as it sets. The material is ordinary volcanic rock, which is denser than water, but the holes bring the average density of the whole lump below one, so it floats. The pumice stone used to rub hard skin off a heel is the same substance, and its usefulness comes from the same holes.
2. Sedimentary rock
Air, water and heat break and wear away the things that are on the surface of the earth. The small things that are broken off in this way are moved from one place to another by rivers, streams, rain and wind, and go on piling up. When hard heat and pressure act on them, those substances stick to one another and become hard rock. The rock made by this process is called sedimentary rock. Fossils are found in some sedimentary rock, and from the study of such rocks it can be known when living things began on the earth.
Some important sedimentary rocks
| Rock | How it forms and what it is like |
|---|---|
| Limestone | After the animals with hard shells that live in the sea die, the pieces of their shells and bones form layer upon layer at the bottom of the sea. Those layers being pressed makes limestone. It is red, brown, white or black with fine grains, and it is used to make cement. |
| Sandstone | The rock made by silica, sticky soil and similar material sticking together in layers of sand. |
| Shale | Particles smaller than the particles of sand are called silt. Soil made of particles even smaller than silt is called clay. The rock made by silt and clay together is shale, and it is black and soft. |
| Conglomerate | This rock is made by small pieces of stone, gravel and sand sticking to one another. It is also called kangad rock. |
Read the four descriptions again and a pattern appears that the book does not point out. Three of them are the same rock at different grain sizes, sorted by how big the pieces were when they settled. Conglomerate is made of pebbles and gravel, which are the largest pieces. Sandstone is made of sand. Shale is made of silt and clay, which are the smallest pieces of all. So the coarser the grain, the closer to fast water it settled, because only fast water can carry a pebble and only still water lets clay sink. That means the grain size of a sedimentary rock tells you how fast the water was flowing when it formed, which is a remarkable amount of information to get from looking at a stone. Limestone is the odd one out of the four, because it is not made of broken rock at all: it is made of the remains of living things.
3. Metamorphic rock
Over a long time, because of heat and pressure, igneous and sedimentary rocks are transformed. When rocks are transformed, some or all of their minerals can change. The rock that is changed from one form and becomes another form is called metamorphic rock. This kind of rock is found in the inner part of the earth.
Rocks and the metamorphic rocks they become
| Rock | Metamorphic rock |
|---|---|
| Silica | Quartzite |
| Limestone | Marble |
| Coal | Graphite, slate |
| Graphite | Diamond |
| Granite, diorite | Gneiss, schist |
Write the table as the book gives it, because that is what an examination will ask for, but two of its rows are worth understanding rather than simply copying. The third row pairs slate with coal, and slate is in fact the metamorphosed form of shale, which the book itself described two paragraphs earlier: press shale hard for long enough and its clay particles rearrange into the flat, hard sheets we split for roofs. Coal metamorphoses into graphite, as the same row correctly says. The fourth row is the astonishing one and it connects straight back to Unit 9. Graphite is the soft black substance in a pencil, and diamond is the hardest natural substance there is, and both are pure carbon. The same element, arranged two different ways, produces one substance you can write with and one that will cut glass, which is exactly the lesson of Unit 9 in its most extreme form: properties come from arrangement, not just from what atoms are present. Diamond forms only very deep in the earth, where the pressure is enormous.
Look at what the table is actually saying: rocks turn into other rocks. Limestone becomes marble. Shale becomes slate. Granite becomes gneiss. Now put that together with the other two ways rock forms, and a circle appears. Igneous rock is broken down by wind and water into small pieces, which are carried away, settle in layers and become sedimentary rock. Sedimentary rock buried deep enough is squeezed and heated into metamorphic rock. Metamorphic rock pushed deeper still melts back into magma, which rises, cools and becomes igneous rock again, and the whole thing starts over. Geologists call this the rock cycle, and it is the single most useful idea in the subject, because it means the three families are not three separate kinds of thing but three stages that the same material passes through, over and over, for as long as the planet lasts. Any rock can become any other rock. It is only a question of where it ends up and how long it waits.
Importance of rock
Rocks are the solid and hard substances found on the surface of the earth, and they are used in various fields. In short, the importance of rock is as follows: to construct houses and buildings; to carve rock and make statues; to roof houses, monasteries and temples; to decorate houses and temples with various kinds of marble; to use in the home as traditional technology such as loharo, silauto, jaato and okhal; to make precious gems; to make cement; and to study the origin of the earth.
Rocks of Nepal: where they are found and what they are used for
| Name of the rock | Place where it is found | How it is used |
|---|---|---|
| Conglomerate | Stream and river banks | In house construction work |
| Sandstone | Stream and river banks | Construction and decoration |
| Silica | Stream and river banks | Jaato, loharo, silauto |
| Limestone | Godavari, Bhainse, Chobhar, Udayapur, Jogimara and other places | Burnt to make lime and cement |
| Marble | Godavari | Laid on floors in houses, buildings and temples |
| Slate | Bandipur in Tanahun, Lalitpur and other places | Roofing houses, and making writing boards in schools |
This is the same habit you built all through the last unit: never learn a use on its own, learn the property that made it the right choice. Slate is used for roofs and for writing boards for one reason, which is that it splits cleanly into thin flat sheets, and it splits that way because it was made from flat layers of clay in the first place. Granite is laid on floors because it is very hard, so feet do not wear it, and because it takes a smooth attractive polish. Silica is used for a jaato and a silauto because it is harder than the spices being ground, so the stone wears away the food and not the other way round. Limestone is burnt for cement because of what it is chemically, which you already know from Unit 10: it is largely the same substance as a seashell, and heating it drives off carbon dioxide. And marble is used for decoration because it is limestone that heat and pressure have turned smooth, even and beautiful. Six uses, six properties, and not one of them is arbitrary.
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