Class 6 Geography Chapter 4 Complete Guide Book — Rocks | PunjabTest.com

Rocks are natural solids made of minerals. A mineral is a natural substance with fixed composition, like quartz or feldspar. Some rocks have one mineral; most have several.

Rocks build mountains, river beds and the ground under our feet. They are the raw material of soil and of many buildings.

Do not mix up the two words, because tests love to catch you on them. A mineral is one single natural substance with a fixed recipe — quartz is quartz wherever you find it, feldspar is feldspar. A rock is a lump made of minerals stuck together: some rocks, like a pure limestone, are mostly one mineral, but most are a mix, like granite, which is quartz plus feldspar plus mica, all visible as different speckles if you look closely. Rocks are everywhere once you notice them: under your feet, in the walls of old buildings, in the bed of a river. They are also the starting point of soil, because soil begins when rock breaks down.

Igneous rocks form when molten rock (magma underground, lava on the surface) cools and hardens. Granite cools slowly underground (large crystals); basalt cools fast on the surface (small crystals).

They are usually hard and strong, and contain no fossils, because their heat would destroy living remains.

Igneous rocks all start as melted rock, and the only question is where it cools. Magma cooling slowly deep underground has time to grow big crystals, and that is granite — hard, speckled, and used for strong buildings and kitchen tops. Lava cooling quickly on the surface has no time for big crystals, so it sets into small, dark grains, and that is basalt, the rock of many volcanic areas. Both are strong, and both are fossil-free for a reason a child can understand: no plant or animal remains could survive the heat that made them. If you ever find a fossil, you are almost certainly not holding an igneous rock.

Sedimentary rocks form from layers of sediment (sand, mud, shells) pressed and cemented together over long times, often under water. Sandstone, shale and limestone are examples.

They often show layers and may contain fossils — the remains of ancient plants and animals — which tell Earth’s history.

Sedimentary rocks are history books written in layers. Picture sand, mud and shells settling at the bottom of a sea or lake, year after year, layer on layer, until the weight above presses the lower layers together and cements them into solid rock. Sandstone is pressed sand, shale is pressed mud, limestone is often pressed shells and tiny sea creatures. That is why fossils live here and almost nowhere else: a shell buried gently in soft mud can harden into rock around it, while the same shell in hot magma would be destroyed. When you see neat layers in a cliff, you are reading pages of Earth's past, oldest at the bottom, youngest at the top.

Metamorphic rocks form when existing rocks are changed by great heat and pressure deep underground, without melting. Limestone becomes marble; shale becomes slate.

Marble is prized for buildings and statues; slate splits into flat sheets used for roofs and writing boards.

Metamorphic means changed shape, and the change happens without melting — that is the key point students miss. Bury a limestone deep underground, squeeze it with the weight of kilometres of rock and bake it with Earth's inner heat, and its crystals grow and lock together into marble, smooth and strong enough for statues and fine buildings. Do the same to a muddy shale and you get slate, which splits into thin, flat sheets, perfect for roofs. The rock never became liquid; it was reshaped while solid, the way clay can be reshaped without melting. So if a question asks how marble forms, the safe answer is always heat and pressure, never melting.

The rock cycle: igneous rocks weather into sediment → sedimentary rocks → buried and changed into metamorphic → melted into magma → igneous again. The cycle never ends.

Uses: granite and marble for buildings, limestone for cement, coal (a sedimentary rock) for fuel, and gemstones for jewellery.

The rock cycle has no beginning and no end, and that is what makes it a cycle rather than a story. Start anywhere: an igneous rock weathers into grains, the grains settle and cement into a sedimentary rock, the sedimentary rock is buried and changed into a metamorphic rock, the metamorphic rock melts into magma, and the magma cools into an igneous rock again. Each step can take thousands or millions of years, so the cycle turns very slowly, but it never stops. And humans dip into it at every stage: granite and marble for buildings, limestone for cement, coal for fuel, and cut gemstones for jewellery. Next time you see a marble floor or a cement wall, you are standing on one turn of that cycle.