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There are three kinds of rocks. These are:

Igneous Rocks

When planet earth first became cool enough to have a solid outer skin, igneous rocks were the first to appear on its surface. Igneous rocks are still being made today as volcanoes erupt molten magma, which cools and becomes solid. Some magma, though, does not get to the surface, instead, it cools as igneous rock underground. Many millions of years later, the igneous rocks which cooled several kilometres (miles) down in the curst may become exposed at the Earth’s types of igneous rocks that crystallize from different magmas, under different conditions. They are grouped by their textures and by the minerals that contain.

If it were possible to put all the rocks of a continent in some kind of giant crushing machine, mix up the crushed rock, melt it, and then let it cool and crystallize, the result would be granite. Granite is one of the most common igneous rocks, found at the core of many mountain ranges. The first continents were made of granite-like rock. Granite contains the minerals quartz and feldspar, along with a small amount of dark minerals such as mica. Their crystals can be seen in the granite on the left : quartz is grey, different feldspars pink and white, and mica is black.

The shiny dark flakes seen in some granites are mica. In granites with large crystals, bits of mica can be flaked off in thin sheets.

An essential mineral in granite, quartz may be transparent but is sometimes a milky blue.

Igneous rocks are classified by the amount and type of feldspar they contain. Feldspar is one of the most common minerals in the Earth's crust.

Some of the rocks formed by recrystallized lava are shown here. The texture of the rocks depends on the type of lava. Some lavas are very got when they are erupted, spreading out quickly as they cool. Others are cooler, and move only slowly, hindered

By the crystals as they shape and grow.

Volcanic gas bubbles out of magma and may be trapped as the lava solidifies. The bubbles in this rock are called vesicles.

Hot basalt lava cools so fast that blobs like this which are thrown out in explosions, have solidified by the time they land on the ground.

Most of the solid surface of the Earth is made of basalt. All the solid ocean crust is basalt, and there are also many huge basalt lava flows on the continents. Basalt is formed by melting in the Earth’s mantle. It is fine-grained and dark in colour because of the dark minerals it contains: principally pyroxene and olivine. If basalt magma cools slowly, it grows larger crystals. Then it is called dolerite, or if the crystals are really large, gabbro.

Gabbro crystallized slowly like granite, but has more dark minerals and usually no quartz.

The dark, heavy rocks on this page contain calcium-rich feldspar. The lighter granites contain sodium-and potassium-rich feldspar.

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Sedimentary Rocks

Layer by layer, sedimentary rocks are made up of material that previously formed other rocks. This material is created by weathering, which breads older rocks down into fragments ---- some small enough to be dissolved in water. These fragments are transported by wind, rivers, and glaciers and eventually deposited as layers or beds of sediment, along with any plant and animal remains trapped within. Over time, the layers are buried and squashed to become gardened, or lithified, into new rocks. Sedimentary rocks contain features which reveal their origins – the kind of rock that weathered into grains, the means of transport and deposition, and all the processes of lithification that turned loose sediment into hard rock.

Pebbles, gravel, and sand in a river bed or on the beach are the raw materials for new sedimentary rock. As they tumble along in the water, pebbles and rock fragments may fracture by impact with each other, while continuing to be chemically weathered or dissolved. Where the grains settle, they may be buried by other layers. Water that percolates through the layers sometimes contains rock material in solution to crystallize around the rock grains, it cements them together to make sedimentary rock.

A pebble beach might become gardened and lithified to made conglomerate. The pebbles with sand grains in between are firmly held together by a rock cement. Breccia forms in the same way, but its fragments are much rougher round the edges

The grains of sand that make a sandstone tell the rock’s history. Those with a polished surface may be quartz grains that gave been rolled around on a beach. Sand grains with a matt surface like ground glass show the sandstone was formed in a desert.

It is easy to see grains of sand on a beach which might one day become sandstone, but limestone’s chemicals are transported invisibly – they are dissolved in water. Sea creatures and plants take carbon dioxide from sea water. This changes the chemical balance of the water, and as a result, the chemicals that make limestone – calcium and magnesium carbonate – separate from the water. They are deposited in thick layers of limy mud on the seabed to made limestone. Carbonates are the same chemicals that made water hard.

Sea creatures such as shellfish help made limestone by taking dissolved calcium carbonate from the water to make their shells. When the shellfish die, they sink into the limy ooze on the sea- or lake bed and help to build up limestone.

Chalk is a soft, pure limestone. Eruope’s chalk cliffs are made from the skeletons of tiny floating plants that lived in the sea more than 65 million years ago.

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Metamorphic rocks

As crustal plates move together, the rocks within are stretched, squeezed, heated – and changed. Metamorphic rocks are made from pre-existing rocks, whether sedimentary, igneous, or metamorphic. When a rock metamorphoses, its minerals recrystallized and its original texture changes. Usually these changes happen deep inside the Earth’s crust, where it is hot enough and there is enough pressure from the overlying rocks to make rocks recrystallized without melting. The recrystallization creates larger crystals and different minerals. At the same time the rocks may become folded or crushed, so they get a new texture in which all the mineral grains are aligned according to the pressures on the rock.

Intense heat changes limestone into marble, and even-grained, sugary-textured rock. Most limestones contain some chemicals other than calcium carbonate. These may be caught in the limestone as grains of sand, or wisps of clay. When the calcium carbonate recrystallizes to marble, it reacts with these other chemicals to make new metamorphic minerals, which come in many colours. The coloured minerals may be in layer which become folded by the pressures of metamorphism. Folds in the green striped marble, for example, show that the original sedimentary layers get both thinned and thickened as they fold.

This igneous rock is made of quartz, feldspar and mica crystals. These are all more or less the same size, and are randomly scattered about the rock.

Muddy grey limestone can be transformed into the multi-coloured marbles seen gere.

When a sandstone which is made entirely of quartz sand grains is metamorphosed, each grain of sand grows to a different shape in response to increased pressure. The once-rounded sand grains interlock and the spaces between are filled with quartz to form a tough new metamorphic rock, quartzite. It is a very much harder rock than marble.

When granite is metamorphosed, the original crystals that make it up recrystallized. If metamorphism is not very intense, the new rock still looks like granite. But it takes on a new texture if there is also directional pressure, like the squeezing that happens in a mountain range. The new rock, gneiss, has a foliated texture. This means the minerals form wispy, more or less parallel bands

This rock formed in heat so intense that parts of it melted. Migmatite shows no sign of the texture of the original rock, which might have been granite. Its banding has been intricately folded

A dull, grey mudstone can be transformed by metamorphism into sparkling, coloured, crystalline rocks. At different temperatures and pressures, different new minerals appear. The rocks here are shown in a sequence from left to right. The rocks on the left have been formed a t the lowest temperatures and pressures, and those on the right, at the highest.

This is a sedimentary rock formed in seas or lakes.

Its pale blue crystal formed in depth and heat of a mountain range.

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