What are rocks?

Rocks are natural, solid Rocks are natural, solid earth materials made of earth materials made of

minerals.minerals.

What are Minerals?

A mineral is a natural inorganic solid that has definite chemical composition.

Most elements do not occur naturally in their pure form but combine with other elements to form minerals.

Only a few minerals are pure elements, such as gold, carbon or sulfur.

Many minerals form crystals. Those crystals can be identified by their shape, color, luster, and hardness.

Rocks are classified into one of three groups based on how they are formed.

Igneous RocksSedimentary RocksMetamorphic Rocks

Igneous rocks

FormationThe inside of the Earth is very hot - hot enough to melt rocks. Molten (liquid) rock forms when rocks melt. The molten rock is called magma. When the magma cools and solidifies, a type of rock called igneous rock forms.

There are two main types of igneous rock, depending on where the magma cooled:

• Intrusive igneous rocks form when the magma comes from deep underground and is forced into the upper layers of the Earth's crust, where it cools slowly.

• Extrusive igneous rocks form when the magma erupts from a volcano and cools quickly on the ground.

What are they like?Igneous rocks contain randomly arranged crystals. The size of the crystals depends on how quickly the molten magma solidified.

If the magma cools quickly, small crystals form in the rock. This can happen if the magma erupts from a volcano.

If the magma cools slowly, large crystals form in the rock. This can happen if the magma cools deep underground.

Granite is an example of an intrusive igneous rock with large crystals.

Basalt is an example of an extrusive igneous rock with small crystals.

A Closer Look at Basalt.Basalt is the main type of rock in

Hawaii.

What is Basalt?

Extrusive rock: forms during eruptions

If it cools as an intrusive rock then its Gabbro.

Types of Basalt Flows

Pahoehoe A’a

Vs.

A’a

Surface texture rough.

Pahoehoe

Common type on HawaiiSurface smooth, billowy, or

ropy

Main Minerals in Basalt

• Olivine• Pyroxene• Plagioclase Feldspar• Nepheline• Biotite

Olivine (Mg, Fe)2SiO4

Pyroxene

Augite

Plagioclase Feldspar

Labradorite

Metamorphic rocks

How are they formed?

Heat and pressure changes one type of rock into another.Earth movements may cause rocks to be deeply buried or squeezed. As a result, the rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks.

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Marble is a metamorphic rock.

So are slate, schist and gneiss.

Sedimentary rocks

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How are they formed?

When a river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.

The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.

The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.

It may take millions of years for sedimentary rocks to form.

What are they like?

Sedimentary rocks have a layered appearance. They may contain fossil remains of animals and

plants that were trapped as the rocks formed. These fossils can be

used to date the rock.

The rock cycle The Earth's rocks do not stay the same forever. They are continually changing because of processes such as weathering and large earth movements. The rocks are gradually recycled over millions of years. This is called the rock cycle.

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