rocks - basics

Basic Introduction to Rocks

Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks

Igneous Rocks

These form from hot, liquid rock ( @ 1500 oC) which cools. When this happens, crystals grow from the chemicals in the liquid.

They form both from either

Magma [ inside the Earth ], example Granite
Granite - cools slowly - big crystals


Lava [ on the surface of the Earth ], example Rhyolite

Rhyolite lava - cools quickly - small crystals

Typical Felsic (light coloured) Igneous Rocks     

Granite & Rhyolite: (shown above)

Both of these contain similar chemicals [ lots of Silicon, making an acid magma ].
This makes them both light coloured.

Typical Mafic (dark coloured) Igneous rocks

Basalt - a mafic lavaBasalt

Much like  Rhyolite, small crystals - cooled quickly on the surface of the earth, so the crystals did not have much time to grow. Unlike Granite & Rhyolite, contain different chemicals [ much less Silicon, making an alkaline magma]. This makes it very dark coloured.

Sedimentary Rocks

Sedimentary Rocks

Most of these are made from Sediment (Wow!) – bits of rock fragments, which have sunk to the bottom of rivers, or more usually the sea. They then get buried & squashed. Over time the fragments get stuck together to form rock. This happens because water also gets trapped with the sediment. Chemicals dissolved in the water make a form of ‘glue’.

Chalk & Limestone, however, are made from the shells of dead sea animals.

Made from gravel, cemented together.

Made from sand, cemented together.

Made from mud, cemented together.

ChalkChalk and LimestoneLimestone:
Both of these are rich in Calcium carbonate [ Ca CO3].
This comes from the animal shells that these rocks contain.
It is an alkali, and so these rocks effervesce - or fizz with dilute acid.
Chalk is made from almost pure animal shells, and so tends to be white in colour.
Limestone is a mixture of both animal shells and mud. This gives it its grey colour.

Metamorphic Rocks

These are usually Sedimentary or Igneous rocks that have been heated and / or squashed by very large amounts.

This can happen by one of the following:

GneissMainly Pressure (example shown is Gneiss pronounced "nice")
Major Earth movements, such as Earthquakes - called Regional Metamorphism


SchistMainly Heat (example shown is called schist)

Being close to an Igneous intrusion [ see Igneous rocks section ] – called Contact Metamorphism

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