rocks - basics
Basic Introduction to Rocks
They form both from either
Lava [ on the surface of the Earth ], example Rhyolite
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
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.
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.
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.
These are usually Sedimentary or Igneous rocks that have been heated and / or squashed by very large amounts.
Mainly Heat (example shown is called schist)