The earth is partly made up with rocks. There are many different sorts of rocks that can differ in colour, texture, density, and hardness. All these can be classified into three, big main groups: Igneous, Sedimentary, and Metamorphic rocks.

Igneous Rocks:

This type of rock is a rock that is from cooled down lava. There are two types of rock within this igneous group. There are Intrusive and Extrusive rocks.

Lump of Granite

Intrusive: This is a rock that is solid (cooled down) lava, but it was cooled under ground. The fact that it was under ground and most probably surrounded by more rock caused it to cool extra slowly. The rock had time to create very big crystals before fully cooling. This causes hard, rough and coarse rocks to form. An example of this Intrusive rock would be Granite. Granite is a multi-coloured, coarse rock that can be found in the Wicklow Mountains (Ireland) and are desirable for their durability and colour diversity. It was used along with limestone to build the Great Pyramid by the Egyptians.

Basalt at the Giant's Causeway

Extrusive: This is still cooled down lava, but only this time it was cooled near or on the surface of the ground. This rock is usually much ‘smoother’ because it cools much more quickly and less crystals have the time to form. An example of Extrusive rock would be Basalt. Basalt is a heavy, black, smooth rock. It forms the Giant’s Causeway. Crushed Basalt is used to make asphalt pavement, cement, and sometimes it is found in the foundations of old italian buildings and also used for the slabs of rock in cobbled streets.

Lump of Basalt

Sedimentary Rocks:

This rock was formed with the sediments of dead plants and animals (sometimes sand) at the bottom of the sea bed.

Lump of Sandstone

With the water pressure pushing all these sediments down, a rocky substance starts to form. An example of this rock is Sandstone or Limestone. Sandstone is mainly made from compressed sand under the sea. Sandstone is coarse and red-ish/brown in colour. It is commonly used as building material because of it’s resistance to weathering, and yet easy to handle.

Metamorphic Rocks:

Lump of Marble

Metamorphic rocks are rocks that were formerly one of the two above, but after constant heat and pressure, the rock ‘morphed’ into a new kind of rock. An example of this changement would be Marble. Marble is formerly Limestone, but after receiving constant heat and grinding pressure, a newer (more expensive) rock is formed. Metamorphic Rocks are usually more expensive because they are rarer. It takes more time to create them. First the actual time to create the first rock and then the time for that rock to change.

Reference: Granite image, Giant’s Causeway image, Basalt image, Sandstone image, Marble image.

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