The Rock Cycle

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Rocks are continuously being formed, changed/modified, destroyed, and reconstructed by forces within the Earth. This is known as the rock cycle.

ROCKS ARE GROUPED ACCORDING TO THEIR FORMATION

Igneous Magma/lava cools e.g. basalt, granite

and becomes solid pumice

Sedimentary When sediments e.g. limestone, coal

(plants, remains of sandstone, shale

animals, rocks), conglomerate

accumulate, compress,

compact, and cement

into a solid.

Metamorphic When a rock is e.g. marble, quartzite

(Latin for subjected to great heat slate, gneiss, schist

change) and/or pressure, it is

changed.

PLUTONIC (INTRUSIVE) ROCKS

Form when magma cools slowly within the Earth's crust. It may cool at a depth of several kilometres e.g. granite. Granite has large crystals because the magma cooled slowly.

Granite may be found in the Mourne mountains Co.Down and the Dublin-Wicklow mountains - The Leinster Batholith that was exposed due to denudation.

Granite is a strong, resistant rock and is commonly used as a building material. It is often polished and used for kitchen worktops and fireplaces. When granite is broken down by weathering it produces clay.

VOLCANIC (EXTRUSIVE) IGNEOUS ROCKS

Form when magma cools outside on the Earth's crust. These have smaller crystals because lava cools quickly when exposed to the elements e.g. basalt

Basalt is composed of quartz so It is a hard, resistant rock and is therefore used for road construction. It is too dark to use for buildings. In the Antrim-Derry Plateau, lava poured out of fissures in the ground (60 million years ago). It cooled and contracted to form six-sided columns of basalt. e.g. The Giant's Causeway.

Limestone --> marble e.g. Connemara Co.Galway

Sandstone --> quartzite eg: Croagh Patrick Co. Mayo

Quartzite

Quartzite is sandstone which changed due to great heat and pressure. The spaces between the grains of quartz are filled with silica (from magma). The quartz and silica are packed tightly together. As a result, quartzite is very resistant to erosion.

Quartzite is a light-coloured rock, which is often white in colour, and is found on the peaks of mountains such as Croagh Patrick, Co. Mayo and the Sugarloaf Co. Wicklow. Because it is hard, it is commonly used for road surfacing.