Friday 15 December 2017

All rocks fall into three great family groups

By Jim Hurley - Nature Trail

Errigal, Co Donegal, a quartzite mountain.
Errigal, Co Donegal, a quartzite mountain.

When you come across a rock face out of doors it is often difficult to tell what you are looking at. An immediate source of help is to access the excellent maps available on the website of the Geological Survey of Ireland.

As a starting point, it also helps to remind oneself that all rocks fall into three great family groups: igneous rocks, sedimentary rocks and metamorphic rocks.

Igneous means 'of fire'. Lava is an obvious igneous rock; it is born of fire. Pictures of lava glowing red hot and flowing from an erupting volcano are always impressive and awe-inspiring. While lave flows out on the surface of the ground many igneous rocks originate underground where hot material cools slowly allowing large crystals to grow. Granite is an example of an igneous rock that forms underground.

When planet Earth was young it is believed that it was extra hot. As it cooled the first rocks that formed had to be igneous rocks, rocks born of fire and intense heat. As time passed, these primordial igneous rocks got worn down through the natural process of weathering and sediment arrived on the scene.

Sediment is the result of weathering. Bits of sediment can be tiny like flat flakes of clay or rounded bits of silt. Grains of sand are bigger but not as big as gravel, stones, cobbles and boulders. All of these bits of sediment can form rock types and these are sometimes named after their major constituent. Mud give rise to mudstone and sand to sandstone.

Other names are not as obvious. A sedimentary rock type that originated from fragments of sediment the size of gravel, pebbles and stones is a conglomerate and a rock type that originated from mixed or poorly sorted sediment is a greywacke.

Igneous rocks are born of fire. Sedimentary rocks result from weathering. The final family of rocks are the most diverse: the metamorphic rocks. They are changed rocks. The parent rock can be an igneous rock, a sedimentary rock or an older metamorphic rock and these can be changed by intense heat, by enormous pressure or by chemical action.

Quartzite is an example of a metamorphic rock. Quartzite is a former sandstone changed by intense heat and enormous pressure. Since it is a very hard rock it resists weathering and forms impressive pointy mountains like the Sugar Loaf in Wicklow, Croagh Patrick in Mayo, Errigal in Donegal and the Twelve Bens in Connemara.

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