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Different rock types explained

By Edited May 3, 2015 4 4

The three types of rock

The three different categories of rock are Igneous, Sedimentary and Metamorphic. Rocks are divided into these categories depending on the way that they were formed

Igneous Rocks

Igneous rocks are formed when volcanoes erupt. Hot magma is pushed to the surface from deep underground and it cools to create igneous rock.

There are two types of igneous rock:

Volcanic igneous rock is formed when the magma cools very fast as it hits the surface. Some volcanic igneous rock can be identified by the glassy appearance. Obsidian is an example of a glassy volcanic igneous rock. Another variation of volcanic igneous rock would be fine-grained basalt

Plutonic igneous rocks are cooled more slowly beneath the surface and this causes the rock to form larger crystals as it cools. This gives plutonic igneous rock a coarse-grained appearance.

The Igneous Rocks include: Andesite, Basalt, Diorite, Granite Granodiorite, Gabbro, Obsidian, Pegmatite, Peridotite  Syenite, Trachyte , Lamprophyre   


Sedimentary Rock

In contrast to the almost immediate creation of igneous rocks, sedimentary rock takes millions of years to form. When weathering breaks down older rocks, wind, rain and ice floes can deposit the silt in layers. After several layers have built up the pressure from the weight of the sea or the immense weight of the silt upon silt presses the particles together. As water trickles through the silt the whole thing begins to gel together over time. When the silt is comprised of living matter such as shells or bones limestone is produced.

The Sedimentary Rocks include: Arkosis, Breccias, Chalk, Cherts and Flints, Dolomite, Fossiliferous Limestone Greywackes, Ironstones, Lithic Arenites, Mudstones, Quartz Arenites, Siltstones, Till and Tillites.


Metamorphic Rocks

Metamorphic rock begins as either igneous or sedimentary rock and is then altered by extreme pressures or temperatures.

There are two types of metamorphism:

Regional metamorphism occurs when movements in the Earth’s crust put extreme pressure on the surrounding rock.

Thermal metamorphism occurs when the extreme temperature of molten lava literally cooks nearby rocks but does not melt them. When this process happens to limestone it becomes marble.

The Metamorphic Rocks include: Amphibolite, Eclogite, Gneisis, Griesen, Garnnet Schist, Hornfels, Marble, Mica Schist, Migmatites, Serpentinite, Skarn, Slate, Spotted Slate, Phyllite and Quartzite.







Jul 18, 2012 12:57pm
That was a nice and informative article! Thumb from me.
Jul 18, 2012 1:13pm
Aug 15, 2012 5:07am
nice article,loved Geography.
Aug 17, 2012 11:24am
Great article especially for an old rock hound like me. Thanks for the information andpictures. Two thumbs up from me
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  1. Snedden, Robert Rocks and Fossils. Bristol: Parragon, 1997.

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