Geology And Geophysics

Rock Types

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From erupting volcanoes to deep ocean seabeds, rocks are being formed all around us.  Rocks are the foundation of the earth and may seem indestructible and everlasting but in truth are constantly changing.  This is reflected in the three basic types of rock, igneous, sedimentary and metamorphic.

Igneous means relating to or resembling fire, a suitable description of rock formed from magma.  Magma is molten rock.  High pressures and temperatures keep magma in liquid form.  Magma is less dense than solid crustal rock, and when given the opportunity will rise.  As magma rises, the pressure acting on it decreases and it begins to solidify. 

Magma chambers are reservoirs under the crutsal surface.  Rising magma can fill a chamber where it will cool slowly.  The rock formed by the slow cooling of magma underground is called intrusive igneous rock.  Intrusive rocks also form when magma rises through cracks in crustal rock but stops before it reaches the surface.  The bands of rock that form this way are termed intrusions. 

As magma slowly cools, the various molecules it contains are able to bond with others of their kind and form large crystals.  The crystals are often visible to the eye.  The most well known rock of this type is granite.  Granites tend to be lighter in color and are composed mainly of silicates, the most common building blocks of rock on earth.

Extrusive igneous rocks form when magma is ejected or flows onto the crust's surface.  Magma cools rapidly at surface temperatures and pressures, resulting in finer crystalline structures.  The extreme example is obsidian, which looks like glass.  The crystals are so fine they cannot be seen even with a magnifying lens.  The most common extrusive igneous rock is basalt.  Basalts, in contrast to granites, are dark, fine-grained and have lower silica content.

Sedimentary rocks are composed of fragments from all rock types. 

Heat, cold, wind and water act together to erode rock.  The resultant fragments, called clasts, are deposited in streams, lakes and oceans.  The deposits are known as sedimentary deposits, hence the name of the rock type they form.  As the years pass, new layers overlie the deposits.  The layers build up, putting greater and greater pressure on the underlying deposits.  Minerals in the surrounding water can precipitate to fill the spaces among the clasts, forming a matrix.  Eventually, the clasts are cemented and squeezed into new rock.

Lakebed deposits usually consist of fine clays and silts that form shale or siltstone.  Sand grains form sandstone.  A mixture of clast sizes ranging from silt to boulders forms a conglomerate.  Conglomerates are most often found in old stream beds.  In oceans are billions of tiny organisms with shells made of calcium carbonate.  As the creatures die, their bodies decompose and the shells settle at the bottom.  Over time the deposit becomes limestone.  Fossiliferous limestone forms when larger shells or skeletal remains of organisms do not completely disintegrate in the deposit.  The fossils are later visible in the rock.

Rock metamorphosis is a change in the rock produced by pressure or heat.  Metamorphic rocks are more difficult to identify than the other types because of the many possible changes the rock can undergo.  There are two methods by which metamorphosis of rocks occurs: regional and contact.  Regional metamorphosis affects large areas and is the result of plate tectonics.  When crustal rock on one plate encounters crustal rock on another plate, the denser rock will be pushed under, or subducted.  When crustal rocks of equal density meet they are folded and pushed upward.  The extreme pressure from these events causes the partial melting of the rocks.  The melting is only partial, so magma has not formed.  As the rocks cool, the mineral components shift according to their bonding preferences.  The rock will have the same chemical composition, but the crystalline structure will have changed.

Contact metamorphism is not as extensive as regional metamorphism.  Magma can move along cracks in solid crustal rock.  The solid rock adjacent to the magma becomes heated and partially melts.  Similarly, when magma is extruded onto the surface the rock it encounters will partially melt.  In both cases, metamorphosis occurs.  Metamorphic rock is usually made from one of the other rock types.  Common metamorphic rocks include: marble (from limestone), quartzite (from sandstone) and slate (from shale).

The earth is a dynamic system of rebirth.  New rock is formed through geologic processes as it is eroded and worn away by time.  An understanding of the basic rock types gives us a glimpse into the complex workings of our planet.

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