Rock types

Rock type is usually defined as a particular kind of rock having a specific set of characteristics1. Rock types are specific assemblages of minerals (most rocks are composed of minerals). Rocks are much more vaguely defined than minerals. Even rocks within one rock type may have a highly variable composition.

Still, there is a fundamentally sound reason to talk about such a vague concept as a rock type because these assemblages occur again and again in many different locations. Rock type is like a biome (desert, savannah, rain forest). One particular desert may greatly differ from another (one is sandy and another rocky) but they both share something that is common (low precipitation). Exactly the same is true with rock types — one granite may be white and another red but they both have a similar composition (major minerals are feldspar and quartz).

Here is a list of major rock types recognized by geologists. Immense number of rock types have been defined (their definitions often overlapping) and many rock classifications exist. However, there seems to be a fair number of terms and classification principles that have successfully stood the test of time.

ANDESITE is a volcanic rock with

a so-called average composition. It is more felsic than basalt and more mafic than dacite. Andesite is a common rock of subduction zones volcanoes. It is generally dark gray and may contain phenocrysts of plagioclase (usually andesine), biotite, and hornblende.

ANORTHOSITE is a

plutonic rock which is composed of plagioclase (>90%). Anorthosite is a fascinating and enigmatic rock type. Some anorthosites display very beautiful display of colors known as labradorescence and are therefore valuable ornamental stones. But anorthosites are interesting for many other reasons as well. They seem to have formed during unique episode of the ancient Earth more than a billion years ago. Anorthosites are associated with valuable mineral resources and last but not least, the same rock type is very widespread on the Moon. The highlands of the Moon which we see as brighter areas are largely composed of anorthosite which is, however, quite different in appearance from its

terrestrial counterparts. Read more…

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BASALT is a very common dark-

colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite). Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of

gabbro. Read more…

BASANITE is a volcanic rock. It is

similar to basalt but contains less silica and usually more alkali metals. Basanite may contain olivine, pyroxene, and feldspathoid phenocrysts.

CARBONATITE is an

igneous rock that is predominantly composed of carbonate minerals. Both volcanic and plutonic carbonatites are known.

Igneous rocks

Igneous rocks have many distinct characteristics. For example, light-colored igneous rocks are

more acidic, and have over 65% silica. Dark-colored rocks are more basic and have a higher

percentage of ferro-magnesian minerals. Igneous rocks are broken up in to three main groups:

acid, intermediate, and basic. Acid rocks have over 65% silica, intermediate rocks have 55-65%

silica, and basic rocks have 45-55% silica. Igneous rock forms when magma (molten rock) cools

and solidifies. Extrusive igneous rocks are those formed when magma reaches the surface (at

which point it is called lava), cooling and solidifying quickly. Intrusive igneous rocks are formed

when magma slowly cools deep below the surface of the earth. Different sized grains form,

depending on the conditions of the rock formation. Intrusive rocks are generally more coarse-

grained than extrusive. Coarse grains are more than 3/16 of an inch; medium grains, 1/64-3/16;

and fine grains, less than 1/64 of an inch. Granite, Rhyolite,Obsidian,Diorite,Felsite,Basalt,and

Pumice are all examples of igneous rocks. All these characteristics are key in identifying igneous

rocks.

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Granite

 

Obsidian Image:

Sedimentary rocks

Sedimentary rock forms when layers of sand and pebbles are compressed enough to form rock.

Fossils are mainly found in sedimentary rock, specifically limestone because limestone is formed

in warm, shallow seas and organisms and shells get fossilized at the bottom. There are three

different grain sizes in sedimentary rock. Coarse which you can see with the naked eye. Medium

which you can see with a hand lens, and fine which you can see with a microscope. Sedimentary

rocks are split into three different categories. Chemical, detrital and biogenic. Chemical refers to

rocks containing minerals produced by chemical precipitation. Detrital refers to rocks which contain

particles from preexisting rocks. While biogenic rocks contain fossil and shell fragments.

Sandstone,Shale,Limestone,and conglomerate are all examples of sedimentary rocks. Igneous

and sedimentary rocks are largely prevalent in the world

Sandstone

 

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Shale

 

Limestone

Metamorphic rocks

Metamorphic rocks form when rocks are subjected to heat and pressure, but not to the point of

melting. Depending on whether it was formed under just heat or heat and pressure the orientation

of the crystals will be different. Contact metamorphoric rocks are formed in just heat and crystals

are randomly arranged. Regional metamorphic rocks are formed in both heat and pressure, and

have crystals that are aligned. The greater the pressure and temperature these rocks are exposed

to the larger the grains. Medium to high grade metamorphic rocks occurs at a minimum

temperature of 480 degrees f and a maximum temperature of 1,472 degrees f but can be much

lower. Some examples of metamorphic rocks are Slate, Marble, and Granulite.

Slate

 

Marble

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