Igneous Rocks

Igneous Rocks

Igneous rocks are rocks that are formed when molten magma cools and becomes solid, either on the surface(lava) as extrusive (volcanic rocks), or below the surface as intrusive (plutonic rocks). There are over 700 described igneous rocks, most forming under the surface.

Types of Magma

There are two general kinds of magma. These magmas are felsic magma and mafic magma. They have temperatures ranging from 600°C to 1200°C. Though these magmas have the same range of heat, they differ in chemical composition. Felsic Magma has a high percentage of silica with a low percentage of calcium, iron, and magnesium. Mafic magma has just the opposite - high levels of calcium, iron, and magnesium with a low percentage of silica. Low-silica magma is hotter, thinner, and more fluid than felsic magma. Felsic magma is thick and flows slowly. When felsic magma hardens, it rocks that have mainly light colored materials. When mafic magma hardens, it forms rocks that contain dark ferromagnesian minerals. Mafic magma forms most volcanic rocks, while felsic magma forms most plutonic rocks.


Obsidian


The obsidian rock is a type of naturally-occurring glass formed as an extrusive igneous rock. It is produced when felsic lava erupting from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian flows where the cooling of lava is quicker and rapid. Obsidian is mineral-like but not truly not. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite.

Obsidian can be found in a lot of places, for example it can be found in different places here in America. Yellowstone National Park has a mountainside containing much obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western U. S. states including Arizona, Colorado, Texas, Utah, and Idaho. Obsidian can also be found in the following countries: Armenia, Turkey, Italy, Mexico, Greece and Scotland.

Obsidian today is used in many ways, they are used in hospitals and microscopes. Obsidian is used in cardiac surgery, because well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, with the edge of the blade being only about 3 mm wide. A study which was done shows that obsidian rocks produced narrower scars, less inflammatory cells, and less granulation tissue in a group of rats.


Pumice


Pumice is an igneous rock characterized by many pores, formed when gas from magma nucleate bubbles cannot readily decouple before chilling to glass. It is considered a glass because it has no crystal form. It is a common production of volcanic eruptions and commonly pale in color, from white, cream, or gray, although it is sometimes found as green, brown, or black. Its average porosity is 90%, and usually floats in water.

After the explosion of Krakatoa, there were many pumice rafts floating in the Pacific Ocean. They drifted there for 20 years. Pumice rafts disperse and support marine life. In 1979, 1984, and 2006, underwater eruptions near Tonga brought forth large pumice rafts, some being 30 km, which floated hundreds of miles to the islands of Fiji.

Pumice is used to make light-weight concrete and insulative low-density bricks .When used as an additive to make cement, pozzolan fine grained pumice) is mixed with lime to form a plaster-like cement. This was used in early Roman times. It is also used to make something called Lava Soap, which is a black soap used by mechanics and people who get very dirty hands.


Rhyolite


Rhyolite is a very light-colored, aphanitic, volcanic rock that is rich in silica and broadly equivalent to granite in composition. Migration of rhyolitic magma through the Earth's crust, which causes most of the earth's hazardous and volcanic activity, represents a major process of chemical fractionation by which continenetal crust grows and evolves.

Rhyolite is formed by moltemnsilica-rich magma flowing toward the Earth's surface. Small differences in this process, notably those related to the release of gas from the magma at shallow depth, produce extremely diverse structural features. The high silica content gives rhyolitic lava a correspondingly high viscosity. This hinders crystallization and often causes young rhyolite to be a mixture of microcrystalline aggregates and glassy material. Because of the glassy nature of most rhyolites, they are best characterized by chemical analysis. They typically have 70 - 75% silicon dioxide and more potassium oxide than sodium oxide.

Some forms of rhyolite have a very nice visual appeal and are used as jewelry.


Scoria


Scoria is a light rock commonly, but not exclusively basaltic or andesitic in composition. It is light because of numerous macroscopic ellipsoidal vesicles. Although it is light, most scoria has a gravity greater than 1, and sinks when in water. The vesicularity of scoria is a result of the exsolution of magmatic volatiles prior to chilling. This rock differs from pumice because it has larger vesicles and thicker vesicle walls, which make it darker in color (usually dark brown, black, or red) and give it a higher density level. Scoria may form as fragmental ejecta or as part of lava. Most scoria is composed of glassy fragments, and sometimes contain phenocrysts.

Scoria is used to make medium weight building blocks.


Granite


Granite is a well known only on Earth where it forms a major part of continental crust. Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in batholiths that are often associated with orgenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations very coarse-grained pegmatite masses occur with granite.

The Origin of this rock is that this rock is formed from magma; this rock is still being studied. Granitic magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometers and up to 50 km depth within thick continental crust. The origin of granite is contentious and has led to varied schemes of classification. Classification schemes are regional; there is a French scheme, a British scheme and an American scheme.

This rock is also favored by many rock climbers, the reason why its steepness, soundness, crack systems, and friction. There are very well know granite that rock climbers go on, it includes Yosemite, the Bugaboos, the Mont Blanc massif (and peaks such as the Aiguille du Dru, the Aiguille du Midi and the Grandes Jorasses), the Bregaglia, Corsica, parts of the Karakoram, the Towers of Paine, Baffin Island, the Cornish coast and the Cairngorms. Granite is used for architectural construction and ornamental stone and momument construction.


Pyroxenite


Pyroxenite is an ultramafic igneous rock, consisting essentially of minerals from the pyroxene group. Some minerals are augite and diopside.They are classified into 3 groups - clinopyroxenites, orthopyroxenites, and websterites which contain both pyroxenes.

Pyroxenite is essentially of igneous origin, although there are some pyroxenites included in the metamorphic complex of the Lewisian of Scotland. Pyroxene-rich rocks which result from the contact metamorphism of impure limestones are called pyroxene hornfelses. Igneous pyroxenites are similar to the gabbros and norites, differing by the absence of feldspar. They are also similar to the peridotites, differing from them by containing over 40% olivine. The connection between these rocks are indicated by their mode of occurance. Pyroxenite is usually found with masses of gabbro and peridotite and are almost never found alone.

Pyroxenite is often very coarse. They usually contain crystals which may several inches in length. In addition to feldspar and olivine, chromite and other spinels are accessory minerals to Pyroxenite.



Diabase


This rock is a mafic rock, or holocrystalline, igneous rock equivalent to volcanic basalt or plutonic gabbro. Diabase is also called dolerite in many references outside North America. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic glass).

This rock can be found in many places such as dikes and sills. Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundreds of individual dikes or sills radiating from a single volcanic center. There is a sill around America it is located in New Jersey Palisades on the Hudson River, near New York City, is an example of a diabase sill. The dike complexes of the Hebridean Tertiary volcanic province which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion region of Ireland, and extends across northern England contains many examples of diabase dike swarms.


Basalt


Basalt is a common extrusive volcanic rock. It is usually gray to black in color. It is usually fine-grained, this being due to rapid cooling of lava on the surface. It may be prophyritic, containing larger crystals in the fined grained matrix, or vesicular like scoria. When basalt is unweathered, it is black or gray.


There are 4 types of Basalt. These are...

1) - Tholeiitic - This basalt is relatively poor in silica and poor in sodium. In this category, most basalts of the ocean floor, most large oceanic islands, and continental flood basalts are found.

2) - High Alumina - This basalt may be silica-undersaturated or silica-oversaturated. It has higher than 17% alumina. It is intermediate in composition, being between Tholeiitic and Alkali basalt. The high relatively high alumina level is due to the lack of phenocrysts of plagioclase in the rock.

3) - Alkali - Alkali basalt is relatively rich in sodium while poor in silica. it is silica-undersaturated and may contain feldspathoids, alkali feldspar, and phlogopite.

4) - Bonenite - Bonenite is a high-magnesium form of basalt or andesite. It usually occurs in back-arc basin. It is distinguished by its low level of titanium and trace element composition.

Basalt can be used in Aggregate.


Gabbro


Gabbro is a dark, coarse-grained. It is chemically equivalent to basalt and is plutonic. Under a vast majority of the earth's surface is underlain by gabbro within the oceanic crust. Gabbro contains varied percentages pyroxene, olivine, plagioclase, and amphibole. When olivine is present in large quantities, this becomes olivine gabbro.

two types of pyroxene are contained in this rock - clinopyroxene, and small amounts of orthopyroxene. If the amount of orthopyroxene is substancially greater than the amount of clinopyroxene, the rock is then Norite.

Gabbro was named by Christian Leopold von Buch, a German geologist. It was named after a town in the Italian Tuscany Region.

Gabbro is used as ornamental facing stones, paving stones, graveyard headstones, and in kitchen countertops.


Diorite

Diorite is an extremely hard rock, making it difficult to carve and work with. It is so hard that ancient civilizations used diorite balls to work granite. Its hardness, however, also allows it to be worked finely and take a high polish, and to provide a durable finished work. major works in diorite tend to be important.

Diorite is a grey to dark grey intermediate intrusive, igneous rock composed principally of pagioclase, feldpar biotite, hornblen and pyexene. Varieties deficient in hornblende and other dark minerals are called leucodiorite. When oliviner and more iron-rich aquite are present, the rock grades into ferrodiorite, which is transitional to qabbro. The presence of significant quartz makes the rock type quartz-diorite (5% quartz) or tonalite (20% quartz), and if orthoclase is present at greater than ten percent the rock type grades into monzodiortie or granodiorite.

Diorites may be associated with either granite or gabbro intrusions, into which they may subtly merge. Diorite results from partial melting of a mafic rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain buldin such as in the andes mountains as large batholiths The extrusive volcanic equivalent rock type is andesite.