Saturday, November 14, 2009


Any aggregate of minerals particles that forms part of earth’s crust is called a rock.

Rocks are ‘divided’ into three principal categories: IGNEOUS, SEDIMENTARY, and METAMORPHIC.

1 .Igneous rock: igneous rock is formed by magma (molten rock) being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

A) Intrusive igneous rocks: Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are coarse grained. These may be either a) plutonic which cool deep within the crust nd have large crystals e.g. granite b) intrusive igneous rocks, which form near the surface, are termed hypabyssal.

Igneous rocks are also classified based on chemical composition.

For e.g. classification based on silica content a)acid rocks (over 66%silica )b) Intermediate rocks (over55- 66%silica) c)basic rocks (over45-55%silica) basalt }d) Ultra basic rocks (less than 45%silica

B) Extrusive igneous rocks these are rocks formed out onto the surface of the earth as magma before cooling and are generally glassy or fine- crystalled. These are also called volcanic rocks or lava e.g. lava

2. SEDIMENTARY ROCKS: Sedimentary rocks are the second great rock class. Whereas igneous rocks are born hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata; hence, they are also called stratified rocks. Depending on what they are made of, sedimentary rocks fall into one of three types.

A) Mechanically formed (Clastic Sedimentary Rocks):The most common set of sedimentary rocks consist of the granular materials that occur in sediment: mud, sand, gravel, and clay. Sediment mostly consists of surface minerals — quartz and clays — that are made by the physical breakdown and chemical alteration of rocks. (Feldspar and other minerals may also be in sediment if they have not had time to break down.) These are carried away by water or wind and laid down in a different place. Sediment may also include pieces of stones and shells and other objects, not just grains of pure minerals. Geologists use the word clasts to denote particles of all these kinds, and rocks made of clasts are called clastic rocks.

B) Organic Sedimentary Rocks: Another type of sediment actually forms in the sea as microscopic organisms — plankton — builds shells out of dissolved calcium carbonate or silica. Dead plankton steadily showers their dust-sized shells onto the seafloor, where they accumulate in thick layers. That material turns to two more rock types, limestone (carbonate) and chert (silica). These are called organic sedimentary rocks, although they are not made of organic material as a chemist would define it.

C) Chemical Sedimentary Rocks: These same ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate), starting with calcite, then gypsum, then halite. The resulting rocks are certain limestone or dolomites, gypsum rock, and rock salt respectively. These rocks, called the evaporate sequence, are also part of the sedimentary clan.

3.Metamorphic rocks

Metamorphic rocks are the third great class of rocks. These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic ("shape-changed") rocks. Metamorphism acts at two scales, the regional scale and the local scale


A) Contact or Local Metamorphism

A type of metamorphism that is important in specific localities is contact metamorphism. This usually occurs near igneous intrusions, where hot magma forces itself into sedimentary strata. The rocks next to the invading magma are baked into hornfels or its coarse-grained cousin granofels, another subject for specialists. Magma can rip chunks of country rock off the channel wall and turn them into exotic minerals, too

B) Dynamic metamorphism (regional) takes place when rock layers undergo strong structural deformation during mountain building.

C) Dislocation metamorphism occurs when pre-existing rocks undergo localized deformation along a fault plane or thrust plane. The mineral composition and the structure of the rock can be altered;


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