Just as any person can be put into one of two main categories of human being, all rocks can be put into one of three fundamentally different types of rocks. They are as follows: 1.Igneous rocks 1.Sedimentary rocks 2.Metamorphic rocks

Igneous rocks are crystalline solids which form directly from the cooling of magma. This is an exothermic process (it loses heat) and involves a phase change from the liquid to the solid state. The earth is made of igneous rock - at least at the surface where our planet is exposed to the coldness of space.

1.Intrusive igneous rocks are formed when magma hardens beneath Earth’s surface. 2. Extrusive igneous rocks are formed when lava hardens.

Igneous rocks are given names based upon two things: composition (what they are made of)and texture (how big the crystals are).

There are various criteria that could be used to classify igneous rocks. Among them are: a.Minerals Present in the Rock (the mode) b.Texture of the Rock c.Color d.Chemical Composition

-The minerals present in a rock and their relative proportions in the rock depend largely on the chemical composition of the magma.

This works well as a classification scheme if all of the minerals that could potentially crystallize from the magma have done so - usually the case for slowly cooled plutonic igneous rocks.

But, volcanic rocks usually have their crystallization interrupted by eruption and rapid cooling on the surface. -In such rocks, there is often glass or the minerals are too small to be readily identified. -Thus a system of classification based solely on the minerals present can only be used.

-Rock texture depends to a large extent on cooling history of the magma. - Thus rocks with the same chemical composition and same minerals present could have widely different textures. -In fact we generally use textural criteria to subdivide igneous rocks in to plutonic (usually medium to coarse grained) and volcanic (usually fine grained, glassy, or porphyritic.) varieties.

Color of a rock depends on the minerals present and on their grain size. Generally, rocks that contain lots of feldspar and quartz are light colored, and rocks that contain lots of pyroxenes, olivines, and amphiboles (ferromagnesium minerals) are dark colored.

But color can be misleading when applied to rocks of the same composition but different grain size. For example a granite consists of lots of quartz and feldspar and is generally light colored. But a rapidly cooled volcanic rock with the same composition as the granite could be entirely glassy and black colored (i.e. an obsidian). Still we can divide rocks in general into felsic rocks (those with lots of feldspar and quartz) and mafic rocks (those with lots of ferromagnesium minerals).

Chemical composition of igneous rocks is the most distinguishing feature. The composition usually reflects the composition of the magma, and thus provides information on the source of the rock.

The chemical composition of the magma determines the minerals that will crystallize and their proportions. A set of hypothetical minerals that could crystallize from a magma with the same chemical composition as the rock (called the Norm), can facilitate comparison between rocks.

Still, because chemical composition can vary continuously, there are few natural breaks to facilitate divisions between different rocks. Chemical composition cannot be easily determined in the field, making classification based on chemistry impractical.

In most places on the surface, the igneous rocks which make up the majority of the crust are covered by a thin veneer of loose sediment, and the rock which is made as layers of this debris get compacted and cemented together. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks.

a. Clastic: your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by compaction and cementation.

b. Chemical: many of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal "playa lakes" occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time.

c. Organic: any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock.

The metamorphics get their name from "meta" (change) and "morph" (form). Any rock can become a metamorphic rock. All that is required is for the rock to be moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions.

In most cases, this involves burial which leads to a rise in temperature and pressure. The metamorphic changes in the minerals always move in a direction designed to restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble.

Foliated Metamorphic Rocks Foliation: and planar fabric element Lineation: any linear fabric elements – They have no genetic connotations – Some high-strain rocks may be foliated, but they are treated separately

– Traditionally: the property of a rock to split along a regular set of sub-parallel, closely-spaced planes – A more general concept adopted by some geologists is to consider cleavage to be any type of foliation in which the aligned platy phyllosilicates are too fine grained to see individually with the unaided eye

– A preferred orientation of inequaint mineral grains or grain aggregates produced by metamorphic processes – Aligned minerals are coarse grained enough to see with the unaided eye – The orientation is generally planar, but linear orientations are not excluded

– Either a poorly-developed schistosity or segregated into layers by metamorphic processes Gneissose rocks are generally coarse grained

Simpler than for foliated rocks Again, this discussion and classification applies only to rocks that are not produced by high-strain metamorphism

a comprehensive term for any isotropic rock (a rock with no preferred orientation) Hornfels is a type of granofels that is typically very fine-grained and compact, and occurs in contact aureoles. Hornfelses are tough, and tend to splinter when broken.

Slate is a metamorphic rock that is created when shale is effected by heat and pressure. Slate has many uses but probably the most common one is for making the roof of a building. Slate can also be used for floors in houses because it is smooth and therefore easy to clean. It is also very hard so it will last a long time.

Marble is also a metamorphic rock. It is created from limestone. Marble can be polished and can be used as a material for buildings, statues, monuments and flooring.

Have you ever wondered what the world would be like without rocks? You may not know it, but if we took all the rocks away, we would have a miserable life. We use rocks for roads, metals, jewelry, tombstones, building materals, cosmetics, and many, many more things. Most of the famous sculptures of the world are made from blocks of stones such as granite and marble. Did you know that the Great Wall of China and the Taj Mahal are made out of rocks? We also collect rocks as a hobby. If we did not have rocks, this earth would be pretty empty. Who wants an empty earth? Not me! To better help you understand the importance of rocks, study the table below of some the most commonly used rocks and minerals.

{ The Importance of Rocks and Minerals Name Type of Rock Use BasaltIgneous Used in road building materials CalciteMineralUsed in cements and mortars and the production of lime GraniteIgneousUsed for buildings, monuments, and tombstones Marble Metamo rphic Used in building, floors, tile in bathrooms ObsidianIgneousUsed in making arrowheads and knives PumiceIgneousUsed in scouring, scrubbing, and polishing materials QuartzMineral Used in making glass, electrical components, and optical lenses Sandsto ne Sedimen tary Used in the building industry for houses Slate Metamo rphic Used for roofs, chalkboards, and patio walks

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