Rock is defined as a naturally occurring solid aggregate of one or more minerals or mineraloids with variable chemical composition and external form.
For example, the common rock granite is a combination of the quartz, feldspar and biotite minerals. The Earth's outer solid layer, the lithosphere, is made of rocks.
BASALT
GRANITE
Rocks have been used by mankind throughout history. From the Stone Age, rocks have been used for tools. The minerals and metals found in rocks have been essential to human civilization
Three major groups of rocks are defined:
•Igneous
•Sedimentary
•Metamorphic
The scientific study of rocks is called petrology, which is an
essential component of geology.
I. Igneous
• Igneous rock (derived from the Latin word igneus meaning of fire, from ignis meaning fire) forms through the cooling and solidification of magma or lava. This magma can be derived from partial melts of pre-existing rocks in either a planet's mantle or crust.
•Typically, the melting of rocks is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition.
•About 64.7% of the Earth's crust by volume consists of igneous rocks; making it the most plentiful category. Of these, 66% are basalts and gabbros, 16% are granite, and 17% granodiorites and diorites.
Igneous rocks are formed by the cooling and hardening of hot molten rock (Magma) from inside the earth.
Igneous rocks, on the basis of the depth of formation, are classified into
(1) Plutonic - When the magma solidifies at great depth, about 2 to 3 mile deep under the surface of the earth, the igneous rock formed is called plutonic e.g. granite, syenite, gabbro.
(2) Intrusive or dyke - When the magma solidifies at moderate depth is called Intrusive or dyke e.g. pegmatite, dolerite.
(3) Extrusive or effusive When the solidification takes place on the surface of the earth as a result
of volcanic activities, the igneous rock formed is called Extrusive or effusive e.g. Rhyolite, pumis, basalt.
On the basis of percentage silica content, igneous rocks are divided into
(a) Acidic : SiO2 content is more than 65% e.g. granite, pegmatite, rhyolite
(b) Intermediate : SiO2 content is between 55 and 65% (subacidic SiO2 60% to 65%, e.g. syenite; sub-basic, SiO2 55 to 60 % e.g., diorite)
(c) Basic : SiO2 content is between 45 and 55 % e.g. basalt
(d) Ultra basic : SiO2 content is less than 45% e.g. picrite
TEXTURES OF IGNEOUS ROCK
A rock’s texture can be classified as glassy, fine-grained or coarse-grained. Glassy- Obsidian Fine grained-Aplite, Tuff Coarse grained- Diorite, Gabbro and granite Texture is affected by:
• Crystal size (which depends on how fast the magma hardens)
• Amount of gas dissolved in the magma (crystals grow faster because it helps ions move around). Magma trapped deep within the crust hardens slowly; magma that reaches the surface as lava hardens very quickly
MAGMA
There are two main kinds of magma which differ in chemical composition
FELSIC/Silicic: high silica content and Fe, Mg, thick and slow-flowing, and forms light coloured rocks (eg. Granite, rhyolite)
MAFIC: lower silica content and high Ca, Mg, hotter, thinner and more fluid, and forms darker coloured rocks (eg. Basalt, gabbro)
II. Sedimentary rocks (Latin sedimentas, means settling)
As soon as igneous rocks are exposed to weathering processes, they start breaking down physically and chemically into soluble and insoluble products. These products when transported by water or glaciers form new deposits which in time become cemented and solidified into new forms of rocks called the sedimentary rocks. Stratification is the most common feature of these rocks, so they are also termed as stratified rocks. e.g. lime stone, sand stone, silt stone, shale and conglomerate. The sedimentary rocks form only 5% of the earth crust
Classification of sedimentary rocks
1. Rudaceous : If the individual grains forming rocks are of the size of boulders and pebbles, called rudaceous. e.g. Grit, Conglomerate, Breccia.
2. Arenaceous : These rocks have individual grains of sand size e.g. sand stone
3. Argillaceous : These rocks have individual grain size of clay in their formation. These may either loose or consolidated rocks various clays are loose sediments. e.g. Kaoline
Sedimentary rocks
•are formed at the earth's surface by the accumulation and cementation of fragments of earlier rocks, minerals, and organisms or as chemical precipitates and organic growths in water (sedimentation).
•This process causes clastic sediments (pieces of rock) or organic particles (detritus) to settle and accumulate, or for minerals to chemically precipitate (evaporite) from a solution.
•The particulate matter then undergoes compaction and cementation during at moderate temperatures and pressures (diagenesis).
•Before being deposited, sediments are formed by weathering or earlier rocks by erosion in a source area, and hen transported to the place of deposition by water, wind, ice, mass movement or glaciers (agents of denudation).
•Sedimentary rocks form under the influence of gravity and typically are deposited in horizontal or near horizontal layers or strata and may be referred to as stratified rocks.
Sedimentary sandstone with iron oxide bands
SEDIMENTARY ROCKS
Sedimentary rocks form when sediments harden into rocks
Three Types:
1. Clastic: formed from fragments of other rocks e.g. conglomerate, sandstone, shales
2. Chemical: formed from mineral grains that come out of solution e.g. halite, selenite, limestone
3. Organic: formed from the remains of plants/animals e.g. coal
- Clastic Rocks
• Fragments come from weathering of existing rocks
• Picked up by wind, waves, glaciers and running water
• Sediments are deposited where water slows down
• Natural cements (silica, calcite, iron oxide) stick the sediments together into firm, cemented rock
Sediment Sorting
Sorting of sediments produce separation of the particle sizes
• Pebbles/gravels are dropped first (form conglomerate)
• Sands (form sandstone)
• Silts, clays are dropped last (form shale)
Chemical Origin
• Water contains dissolved minerals, which can fall out of solution (precipitate) due to evaporation or chemical action
• Limestone can be formed from tiny grains of calcite deposited from sea or lake waters
• Other examples are rock salt (halite) and rock gypsum
III. Metamorphic rocks
Metamorphic rocks are formed from igneous and sedimentary rocks through action of heat, pressure and chemically active liquids and gases. Metamorphism may result in changes mainly physical, chemical or both. Heat, pressure and water are called agents of metamorphism. Thus, when igneous and sedimentary rocks are subjected to tremendous pressure and high temperature, metamorphism takes place and metamorphic rocks are formed. e.g.
• (1) Sand stone Heat & Pressure Quartzite
• (2) Shale Pressure Slate
• (3) Lime stone Heat Marble
• (4) Granite Pressure Gneiss
• (5) Basalt Pressure Schist
Metamorphic rocks
1. are formed by subjecting any rock type— sedimentary rock, igneous rock or another older
metamorphic rock—to different temperature and pressure conditions than those in which the
original rock was formed.
2. This process is called metamorphism; meaning to "change in form".
3. The result is a major change in physical properties and chemistry of the stone.
4. The original rock, known as the protolith,transforms into other mineral types or otherforms of the same minerals, byrecrystallization.
5. The temperatures and pressures required forthis process are always higher than thosefound at the Earth's surface: temperaturesgreater than 150 to 200 °C and pressures of1500 bars.
6. Metamorphic rocks compose 27.4% of thecrust by volume.
Types of Metamorphism
Regional Metamorphism: large areas of rockare under intense heat/pressure, which occursduring mountain-building
Heat comes from friction of rocks, andpressure from overlying weight and thesqueezing of moving rocks
Contact Metamorphism: when hot magmaforces itself into overlying rock (intrusions)and bakes the rock
Depending on the structure, metamorphic rocks are divided into two general categories.
1. foliated
A gneiss has visible bands of differing lightness, with a
common example being the granite gneiss. Other varieties
of foliated rock include slates, phyllites, and mylonite.
Schists are foliated rocks that are primarily composed of
lamellar minerals such as micas
2. non-foliated:
Familiar examples of non-foliated metamorphic rocks include
marble, soapstone, and serpentine. This branch contains
quartzite—a metamorphosed form of sandstone—and
hornfels
Gneiss-------------Schist
Granite---------------Gneiss
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