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Geological classification of Rocks

Rocks

Igneous rocks

Sedimentary rocks

Metamorphic rocks

Igneous rocks

Intrusive Rocks

Plutonicrocks

Hypabyssalrocks

Extrusive Rocks

Igneous Rocks* Formed by solidification of magma.

Extrusive Rocks : ( Above the earth’s surface)

• Lava from volcanic eruptions on cooling and solidification forms extrusive rocks or volcanic rocks whose texture is glassy.

• Due to rapid crystallization, cavities are formed in rocks which are called vesicles.

Intrusive Rocks : (Beneath the earth’s surface)

1. Plutonic rocks- magma crystallizes inside the earth’s crust which cools slowly and minerals of these rocks are coarse grained structure.

2. Hypabyssal rocks- magma solidifies close to earth’s crust. They are finer than plutonic rocks and coarser than volcanic rocks and generally show porphyritic structure.

Chemical composition :~ SiO2 40-75%, Al2O3 10-20%, others like Ca, Mg and Fe 10%.Texture of Igneous rocks:1. Crystallinity

a) Holocrystalline: It is the rock which is fully composed mostly of crystals.b) Holohyalite: It is the rock which is fully composed of glazzy matter.c) Hemicrystalline: It is the rock which is partly of crystals and partly of glazzy matter.

b)

c)

2. Granularity

Mineral grain size is dependent on cooling and viscosity of magma. Grain sizes are bigger in plutonic rocks than in volcanic rocks.

3. Shape

Rock depends on the shape as well as relative size and arrangement of minerals. They are classified into three types namely :

a)

a) Euhedral: minerals are developed equally in size and shape and are completely bonded with faces.

b) Subhedral: minerals are developed partially or to an intermediate state.

c) Anhedral: minerals develop irregular shape because their growth is controlled by neighbouring materials.

a)

b)c)

Based on acid

magma

Based on free silica

Oversaturated rocks

Saturated Rocks

Undersaturated Rocks

Ultrabasicrocks

Basic rocks

Intermediate rocks

Classification of Igneous rocks

Examples of Igneous rocks

Andesite Basalt Gabbro

Granite Obsidian Pegmatite

Peridotite Pumice Quartz-Dolerite

Rhyolite Scoria Welded-Tuff

~Anna University Questions:-

1. Write about the mineral composition, origin and properties and uses of Basalt.

2. What is dolerite? Write its uses.3. How are rocks classified? Give the main characteristics and geological classification of rocks.4. Write short notes on Granite and few textures of Igneous rocks.

Engineering Geology is a subject whose knowledge and ideas are needed to a Civil Engineer before starting any project. E.g. if a dam construction project is to be started then a Civil Engineer should know about the type of rock in which the basement of dam is located whether it's Igneous rock or Sedimentary or Metamorphic. Is there any faults, folds or any discontinuities that a Civil Engineer should be aware of.

# Why a civil engineer should know about the types of rocks and its importance ???

Sedimentary Rocks Pre-existing rocks form sediments through the process of

erosion, transportation and deposition by various natural agencies like wind, water, glacier, etc. These sediments undergo compaction and consolidation resulting in sedimentary rocks.

(1)

•Mechanically formed rocks

(2)

•Chemically formed rocks

(3)

•Organically formed rocks

Sedimentary rocks are grouped into three classes based on the mode of formation, viz.,

Clastic Sedimentary rocks

EROSION: Hard rocks erode, decompose and decay into smaller grains and particles by natural agencies like atmosphere, water and ice. This disintegrated and loosened material accumulates near the source which is called detrius and are known as detrial rocks.

TRANSPORTATION: The eroded material is transported for deposition by agents like by wind, running water and ice. Wind transports sediments from deserts, ploughed fields to forest, etc in loads. Streams and rivers transport the load as bed-load, suspended load and dissolved load which are dumped at the settling basins. Ice in the form of glaciers breaks the rocks along their bases and sides and deposit millions of land mass into seas and oceans.

DEPOSITION: Based on the deposition under continental

environments they may be the glacial deposits, the fluvial deposits, the glacio-fluvial deposits and the eolian deposits and marine deposits in the marine environment.

DIAGENESIS: The sediments deposited in the settling basins gradually get converted to cohesive, hard and massive rock formation through the process of compaction, consolidation and cementation which is collectively known as diagenesis. Cementation is the process by which loose grains of the sediments in the settlement basin get hold together by a binding material.

Non-clastic sedimentary rocks

CHEMICALLY FORMED ROCKS: Water is the best solvent which from rains, springs, streams, rivers, lakes and underground water bodies dissolve many compounds from rocks. In most of the cases these dissolved salts are carried by the running water to sea, the ultimate destination. At some stage, the dissolved salts get crystallized out either through evaporation or through precipitation. Chemically formed rocks may be precipitated one and evaporated one, e.g., limestone and rock salt respectively.

ORGANICALLY FORMED ROCKS: The extensive water bodies viz., oceans and seas, sustain a great variety of animal and plant life. Many sea organisms are formed mainly of calcium and/or magnesium carbonates. Death and decay of these organisms within the water bodies gradually results into huge accumulations of carbonate materials. These materials get compacted and consolidated in the same manner as other sediments. E.g., limestone.

TEXTURE OF SEDIMENTARY ROCKS

1. Origin of grainsA sedimentary rock may be partly or wholly composed of clastic or non-clastic grains.

2. Size of grainsCoarse grained rocks : average grain size > 5mmMedium-grained rocks: average grain size between 5 and 1 mm.Fine grained rocks : average grain size < 1mm.3. Shape of grains

The shapes may be rounded, surrounded, angular and subangular. The causes for sphericity are due to various degree of transport and abrasion.4. Packing of grains

Sedimentary rocks may be porous or dense depending on the degree of packing, which is related to the load of the overlying sediments during the process of deposition.5. Fabric of GrainsThe orientation of the particles are defined with reference to the orientation of their longer axes. The rock is said to be of preferred orientation when most of the elongated particles are arranged in the same direction.

6. Crystallisation TrendGenerally, the texture of sedimentary rocks of chemical origin is defined by the degree of crystallized grains. Rocks with interlocking grains are called crystalline granular texture or they may be of non-crystalline colloidal particles when they are termed as amorphous.

CLASSIFICATION OF SEDIMENTARY ROCKS Based on the size and origin of sediments they are classified as namely,1. Rudaceous DepositsRocks consisting mainly of gravels, pebbles, cobbles or boulders and properly cemented, e.g., conglomerate and breccia.

2. Arnaceous DepositsRocks containing mainly sandy material with quartz and other minerals like apatite, garnet, zircon, tourmaline and magnetite, e.g., sandstones.

3. Argilaceous DepositsClay and shale are used for making brick and tiles.4. Carbonaceous Deposits.Formed by the accumulation of organic materials consist of plant debris e.g., coal, peat, lignite, etc.5. Calcareous Deposits.Calcareous deposits are limestone and dolomite.

STRUCTURE OF SEDIMENTARY ROCKS

Stratification~ Layered arrangement in the form of beds or strata that may be similar or dissimilar in colour, composition, grain size and texture is called stratification. Further these layers may be horizontal or slightly inclined or steeply inclined or broken.

Lamination~ Similar to stratification but the individual layers are quite thin. It is the characteristic structure of fine-grained sedimentary rocks like clays and shales.Cross Bedding~ Here, the layers are lying one above another and not parallel but irregular or inclined. Such structure often results from deposition having taken place in a shallow-water environment and are referred to as false bedding.

Bed ABed B

Bed CBed D

A

BC D

Stratification, beds and bedding

Laminae

Cross bedding a) tabular b) lenticular c) wedge shaped

a) b)c)

Lamination

Examples of sedimentary rocks

Breccia Chert Coal-anthracite

Conglomerate Dolomite Flint-nodule

Halite Hematite Limestone

Oil shale Shale Siltstone

~Anna University Questions:-

1. What are the texture of sedimentary rocks?2. What are the rocks used as building

materials? Give an account of the properties of rocks as building materials.

3. Write short notes on the following rocks i) Limestone ii) Sandstone and iii) Shale.

# Why a civil engineer should know about the sedimentary rocks and its importance ??? The study of sedimentary rocks and rock strata provides information about the subsurface that is useful for civil engineering, for example in the construction of roads, houses, tunnels, canals or other constructions. 

Meta = changeMorphic = form/shape

~ Formed under tremendous heat, great pressure, and chemical reactions .~ Can make new minerals .~ Most are found on or near plate boundaries.~ Metamorphism is changing one type of rock into another as a result of heat(from mantle), pressure (from layers above) and/or chemical reactions.

Garnet crystalMica garnet schist

Rocks that have been changed from an original “parent” rock are known as

metamorphic rocks.

The heat comes from: 1. Mantle.2. Pressure of the layers.3. Movement of the Plates on

Earth.

Agents of metamorphism

1. HeatAt greater depths in earth the temperature is very high. In the other parts of the earth, the common cause for the elevated temperature is the intrusion of hot igneous bodies.2. Uniform pressureBecause of deep burial, static pressure occurs. This pressure is due to the weight of the overlying rocks. This static pressure is usually associated with high temperatures.3. Directed PressureDirect pressure is caused during folding movement of rocks.

Directed pressure has influence only near the earth’s surface. As the depth increases, the effect of the directed pressure decreases and that of uniform pressure increases.4. Chemically Active Fluids and GasesChemically active fluids and gases bring about the changes in their original composition when they pass through the pores of rocks. The source of these chemical agents is generally the intrusive igneous body within the country rocks.

PROCESS OF METAMORPHISM1. GranulationBecause of high pressure and high friction, rocks are shattered and melted. The process of crushing of rocks takes place without loss of coherence is called granulation.2. Plastic DeformationThe shape of solid changes when it is subjected to stress. On the removal of stress the solid should regain its shape, otherwise it would have deformed permanently which is called plastic deformation.3. RecrystallizationFormation of new minerals or crystals from the pre-existing minerals is called recrystallization which causes mineralogical and texture changes.4. MetasomatismIt is the process by which original composition of rocks are changed primarily by the addition or removal of material due to the movement of hydrothermal fluids through rocks under high temperature and pressure.

TYPES OF METAMORPHISM:1. Cataclastic metamorphismCaused due to direct pressure or lateral pressure produced due to earth movements such as folding and faulting. As a result, rocks are crushed, ground and deformed.

2. Dynamic metamorphismIt is associated with low temperature and the rocks are formed partly by mechanical effects and partly by growth of new minerals e.g., slates.3. Contact metamorphismCaused due to local heating of rocks by the intrusion of heat igneous rock bodies and is also known as thermal metamorphism. It’s general effect is to promote recrystallization.4. Plutonic metamorphismCaused due to high static pressure and high temperature at great depths in earth. These rocks have an even grained texture and denser minerals.5. Regional metamorphismWhen directed pressure and heat act together in presence of migrating hydrothermal fluids at great depths, the rocks are metamorphosed over wide areas. It is also known as dynamothermal metamorphism.

6. Metasomatism It is a type of contact metamorphism. The composition of parent rock remains changed substantially but its volume remains unchanged. It is the process of introducing other elements like circulating fluids derived from igneous magma during recrystallisation.7. Retrogressive metamorphismWhen high temperature metamorphic mineral assemblages are changed to a low temperature mineral assemblages, the process is called the retrogressive metamorphism. Such changes take place when an intensely metamorphosed rock is subjected either to strong differential movement or hydrothermal activity.TEXTURE OF METAMORPHIC ROCKS1. Crystalloblastic TexturesThese textures include all those textures that have been newly imposed upon the rock during the process of metamorphism and are, therefore, essentially the product of metamorphism.2. Palimpset TexturesThese textures are similar in essential details as in the parent rock with little or no modifications taking place during metamorphism.

Examples of Metamorphic rocks

AMPHIBOLITE GNEISS HORNFELS

MARBLE MUSCOVITE-SCHIST PHYLLITE

QUARTZITE QUARTZ-NOVACULITE

SLATE TALC-SOAP STONE

~Anna University Questions:

1.Write short notes on mineral composition, texture, origin, engineering properties and uses of i) marble ii) quartzite iii) schist iv) slates and v) gneiss. # Why a civil engineer should know about the metamorphic rocks and its importance ???Geotechnical engineers belong to a branch of civil engineering; these engineers study the rocks and minerals in the earth's crust. They understand the rock cycle and the effects that a change in environment, including heat and pressure, might have on different rocks and soils. They use their knowledge to create technologies to help predict natural hazards, such as landslides and earthquakes, as well as assess the impacts and risks to humans associated with the development of housing and industrial areas.

THE END

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Prepared by,D. DEEPAK KUMAR.