rocks

geography chapter 6

difference between rocks and minerals.

rocks	minerals
they are aggregates of minerals.	they are solid inorganic substances occurring naturally.
has no definite chemical composition.	have definite chemical compositions.
minerals are organised to form rocks.	they are pure substances (elements), cannot be divided any further.
chief types - igneous, metamorphic, sedimentary.	four chief mineral groups - silicates, carbonates, sulphides, metallic minerals.
examples: basalt, granite, sandstone, slate, quartz.	examples: iron, silicon, magnesium, nickel, calcium, potassium, sodium (abundant).

three types of rocks?

igneous, sedimentary, metamorphic

the word ‘igneous’ is derived from?

latin word ‘ignis’ meaning fire.

how were igneous rocks formed?

formed due to cooling, solidification and crystallisation of hot molten material of the earth known as magma found at great depths of the interior of the earth.

describe the chemical composition of igneous rocks.

the chemical composition of I rocks varies from alumino-silicate minerals (like quartz, feldspar) to granite and rhyolite - collectively known as sialic rocks.

which rocks were the first to form?

igneous rocks were the first to be formed - hence they are known as primary rocks. they form the basis of formation of other rocks.

give a few characteristics of igneous rocks.

• hard and compact.

• formed by solidification of magma.

• granular and crystalline.

• most of them consist of silicates.

• they do not have layers and are generally weathered by mechanical weathering.

• they are associated with volcanic activity, found mostly in volcanic regions.

two types of igneous rocks (based on origin)?

extrusive and intrusive.

how are extrusive igneous rocks formed?

• they are formed by cooling of the molten magma on the surface.

• the magma which is brought onto the surface through fissures or volcanic eruptions solidifies to form rocks.

• these rocks are smooth and fine-grained.

• also called volcanic rocks.

how are intrusive igneous rocks formed?

when the magma solidifies inside the earth’s crust it cools slowly, forming coarse textured, hard rocks with large crystals. eg. granite, gabro, dolomite.

different types of intrusive rocks are formed - batholiths, laccoliths, sills, dykes, necks.

what are batholiths?

• they are deep-seated intrusion of igneous rocks.

• usually dome-shaped with no definite floor, forming cores of mountain ranges.

• may be exposed to earth’s surface due to continued erosion.

• chiefly composed of granite.

what are laccoliths?

• magma does not reach the crust of the earth - becomes solid just below the crust.

• upper surface is dome-shaped and bottom is flat.

what are sills?

• when magma flows between layers of rocks horizontally and hardens there.

what are dykes?

the magma is forced upwards and fills vertical cracks or fissures in existing rocks, then hardens there to form dykes.

what are necks?

when the passage of an extinct volcano is filled with magma, it hardens there and is known as a volcanic neck or plug.

what are the two types of igneous rocks (based on chemical composition)?

acid igneous rocks and basic igneous rocks.

what are acid igneous rocks?

• silica content in these rocks - about 65-85%.

• lack in iron and magnesium.

• granite is an acid igenous rock - made of large crystals of quartz, feldspar and mica - commonly used as building material.

what are basic igneous rocks?

• they have higher percentage of oxides of denser elements.

• silica content varies from 40-60%.

• heavy and dark in colour.

• eg - basalt, gabro and dolerite.

how are sedimentary rocks formed?

• when igneous rocks are exposed on the earth’s surface, they are broken down by weathering and carried away by forces of gradation like rivers, glaciers and ocean waves.

• they are then deposited as sediments.

• any exposed rock, due to agents of erosion, may weather and disintegrate.

• the sediments are deposited over long periods of time. subsequent layers are added and due to their own weight from above, the sediments get cemented together to form sedimentary rocks.

give 5 characteristics of sedimentary rocks.

1. they are formed from materials derived from other rocks, may also have plant and animal remains. thus they may contain fossils.

2. they may be formed through different processes and are also classified based on their formation.

3. they are most widespread on the earth’s surface, constitute about 75% of the surface area of the globe.

4. they are generally not crystalline - they are soft and layered as they are formed by deposition of sediments.

5. they make extensive landforms - do not make massive rock forms like batholiths and laccoliths.

define lithification.

lithification is the process that turns loose sediments into rocks.

what are the different processes involved in lithification?

lithification means turning loose sediments into hard rock. transported sediments are deposited layer by layer, building up pressure and temperature. 3 processes involved:

1. evaporation: water from loose materials is evaporated (in the case of rock salt). accumulation of rock salt mainly takes place through evaporation before being compacted and cemented.

2. compaction: sediments after piling up are gradually squeezed by the weight of overlying layers and hardness of underlying layers. (like how sand is compacted into sandstone).

3. cementation: bonding together of the compacted sediments by natural materials like calcium compounds etc which have the properties of bonding on the loose materials.

what are the different types of sedimentary rocks on the basis of their formation?

mechanically formed, chemically formed, organically formed.

what are mechanically formed sedimentary rocks?

• contain fragments of breaking up of other rocks.

• these sediments are called clastic sediments.

• breaking up of older rocks —> through denudation by agents of erosion.

• in mechanically formed rocks, the sediments are slowly squeezed by the weight of overlying sediments and the lower layers harden to form rocks.

• such rocks are formed in layers, hence called stratified rocks.

• eg of mechanically formed rocks - shale, sandstone, clay, loess etc.

what are chemically formed sedimentary rocks?

• they are formed by the direct precipitation of mineral matter from solution.

• accumulation takes place in lakes and lagoons.

• compacted through evaporation.

• eg: gypsum - formed by evaporation of sea water, found in sedimentary rocks all over the world. rock salt - found in dead sea, aral sea and in sambhar lake in rajasthan.

what are organically formed sedimentary rocks?

• contain remains of dead plants and animals.

• limestone - formed by skeletons, shells and animal remains.

• rocks like peat, lignite, bituminous and anthracite - carbonaceous rocks.

• rocks formed due to dominance of silica content - siliceous rocks.

• chalk - form of carbonate rock, formed due to precipitation of carbonate minerals.

what are the types of sedimentary rocks based on agents formation?

riverine, lacustrine, glacial, aeolian, marine.

what are riverine rocks?

formed by alluvial deposits brought by flowing water of streams.

what are lacustrine rocks?

found on the bed of a lake corresponding to successive periods of deposition.

what are glacial rocks?

• formed by glacial deposits in form of debris or tills.

• glaciers erode the surface and sides of a valley and transport the eroded matter further.

• glacier melts due to heat —> debris brought by it is left behind in form of moraines, which form glacial rocks.

• glacial rocks include boulders, gravels, sand etc.

what are aeolian rocks?

• formed with sand particles brought by winds.

• deposition of sand particles, one over the other, make them hard and form sedimentary aeolian rocks.

• found in deserts.

• loess - important aeolian deposit.

what are marine rocks?

formed at the ocean floor.

1. calcarious sedimentary marine rocks: formed by deposition of shells and skeletons of sea organisms. live on ocean floor and extract calcium carbonate from ocean water. eg - chalk and limestone.

2. carbonaceous sedimentary rocks: formed by sea plants which remain buried for very long. long preserved remains - fossils. they are converted into coal, lignite and peat due to pressure of overlying rocks on them

how are metamorphic rocks formed?

• word ‘metamorphic’ is derived from ‘metamorphose’ meaning change in form.

• metamorphism is the alteration of the composition or structure of a rock by heat, pressure or other natural agency.

• rocks which were once igneous or sedimentary underwent change through physical and chemical processes.

• temperature and pressure take part in changing the form of rocks.

give 5 characteristics of metamorphic rocks.

1. harder and more compact than their original form.

2. most are impermeable (do not allow water to pass through them).

3. do not have fossils.

4. formed due to change in texture and composition of the pre-existing rocks.

5. new minerals are formed during metamorphism.

explain thermal metamorphism.

• occurs when the transformation of the original rock takes place due to the influence of high temperature.

• heat may be from hot magma or from friction of moving rock layers.

explain dynamic metamorphism.

• occurs when the transformation takes place because of pressure at a great depth within the earth’s crust.

• chemically active hot gases while passing through the rocks change their chemical composition.

• under conditions of this change, minerals get arranged in a series of bands called foilation.

what is regional metamorphism?

• due to mountain building, igneous and sedimentary rocks are buried deep inside the crust.

• pressure of overlying rocks and intense heat caused by large-scale earth movements change such rocks

• on large scale - regional metamorphism, on small scale - local metamorphism.

what is the rock cycle?

• the continuous process of transformation of old rocks into new ones is called rock cycle.