Minerals, Rocks and Mountains

Start music   Jan 3 2017 || 8:46 AM

MINERALS AND ROCKS

 

  • The earth is composed of various kinds of elements. These elements are in solid form in the outer layer of the earth and in hot and molten form in the interior.
  • About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium, and the rest is constituted by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements.
  • The elements in the earth’s crust are rarely found exclusively but are usually combined with other elements to make various substances.

 

  • A mineral is a naturally occurring inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.
  • A mineral is composed of two or more elements.
  • But, sometimes single element minerals like sulphur, copper, silver, gold, graphite etc. are found.

 

Source:

  1. The basic source of all minerals is the hot magma in the interior of the earth.
  2. Minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms.

 

PHYSICAL CHARACTERISTICS:

    1. External crystal form — determined by internal arrangement of the molecules
    2. Cleavage — tendency to break in given directions producing relatively plane surfaces, result of internal arrangement of the molecules
    3. Fracture — internal molecular arrangement so complex there are no planes of molecules.
    4. Lustre — appearance of a material without regard to colour.
    5. Colour —Minerals have characteristic by
  • colour determined by their molecular structure
  • coloured by impurities.
    1. Streak — colour of the ground powder of any mineral.
    2. Transparency — transparent; translucent; opaque.
    3. Structure — particular arrangement of the individual crystals.
    4. Hardness — relative resistance being scratched.

 

Ten minerals are selected to measure the degree of hardness from 1-10. They are:

1. Talc; 2. Gypsum; 3. Calcite; 4. Fluorite; 5. Apatite; 6. Feldspar; 7. Quartz; 8. Topaz; 9. Corundum; 10. Diamond.

 

    1. Specific gravity — the ratio between the weight of a given object and the weight of an equal volume of water.

 

 

SOME MAJOR MINERALS AND THEIR CHARACTERISTICS:

 

Name of the mineral

Common elements

Composition

Colour

Uses

Feldspar

 

Silicon and oxygen

sodium, potassium,

calcium, aluminium etc. (are found in specific

feldspar variety)

Half of the earth’s crust

Salmon Pink

ceramics and

glass making.

Quartz

 

Sand, Silica and Granite

 

Virtually insoluble in water

 

white or colourless

Radio and Radar

Pyroxene

 

Calcium, Aluminum,

Magnesium, Iron and Silica

 

Found in meteorites

 

10 per cent of the earth’s crust

Green or Black

 

Amphibole

Or Hornblende

Aluminum, Calcium, Silica, Iron, Magnesium

7 per cent of the earth’s crust

Green or Black Colour

Asbestos

Industry

Mica

 

Potassium, Aluminium,

Magnesium, Iron, Silica etc

 

Found in igneous and metamorphic rocks

4 per cent

of the earth’s crust

 

Electrical Instruments

Olivine

Magnesium, iron and silica

 

Found in basaltic rocks

 

Greenish Crystal

Jewellery

 

 

Metallic Minerals

These minerals contain metal content and can be sub-divided into three types:

    1. Precious metals : gold, silver, platinum etc.
    2. Ferrous metals : iron and other metals often mixed with iron to form various kinds of steel.
    3. Non-ferrous metals : include metals like copper, lead, zinc, tin, aluminium etc.

 

Non-Metallic Minerals

  1. These minerals do not contain metal content.
  2. Sulphur, phosphates and nitrates are examples of non-metallic minerals.
  3. Cement is a mixture of non-metallic minerals.

 

 

ROCKS

  • The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals.
  • Rock may be hard or soft and in varied colours. For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white.
  • Rocks do not have definite composition of mineral constituents.
  • Feldspar and quartz are the most common minerals found in rocks.

 

  1. Petrology is science of rocks.
  2. A petrologist studies rocks in all their aspects viz., mineral composition, texture, structure, origin, occurrence, alteration and relationship with other rocks.

 

There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation.

They are:

    1. Igneous Rocks — solidified from magma and lava
    2. Sedimentary Rocks — the result of deposition of fragments of rocks by exogenous processes
    3. Metamorphic Rocks — formed out of existing rocks undergoing recrystallisation

 

Igneous Rocks:

 

  1. As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rocks.
  2. The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools and solidifies.
  3. The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
  1. Plutonic rocks – Cools slowly forms rocks such as granite, diorite, gabbro which are exposed at surface by the process of denudation and erosion.
  2. Volcanic rocks – Solidify rapidly on the earth surface.
  1. Most of the igneous rocks are extremely hard and resistant, for this they are quarried and used for road making, polished monuments and Grave stones.

 

Sedimentary Rocks:

 

  1. Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments.
  2. Such fragments are transported by different exogenous agencies and deposited.
  3. These deposits through compaction turn into rocks. This process is called lithification. In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification.
  4. Hence, we see a number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.
  5. Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
      1. Mechanically formed — sandstone, conglomerate, limestone, shale, loess etc. are examples;
      2. Organically formed— geyserite, chalk, limestone, coal etc. are some examples;
      3. Chemically formed — chert, limestone, halite, potash etc. are some examples.

 

Metamorphic Rocks

  1. Metamorphic means ‘change of form’.
  2. These rocks form under the action of pressure, volume and temperature (PVT) changes. Metamorphism occurs when
  1. Rocks are forced down to lower levels by tectonic processes
  2. When molten magma rising through the crust comes in contact with the crustal rocks
  3. The underlying rocks are subjected to great amounts of pressure by overlying rocks.
  1. Mechanical disruption and reorganization of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.
  2. The materials of rocks chemically alter and recrystallise due to thermal metamorphism. There are two types of thermal metamorphism
  1. Contact metamorphism - the rocks come in contact with hot intruding magma and lava and the rock materials recrystallise under high temperatures.
  2. Regional metamorphism - rocks undergo recrystallisation due to deformation caused by tectonic shearing together with high temperature or pressure or both.
  1. In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.
  2. Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades. Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks.
  3. Types of metamorphic rocks depend upon original rocks that were subjected to metamorphism.

 

Before metamorphism

 

After metamorphism

  1. Clay
  2. Limestone
  3. Sandstone
  4. Granite
  5. Shale
  6. Coal

Slate

Marble

Quartzite

Gneiss

Schist

Graphite

 

 

Types of Mountains:

 

  1. Fold Mountains:
  • Most widespread and important mountain system.
  • Caused by large scale earth movements, when stresses are set up on earth’s crust.
  • Stresses can be due to increased loading of overlying rocks, flow movements in the mantle, magmatic intrusion into the curst, expansion or contraction of some part of the earth.
  • Stress cause folding along lines of weakness.
  • Create series of waves, upfolded waves – anticlines, downfold wave – synclines.
  • Examples – Himalayas, Rockies, Andes and Alps

 

  1. Block Mountains:
  • Faulting caused by compression or tension.
  • Forces which lengthen or shorten the earth’s crust, causing a section to subside or to rise above the surrounding level.
  • Example – The Hunsruck Mountains, The Vosges and Black Forest of Rhineland

 

  1. Volcanic Mountains:
  • They are built up from material ejected from fissures in the earth’s crust.
  • Materials are molten lava, volcanic bombs, cinders, ashes, dust and liquid mud.
  • They are also called as mountains of accumulation.
  • Common in circum-Pacific belt (Ring of Fire) includes – Mount Fuji(Japan), Mount Mayon(Philippines), Mount Merapi(Sumatra), Mount Agung(Bali).

 

  1. Residual Mountains:
  • Evolved by denudation.
  • General level of land is lowered by agents of denudation.
  • Example – Mount Manodnock (U.S.A).

 

 

Types of Plateau:

 

They are elevated uplands with extensive level surface, and descend steeply in the surrounding lowland. They are also called as tablelands.

 

  1. Tectonic Plateau - Formed by earth movements cause lift; ex – Deccan plateau
  2. Volcanic Plateau – Molten lava spread over surface to form successive sheets of basaltic lava; ex – Antrim Plateau of Northern Ireland
  3. Dissected Plateau – Formed due to weathering and erosion by water, ice and winds; ex – Scottish Highland

 

 

Types of Plains:

 

A plain is an area of lowland, either level or undulating.

They form the best lands in the country used for

  • Cultivation
  • Settlement

Example: Indo-Gangetic Plains, Mississippi Plain etc,

 

  1. Structural Plains - They are structurally depressed areas of the world; ex – Great Plains of USA.
  2. Depositional Plains – Formed due to deposition of material brought by the various agents of transportation; ex – Nile Delta of Egypt.
  • Rivers form – Alluvial Plains, Flood Plains, Deltaic Plains
  • Glaciers form – Outwash Plains, Tilt Plain or Drift Plains
  • Waves and Winds form – Coastal Plains
  • Winds form – Aeolian deposits, loess plains
  1. Erosional Plains – caused by agents of erosion; Such plains are called as peneplains