Rocks and
Rocks and Mountains
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Rocks and
The earth's crust is formed of mineral materials called rocks. The rocks which form the substructure of our lithosphere are grouped into three broad categories:

  1. Igneous Rocks
  2. Sedimentary Rocks
  3. Metamorphic Rocks
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Igneous Rocks
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Igneous rocks are formed by the solidification of molten magma from the interior of the earth. About 95% of the earth's crust is made of this type of rock.
In fact, all other types of rocks originate from these rocks and. therefore, they are also called primary rocks. Igneous rocks are of three types.
Examples of lgneous Rocks:
(1) Granite: These rocks are the major continental rocks.
(2) Basalt:These rocks are found on ocean beds.
(3) Volcanic:These rocks are formed by the solidification of molten lava ejected by the volcanoes.
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Igneous rock bodies:
The Igneous rocks bodies are the igneous bodies which are made by the solidification of lava in the interior of the earth.
They are found nearly in every fold mountain system. They differ in names which are given to them on the basis of shape, size, position and the surrounding rocks.
The Batholith, Laccolith, Stock, Sill and Dyke etc..are the examples of Igneous rock bodies. The volcanoes found on the surface of continents and ocean floor are the examples of the Extrusive Igneous rocks bodies.
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Types of Igneous Rocks on The basis of structure:
On the basis of structure the Igneous rocks are divided in to two groups:

  1. Extrusive Igneous Rocks
  2. Intrusive Igneous Rocks
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Extrusive Igneous Rocks:
The extrusive igneous rocks are those which are made by the solidification of magma over the earth surface.
They are bright and non-crystalline. Gabbro and Basalt are the examples of the 1st type of Igneous Rocks
These rocks may further be subdivided into two parts:

  1. Acid Igneous Rocks
  2. Basic Igneous Rocks
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Acid Igneous Rocks:
On account of silicon contents about 65 %, the solidification takes place soon. They are unable to flow long.
The material goes on accumulating with the result the Acid lava cones are formed.
They possess very small quantities of Iron and Magnesium. Granite is the suitable example of such rocks.
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Basic Igneous Rocks:
They contain more quantity of magnesium and less of silicon.
On account of less silicon contents the fluid material can flow upto longer distance.
They do not form conical hills or mountains but make Plateau like shape. They are of grey colour.
Basalt is the suitable example.
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Intrusive Igneous Rocks:
At the time of volcanic eruption, the magma does not reach the upper surface and remains accumulated in the intermittent fissures and solidifies there, it f0rms various types of internal rock features.
The intrusive igneous rocks also can be divided into two groups:

  1. Plutonic Igneous Rocks
  2. Hypabyssal Igneous Rocks
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Plutonic Igneous Rocks:
These rocks are found very deep inside the earth.
The solidification process is very slow.
It contains crystals.
Granite is the most suitable example of it.
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Hypabyssal Igneous Rocks:
When the interior Magma flow finds some obstacle, it solidifies in the pits and fissures between plutonic and volcanic rocks.
This type of rock is called the hypabyssal rocks.
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Shapes of Igneous Rocks
(a) Laccoliths:These features are produced where tongue like lateral intrusion of magma have forced the overlying strata into a dome shaped feature. In simplest form magma solidifies in a cake like mass. Such features are called Laccoliths.
(b) Batholiths:When solidification of magma goes up and down, there are creeks in the upper layers and the molten lava or magma enters in them. These are known as the Batholiths.
(c) Sheets and Sills:When the magma is trying to get out and enters into the minor fissures it is called the sheet. The border sheets are called sills.
(d) Dykes:Dykes are formed when magma has risen through near vertical fissures solidifying to form walls of rocks.
^ico-hand-right-pointer (e) Stalks:When magma solidifies in round or in egg shape, it is known as stalk.
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Characteristics of Igneous Rocks:
The characteristics of Igneous rocks are as follows.
1. The igneous rock do not contain layers.
2. They are without fossils
3. The upper part of the rock contains joints.
4. They are formed by volcanic lava
5. They are crystalline rocks.
6. They are impervious rocks.
7. They are massive.
8. Weathering effect is difficult over them.
9. They are primary rocks of the crust hence are also called the primary rocks
There are so many useful minerals associated with igneous rocks such as iron Nickel. Copper Zinc, Manganese, Gold, Platinum, Lead, Tin etc.
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The soil which is derived from basaltic lava flows is known as Regur soil.
It is highly argillaceous and somewhat calcareous it is compact and highly retentive of moisture.
It is very fertile and suitable for cotton, wheat and linseed products.
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Sedimentary Rocks
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These rocks are formed from the sediment deposits on the ocean beds. They comprise only about 5% of the earth's crust but cover about 75% of the total land surface.
These rocks are made up of the weathered remains of igneous rocks but also contain organic matter from the remains of marine organisms.
Sedimentary rocks are formed in horizontal layers called strata and take millions of years to harden into rocks. These rocks are also known as strati red rocks because of these layers.
Although three-fourths of the earth's surface is covered with sedimentary rocks, they make up only about 5 per cent of the volume of earth's crust. Any rock on earth's surface is exposed to weather changes and to the agents of erosion.
It thus gets broken into fragments. further reduced into small particles called sediments. These are derived from igneous, metamorphic or older sedimentary rocks. For this reason these are known as sedimentary or fragmental rocks.
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Introduction Cont.,
Some sediments are of sand, some of clay and others are of different bits of rocks making pebbles or gravel.All sediments of one or the other type are carried by running water, wind or ice.
These are left buried layer by layer at favorable sites,mostly setting down along silent water of a stream, a lake or sheltered parts of sea and its beaches.
The sediments get sorted by the transporting agents like running water, winds, waves,the larger and heavier particles being deposited first after moving for a relatively short distance. The smaller and finer particles are carried far away.
Examples of Sedimentary Rocks:
(i) Gypsum, chalk and limestone.These rocks are formed by the deposition created by chemical action or chemical sedimentation
(ii) Peat, Ignite bituminous coal anthracite. These rocks are formed by deposition of organic matter or marine remains.
(iii) Conglomerates gravel pebbles and shingle sandstone and shale, layered clay or claystone.These rocks are formed by the deposition of sediments in water.
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Types of Sedimentary Rocks on the Basis of Structure
On the basis of Structure the Sedimentary Rocks divided into four types:

  1. Arenaceous Rocks
  2. Argillaceaus Rocks
  3. Carbonaceous Rocks
  4. Calcareous Rocks
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Arenaceous Rocks
These rocks are generally formed of sand and possesses minute holes, hence is porous and erosion on these rocks is difficult.
They are easy to be picked up by the wind.
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Argillaceous Rocks
They contain more particles of gypsum. It is soft and compact and do not contain pores or tiny openings through which water can pass.
Therefore, it is impervious and easy to be eroded due to softness.
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Carbonaceous Rocks
In the carboniferous age the huge forest vegetation sub-merged under water due to upheaval in earth surface.
This forest vegetation under great heat and pressure turned into coal. These rocks are called carbonaceous rocks
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Calcareous Rocks
These rocks are formed by the fossils of oceanic creature and animals. These rocks are hard but easily soluble in water.
Limestone, dolomite and chalk are the good examples of calcareous rocks. The Great Bareer Reef of Eastern part of Australia is formed in this way.
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Types of Sedimentary Rocks on the Basis of Formation
On the Basis of Formation types of Sedimentary Rocks are divided into:

  1. Aqueous Rocks
  2. Glacial Rocks
  3. Aeolian Rocks
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Aqueous Rocks:
These rocks are formed by the action of water. They are divided in three sub-categories - Riverine, Lacustrine and Oceanic.
The riverine rocks are originated in two ways. The rivers when low they pick up rock particles by erosive force of running water and secondly they deposit the eroded particles elsewhere.
In this way riverine rocks are formed by the work of river erosion, transportation and deposition. The rocks of Gangetic plains are made by this system.
The ruins also deposit the eroded material in the oceanic floor where layer after layer the sedimentary rocks are found Sometimes sediments are deposited in lake bed where layers of rock are formed.
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Glacial Rocks:
These rocks are formed by the action of glaciers over the land.
When a glacier advances forward it also erodes, transports and deposits the rock particles in form of moraines.
This gives rise to glacial rocks of mucus forms.
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Aeolian Rocks:
The wind picks up, transports and deposits the sand particles by this activity in sandy deserts there is formation of different types of land forms.
The land forms under pressure and weight of layersthey take form of Aeolian rocks. Such rocks are seen in Loess Plateau.
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Characteristics of Sedimentary rocks:
The characteristics of Sedimentary Rocks are as follows:
1. These rocks are porous.
2 They are stratified and without pores.
3. They are affected by erossive forces easily.
4. These rocks are formed by the rocks sediments fossils and vegetative material so they contain more calcareous and carbonaceous contents.
5. They possess joints and stratification.
6. Majority of the rocks of this type are formed by the sedimentation on the floors of water bodies
7. These rocks are easily breakable due to softness.
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Metamorphic Rocks
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Change is the rule of nature. This change is also seen in rocks. The pressure and heat bring a partial or whole change in rocks.
Sometimes the physical properties of the original rock are entirely changed from that of newly formed rock. This type of change is known as the Metamorphismand the rocks formed by this process are known as Metamorphic Rocks.
The word Metamorphic is derived from Greek word Metamorphoswhich means change. This change is found in texture, colour, hardness and minerals composition of rocks.
This metamorphism is brought by the pressure and heat of the layer mass over each other in Sedimentary and Igneous rocks.
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Examples of Metamorphic Rocks
1. Slate: Is formed by compression of sedimentary rocks like shale and mudstone.
2. Quartzite: Is formed from sedimentary rocks like sandstone.
3. Gneiss: Is formed by the metamorphosis of igneous rocks like granite.
4. Marble: Sedimentary rocks like limestone turn into marble under intense heat.
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Major rocks types by depth
The classification of these rocks are of two types
On the basis of Agents:
On the basis of Regional effect:
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On the basis of Agents:
(i) Rocks formed by Thermal metamorphism:Metamorphism due to intense heat. This heat is obtained from Lava.
(ii) Rocks formed by the Dynamic metamorphism:The formation of metamorphic rocks under the stress of pressure is known as Dynamic metamorphism. In this case granite is converted into Gneiss, clay and shale into schist at great depth.
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On the basis of Regional effect:
(i) Rocks formed by contact metamorphism:This metamorphism is under heat produced by magma. This change allows the rocks minerals in great quantity. Under this process the lime stone is converted into marble.
ii) Rocks formed by Regional metamorphism:In this process the metamorphism takes place under pressure and heat due to various layers of rocks cleavage and plastic are the examples of this type.
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General characteristics
General characteristics of Metamorphic Rocks are as follows :
1. These rocks are formed by intense pressure heat and the chemical action over the rocks.
2. They are formed by the metamorphism in sedimentary and Igneous rocks.
3. They are formed in solid state.
4. They are formed at the depth of thousands of kilometres.
5. There are no fossils in them
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Largest and Highest mountain range- Himalaya-Karakoram in Asia, with the highest peak. Mt. Everest (8848 m)
Other important mountain ranges:
Name Continent
Andes South America
Alps Europe
Rockies North America
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Introduction Cont.,
Young or new mountains have come into being after the continental drift started, with the break up of the large land mass of Pangaea.
The Himalayas, the Andes, the Rockies and Alps are examples of new or young mountains.
Old mountains are those which were formed in the pre-drift era long before the continental masses came together to form Pangaea.
The Pennines (Europe), Appalachians (America) and the Aravallis (India) are examples of old mountains.
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Types of Mountains
There are broadly four types of mountains which are differentiated on the mode of their origin or formation, viz.,

  1. Fold Mountains
  2. Block Mountains or Horst
  3. Volcanic Mountains
  4. Residual Mountains
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Fold Mountains
The mountains formed by the crumbling or folding of rocks resulting in huge wrinkles or folds upon the earth's surface as well as the ocean floors are called Fold Mountains.
These are formed as a result of a series of earthquakes.
One part is ridged up, to form mountains often of great height, while the other part is covered by the waters of the ocean.
All big mountain systems have been formed in this way- the Himalayas Alps, Andes, Rockies, etc.
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Block Mountains or Horst
A mountain mass formed by the lifting up of land between faults (cracks in the rock strata) or by the sinking of land outside the faults, are called block mountains.
They are formed when a mass of elevated land under strain, cracks, leaving a higher elevation standing between two areas of lower elevation.
It may happen that the outer part sinks leaving an elevated central part, a crust block or block mountain. These are usually steep-sided.
The Vosges in France and the Black Forest mountains in western Germany Come under this class of mountains.
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Volcanic Mountains
A volcano is a mountain formed of material that has erupted from inside the earth through an opening in the earth's crust.
Super-heated molten rock matter called lava is ejected, forming a hill, conical in shape with a funnel-shaped hollow at its top called a crater.
It is estimated that there are about 850 active volcanoes. 80 of which are on the ocean floor. Mt. Fujiyama in Japan, Mt. Vesuvius in Italy and Chimborazo and Cotopaxi in Andes (South America) are examples of volcanic mountains. They are also called mountains of accumulation.
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Residual Mountains
Mountains that are deeply dissected and reduced by weathering and river action, are called residual mountains. The Catskill mountains in New York are residual mountains. They are also called mountains of denudation.
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It is the eruption of molten rock at the earth's Surface, often accompanied by rock fragments and explosive gases.Vulcanism and the degassing of Earth gave Earth its atmosphere. In the earliest stages of evolution, steam emitted through Volcanoes condensed to fall as rain and inundate the depressions on the surface of the Earth.
Lava, dust and pyroclasts spewed out from volcanoes cause widespread devastation, but for geologists a volcanic eruption is one of those rare Occasions When the earth's inner secrets become amenable to close study.
All questions as to how molten matter wells up to the surface or why most explosive volcanoes occur along particular belts of Earth, have been answered by the theory of Place Tectonics.
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Introduction Cont.,
Greatest volume of volcanic matter is produced along the oceanic ridge systems that are essentially divergent plate boundaries. Such volcanic activity had led to the sudden formation of the Surtsey Island, south of lceland, in I963.
The 'Ring of Fire', which is the most active volcanic belt bordering the Pacific Ocean is associated with subduction plate boundaries.
The numerous volcanoes dotting the sea off Philippines, Japan, the Aleutian Islands and the volcanoes of the Andes mountains, are all examples of subduction zone vulcanism.
Volcanoes may also occur within a rigid plate, but the processes that lead to such in-plate vulcanism are difficult to explain.
The volcanic Islands of Hawaii in the mid-Pacific, and the famous hot water geysers of the Yellowstone region of North America are examples of such in-plate volcanoes.
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Distribution of volcanic activity
Most vulcanism not associated with seafloor spreading is related to subduction zones and is concentrated at convergent plate boundaries. But some volcanic activities are associated neither with mid-oceanic ridges nor with subduction zones.
These are regions of local magmatic extrusions, which are popularly known as the hotspots. The island of Hawaii, for example, lies in an archipelago in the middle of the Pacific. On the African Plate, where West Africa and Equatorial Africa meet (at Africa's so-called 'armpit'), lies Mt. Cameroon, another active volcano far from spreading ridges and subduction zones.
Volcanoes have been traditionally differentiated into active, dormant and extinct. An active volcano is one that has erupted in recorded history, like Barren, Islands in India and Mt. Stromboli in Italy. A dormant volcano has not been seen to erupt, but it shows evidence of recent activity, like Mt. Kilimanjaro.
When a volcano shows no sign of life and exhibits in evidence of long-term weathering and erosion, it is tentatively identified as extinct. Interestingly, there is no clear cut definition of an extinct or a dormant volcano.
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An earthquake is basically the vibration of Earth produced by the rapid release of energy. This energy radiates in all directions from the source, focus, in the form of waves. Just as the impact of the stone sets water waves in motion, an earthquake generates seismic waves that radiate outwards from the earth's interior.
Even though, energy dissipates rapidly with increasing distance from the focus,-seismic sensors, called seismographs, located throughout the world can record the event. It is estimated that over 30,000 earthquakes, strong enough to be felt, occur worldwide annually.
Fortunately most of these are minor tremors and do very little damage and only about 75 significant earthquakes take place each year, many of which occur in remote regions.
The shaking of the ground, coupled with the liquefaction of soil, wreaks havoc on buildings. The intensity of an earthquake and the destruction it causes are greater over softer soil regions, for example on alluvial soils, than on more rocky areas.
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Reasons of Earthquakes
The Himalayas, making the great mountain wall of the north, also happen to form the northern margin of the Indian plate and hence are marked by frequent and severe earthquakes.
One such zone of in-plate seismicity is the Narmada-Sone lineament, cutting across the northern plane. This zone, though away from the active Himalayan region, frequently witnesses tremors of low magnitude.
In fact, the Jabalpur earthquake can be rightly attributed to this. The same reason also explains the 'unimaginable ' seismic accident that was witnessed in the wee hours of September I993, in Latur.
This area is thought to be one of the most stable blocks on the earth's surface with almost negligible seismic activity. But the incident has thrown up new challenges altogether and calls for a new line of study into earthquake dynamics. Latur has, in fact, changed the perspective of threats in India's oldest region, the peninsula.
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The recent earthquake of January 26, 2001 which flattened parts of Gujarat was also caused by the growing “in-plate stress” in the region. In fact, the whole Kutch is a fault.
Bhuj, which bore the brunt of the nature's fury, lies close to the Allah bund fault. Hence, in-plate seismicity is due to the reactivation of concealed shields (stable parts of the earth) and release of energy due to the continuous movement, and such realignment of plates is facilitated by the presence of faults.
Earthquakes are at present studied by a science known as SEISMOLOGY. Hence, all the phenomena related to the emergence and manifestation of Earthquakes are called SEISMIC.
The term EARTHQUAKE covers any vibration of the Earth's surface brought about by natural causes.
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Types of Earthquakes
Earthquakes can be divided into three main groups:

  1. Volcanic Earthquakes
  2. Denudation Earthquakes
  3. Tectonic Earthquakes
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Volcanic Earthquakes
VOLCANIC EARTHQUAKES are connected with the process of volcanism and are thus developed only in the regions of contemporary volcanic activity, either accompanying of preceding the eruption of volcanoes.
They emerge as a result of deep explosions of gases, emitted from the magma and hydraulic shocks of magma, which moves along the channels of complex form, etc.
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Denudation Earthquakes
DENUDATION EARTHQUAKES or Earthquakes DUE TO THE COLLAPSE, are spread less widely than the volcanic ones.
They result from the collapses of considerable masses of rocks, mainly in the mountain regions, the sinking of underground cavities.
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Tectonic Earthquakes
The third group of Earth-Quake is called TECTONIC EARTH-QUAKES. Earth quakes belonging to this group are characterised by maximum force and account for 95 per cent of all the earthquakes that are registered.
According to current judgements tectonic earthquakes are connected with the short relaxations of mechanical stresses that have continuously been accumulated in the depth of the earth and that have emerged during the reciprocal displacements of individual blocks of Lithosphere.
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Duration of Earthquakes
The duration of Earthquakes can vary from several seconds to some months (and even years). Owing to the gradual intermittent release of mechanical stresses there takes place a recurrence of underground shocks.
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Earthquakes Intensity
The FORCE, or INTENSITY, is taken to be the external (outside) effect of an earthquake that is, "its manifestation on the Earth surface". The "force" of earthquakes is estimated by the value of acceleration of the particles constituting the Earth's surface under the impact of the shock produced by earthquakes.
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