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A Volcano is a vent (opening) in the earth’s crust through which molten material erupts suddenly.  Volcanism is the phenomenon of eruption of  molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where Lava, Pyroclastics And Volcanic Gases erupt through a break in the surface called a Vent.


The molten rock material found in the interior of the earth is called Magma. When it reaches the earth’s surface, it is known as Lava. The material that reaches the ground includes Lava Flows, Pyroclastic Debris, Volcanic Bombs, Ash & Dust ,gases such as Nitrogen Compounds, Sulphur Compounds and minor amounts of Chlorine, Hydrogen And Argon. Crater is a saucer shaped depression in the mouth of a volcano. When the crater is widened, it is called as Caldera. When they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas.


Based on the Frequency of Eruption, there are three types of volcanoes.
  1. Active Volcanoes: Volcanoes which erupt frequently are called active volcanoes. Generally, their vent remains open.
Ex :- Mount Etna of Italy; Cotopaxi in Ecuador; Mt Mayon – Philliphines. Barren Island is an island located in the Andaman Sea. Dormant Volcanoes: These volcanoes may not have erupted in the recent past but there is a possibility of eruption at any time. In other words, they may lie dormant awaiting active eruption anytime. Sometimes gases and steam come out of them. They cause great destruction to life and property once they become active again. Ex:-  Mt. Vesuvius of Italy ; Mt. Fujiyama of Japan. Extinct Volcanoes: These volcanoes have exhausted their energy   and have not erupted during the known geological period. The vent of these volcanoes remains closed with solidified lava. The formations such as craters may be filled with water. Ex :  Popa in Myanmar &  Mt. Kenya in Eastern Africa. On the basis of nature of eruption and form developed on the surface, they are classified into following types: Shield Volcanoes: These are made up of basalt, a type of lava that is very fluid when erupted. They become explosive when water gets into the vent. They develop into a cinder cone.  Ex :-  Hawaiian volcano. Composite Cone Volcanoes: They are also called ‘strato volcanoes’. These volcanoes are characterized by eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions. They are cone-shaped volcanoes composed of layers of lava, ash and rock debris. Examples: Mt Shasta , Mt Ranier , Mt Mayon of Philliphines

Cinder Cone Volcano:

They are usually of low height & are formed of Volcanic dusts & ashes , Pyroclastic matter .  It forms when magma is thrown out to the surface, cooled in to ash and cinders and settled around the mouth of volcano. It is less dangerous than other volcanoes. Lava Cone : Unlike composite and shield volcanoes, lava domes are of significantly smaller structure. They are formed when the lava is too viscous to flow to a great distance. As the lava dome slowly grows, the outer surface cools and hardens. Flood Basalt Provinces These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of sq. km of thick basalt lava flows. Example:  The Deccan Traps, India. EFFECTS OF VOLCANIC ACTIVITIES Destructive Effects Of Volcanoes: It creates damage to life and properties. The volcanic gases pose 
  • Potential Hazard To People, 
  • Animals 
  • Agriculture, while sulfur dioxide gas can lead to acid rain and air pollution.
POSITIVE EFFECTS OF VOLCANOES: Volcanism creates new landforms. Volcanic rocks yield very fertile soil upon weathering and decomposition.  Ex:- The Kimberlite rock of South Africa, the source of diamonds. DISTRIBUTION OF VOLCANOES The major regions of volcanic distributions are World Map from DK, Oxford etc.. CIRCUM-PACIFIC REGION :- It is popularly termed the ‘Pacific Ring of Fire’, has the greatest concentration of active volcanoes. Volcanic belt and earthquake belt closely overlap along the ‘Pacific Ring of Fire’. It is estimated to include two-thirds of the world’s volcanoes.

Mid Atlantic Region

The Mid Atlantic Region coasts have comparatively fewer active volcanoes but many dormant or extinct volcanoes. Ex:- St. Helena, Cape Verde Islands and the Canary Islands. The Great Rift valley of Africa In Africa some volcanoes are found along the East African Rift Valley.   Kilimanjaro and Mt. Kenya are extinct volcanoes.The only active volcano in West Africa is Mt. Cameroon.

Mediterranean Region :-

Volcanoes of the Mediterranean region are mainly associated with the Alpine folds. Ex: Mt. Vesuvius, Mt. Stromboli (known as the Light House of the Mediterranean Sea).


Barren Island is the only island in India having confirmed active volcano named Barren volcano, which erupted recently in 2017.However, the other volcanic island in Indian Territory is Narcondam (Andaman and Nicobar Islands). It is probably extinct. Its crater wall has been completely destroyed.


Rocks and Minerals :- Any natural mass of mineral matter that makes up the earth’s crust is called a rock. Rocks can be of different colour, size and texture.  There are three major types of rocks: 
  • Igneous Rocks 
  • Sedimentary Rocks
  • Metamorphic rocks.
When the molten magma cools, it becomes solid. Rocks thus formed are called igneous rocks or primary rocks.  There are two types of Igneous Rocks:     1 . Intrusive Rocks 
  1. Extrusive Rocks.
 If the magma cools slowly at great depths, mineral grains increase in their size.  Examples: Granite (intrusive). If the magma gets sudden cooling (at the surface) results in small and smooth grains. Example: Basalt (extrusive) etc. The igneous rocks are the oldest of all the rocks. INTRUSIVE IGNEOUS ROCKS
  1. Batholiths : Batholiths are large rock masses formed due to cooling and solidification of hot magma inside the earth. It is granitic in origin.
  1. Laccoliths : Laccoliths are large dome-shaped intrusive rock connected by a pipe-like conduit from below. These are basically intrusive counterparts of exposed domelike batholiths. The Karnataka plateau is spotted with dome hills of granite rocks.
Most of these, now exfoliated, are examples of laccoliths.
  1. Lapoliths :- When the magma moves upwards, a saucer shape, concave shaped body called Lapolith is formed.
  1. Sill :- Sill is a solidified sheet-like horizontal lava layer inside the earth. The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.
  1. Dyke :- When the magma makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes.
These are intrusive forms led to the development of the Deccan traps. Extrusive igneous formations like caldera, crater, etc. are already discussed above in detail. EARTHQUAKE Earthquake  “ is a form of energy of Wave Motions transmitted through the surface layer of the earth in widening circles from a point of sudden  energy release, the Focus”.   It is a natural event. Earthquake results from the sudden release of pressure which has slowly built up within the earth’s crust. Energy is released in the form of shockwaves known as Seismic Waves.It is recorded by an instrument called “Seismograpgh or seismometer at the epicenter. Focus or Hypocenter is the location inside the earth where the earthquake originates.Epicenter is the point on the earth’s surface vertically above the focus of an earthquake. There are two types of earthquake in nature.
  1. TECTONIC – associated with faulting movement of rocks.
  2. VOLCANIC – associated with major volcanic eruptions.
On the basis of depth, tectonic earthquakes are categorized into 3 types.
  1. SHALLOW earthquakes – where the depth of focus is less than 70km.
  2. INTERMEDIATE earthquakes – where the depth of focus is 70 to 300km.
  3. DEEP EARTHQUAKES – where the depth of focus is 300 to 700km.
About 90% of earthquakes are within a focus less than 100km deep. MEASUREMENT OF AN EARTHQUAKE
  • INTENSITY SCALE: This scale was prepared by Italian scholar Giuseppe mercalli in 1903. He measured earthquake   on the basis of intensity & intensity reflects the impact of earthquake. The severity of earthquake shaking is assessed using a descriptive scale – the Modified Mercalli Intensity Scale. The range of intensity scale is from 1-12.
  • MAGNITUDE SCALE: This scale was prepared by scholar named C.F. Richter in 1935. Earthquake size is a quantitative measure of the size of the earthquake at its source. The Richter Magnitude Scale measures the amount of seismic energy released by an earthquake. The magnitude is expressed in numbers from 0-10.
  • CIRCUM PACIFIC BELT :- This belt includes Western Sides of North and South Americas & Countries like Indonesia, Philippines, Japan Etc. This belt registers about 80% of quakes in the world.This belt registers all three types of earthquakes (shallow, intermediate & deep). The earthquake proneness is because of subduction boundaries.
  • TRANS-EURASIAN MOUNTAIN BELT :-  This belt extends from Mediterranean Sea through southwest Asia & Himalayas into the Southeast Asia where it meets circum pacific belt. This belt registers about 15% of world’s earthquakes and experiences only shallow and intermediate earthquakes. The earthquake proneness is because of convergent boundaries.
  • The third zone of earthquake is associated with global system of mid oceanic ridges with an extension to include East African rift valley system. This belt registers only shallow earthquakes.
SEISMIC WAVES Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs. What Is Seismology? Seismology is the study of earthquakes and seismic waves that move through and around the earth. A seismologist is a scientist who studies earthquakes and seismic waves.     1).Body waves      2).Surface waves.  BODY WAVES:- Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. They are classified in two types 
  • P or Primary wavesare the fastest seismic waves (6 km/sec) are the first to arrive at the surface. They causethe matter to oscillate forward andbackward, parallel to the motion ofthe seismic wave front. This exerts pressure on the material in the direction of the propagation. They pass through solid, liquid and gaseous materials.
  • S or Secondary waves are slower than the primary waves (3.5 km/sec. in the upper crust). They cause matter to oscillate side to side, perpendicular to the motion of the wave front. Hence, they create troughs and crests in the material through which they pass. They pass through only solid medium.
Surface Waves :Surface Wavesare the waves that travel along the earth’s surface. They are slower than body waves. These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs. ANTIPODE The antipode of any place on the Earth – the place that is diametrically opposite it, so a line drawn from the one to the other passes through the centre of the Earth and forms a true diameter. For example, the antipodes of New Zealand’s lower North Island lie in Spain. FOR EXAMPLE: if A is epicenter; is anticentre. The term anticentre refers to antipode of epicenter of an earthquake. SHADOW ZONE Earthquake waves get recorded in seismographs located at far off locations. However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’.    Shadow zone for p waves                                 Shadow zone for s waves Shadow zone for p waves: 103° to 142° on both sides. Shadow zone of S-wave: entire zone beyond 103°. The shadow zone of S-wave is much larger than that of the P-waves. CAUSES  &  IMPACT  OF  EARTHQUAKES :-   There are many factors controlling the occurrence of the earthquake. Some of the major factors include:
  1. Plate Tectonic Movements 
  2. Volcanic Eruptions.
  3. Construction of large dams results in earthquake. 
Example :- Koyna dam, Maharashtra.
  1. Other Reasons: The nuclear explosions also release massive energy to cause tremors in the earth crust. When underground cave collapses, earthquake may occur.
  1. Damage to buildings, roads, rails, factories, dams, bridges etc.
  2. Landslides caused by earthquakes damage infrastructure.
  3. Fires in the forest and urban areas.
  4. Flash floods.
  5. Tsunami.
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