Endogenic movements :Volcanoes

Volcanoes

A volcano is a vent or an opening in the earth’s crust. Through this opening or vent the molten rock material, ash, steam ,rock fragments, and other hot gases are emitted slowly or forcefully in the course of an eruption.

The volcanism is a geologic processes which give rise to volcanoes and volcanic rocks .

What is Magma and Lava ?

• The molten rock material below the surface is called ‘magma’.

• When Magma  comes on the surface it is called ‘Lava’.

• Lava can be felsic (means viscous) or mafic (less viscous or fluid).

What type of gases are ejected from volcanoes ?

• Generally water vapour is most dominating part of volcanic gases and consists of 90% of it.

• Some of the gases are- NH3, CO2, N2, SO2, Cl2, HCl, H2S, Hydrocarbon gases and arsenic vapours.

Causes of Volcanoes-

• The cause of volcanic eruption is the excessive pressure exerted by the magma and hot gases on the earth’s crust.

• Friction and fault movements at greater depths may cause melting and forcing of the molten material to the surface.

A.On the basis of the frequency of eruption, volcanoes are of three types-

(i) Active-

• Active volcanoes are erupting currently or have erupted recently.

Examples-

• Stromboli in Mediterranean.

• Krakatoa in Indonesia.

• Mayon in Philippines.

• Mauna loa in Hawai Islands.

• Barren Island in India.

(ii) Dormant -

• Dormant are those volcanoes which have erupted at least once in human history and are not active now.

Examples-

• Vesuvious of Italy.

• Cotopaxi in South America.

(iii) Extinct-

• Extinct volcanoes are those which have not erupted during long human history.

Example- 

• Mount Popa of Myanmar.

•  Kilimanjaro of Tanzania.

It is not, always very simple to categorize a volcano as dormant or extinct.

For example the Vesuvious and Krakatoa are the volcanoes which were dormant for hundreds of years and became suddenly active .

B. On the basis of mode of eruption, volcanoes are divided into two types:

(i) Central type of volcanoes -

• When the eruption in a volcanoes takes place from a vent or a hole, it is called a central type of volcano.

• Depending on the nature of erupted materials there is formation of different types of domes or conical hills . 

• The magma is more viscous in Central type volcanoes and tends to build a volcanic landform on the flanks of its central vent.

(ii) Fissure type volcanoes-

• There is formation of plateaus and shields because fissure type of volcanoes erupt through a crack or fissure and cause.

• Magma is less viscous in these types of volcanoes.

• Example- Deccan Traps of India.

C.On the basis of the fluidity of lava there are two types of volcanoes :

(i) Volcanoes of basic lava -

• Basic lava has low melting point and high fluidity and also have low silica content.

• Due to these reasons lava flows quietly and spread out in thin sheets.

(ii) Volcanoes of acid lava-

• Acidic lava has high silica content and have high melting point.

• It is highly viscous and solidifies quickly.

• Due to above reasons it forms large land forms with steep slopes.

D.On the basis of explosive activity and nature of eruption-

(i)Hawaiian types-

• In this type of volcanoes there is less explosive activity.

• Lava flow is mobile and thin and it spreads out.

• Little drops of lava form thread like masses blown up by wind called Pele’s hair.

• Pele’s hair is the name given after Pele - the Hawaiian Goddess of Fire.

(ii)Strombolian Type-

• Stromboli volcano is located in Sicily-Italy.

• It is a bit violent type volcano.

• Lava is more viscous and the fragments of lavas are blown up into air forming bombs and scoriaceous fragments.

• Stromboli is known as lighthouse of Mediterranean because the incandescent masses of gases are given off due to volcanic activity

(iii)Vulcanian Type-

• This volcano is located in Lipari, Sicily-Italy.

• It is sometimes violent and sometimes silent.

• Huge quantities of gases rise upwards vertically like cauliflower.

• In this type of volcano the lava is mbit viscous due to which violent eruption can be seen here.

(iv)Vesuvian Type-

• This volcano is located in Vesuvius near Naples.

• In this volcano the magma is highly viscous and due to highly viscous magma there is violent eruption in this type of volcano.

(v)Pelean Type

• It is located at Mont Pelee, Martinique, West Indies.

• In term of explosive nature it is  characterized as Most violent due to highly viscous magma.

(vi)Plinian Type

• It is extremely violent volcano located in Pompeii, Rome.

• Plinian type is the name given after Pliny, the younger, who in his letters to Tactius, described a violent explosion of the Vesuvius that caused the destruction of cities of Pompeii and Herculaneum.

E.Some other types of volcanoes

Shield Volcanoes-

• There is eruption of fluid basaltic lava  in Shield volcanoes .

• This lava flows out of vents in all directions, traveling long distances before solidifying.

• These volcanoes are e the largest of all the volcanoes on the earth.

• Although they are characterized by low-explosivity but they become explosive if somehow water gets into the vent.

• Example- The Hawaiian volcanoes.

Examples: 

• Deccan Traps.

• Siberian Traps.

• Snake Basin.

• Canadian Shield etc.

Cinder cone volcanoes 

• These are much smaller than either composite or shield volcano.

• They are typically circular, with a large bowl-shaped cater at the summit.

• The Paricutin volcano, in Mexico, is an example of a cinder cone cone.

Composite Volcanoes(tall cones)-

• There is eruption of cooler and more viscous lavas than basalt in composite volcanoes .

• Along with lava, there is also eruption of large quantities of pyroclastic material and ashes.

• These volcanoes often result in explosive eruptions. 

• This material accumulates in the vicinity of the vent openings leading to formation of layers.

Examples-

• Mount St. Helens.

• Mount Fuji in Japan.

• Mount Cotopaxi in Ecuador.

• Mount Shasta in California.

• Mount Hood in Oregon .

Mid-Ocean Ridge Volcanoes

• These volcanoes occur in the oceanic areas.

• The mid-ocean ridges are the Earth's largest volcanic system, accounting for more than 75% of all volcanic activity on the planet.

• Mid-ocean ridges are created by the upwelling of basaltic lava and lateral rifting of ocean crust .

It is broadly of two types-

Intrustive landforms-

• Theses landforms are formed due to colling of lava before reaching the earth surface.

Extrusive land forms-

• When some material is thrown out to the surface during volcanic activity then there is formation of extrusive landforms.

• The materials which is thrown out during the volcanism  includes lava flows, pyroclastic debris, volcanic bombs, ash, dust and gases such as nitrogen compounds, sulphur etc. .

• There is formation Plutonic and Volcanic rocks and Plutonic rocks are formed by the cooling of magma within the crust .

• Rocks formed by the cooling of lava above the surface are called volcanic rocks.

Concordant body: When there is no cutting or breaking of rock layers and the igneous body runs parallel to the bedding or foliation plane of the country rock ,it is known as Concordant body.

Discordant body: When the igneous body cuts across the bedding or foliation plane and penetrates through the country rock.

1.Batholiths-

• These are cooled portion of magma chamber.

• When the large body of magmatic material cools in the deeper depth of the crust then there is formation of large domes.

• When there is Denudation on the surface then they become exposed on the surface of the earth .

2.Lacoliths-

• These are lens-shaped, that has been intruded between rock strata causing uplift in the shape of a dome.

Example-

• Domal hills in karnataka pleatue.

3. Lopolith-

• In lopolith there is a depressed central region in which there is a large igneous intrusion which is lenticular in shape.

4.Phacolith-

• It is a pluton of igneous rock which runs parallel to the bedding plane or foliation of folded country rock. 

5.Sills-

• These are flat intrusion of igneous rock and they are formed between the layers of rock.

• Sill is a concordant igneous bod.

6.Dykes-

• Dikes are tabular or sheet-like bodies of magma and the main feature of these bodies are that they cut through and across the layering of adjacent rocks.

• Dike is a discordant igneous body.

• It can further divided depnding on its shape like Radiating dyke,cones dyke,ring dyke etc.

7.Bysmalith

• It is a special type of laccolith which is a roughly vertical cylindrical igneous intrusion bounded by steep faults.

• It results into a cylindrical plug like mass. Bysmalith is a body where the roof uplifts along a circular or arcuate fault.

8.Lava dome-

• It is a dome shaped  steep-side mass of rock. It looks as if lava has squeezed out like toothpaste, with very little lateral spreading.

9.Lava cone-

• The Lava cone are formed due to the Lawa flow .They are also known as ‘plug-dome’ due to heaping of lava during quiet type eruption.

10.Crater-

• It is a bowl-shaped pit, found at the summit of most volcanic mountains, surrounding the central vent.

11.Chonolith -

• It is an igneous intrusion whose form is so irregular that it can not be classified as a laccolith, dike, sill or any other recognised body.

12.Stock-

• It is an intrusive mass of plutonic igneous rocks, but, smaller in size than the batholith and usually possessing more or less circular or elliptical cross section.

13.Boss-

• Boss is a mass of plutonic igneous rocks (or stocks) which has circular outline upon the surface.

14.Caldera-

It is a hollow shaped structure that is formed shortly after the emptying of a magma chamber in a volcano eruption.

It is of two types: 

Explosive type-

• They are produced by explosive violence of volcanic activity  The top of the cone is blown up in the air producing the caldera.

Subsidence type-

• It is produced due to submergence of the top because of the circular fractures produced as a result of erupting magma from below.

15.Caldera lakes

• When the volcanic activity ends water may be filled up in the calderas producing caldera lakes.

Examples-

• Lake Toba of North Sumatra(Largest in the world).

• Crater Lake and those in Yellowstone National Park.

Pyroclastic material-

• These are collection of ash, hot gases, and rock fragments that flow down the slope of the volcano.

Agglomerate-

• They are pyroclastic rocks consisting mainly of fragments larger than 20 mm in diameter.

Volcanic Ash and volcanic dust-

• They are fine grained and remain suspended in the atmosphere for a long time.

Lapilli

• Lapilli is also known as cinders.

• Lapilli are Small rock fragments which are ejected from a volcano .The size of these lapilli are smaller then Volcanic Bombs, but larger than Volcanic Ash. 

Tuff-

• Tuff are Rocks that are made of ash and fine ash .

Lahar -

• It is a mudflow composed of pyroclastic ash.

There are more than 500 volcanoes in the world. The most dominant areas of the volcanoes can be seen majorly in three bels-

The Circum-Pacific belt-

• This belt has the greatest concentration of volcanoes,due to large distribution of such volcanoes this belt is called ‘Pacific Ring of Fire’.

• Distribution of This Belt : from the Aleutian Islands to Japan, Philippines, Indonesia to New Zealand and From Andes mountains of south America to Alaska and .

1.The Mid-World Mountain belt -

• It occupies the second position with regard to the numbers of volcanoes.

• Distribution : Europe to Asia Minor and than crossing through Himalayan region and joins the Circum-Pacific belt.

2.The African Rift Valley belt-

• The African rift valley region ranks third. and majority of volcanoes are not active and are Extinct .

• Mt. Cameroon is the only active volcano and it is situated in Central West Africa.

3.Volcanoes in India-

At present there is only 1 active volcano in India known as Barren island volcano.

Barren Island-

• It is located in the Andaman Sea. Its was 1stly erupted in the year 1787 and lastly in the year 2017. 

Other inactive/Extinct volcanoes are-

• Narcondam Island(andaman).

• Deccan Traps.

• Baratang(andaman).

• Dhinodhar Hill(Gujrat) etc.

Negative Effects of Volcanoes-

• Damage to human life.

• Deterioration of water quality due to harmful chemicals in it.

• They can cause Earthquake on earth.

• They can change landscape of the earth.

• They can cause Air pollution like acid rain etc.

Positive effects of volcanoes-

• Volcanoes provide materials which can enhance the fertility of the soil.

• Volcanism creates new landforms like islands, plateaus,volcanic mountains etc. For example: Deccan plateau, Mt. Vesuvius.

• Precious and semi-precious stones are found in volcanic rocks e.g. diamonds are found in South Africa.

• Crater lakes and caldera lakes enhances tourism and our water resources.

Important One Liners related to Volcanoes

What is a volcano?

Answer: A volcano is a geological formation that allows magma, ash, and gases to escape from below the Earth's surface.

What is the name of the volcanic rock that is formed from solidified lava?

Answer: Basalt

What is the name of the type of volcano that has a broad, gently sloping cone and is made up of layers of lava and ash?

Answer: Shield volcano

What is the name of the type of volcano that has a steep, conical shape and is made up of layers of ash and lava?

Answer: Stratovolcano

What is the name of the type of volcano that forms when magma erupts underwater?

Answer: Submarine volcano

What is the name of the molten rock that is found below the Earth's surface?

Answer: Magma

What is the name of the molten rock that has reached the Earth's surface and is no longer under pressure?

Answer: Lava

What is the name of the largest active volcano on Earth?

Answer: Mauna Loa, located in Hawaii.

What is the name of the process where a volcano collapses in on itself after an eruption?

Answer: Volcanic collapse

What is the name of the gas that is commonly released during a volcanic eruption and can pose a health hazard to humans and animals?

Answer: Sulfur dioxide

What is the name of the volcanic eruption that occurred in AD 79 and buried the city of Pompeii?

Answer: Mount Vesuvius eruption

What is a shield volcano?

Answer: A shield volcano is a type of volcano with a broad, gently sloping cone and is made up of layers of lava and ash.

What is a stratovolcano?

Answer: A stratovolcano is a type of volcano with a steep, conical shape and is made up of layers of ash and lava.

What is a cinder cone volcano?

Answer: A cinder cone volcano is a small, steep-sided volcano that forms from explosive eruptions of volcanic fragments.

What is a fissure volcano?

Answer: A fissure volcano is a type of volcano that erupts from a long, narrow crack or fissure in the Earth's surface.