9A7 -VOLCANOES

Hello Friends,
We had studied in the lesson 'Lithosphere" about the structure of the earth. You know that there are various forces are actively working inside the earth. Today we shall study about the volcanoes.

Volcanic eruptions are among the Earth's most powerful and destructive forces. Imagine hearing a volcano erupt thousands of miles away. Imagine looking through binoculars and seeing the top of a mountain collapse. Imagine discovering an ancient Roman city that had been buried in volcanic ash.
Volcanoes are also creative forces. The Earth's first oceans and atmosphere formed from the gases given off by volcanoes. In turn, oceans and an atmosphere created the environment that made life possible on our planet. Volcanoes have also shaped the Earth's landscape. Many of our mountains, islands, and plains have been built by volcanic eruptions.

Why Do Volcanoes Erupt?
Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Because it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually some of the magma pushes through vents and fissures in the Earth's surface. A volcanic eruption occurs! Magma that has erupted is called lava.
Some volcanic eruptions are explosive and others are not. How explosive an eruption is depends on how runny or sticky the magma is. If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano. Lava flows rarely kill people, because they move slowly enough for people to get out of their way. Lava flows, however, can cause considerable destruction to buildings in their path.
If magma is thick and sticky, gases cannot escape easily. Pressure builds up until the gases escape violently and explode. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.
Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountainsides destroying almost everything in their path. Ash erupted into the sky falls back to Earth like powdery snow, but snow that won't melt. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.
Because there may be hundreds or thousands of years between volcanic eruptions, people may not be aware of a volcano's dangers. When Mount St. Helens in the State of Washington erupted in 1980, it had not erupted for 123 years. Most people thought Mount St. Helens was a beautiful, peaceful mountain and not a dangerous volcano.
•Volcanoes are the result of hot spots within the crust or mantle of the earth.
•The hot, liquid rock will break through weak spots in the surface and form volcanoes or flood basalts.
•Many volcanoes do not release lava, instead they spit ash and small bits of lava called lapilli.
•Some eruptions are quiet with very fluid (low viscosity) lava flows while others are explosive

Why do volcanoes blow their tops?
•Volcanoes are mountains that spit fire. Deep under the Earth, there is red-hot, runny rock called magma.
•Sometimes the magma bursts up through a crack in the Earth’s crust and a volcano erupts.
•Magma (melted rock form inside the Earth) squeezes into the split, adding new rock to the plates and forming huge undersea mountain chains.

Eruptions are Classified as:
•Quiet………or Violent!

How are volcanoes formed?Deep in the earth it is very hot. It is so hot that rocks melt. The melted rock is called magma. The magma is lighter than the rocks around it so it rises. Sometimes it finds a crack or hole in the earth’s crust and bursts through. This is how a volcano begins.

In other words, what are volcanoes?
•The openings of the crust of the Earth through which hot magma along with rocks, gases and steam erupts forcefully are known as volcanoes.

What is magma?
•The very hot molten rock material is known as magma.

•What do you understand by ‘crater of a volcano’?
Funnel shaped depression seen at the top of the volcanic cone is termed as crater of volcano.

What are the different kinds of volcanoes?

Volcanoes do not all look alike. Their shape is based on what type of materials they erupt. There are three main kinds, or shapes, of volcanoes.

1. On the basis of Frequency & Intensity of Eruption:

•3 major types
1.Active Volcanoes
2.Dormant Volcanoes
3.Extinct Volcanoes

1. ACTIVE VOLCANOS:-
•Are those which have erupted lava, gases, cinder, pumice, etc in the recent historic periods.
•About 500 active volcanoes in world.
•Mt. St. Helena (USA), Mt. Stromboli, Mt. Etna in Italy, Mauna Loa in Hawaii
•Mt. Stromboli – frequently erupts – making the summit glow
•Known as ‘Lighthouse of the Meditarranean’
2. DORMANT VOLCANOS:-
•those that have erupted in historical times but are now quiet called dormant.
•It can become active any time
•Called sleeping volcano
•Eg. Mount Ranier of USA, Lassen of California

3. EXTINCT VOLCANOS:-
•If a volcano once erupts, and completely stops erupting – extinct volcano
•Known as dead volcano
•E.g. Chimborazo of S. America

STRATO OR COMPOSITE VOLCANO:

Stratovolcanoes, also known as composite cones, are the most picturesque and the most deadly of the volcano types. Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in an upward direction. The summit area typically contains a surprisingly small summit crater. This classic stratovolcano shape is exemplified by many well-known stratovolcanoes, such as Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Mt. Agua in Guatemala.

In detail, however, stratovolcano shapes are more variable than these classic examples, primarily because of wide variations in eruptive style and composition. Some may contain several eruptive centers, a caldera, or perhaps an amphitheater as the result of a lateral blast (e.g., Mt. St. Helens).
Typically, as shown in the image to the left, stratovolcanoes have a layered or stratified appearance with alternating lava flows, airfall tephra, pyroclastic flows, volcanic mudflows (lahars), and/or debris flows. The compositional spectrum of these rock types may vary from basalt to rhyolite in a single volcano; however, the overall average composition of stratovolcanoes is andesitic. Many oceanic stratovolcanoes tend to be more mafic than their continental counterparts. The variability of stratovolcanoes is evident when examining the eruptive history of individual volcanoes. Mt. Fuji and Mt. Etna, for example, are dominanted by basaltic lava flows, whereas Mt. Rainier is dominated by andesitic lava, Mt. St. Helens by andesitic-to-dacitic pyroclastic material, and Mt. Lassen by dacitic lava domes.
Stratovolcanoes typically form at convergent plate margins, where one plate descends beneath an adjacent plate at the site of a subduction zone. Examples of subduction-related stratovolcanoes can be found in many places in the world, but they are particularly abundant along the rim of the Pacific Ocean, a region known as Ring of Fire. In the Americas, the Ring of Fire includes stratovolcanoes forming the Aleutian islands in Alaska, the crest of the Cascade Mountains in the Pacific Northwest, and the high peaks of the Andes Mounains in South America.



* composite volcanoes, erupt with molten lava, solid rock, and ash. The layers pile up much like layers of cake and frosting. The layers form into symmetrical cones, and the slopes are steep.

CINDER CONE VOLCANO:-

Cinder cones are mounds of basaltic scoria that forms by streaming gases that carry lava blobs and ribbons into the atmosphere to form lava fountains. The lava blobs commonly solidify during flight through the air before landing on the ground. If gas pressure drops, the final stage of building a cinder cone may be a lava flow that breaks through the base of the cone.

If there is abundant water in the environment, magma interacts with water to build a maar volcano rather than a cinder cone. The longer the eruption the higher the cone. Some are no larger than a few meters and others rise to as high as 610 meters or more, such as Paricutin volcano, Mexico that was in nearly continuous eruption from 1943 to 1952. Accompanying the pyroclastic activity were lava flows that emerged from its base to destroy the village of Paricutin. Cinder cones can occur alone but commonly occur in groups or fields.
Cinder cone volcanoes form when solid rock and ash shoot up into the air and fall back around the volcano opening. The cinder cone volcano has steeply sloped sides.
SHIELD VOLCANO:-
Shield volcanoes are large volcanic forms with broad summit areas and low-sloping sides (shield shape) because the extruded products are mainly low viscosity basaltic lava flows. A good example of a shield volcano is the Island of Hawaii (the "Big Island"). The Big Island is formed of five coalesced volcanoes of successively younger ages, the older ones apparently extinct.
Mauna Loa, one of the main volcanoes, has a higher elevation than any mountain on Earth -- 9090 meters (30,000 feet) from the floor of the ocean to its highest peak. Shield volcanoes have summit calderas formed by piston-like subsidence. Subsidence occurs when large volumes of lava are emptied from underground conduits; withdrawal of support leads to collapse. Many smaller pit craters also occur along fissure zones on the flanks of the volcanoes. These form by collapse due to withdrawal of magma along conduits.

Volcanoes that build up from many slow, steady, flows of hot lava, are called shield volcanoes. This kind of volcano is low and broad with gently sloping sides. They look like a warriors shield.

On the basis of nature of erruptions
QUIET OR EXPLOSIVE (VIOLENT)
What causes quiet or violent eruptions?
•The amount of silica in the magma.
•The amount of water in the magma.
•Other gases in the magma.

QUIET ERUPTIONS:
•Some erupt magma quietly without any explosion.
•Volcanoes of Hawaiian islands erupt magma quietly.
PRODUCTS OF ERUPTION
1. Molten Magma – when the magma reaches the earth’s surface, it is called lava.
•A narrow pipe connects the volcanic vent with a magma chamber below.
•As magma rises through the pipe, gases and steam dissolved in the magma escape explosively through pipe.
Volcano occur in high frequency; lava fills cracks during eruptions

Grantitic Magma:-Magma is composed of granite.
•Contains large quantities of silica.
•Contains water and other gases.
•This magma is highly viscious.
•Eruptions are typically violent. Basaltic Magma :-Magma composed of basalt.
•Contains small quantities of water and other gases.
•Contains small quantities of silica.
•Minerals contain quantities of iron.
•Typically quiet eruptions.
2. Steam – abundant gas erupted from volcanoes
Other gases – sulphur dioxide, hydrogen sulphide, hydrogen, carbon dioxide, etc
Some gases inflammable and they burn near the volcanic vent.
•Explosive eruptions result in particles of dust, ash and rocks being thrown out in large quantities.
•Thus volcanoes erupt materials in gaseous, liquid and solid states.
•Some volcanic eruptions are accompanied by rainfall caused by eruption of large quantities of steam, which form huge clouds.








Landforms in Volcanic regions
•Landforms found in volcanic regions are classified into two major types.
1.Extrusive forms
2.Intrusive forms

EXTRUSIVE FORMS

•Are those formed by the solidification of lava on the surface of the land.
•Most typical extrusive landform found – a cone-shaped hill called volcanic cone
•When a volcano erupts explosively, a portion of the summit gets blown up to form a depression called crater.

•The volcanic cone is classified into various types based on type of materials forming the cone.
•Volcanic cones are called cinder cones when the material erupted consists of cinder and other solid particles.
•Cinder cones have steep slopes because they consist of particles of larger size.
•Most volcanoes start as cinder cones and grow into large volcanic hills with alternating layers of lava and ash.
•Such cones are called composite cones.
•Formed due to an explosive eruption followed by eruption of lava.
•Explosive eruption leads to the formation of a layer of ash
•This is followed by a quiet period and then the process gets repeated.

INFLUENCES OF VOLCANOES

Volcanoes cause distruction of life and property in the area around the site of volcanic eruptions. hot magma erupts in large quantities and flows on the slopes of the volcano destroying settlements, roads, and other structures.But there is also some benefits due to volcanoes like the soils derived from sheets of lava are very fertile,which helps in growth of good crops.
DESTRUCTION CAUSED DUE TO VOLCANO.
BENEFITS DUE TO VOLCANOES.

DESTRUCTION:
Volcanoes are the great threat to lives and properties of people.
Large quantities of ash get deposited over large areas of cultivated land rendering them unsuitable for cultivation
Huge clouds are formed and heavy rainfall may result after the eruption.
Volcanic eruptions in oceanic islands may lead to the formation of high waves and large-scale flooding of coastal areas destroying settlements and cultivated lands.
These are often accompanied by earthquakes which caused additional damage to area.
BENEFITS
The soil s derived from sheets of lava are quite fertile.
The black soils of the Deccan Plateau are retentive of moisture and suitable for the cultivation of cotton. Such fertile soils occur in java and southern Brazil.
Volcanic activity has resulted in the concentration of certain minerals. example the diamond mineral Kimberley in south Africa.
The springs and geysers in volcanic regions contain water rich in mineral salts which are useful medical value.
Volcanic regions are of scenic beauty and attract tourists. The yellow stone national park in the united states is region of tourists attraction.

QUIZ1. What is the name of molten rock that erupts from volcanoes? MAGMA - LAVA - VENT
2. What is the name of molten rock within the Earth's crust? MAGMA - LAVA - VENT
3. What is the name of the tube through which molten rock flows? PARASITIC - CONDUIT - BASE
4. In which part of the Earth would you find a magma reservoir? CRUST - PARASITIC - CONDUIT
5. Composite volcanoes are made up of layers of lava and ______. CONDUIT - ASH - MAGMA
6. What is the name of a smaller vent-structure on the side of some volcanoes? SUMMIT - MAGMA RESERVOIR - PARASITIC CONE
7. What is the name of the bowl-like opening of a volcano? SILL - CRATER - ASH
8. Are ash clouds emitted from sills? YES - NO
9. What is the name of an opening through which molten rock and gases escape from a volcano? CONDUIT - VENT - FLANK
10. The sides of a volcano are called its flanks. YES - NO

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