VOLCANO and TYPES

 

VOLCANO

Volcano is a landform, a mountain, where molten rocks erupt through the surface of the planet. The volcano mountain opens downwards to a pool of molten rocks underneath the surface of the earth. A volcano is a vent in the earth's crust through which Lava, Steam, ashes etc. are expelled. 

The volcano eruption begins with the formation of magma in the lower section of the earth's crust. The earth's crust is made up of massive slabs called plates, which fit together like a jigsaw puzzle. The friction during the movement of plates causes earthquakes and volcanic eruptions with pressure, it travels upwards with tremendous force hitting solid rocks and other material and creates a new passage to the earth's surface. Once the magma reaches the air it is called lava

 

RING OF FIRE

A zone along the edge of Pacific Ocean that has many Volcanoes and Earthquakes. The horseshoe shaped 40,000 kilometre long belt is characterized by higher volcanic activity. The Ring of Fire is a string of volcanoes that runs around the edge of the Pacific Ocean. A string of 452 volcanoes stretches from the southern tip of South America, up along the coast of North America, across the Bering Strait, down through Japan, and into New Zealand.

 

HOW ARE THEY FORMED:

When tectonic plates collide and go through the process of subduction, it sets the foundation for a volcano. The overlapping of the tectonic plates causes the magma to break through the crust, which is the cause of a volcanoes' birth. When temperature and pressure rises, the rock melts and moves through the surface and crust, and releases gases and magma, volcanic eruption occurs.

Definitions Of Volcanic Terms:

1.  The narrow opening of a volcano is called Vent.

2.      The upper part of vent is a cup shaped depression called Crater.

3.      The rock material ejected during volcanic activity is deposited on surface of Earth. It is called Lava.

4.  Pyroclastic Flow is fluidized mixture of solid to semi-solid fragments. It is characterized by hot expanding gases that flows down the flank of volcanic edifice. Pyroclastic flows are mixtures of hot gas, ash and other volcanic rocks travelling very quickly down the slopes of volcanoes. They are one of the most dangerous hazards posed by volcanoes.

5.      Viscosity - the property of a fluid that resists the force tending to cause the fluid to flow

6.      Magma - molten material beneath or within the earth's crust, from which igneous rock is formed.

7.      Ash- the powdery residue of matter that remains after burning

8.      Caldera - A large crater formed by volcanic explosion or by collapse of a volcanic cone.

9.       A natural spring of mineral water at a temperature of 21°C (70°F) or above, found in areas of volcanic activity is called Hotspring.

10.   A hot spring that intermittently sends up fountain like jets of water and steam into the air is known as a Geyser.

 

VOLCANOES CAN BE IN ONE OF THREE PHASES: ACTIVE, DORMANT, AND EXTINCT.

·         Active volcanoes are ones that have erupted in the past 10,000 years, and that are likely to erupt again. An example is Mt. Saint Helens in U.S.A.

·      Dormant volcanoes are ones that have not erupted in the past 10,000 years, but still have the possibility, though unlikely. An example is Mt. Rainier in the U.S.A.

·         Extinct volcanoes are one which no eruption has occurred within historic times and future occurrences are highly improbable. An example is Mt. Ashitaka in Japan.

Volcanic Landforms

Volcanic landforms are controlled by the geological processes that form them and act on them after they have formed. Thus, a given volcanic landform will be characteristic of the types of material it is made of, which in turn depends on the prior eruptive behavior of the volcano. Here we discuss the major volcanic landforms and how they are formed Most of this material will be discussed with reference to slides shown in class that illustrate the essential features of each volcanic landform.

Shield Volcanoes

 A shield volcano is characterized by gentle upper slopes (about 5o) and somewhat steeper lower slopes (about 10o).

 Shield volcanoes are composed almost entirely of relatively thin lava flows built up over a central vent.

 Most shields were formed by low viscosity basaltic magma that flows easily down slope away from the summit vent.

 The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity increases, its thickness builds up on the lower slopes giving a somewhat steeper lower slope.

 Most shield volcanoes have a roughly circular or oval shape in map view.

 Very little pyroclastic material is found within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic material accumulate as a result of fire fountaining events.

 

Stratovolcanoes (also called Composite Volcanoes)

 Have steeper slopes than shield volcanoes, with slopes of 6 to 10o low on the flanks to 30o near the top.

 The steep slope near the summit is due partly to thick, short viscous lava flows that do not travel far down slope from the vent.

 The gentler slopes near the base are due to accumulations of material eroded from the volcano and to the accumulation of pyroclastic material.

 Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite volcanoes. Pyroclastic material can make up over 50% of the volume of a stratovolcano.

 Lavas and pyroclastics are usually andesitic to rhyolitic in composition.

 Due to the higher viscosity of magmas erupted from these volcanoes, they are usually more explosive than shield volcanoes.

 Stratovolcanoes sometimes have a crater at the summit that is formed by explosive ejection of material from a central vent. Sometimes the craters have been filled in by lava flows or lava domes, sometimes they are filled with glacial ice, and less commonly they are filled with water.

 Long periods of repose (times of inactivity) lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.

 

Cinder Cones

 Cinder cones are small volume cones consisting predominantly of ash and scoria that result from mildly explosive eruptions. They usually consist of basaltic to andesitic material.

 They are actually fall deposits that are built surrounding the eruptive vent.

 Slopes of the cones are controlled by the angle of repose (angle of stable slope for loose unconsolidated material) and are usually between about 25 and 35o.

 They show an internal layered structure due to varying intensities of the explosions that deposit different sizes of pyroclastics.

 On young cones, a depression at the top of the cone called a crater, is evident, and represents the area above the vent from which material was explosively ejected. Craters are usually eroded away on older cones.

 If lava flows are emitted from tephra cones, they are usually emitted from vents on the flank or near the base of the cone during the later stages of eruption.

 Cinder and tephra cones usually occur around summit vents and flank vents of stratovolcanoes.

 

Craters and Calderas

 Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and ash.

 Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcanic structure. The collapse results from evacuation of the underlying magma chamber.

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