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Volcanoes and Volcanic Eruptions

The term volcano means the vent from which magma and other substances erupt to the surface Or when molten rock erupts from an opening on Earth's surface, volcanoes develop.

Note: 75% of the world's active volcanoes are underwater.
. The ash cloud from a large volcanic eruption can be up to 60 kilometres (40 miles) high.

   Mount St. Helens Volcano
   (Copyright: Getty Images)
Volcanoes are classified as active, dormant or extinct, based on how they erupt. An "active" volcano is one that is known to have erupted in recent history. A volcano that has not erupted recently but might erupt again is called "dormant". A volcano that has stopped erupting altogether is called "extinct".

Since volcanic eruptions are caused by magma (a mixture of liquid rock, crystals and dissolved gas) expelled onto the Earth's surface.
Three Basic types of magma:
1. Mafic or Basaltic- SiO₂ 45-55 wt%, high in Fe, Mg, Ca, low in K, Na
2. Intermediate Or Andesitic- SiO₂ 55-65 wt%, intermediate in Fe, Mg, Ca, Na, K 
3. Felsic Or Rhyolitic- SiO₂ 65-75%, low in Fe, Mg, Ca, high in K, Na

Temperature of Magmas:
. Mafic/Basaltic: 1000-1200 degree celsius
. Intermediate/Andesitic: 800-1000 degree celsius
. Felsic/Rhyolitic: 650-800 degree celsius

* Products Of Volcanic Eruptions:
When magma reaches the surface of Earth, it is called lava. Since it is a liquid, it flows downhill in response to gravity as a lava flows. Different magma types behave differently as lava flows, depending on their temperature, viscosity and gas content.

. Pahoehoe Flows: Basaltic lava flows with low viscosity, start to cool when exposed to the low temperature of the atmosphere. Lava flows that initially have a smooth surface are called pahoehoe flows. Pahoehoe flows tend to be thin and, because of their low viscosity travel long distance from the vent.
    Pahoehoe Lava Flow, Kilauea Volcano, Hawaii,      November 1985 
    (Copyright: J.D Griggs, U.S. Geological Survey)

. A'A' Flows: Higher viscosity basaltic and andesitic lavas also initially develop a smooth surface skin, but this is quickly broken up by flow of the molten lava within and by gases that continue to escape from the lava. This creates a rough, clinkery surface that is characteristic of an A'A' flow.
                                                                                               (Copyright: Pinterest.com)

. Lava Tubes: Once the surface skin becomes solid, the lava can continue to flow beneath the surface in lava tubes. The surface skin insulates the hot liquid lava form further cooling. When the eruption ends, liquid lava often drains leaving an open cave.
                                                                                              (Copyright: Forbes.com)

. Pillow Lavas: When lava erupts on the sea floor or other body of water, the surface skin forms rapidly, and like with pahoehoe toes inflates with molten lava. Eventually these inflated balloons of magma drop off and stack up like a pile of pillows and are called pillow lavas.
 Copyright: blog.education.nationalgeographic.org

* Types Of Volcano:
When magma erupts at the surface it can form different types of volcanoes depending on the viscosity, or stickness, of the magma, the amount of gas in the magma, and the way in which magma reached the surface.

. Shield Volcanoes: It forms where a volcano produces low viscosity, runny, lava it spreads far from the source forming a volcano with gentle slopes. This type is called shield volcano.
- A shield volcano is characterized by gentle upper slopes (about 5 degree) and somewhat steeper
lower slopes (about 10 degree).
- 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.
    (Copyright: scientiafantastica.wixsite.com)

. Stratovolcanoes (also called Composite Volcanoes): These volcanoes are majestic giants with steep sides and a symmetrical cone shape. They form from very thick, viscous, or sticky lava that won't flow easily. 
- Have steeper slopes than shield volcanoes, with slopes of 6 to 10 degree low on the flanks to 30 degree 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.
                                                                                               (Copyright: earthhow.com)

. 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.
- An example of cinder cone is Paricutin Volcano in Mexico. This volcano was born in a farmers corn field in 1943 and erupted for the next 9 years.
   (Copyright: maggiesscienceconnection.weebly.com)

. Craters and Calderas: Calderas 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.
                                                                                               Crater (Copyright: lpi.usra.edu)

                                                                                                Caldera (Copyright: study.com)

* Can we Predict Volcano Eruptions?
Volcanoes give warning of pending eruption like, small earthquakes, swelling or bulging of the volcanoes sides, and increased emission of gases from its vents. None of these signs mean an eruption is imminent, but they can help scientists evaluate the state of the volcano.

However it's impossible to say exactly when, or even, if any volcano will erupt.


Reference:
 



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