How does an eruption happen?
     An eruption begins when pressure on a magma chamber forces magma up through the conduit and out the volcano's vents.  When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma.  The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam.
    Obstacles also influence the type of eruption.   When the pipe is blocked by a stopple or an accumulation of pumice, the pressure in the pipe will build up very high resulting in an explosion.      When magma reaches earth's surface it is called lava.  It may pour out in gentle streams called lava flows or erupt violently into the air.  Rocks ripped loose from the

inside of the volcano or torn apart by the gas may be shot into the air with the lava.   These rocks blown out of a volcano are called pyroclastic rocks.  The rock fragments fall back to earth in many different shapes and sizes:

Dust - particles less than 1/100 inch in diameter
Dust particles may be carried great distances. In a powerful eruption they may be carried around the earth several times.
Ash - fragments less than 1/5 inch in diameter
Most volcanic ash falls to the surface and cemented together by water to form a rock called volcanic tuff.
Bomb - A rounded piece of newly hardened lava which takes shape while flying through the air.

Block - A piece of lava that has sharp corners.
Cinder - Bubbly rock formed by liquid lava cooling in the air.
Pumice - Cinder so bubbly that it floats in water.

     Volcanic activity is classified by how often a volcano erupts.  A volcano may be active, intermittent, dormant, or extinct.  Active volcanoes erupt constantly.  Intermittent volcanoes erupt fairly regularly.  Dormant volcanoes are inactive, but not long enough to determine whether they will erupt again or not.  Extinct volcanoes have been inactive since the beginning of recorded history.

 

Different Types of Eruptions
     Since volcanoes erupt different from eachother, depending on the kind of rising magma as well as other factors , we can distinguish various eruption types/styles. 
     Submarine eruptions occur under the sea chiefly emerging from vents aligned along fissures of mid-ocean ranges.
     Because the ranges lies far beneath sea level the eruption probably won't even be noticed from the surface.  Because of the pressure of the sea, gases and steam remain in solution which stifles any potential explosions.  The lava will roll down off the slopes quietly in blocks, called pillow-lava.
     If submarine activity continues long enough a volcanic island may emerge.  Volcanic islands, such as Stromboli in Hawaii and Volcano in Italy began their lives as seamounts.  They are particularly common in the South Pacific Ocean.      Hydrovolcanic refers to the activity of volcanoes associated with water, HYDRO.  The two main groups are:
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     Surtseyan eruptions take place mainly in shallow seas and lakes.  Because the water is shallow, as opposed to submarine eruptions, the pressure exerted by water is relatively low.
     Hydrovolcanic explosions can easily develop when the water is less than 100m deep.  The sudden transfer of heat from magma at a temperature of 1200C to water at less than 20C causes the water to transform into steam.   The expansion of water releases energy which shatters the magma.   The explosive expansion of the steam thus takes place near the surface in an open vent.  The generated energy can be dispersed into eruptions which may last until the magma stops rising or until a mass of fragments prevent water to invade the conduit.   Shattered fragments of magma accumulate around the vent in fine and thin beds known as tuffs.
     During the eruption ash and steam clouds may form reaching to 5km in height.  However, particular to Surtseyan eruptions are the thick,dark-pointed jets of fragments, often headed by bombs, shot out of the vent.  
     Surtseyan eruptions are named after activity of Surtsey, a volcano situated in Iceland which grew from the ocean floor at a depth of 130m to the surface.      Phreatomagmatic eruptions are eruptions characterized by violent steam explosions.
     Surface water may flow through fissures in the Earth's crust and interact with the rising magma causing the explosions.  Country rocks, steam and shattered pieces of magma  are ejected.
     Such eruptions are often brief because the water is limited compared with the amount available during Surtseyan activity.  Nevertheless, phreatomagmatic eruptions are violent and they may form maars on the surface.
     The phreatomagmatic eruption ends when the water supply is exhausted, and not, as in most other eruptions when the magma stops rising. 		 

 

 

 

 

 

 

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