Magma
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  During a volcanic eruption heated rock rises from the asthenosphere to levels with lower overlying pressure. As it melts to form magma, the volcano expands and the surrounding rocks become heated and may melt and crack due to the pressure. As the magma moves upwards, the pressure decreases and it may become even more fluid. The gases are dissolved  in the magma but, as the pressure continues to fall as the magma nears the surface, the gases come out of solution and form bubbles. These expand and help to propel the magma on to the surface. Volcanic eruptions can occur through long cracks in the ground called fissures in the earths crust, as a fissure eruption (below) or through a more concentrated area to form a volcano.

73% of magma which reaches the earth's surface occurs on Constructive Plate Boundaries. The main areas are mid-ocean ridge systems where melting of the upper mantle produces basaltic magma.  The eruptions tend to be non-violent and frequent and as they occur on the sea floor they do not represent a major hazard to people except where portions of the ocean ridge emerge above sea level to form inhabited islands like Iceland.
Doming under the continental crust like in East Africa - which may be the birth of a Constructive Plate Boundary - tends to produce unusual ultrabasic lavas.

  
 

Fissure Eruption typical of Iceland (Constructive Plate Boundary)

  
  Magma is a mixture of molten rock and gases, and a key characteristic is its viscosity.The is a measure of its internal resistance to flow. The lower the viscosity the more fluid the magma. Three factors affect viscosity....
1. Higher temperatures cause atoms to spread out and decrease in density, so magma at higher temperature is less viscous and will flow more easily.
2. Dissolved gases make the magma more fluid and buoyant. The higher the temperature the more the gases remain dissolved and the less viscous the magma.
3. the amount of silica in the magma. Silica has close jointing and packing of atoms and makes flow slow, and it is more difficult for gas to escape.
Basalt lava is low in silica which is called a 'basic' chemical composition.  They are hotter at 1000*C to 1200*C and have low viscosity. Gases are released more easily and the eruptions tend to be less explosive like the fissure eruptions of Constructive Plate Boundaries like Heimaey 1973 in Iceland or like lava flows from Kilauea in Hawaii at an Intra-Plate Hot Spot which builds shield volcanoes. Shield Volcanoes have gentle sides because the lava is runny and moves away from the vent quite freely before solidifying. They are very tall wide structures compared to conical (composite) volcanoes - although in places like Hawaii much of the volcano may be below sea level.		  
   
  Hot Spots are small areas of crust with an unusually high heat flow and are found away from plate boundaries. Slowly rising mantle rocks create volcanic activity on the surface. The movement of lithospheric plate over the hot spot produces a chain of what are mainly now extinct volcanoes. Hawaii is the most famous example and Kilauea at the southern edge of the main island is the world's most active volcano. The eruptions are of low viscosity basalt lavas and are relatively 'safe'.  
   
  Lavas with high silica content have an acidic chemical composition and form rocks called rhyolites. They are viscous and have relatively low temperatures of 600*C to 1000*C. The lavas flow slowly, the gases do not escape easily and build up to produce more explosive eruptions. They are typical of Destructive Plate Boundary volcanoes either ocean v. ocean plate collisons or ocean v. continent plate collisions.
Andesite lava is intermediate but rather silica rich and therefore also likely to be quite explosive in eruption. Andesite volcanoes are also typical of Destructive Plate Boundary subduction zones. These volcanoes have steeper sides and are smaller in scale tahn shield volcanoes - they have the typical volcano  conical shape and are often alternating layers of ash and pyroclastic lavas called a composite volcano.

Some 80% of the world's active volcanoes occur along Destructive Plate Boundaries. As the ocean plate descends into the mantle and melts the magma rises up and mixes with continental crust (i.e. at an ocean plate v. continental plate Destructive Plate Boundary). The magma has a higher silica content than along ocean ridges. These andesitic and rhyolitic magmas produce the most violent volcanic activity (but of lower frequency e.g. 500 years of dormancy). Volcanic island arcs form where two ocean plates collide (i.e. ocean plate v ocean plate Destructive Plate Boundaries) and produces both andesitic and basaltic magmas.

  
 
Composite Volcano (Destructive Plate Boundary)