Re: what makes volcanos erupt

The simple answer is that magma finds its way to the surface and as soon as 
it gets to the surface, there is an eruption (by definition).

The path to the surface is not so simple.  Magma (molten rock still in the 
ground) rises as bouyant blobs from source areas in the earth's mantle or 
lower crust.  A typical magma blob might be from 1 to 10 km or more across. 
 These blobs rise until they encounter rocks that are too stiff for the 
magma to flow through.  Those stiff rocks must be fractured before the 
magma can move up any farther.  Fractures may already exist that the magma 
can rise up, or the rock may need to broken.  Either way, pressure within 
the rising magma can help the formation or the opening up of cracks.  
Sometimes a single large crack or a set of en-echelon cracks will open up 
from the magma chamber all the way to the surface.  Other times, a network 
of cracks will form, allowing blocks of rock to fall into the magma, making 
room for the magma to move upward.  Even in this case, a crack eventually 
has to be formed that goes all the way to the surface. The activity leading 
up to the formation of this final crack can take a very long time and is 
often characterized by a variety of small earthquake swarms (from cracking 
rocks) and by the venting of gasses (Which move through cracks more easily 
than the magma.)  There may also be tilting of the slopes of the volcano as 
magma gets closer to the surface and the magma chamber enlarges.

Once the magma chamber is connected to the surface by a crack, things can 
proceed quite rapidly.  Before this final crack existed, the magma was 
being pushed down on by the weight of the overlying rock.  Even for shallow 
magma chambers, the pressure can be hundreds to thousands of atmospheres.  
As soon as the crack is open, however, the top surface of the magma is 
experiencing only 1 atmosphere of pressure, coming from the air in the 
crack.  

Almost all magmas contain dissolved gasses and the sudden pressure drop 
leads to those gasses coming out of solution, just like CO2 comes out of 
solution when you open a soda bottle.  In magmas, the formation of bubbles 
is known as vessiculation.  The big pressure drop in volcanos is like what 
happens when you shake a soda bottle and then open it.  Also, magma is 
stickier than soda and so the bubble tend to stay trapped in the magma.  
All that gas takes up a LOT more space as a gas than it did in solution and 
so the volume of the magma is suddenly increased by 10 or 100 times.  The 
magma chamber is surrounded by solid rock and cannot expand, so that extra 
volume must get out of the chamber somehow and it does so by squeezing out 
the crack and onto the surface.  Now the volcano is erupting!  If the magma 
is not too sticky and doesn't have too much gas, the excess can splat and 
burble out fairly quietly, like it does on Hawaii.  If the magma is sticky 
and has lots of gas, then the foaming magma will come out explosively, like 
at Mt Pinatubo, Soufrierre, or Mt St. Helens.

So, the formation of a crack leads to a pressure drop, which leads to 
foaming, whcih leads to volume increase, which drive magma out of the 
chamber up the crack that led to the pressure drop to begin with.  The 
eruption will continue as long as the pressure in the magma chamber is 
greater than atmospheric pressure and can keep pushing magma out of the 
chamber.

Some basic information about volcanos and volcanic eruptions can be found 
at:http://www.cotf.edu/ete/modules/volcanoes/volcano.html

David Smith
La Salle University, Philadelphia, PA