Notes on where volcanoes get their magma!

A common mistake is that the Earth's Mantle is liquid and that volcanoes get their magma when
the Crust breaks and this magma leaks through....

     
NOT TRUE!!!!  The Mantle is SOLID very HOT rock that is under a LOT of pressure.
                              It is under TOO MUCH PRESSURE to melt even though it is very hot.



Magma DOES come from the Mantle and the Crust though, but what could cause these
solid rocks to melt?

There are three different things that allow the rocks to melt.....

1)  Lowering the pressure will allow hot rocks to melt in the Mantle.


The Crust can get stretched and thinned at divergent boundaries.  When this happens, the
"overpressure" on the Upper Mantle lowers and the hot rocks in the Upper Mantle can melt.  
Blobs of magma rise up and can come to the surface and produce volcanoes and fissure flows.  
(Examples:  Mt. Tom, Mt. Kilomanjaro, the Fissure Flow at Turners Falls).  If the divergent plates
above keep spreading out, the Asthenosphere will come up to the surface and start creating
oceanic crust.  (Example:  The Mid-Atlantic Ridge)

2)  Raising the temperature will allow rocks in the Mantle to melt, even though they are
under pressure.  It will also allow rocks in the Crust to melt.


There are two ways to raise the temperature:
Friction and Heat Transfer from further below.

Friction happens when plates converge.  If an oceanic plate subducts, it will add heat to the
Upper Mantle and lower Crust, causing it to become hot enough to melt.  This makes silica rich
magma, and when this magma makes it to the surface, it makes very explosive volcanoes.  
(Examples:  Mt. St. Helens, Mt. Merapi).  When continental plates converge, the friction of
mountain building will melt rocks, but the magma rarely makes it to the surface before it cools.  

Heat can be transferred from the Outer Core to the Mantle and rise up in plumes of superheated
mantle rocks which melt and rise up through the Mantle.  This creates a HOTSPOT volcano (like
the ones in Hawaii).


3)  Changing the chemistry of the rocks in the Mantle will LOWER the melting
temperature.  Particularly ADDING WATER to the Mantle will lower the melting
temperature of those rocks.


How does the earth get water down to the Mantle?  SUBDUCTION.  Oceanic plates are soaked
with ocean water, and carry that water down into the Mantle with them.  The water lowers the
melting temperature of the rocks being subducted and the rocks around them, making it easier
for the friction of subduction to melt the rocks of the Upper Mantle and lower Crust.