MadSci Network: Earth Sciences

Re: what is the pressure for metamorphism generated by?

Date: Sun Jun 1 14:17:25 2003
Posted By: David Scarboro, Associate Lecturer
Area of science: Earth Sciences
ID: 1054245250.Es

Dear Christin,

Metamorphism takes place under conditions of great heat and pressure deep 
in the Earth’s crust.  There are two types of metamorphism, 
called “regional metamorphism” and “contact metamorphism”.  It is in 
regional metamorphism that the role of pressure is vital.

Regional metamorphism gets its name from the fact that it usually takes 
place over a wide area, typically an area in which a collision is taking 
place between the tectonic plates that comprise the Earth’s crust.  As 
tectonic plates move very slowly across the globe they pull apart in some 
areas (the mid-ocean ridges) and collide in other areas.  Where two 
continents collide the result is the formation of a new mountain range.  
Imagine two great crustal plates slowly pushing together, as is happening 
today in the Himalayan region of south Asia.  Tectonic collisions generate 
enormous pressures, as the two plates press together and compress the 
rocks in the collision zone, and it is this pressure that is responsible, 
along with heat, for regional metamorphism.  This happened in the late 
Palaeozoic era when Africa and North America collided and the Appalachian 
Mountains were formed.  Regional metamorphism was responsible for the 
formation of a long belt of metamorphic rocks along the eastern side of 
the Appalachians, in what is now the piedmont between the mountains and 
the coastal plain from Maryland to Georgia.
Rocks deep in the crust (up to 10s of kilometres deep) that are caught up 
in a tectonic collision will be slowly squeezed under tremendous 
pressure.  They will also be hot because heat increases with depth in the 
Earth.  The heat and pressure that cause metamorphism are not enough to 
cause rocks to melt, but are enough to cause the atoms in the crystals of 
the minerals that compose the rocks to become mobilized and rearrange 
themselves to form new minerals.  The end result is a rock that has 
recrystallized and changed from whatever it was to start with to become a 
metamorphic rock, such as gneiss, schist or slate.

The other type of metamorphism, contact metamorphism, takes place at much 
shallower depth in the crust, typically only a few kilometres at most.  
Magma chambers, such as beneath volcanoes, can exist for thousands or even 
millions of years, and the great heat contained in the molten magma 
will “bake” the surrounding rocks, creating an aureole of metamorphosed 
rock around the chamber.

I hope this answers your question.

Best wishes,

David Scarboro

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