Making room for magma in cross sectional views. |
The back-country hiker who stands in the upper Baker River Valley will
look up at forbidding cliffs of granitic rock of the Chilliwack batholith
all around and, realizing that he or she stands in the middle of a once
vast chamber of molten rock, might wonder (especially with prompting from
this website) how all this granitic rock found room for itself in the rocks
that were here before the magma arrived. No vast cave could exist in the
crust at the depths and pressures where this magma cooled. Geologists have
proposed a number of scenarios including having the invading magma punch
the roof rocks above the pluton up like a piston, or crowding the invaded
rocks aside. If the older rocks just melted to make the magma, not much
new room would be necessary but that does not seem to work. We know that,
for the most part, not much of the surrounding country rock (at the level
where we now see the batholith) was melted and incorporated into the plutons.
The chemical compositions of the plutons do not reflect additions from
the surrounding rocks.
Multiple diapirs after Paterson, Fowler, and Miller (1996). |
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