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North Cascades National Park:
Volcanoes and Their Roots: Making room for magma
Making room for magma in cross sectional views
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).
Making room for magma in cross sectional views
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This site is a cooperative endeavor of the
US Geological Survey Western Earth Surface Processes Team
and the National Park Service.
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This page was last updated on 12/1/99
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Material in this site has been adapted from a new book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS and published by The Mountaineers, Seattle