UMass Sesquicentennial

Magma Mixing

What are those dark blobs? They are what geologists call "magma pillows". What we are looking at in the photo is part of a preserved magma chamber (the volcanoes it is related to have eroded away). In some magma chambers, you can more than one type of magma-- each having their own chemical composition, and, therefore, different physical properties when molten, including density, buoyancy, etc... Magma pillows such as this are a classic "magma mingling" texture seen in granites and associated rocks and are evidence of magma of one type of composition being injected into a chamber filled with magma of a different composition-- the darker blobs consist of a much more iron and magnesium rich magma, relatively low in silica content, and the white rock around it being a much more aluminum and silica rich magma. The two don't mix particularly easily under certain conditions, and pillow-shaped blobs, such as these, form.

Image taken during the 2004 NEIGC field conference. Stay tuned for information regarding this year's NEIGC field conference.


The MA Geological Survey
Department of Geosciences
269 Morrill Science Center
University of Massachusetts
611 North Pleasant Street
Amherst, MA 01003-9297

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