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.

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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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