This image was compiled from a series of photos taken through a microscope
under cross-polarized light (actual size: 27 x 43 mm.). Polarized light
is a special filtering effect allowing one to identify specific mineralogies
by their inherent optical properties. The coarser crystals suggest the
magma from which the rock crystallized underwent an initial period of slow
cooling. Slow cooling occurs only at depth and hence we can assume there
likely existed a magma chamber deep below the volcano containing the originally
liquid magma. Once this magma reached the surface it cooled much more rapidly
producing the fine-grained crystal matrix surrounding the coarse crystals.
The overall chemical composition of the sample, as inferred from the minerals
present, suggests that the magma derived from partially remelted ocean
crust. The average ocean crust is similar in composition to an andesite
except that it contains heavier elements such as iron and magnesium. When
oceanic crust is partly melted these elements remain while the silica enriched
liquid rises up through the crust and crystallizes as an andesite volcanic
rock.