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.