Rare fragments of olivine-rich metamorphic rock were brought to the surface in a few Pinacate eruptions. This microscopic view of an area 4 mm across uses polarized light to illuminate a paper-thin slice of such a fragment that contains five different minerals. The chemistry of these mineral grains indicates that they originated in the earth's mantle about 30 km (18.6 mi.) below the surface, where the temperature is 870° C (about 1600° F). Pinacate lavas originated by partial melting of the mantle, albeit at far greater depth and higher temperature.