lava ultramafic lava 2013-05-05T16:56:29Z ENVO ENVO:01000235 PLB Ultramafic lava is lava which, relative to other lava types, has very high magnesium oxide content and less than 45 percent silica by weight is thought to have erupted at temperatures of 1600 degrees Celsius. Due to its high temperature and composition, the viscosity of ultramafic lava is very low.