Komatiites erupted billions of years ago as pulsating streams of white-hot lava. Their unusual chemical compositions and exceptionally high formation temperatures produced highly fluid lava that crystallized as spectacular layered flows. Investigation of the extreme conditions in which komatiites formed provides important evidence about the thermal and chemical evolution of the planet, and the nature of the Precambrian mantle. This monograph, written by three experts with long experience in the field, presents a complete account of the characteristics of komatiites including their volcanic structures, textures, mineralogy and chemical compositions. Models for their formation and eruption are evaluated (including the anhydrous vs. hydrous magmas controversy). A chapter is also devoted to the valuable nickel and copper ore deposits found in komatiites. Komatiite is a key reference for researchers and advanced students interested in petrology, Archaean geology, economic geology, and broader questions about the evolution of the Earth's crust and mantle.