An ophiolite is a slice of oceanic crust and underlying mantle that is thrust onto a continent by obduction. The stratigraphic sequence observed in an ophiolite corresponds to the layering observed of oceanic crust (from top to bottom): an upper layer of oceanic sediment (siliceous or carbonate ooze); a layer of pillow basalt followed a layer of sheeted near-vertical dykes; layers of gabbro and plagioclase or mafic cumulates; and a basal layer of “tectonite,” the deformed harzburgite (a type of peridotite) of the uppermost mantle. Many ophiolites contain only some of these lithologies.
Debate surrounds the question of whether any ophiolite represents normal oceanic crust formed at a mid-oceanic spreading centre. The geochemical compositions of basalt rocks indicate that many ophiolites are segments of crust that formed in back-arc basins. Reactions between water and ophiolite at high temperatures (300 °C) can produce low-molecular hydrocarbonsby abiotic...