The ophiolite sequence in a set of pebbles. When slabs of oceanic crust and upper mantle are emplaced on land during continental collisions, say, they are called ophiolites. They are evidence for the presence of a former deep ocean. From top to bottom here: red chert (former siliceous sediment on the seafloor), basalt (lava on the seafloor), diabase (with a white quartz vein; these were the feeder dikes for the lavas), gabbro (rock formed in the magma chamber), and harzburgite (mantle rock from which the basaltic magma was extracted). The white rock at the bottom is limestone representing continental crust on which the ophiolite was emplaced. (Pebbles are from a beach in Turkey, a country with several ophiolites.)
