The Penrose field conference on ophiolites in 1972 redefined the term ophiolite to include only the igneous rocks listed above, excluding the sediments formed independently of the crust they sit on.[1] This definition has been challenged recently because new studies of oceanic crust by the Integrated Ocean Drilling Program and other research cruises have shown that in situ ocean crust can be quite variable, and that in places volcanic rocks sit directly on peridotite tectonite, with no intervening gabbros.