Beneath the seafloor Beneath marine sediments, oceanic crust records how magma cools into basalt, dikes and gabbro. Below the crust lies solid mantle rock. Sedimentary basins can also preserve organic matter and trap oil and natural gas.
Mud, sand and biological remains accumulate on the seabed. Burial compacts them into sedimentary rock. In some basins, organic-rich source rocks generate hydrocarbons, which migrate into porous reservoirs beneath sealing rocks.
Organic matter buried in sedimentary basins can generate oil and natural gas over geological time. Hydrocarbons migrate into pores in reservoir rock and accumulate beneath a low-permeability seal. A conventional trap may contain gas above oil, with formation water below. Deposits occur locally where source, reservoir, seal and burial history coincide.
Basaltic lava cools rapidly in seawater, often forming rounded pillows with chilled rims. These solid lava flows make up the upper volcanic part of much oceanic crust.
At spreading ridges, magma rises through fractures. Repeated intrusions cool into closely spaced, steep dikes that once fed eruptions on the seabed. The dikes preserve the crust’s former magma pathways.
Magma cooling slowly at depth grows larger crystals and forms gabbro. This coarse-grained rock makes up much of the lower oceanic crust. Its structure varies with magma supply and tectonic setting.
The Mohorovičić discontinuity, or Moho, is a seismic boundary between crust and mantle. Beneath typical gabbroic ocean crust lies peridotite, rich in olivine and pyroxene. This uppermost mantle is predominantly solid rock.
Beneath the seafloor Marine sediments Oil and natural gas Pillow basalt Sheeted dikes Deep gabbro Moho and uppermost mantle Ocean trench Earth’s interior Beneath the seafloor · Wikipedia 