9.2 The Geology of the Oceanic Crust

GEOL1340

Decompression Melting

The process by which hot asthenospheric mantle rock rises upward at divergent plate boundaries and partially melts due to decreasing pressure rather than added heat, producing mafic (basaltic) magma.

Oceanic Crust

The thin (~7-8 km), dense (~3.0 g/cm³) crust underlying ocean basins, composed primarily of mafic igneous rocks (basalt and gabbro) that float lower on the mantle.

Layer 1 (Deep-Sea Sediments)

The topmost pelagic layer of loose mud, clay, and siliceous/calcareous oozes that gradually thickens with distance and age from mid-ocean ridges.

Layer 2A (Pillow Basalts)

Extrusive igneous basalt formed when mafic lava erupts subaqueously and rapidly quenches into smooth, bulbous, pillow-like structures upon contacting cold seawater.

Layer 2B (Sheeted Dikes)

A dense network of vertical, tabular igneous intrusions representing the feeder conduits where rising magma filled extensional fractures at spreading centers.

Layer 3 (Intrusive Gabbro)

The thickest lower crustal layer composed of coarse-grained, phaneritic plutonic gabbro that cooled slowly in subterranean magma chambers.

Mohorovičić Discontinuity (Moho)

The seismic velocity boundary marking the transition between the base of the oceanic crust (gabbro) and the underlying upper mantle (peridotite).

Upper Mantle Peridotite

The dense, ultramafic rock layer located directly beneath the crust-mantle boundary.

Ophiolite

A complete, tectonically uplifted and emplaced sequence of ancient oceanic crust and upper mantle preserved on land (e.g., Gros Morne, Newfoundland).

Hydrothermal Metamorphism

The chemical alteration of hot ocean crust by circulating seawater superheated near magmatic chambers, producing hydrous green minerals (chlorite, epidote).

Black Smokers

Hydrothermal vents that discharge superheated, metal-rich water into the deep ocean, instantly precipitating dark sulfide minerals (iron, copper, zinc) to form chimney-like structures.

White Smokers

Hydrothermal vents that emit cooler, lighter-colored fluids rich in barium, calcium, and silicon sulfates.

Wilson Cycle

The tectonic life cycle of ocean basins involving continuous creation at divergent mid-ocean ridges and destruction at convergent subduction zones.

Age of Oceanic Crust

The maximum age of the seafloor is relatively young (~180 to 280 Ma) because older crust is continually recycled via subduction or obducted onto continents.

Sea-Floor Spreading Rates

The velocity at which tectonic plates diverge; fast spreading occurs in the Pacific (~10 cm/yr per side), while slow spreading occurs in the Atlantic (<2 cm/yr per side).

Magnetic Reversal Chronology

The pattern of normal and reversed magnetic polarity recorded in iron-bearing minerals within basaltic seafloor crust, used to date oceanic lithosphere.

Seamounts and Ocean Islands

Submerged or emergent underwater volcanoes formed either at divergent boundaries, subduction zones, or above stationary mantle plumes (e.g., Hawaii-Emperor chain).

Fringing Reef

A coral reef that forms directly along the shoreline of a tropical volcanic island.

Barrier Reef

A coral reef separated from a subsiding volcanic island by a deep lagoon of water.

Atoll

A ring-shaped coral reef enclosing a lagoon that remains after a volcanic island has completely subsided beneath sea level.