Geography ocean features

Seamount

An underwater volcano that stays below the ocean surface.

Volcanic island

A seamount that has grown above the ocean surface.

Guyot

A flat-topped seamount with its summit below sea level.

Island chain

A line of volcanic islands formed as a plate moves over a hotspot.

Atoll

A ring of coral around a lagoon, left after a volcanic island has sunk or eroded away.

Mid-ocean ridge

An underwater mountain chain where plates move apart and new crust forms.

Slow-spreading ridge

A ridge spreading 2-4cm per year with a rift valley and rough relief.

Fast-spreading ridge

A ridge spreading ~16cm per year with no rift valley and smooth relief.

Ocean trench

A long, narrow, very deep depression where oceanic crust is subducted.

Island arc

A curved line of volcanic islands formed along a subduction zone.

Abyssal plain

A flat area of the deep sea floor covered by fine sediment (clay and silt).

Submarine canyon

A steep-sided valley cut into the continental slope by rivers and turbidity currents.