Vent Formation

Hydrothermal vents form when seawater seeps into cracks in the earth's crust, becomes superheated by magma, picks up and loses minerals on its way, then rises and bursts out into the ocean again through holes in the seafloor.

Vent openings range from 1.27 cm to 1.83 m (one-half inch to six feet). Some occur alone, while others appear in clusters or multi-cluster fields. In the Trans-Atlantic Traverse (TAG) scientists have counted hundreds of active vents and thousands of extinct chimneys in a vent field that is the size of the Houston Astrodome. It is located about 3.2 km (2 miles) under the ocean's surface, 2400 km (1500 miles) southwest of the Azores.

Why do cracks form? The theory of plate tectonics helps explain this common occurrence.

Plate Tectonics Theory

About twenty separate, cool plates of rock of various sizes make up the earth's crust. The Mid-Ocean Ridge marks these plate boundaries. The plates float above fiery magma, whose heat makes them come together, move apart, and slide past each other. Sometimes the plates come together until one slips under the other, or subducts, to be recycled in the magma; both deep ocean trenches and high mountains are formed in the process.

Oceanic crust is usually basalt, while continental crust is lighter but thicker granite.

When the plates under the ocean move apart, magma rises, often violently, from inside the earth's mantle to fill the gap, solidifies quickly in the icy sea water, and forms new crust. This process is called seafloor spreading.

The white arrows show the separation caused at the Mid-Ocean Ridge as the earth's tectonic plates move apart.

Image courtesy of NOAA.

Cross-section of geological formations under the ocean.

1 - Ocean surface. 2 - Continental shelf. 3 - Continental slope. 4 - Continental rise. 5 - Abyssal plain. 6 - Mid-Ocean Ridge. 7 - Ridge valley. 8 - Hydrothermal vent. 9 - Magma chamber.