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.
This image is a graphic of seafloor spreading. At the top is the ocean floor with three
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.
Neat Fact: 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
Plate Tectonics Theory
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.
Neat Fact: Oceanic crust is usually
while continental crust is lighter but thicker
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.
ridges or mountains. In the front of the graphic we see the ocean floor that is the top
of two of the earth's tectonic plates spreading apart and causing a gap. Magma from below
is welling up to "repave" the gap with hot lava that will push aside old floor to form new
An estimated 20 cubic kilometers (4.8 cubic miles) of molten basalt are produced each year by
this spreading that "repaves" parts of the ocean floor, or enough to cover all the U.S. interstate
highways with a layer of rock 3 meters (10 feet) deep.
Just as cracks eventually develop in asphalt highways and driveways, cracks form in the ocean's
pavement due to the shifting of the earth's plates. Tectonic plates move 1.27 to 15 cm (0.5 to 6
inches) a year, but the movement occurs in bursts, and different plates move in different motions
at different speeds. For example, in the
East Pacific Rise, the
Pacific plate moves west at 13 cm
(5 inches) per year, while the Nazca plate
is shifting east about 7.6 cm (3 inches) per year. This movement eventually causes cracks to appear
in the new crust. Seawater seeps in and the process of
The new pavement shoves the older pavement to either side to form ridges.
The theory of plate tectonics helps us understand why hydrothermal vents are often found in areas of
active seafloor spreading along the earth's Mid-Ocean Ridge.
This graphic shows the basic geological formations under the ocean.
This image is a graphic showing what is below the sea: the gradual descent of land known as
the continental shelf, slope and rise that leads to the abyssal plain, or relatively flat ocean
bottom on either side of the mountainous Mid-Ocean Ridge. The graphic shows the
magma deep in the earth, the ridge valley between two peaks, and the hyrothermal vents that
have formed on these peaks.
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.
Underwater Chimneys? -->
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