Page 176 - (DK) Ocean - The Definitive Visual Guide

174 THE OPEN OCEAN AND OCEAN FLOOR

Seamounts and Guyots

SEAMOUNTS ARE TOTALLY submerged, undersea PEOPLE
mountains that rise at least 3,300 ft (1,000 m) from HENRY GUYOT
the sea floor; smaller ones are called sea knolls. Guyots
are seamounts that once rose above sea level—as a Arnold Henry Guyot (1807–1884)
was the first professor of geology
result, they have a flat top caused by erosion. Often at Princeton University. He set up
isolated in deep ocean, seamounts and guyots provide a system of weather observatories
that led to the formation of the
a habitat for marine life adapted to shallower water. US Weather Bureau. Guyots were
The obstruction of a seamount forces nutrient-rich, named in his honor by a later
Princeton geology professor,
deep-sea currents to rise closer to the surface, forming Harry Hass. Hass discovered guyots
using echo-sounding equipment
eddies above the seamount. These trap nutrients and
during World War II.
support plankton, which in turn attract shoals of fish.

Geological Origins EVOLUTION OF A GUYOT
Seamounts start as undersea volcanoes, where a rift in
the sea bed allows volcanic eruptions. Many arise at direction of plate
movement
rifts on the crest of mid-ocean ridges, formed by the
movement of tectonic plates (see p. 185). Because
these rifts are generally linear, seamounts tend to be
elliptical or elongated in shape. They are made of A B A C B A
volcanic basalt rock, but a thin layer of marine
sediment accumulates over time. Seamounts often
occur in chains or elongated groups, either because
there are several weak spots along a rift, or because a
1 A guyot (A) begins life when a 2 Over millennia, erosion reduces 3 As the island moves farther, it
series of seamounts originated sequentially at a single, volcano erupts above a “hotspot,” the island to a flat top at sea level, sinks and forms a guyot. New
stationary volcanic hotspot. Sometimes volcanic creating a small volcanic island. while it (A) moves away from the islands (B and C) erupt from the
eruptions break above the ocean surface to form hotspot. A new island (B) forms. hotspot.
island chains, and these may continue out to sea as
a line of guyots, or tablemounts. Newly formed
volcanic rock is easily eroded, so over time, the Upwellings
above-water peak of the volcanic island is eroded
down to a flat top. Then, as the ocean plates carry The open ocean is mainly barren, because cold,
it away from the zone of nutrient-rich currents are confined to deep water, far
volcanic activity, the SEAMOUNT FORMATION beneath the reach of plankton. Seamounts—which
A seamount forms from an
flat-topped guyot sinks stand up to 13,000 ft (4,000 m) above the sea bed—
underwater volcanic eruption.
beneath the surface. Erosion here is slower than on form a major obstruction to these currents, diverting
land, so it remains conical. them and pushing them upward. This brings an
upwelling of nutrients into the sunlit zone, and allows
World Distribution phytoplankton to flourish. As these nutrient-rich currents rush over
the top of the seamount, they split in two and sweep around it. This
There may be 100,000 seamounts and guyots in the oceans, makes the water above the seamount rotate, encircling a cylindrical
but few have been mapped or explored and the total number is column of still water that trapped nutrients spiral
unknown. Seamounts may occur either singly or in clusters or extends high above the height and plankton flow
still
chains, reflecting zones of past volcanic activity. The Pacific, with of the seamount. This “virtual”
water
its Ring of Fire, is the most volcanically active ocean, containing cylinder is called a Taylor
over 30,000 seamounts and guyots. Pacific chains typically form in Column. Above a seamount,
a northwesterly direction, matching the direction of plate movement, it forms an area of back- upwelling
OCEAN ENVIRONMENTS DISTRIBUTION MAP This creates a zone of deep-water
eddies and still water in
with 10 to 100 seamounts in each chain, sometimes connected by
which nutrients accumulate
an undersea ridge. In the Atlantic and Indian oceans, by contrast,
seamounts mostly occur singly.
and plankton get trapped.
incredible richness and
OF SEAMOUNTS
seamount
productivity above the
Some seamounts
seamount—an “oasis”
and guyots arise
over volcanic hotspots,
in the nutrient desert
flow splits
often in chains.
of the open ocean.
Others form singly
along mid-ocean
current
ridges. Total numbers
WATER COLUMNS
are unknown.
The currents spiraling around and
over a seamount create a column of
still water above it. Plankton thrive
on the nutrients trapped there.