Download Earth Science 14.1 The Vast World Ocean

Earth Science 14.2 Features of the Ocean Floor
Ocean Floor features
Oceanographers studying the
topography of the ocean floor
have divided it into three
major zones
 Continental margins
 The ocean basin floor
 The mid ocean ridge
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The illustration at right shows
the typical topography of an
ocean; the continental margin,
ocean basin, mid-ocean ridge
and again to the next
continental margin.
Scientists have discovered
that each of these areas has
it’s own distinct features
Ocean Floor features
Continental Margins:
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The zone of transition between
a continent and the adjacent
ocean basin floor is known as
the continental margin.
In the Atlantic, thick layers of
undisturbed sediment cover
the continental margin.
This region has very little
volcanic or earthquake activity.
This is because the continental
margins in the Atlantic Ocean
are not associated with plate
boundaries, unlike the Pacific
Ocean.
Ocean Floor features
Continental Margins:
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In the Pacific Ocean, where
plate boundaries converge,
oceanic crust is plunging
beneath continental crust in a
subduction zone.
This force results in a narrow
continental margin that
experiences both volcanic
activity and earthquakes.
Ocean Floor features
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If you could travel from one
coast to another across the
ocean floor, the first zone one
would pass through upon
leaving land would be the
continental shelf.
The continental shelf is the
gently sloping submerged area
extending from the shoreline.
This shelf is almost
nonexistant on some
shorelines.
Ocean Floor features
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However, on other shorelines in
the world, the continental
shelf can extend outwards as
much as 1500 kilometers.
On average, the shelf is about
80 kilometers wide 130 meters
deep at it’s seaward edge.
The average steepness of the
drop is only about 2 meters per
kilometer drop; a slope so
slight that the human eye can
barely perceive it.
Ocean Floor features
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Continental shelves have
political and economic
significance as well.
Continental shelves contain
important resources such as
mineral deposits, oil and
natural gas deposits, and
enormous sand and gravel
deposits.
The waters of the continental
shelf also contain important
fishing grounds, which are a
significant source of food.
Ocean Floor features
Continental Slope:
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Marking the seaward edge of the
continental shelf is the
continental slope.
The slope is steeper than the
shelf and it marks the transition
from continental crust to
oceanic crust.
Although the steepness of the
continental slope varies from
location to location; the average
slope is 5 degrees.
In some places, the slope can
exceed 25 degrees. The
continental slope is a relatively
narrow feature, averaging only
about 20 kilometers in width.
Ocean Floor features
Continental Slope:
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Deep, steep sided valleys
known as submarine canyons
are cut into the continental
slope.
These canyons may extend to
the ocean basin floor.
Most information suggest that
submarine canyons are formed
by erosion, at least in part, by
turbulent underwater currents.
Ocean Floor features
Continental Slope:
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Turbidity currents are
occasional movements of dense
sediment-rich water down the
continental slope.
They are created when sand
and mud on the continental
shelf are disturbed, perhaps
by an earthquake, and become
suspended in the water.
Ocean Floor features
Continental Slope:
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Because the muddy water is
denser than sea-water, it flows
down the slope.
As it flows down, it erodes the
bank away, accumulating more
sediment, and eventually cuts
these deep canyons into the
shelf’s surface.
Erosion from these muddy
torrents over time is believed
to be the major force behind
the carving of these large
canyons.
Ocean Floor features
Continental Slope:
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Narrow continental margins,
such as the one along the
California coast, are marked
with numerous submarine
canyons created by runoff
sediment from the land.
Turbidity current are know to
be an important mechanism of
sediment transport in the
ocean.
Turbidity currents erode
submarine canyons and deposit
sediments on the deep-ocean
floor.
Ocean Floor features
Continental Rise:
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In regions where trenches do
not exist, the steep
continental slope merges into a
more gradual incline known as
the continental rise.
Here the steepness of the
slope drops to about 6 meters
per kilometer.
Where the width of the
continental slope averages only
about 20 kilometers wide, the
continental rise may be
hundreds of kilometers wide.
Ocean Floor features
Ocean Basin Floor
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Between the continental
margin and mid-ocean ridge,
lies the oceanic basin floor.
The size of this region, nearly
30 percent of Earth’s surface,
is comparably equal to the
percentage of land above sea
level.
This region includes deep ocean
trenches, very flat areas
known as abyssal plains, and
tall volcanic peaks called
seamounts and guyots.
Ocean Floor features
Deep Ocean Trenches:
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Deep ocean trenches are long,
narrow creases in the ocean
floor that form the deepest
parts of the ocean.
Most trenches are located
along the margins of the
Pacific Ocean, and many
exceed 10,000 meters (almost
30,000 feet) in depth.
A portion of one trench, the
Challenger deep in the Mariana
Trench has been measured at a
record 11,022 meters below
sea-level.
Ocean Floor
Abyssal Plains:
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Abyssal plains are deep,
extremely flat features. In
fact, these regions may be the
most level areas on Earth.
Abyssal plains have
accumulations of thick
accumulations of fine sediment
that have buried an otherwise
rugged sea floor.
The sediments that make up
the abyssal plain are carried
out there by turbidity currents
or as a result of suspended
sediments settling.
Ocean Floor
Abyssal Plains:
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Abyssal plains are found in all
oceans of the world.
However, the Atlantic Ocean
has the most extensive abyssal
plains because it has few
trenches to catch sediment
carried down the continental
slope.
Ocean Floor
Seamounts and guyots:
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The submerged volcanic peaks
that dot the ocean floor are
called seamounts. They are
volcanoes that have not
reached the ocean surface.
These steep sided cone shaped
peaks are found on the floors
of all oceans. However, the
greatest number can be found
in the Pacific.
Some can be found forming at
volcanic hot spots such as the
Hawaiian-Emperor Seamount
chain.
Ocean Floor
Seamounts and guyots:
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Once underwater volcanoes
reach the surface, they form
islands. Over time, running water
and wave action erode these
islands to near sea-level.
Over millions of years, these
islands gradually sink below sealevel.
This process occurs as the
moving plate carries the island
away from the elevated oceanic
ridge or hot spot where they
originated.
These once active flat-topped
but now submerged structures
are called guyots.
Ocean Floor
Mid-ocean ridges:
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The mid-ocean ridge is
located near the center of
most ocean basins.
The mid-ocean ridge
system is an
interconnected system of
underwater mountains that
have developed on newly
formed ocean crust.
This system is the longest
topographic feature on
Earth running 70,000
kilometers around the
world’s oceans.
Ocean Floor
Mid-ocean ridges:
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The term ridge may be
misleading because the midocean ridge is not narrow.
It has widths from 1000 to
4000 kilometers and may
occupy as much as a total of
one half the total area of
the ocean.
The mid ocean ridge is
broken into sections. These
segments are offset by
transfer faults where plates
slide past each other
horizontally, resulting in
shallow earthquakes.
Ocean Floor
Sea-floor spreading:
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A high amount of volcanic
activity takes place along
mid-ocean ridges.
This activity is associated
with sea-floor spreading.
Sea-floor spreading occurs
where divergent plate
boundaries are moving
apart from each other.
New ocean is formed at
mid-ocean ridges as magma
rises between diverging
plates and cools.
Ocean Floor
Hydrothermal vents:
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Hydrothermal vents form
along mid-ocean ridges.
These are zones where
mineral-rich water, heated
by the hot, newly formed
oceanic crust, escapes
through cracks in the
oceanic crust into the water.
As the super-heated
mineral-rich water comes in
contact with the surrounding
cold water, minerals
containing metals such as
sulfur, iron, copper, and zinc
precipitate out and are
deposited.