Download Science 8 CLASS COPY - DO NOT WRITE ON Ocean Floor Features

Science 8
CLASS COPY - DO NOT WRITE ON
Ocean Floor Features
Continental Shelf:
Continental shelves
​
are found near most of the
Earth’s coasts, angling slowly away from the shore to
a depth of about 200 meters (650 feet). Rivers build
the continental shelf as time passes. The outgoing
river water carries ​sediment to the edge of
continents, which piles up to create the continental
shelf. Off many coasts, especially on the Atlantic, the
shelf can be 100 miles wide. The widest shelf
anywhere stretches 800 miles into the sea off the
Atlantic shore of Siberia (Northern Asia).
Off Florida, the Gulf Stream current runs like a swift underwater river, carrying away debris from
rivers. This leaves behind little to no continental shelf. Off mountainous shores like the Pacific coast of
America, there are fewer rivers to bring sediment. Here, the continental shelf is very narrow.
Continental Slope:
At the outer edge of the continental shelves,
the land plunges down quickly. This is often
where ​continental rock meets ​oceanic rock.
The slope sometimes falls unbroken for two
to three miles past the continental shelf.
These ​continental slopes are among the
most imposing features of the entire earth.
They are the highest and longest walls in
the world, spanning a total of 200,000 miles
across and up to 10,500 feet high.
Nearly everywhere that oceanographers have mapped the continental slope, they have found deep
canyons cut into its face. For a long time, the force that carved them was a mystery. Many were too
far off shore, and in water too deep, to be old riverbeds. ​Earthquakes had opened a few but not all of
these canyons. Scientists now believe they were cut into the continental slopes by powerful
underwater avalanches called ​turbidity currents​. These are high density currents full of sediment that
move very quickly down the slope. An ​earthquake or build up of sediment on the continental shelf can
send sediment rushing down the slope. The sediment, flowing at very high speeds, gouges out a
canyon as it goes. Turbidity currents can flow for hundreds of miles. As they slow down, they spread
out into thick layers of sediment.
The Deep-Ocean Basin or Floor:
The deep ocean floor begins at the foot (bottom) of the ​continental slope​. It is the biggest area of the
ocean floor, comprising over 70% of the total sea area. In this domain of darkness, humans have made
our most startling discoveries about the earth’s hidden surfaces.
The flat sections of the deep-ocean floor turned out to be flatter than anything ever seen on land.
Called ​abyssal plains​, these areas are as smooth and flat as a dance floor. Samples from these ​abyssal
plains show that sediments from ​turbidity currents formed them. After carving canyons in the
continental slope​, the ​turbidity currents spread out when they reached the sea floor. ​Sediments
carried by the currents filled every seabed crevice with sand, mud and silt from rivers along coasts.
However, the ocean floor isn’t just flat. There are peaks, cones, ridges and cliffs that have formed
beneath the sea. Many of these have been preserved largely unchanged because there is no wind,
rain, and frost to wear away, or erode, these features. On the Pacific floor, for example, there is a cliff
that is one half a mile to one mile high and 3,000 miles long. It is as sharp and jagged, as if it had been
formed yesterday. In spite of its appearances, the seafloor as it exists today is 70 million years old in
places and almost unchanged.
Several other interesting features found on the ocean floor are ​seamounts, guyots (“gee-oh”), and
atolls​. Many of these features were also once active ​volcanoes​. ​Seamounts are cone-shaped submarine
(under water) ​volcanoes​, which rise from the floor. In some areas, the lava flow has caused a
seamount to rise above the surface of the ocean. These ​seamounts are then called ​volcanic islands​.
The Hawaiian Islands are examples of volcanoes rising from the ocean floor, or volcanic islands.
Some of the ​seamounts had their tops
eroded away by ocean waves. These
“flat-topped” volcanic peaks are called
guyots​. Some ​seamounts close to the
surface support the growth of coral
animals. The coral can form a ring
made of calcium carbonate rock
around the island or a ring on top of a
guyot. These ringed coral islands are
called ​atolls​.
The rift valleys and canyons we see on
continents have counterparts found in the deep ocean basin. Rises in the sea floor, called ​mid-ocean
ridges​, are areas where the sea floor appears to be spreading and growing. These rises take the form
of long, narrow cracks in the sea floor surrounded by jagged peaks. These peaks are formed by
magma rising from a crack in the sea floor, then cooling and solidifying. Deep ​trenches occur where
the ocean floor meets a continent in a specific way. The ocean floor pushes underneath the
continental floor, forming a ​trench.​ Some of these steep-sided canyons/trenches are ten times deeper
than the Grand Canyon!
Scientists have measured the ocean’s depths and found that the shape of the ocean floor, or
topography​, is very much like that on land. The underwater world is full of peaks, valleys, volcanoes,
and plains just as like on land. The illustration below shows all ocean floor features discussed so far.
Continental (land) features
Plateaus
A ​plateau is a relatively level, large area of land that rises
1,500 feet (457 meters) or more above its surroundings and
has at least one steep side. A plateau may cover an area as
small as several square miles or as large as half of the United
States! Other features similar to plateaus include ​mesas and
buttes​, both of which are smaller.
Some plateaus formed as a result of uplift, or the slow,
upward movement of large portions of rock. Others formed
as a result of repeated lava flows, spreading out over long distances and building up the land surface.
Some plateaus can form simply when the side of a land region is weathered away through erosion by
wind and water.
Valleys
Valleys ​are one of the most common surface features on
Earth. They are carved by flowing water or ice, which
gradually wears away Earth surfaces through erosion. Valleys
take on a wide variety of forms — from steep-sided canyons,
such as the Grand Canyon on the Colorado River, to broad
plains​, such as the lower Mississippi River valley. The form of
a valley depends on many factors, including what is eroding
it, the slope of the land surface, the nature of the soil or rock
where the valley is being created, and time.
Canyons
A ​canyon is a deep, narrow valley with steep sides. The
movement of rivers, the processes of ​weathering and
erosion​, as well as tectonic activity create canyons.
A river canyon forms when the water pressure of a river
cuts deep into a river bed. Sediments from the river bed
are carried downstream, creating a deep, narrow
channel. Weathering and erosion also contribute to the
formation of canyons. In winter, water seeps into cracks
in the rock. This water freezes, which expands as it turns
into ice. Ice then forces the cracks to become larger and larger, sometimes breaking the rock into
smaller pieces in the process. During brief, heavy rains, water rushes down these cracks, eroding even
more rocks and stone.
Shoreline
A ​shoreline is where land and water meet. Most shorelines contain sand. The motion of waves helps
shape shorelines through erosion as waves remove sand away. The energy in waves is constantly
breaking rock into smaller and smaller pieces. Crashing waves can break solid rock and throw the
pieces back toward the shore. Breaking waves can enter cracks in the rock and break off large
boulders. Waves also pick up fine grains of sand, which then rubs against rocks on shore to create
more erosion. During deposition, waves add sand to shorelines as they carry many materials, such as
sand, shells, and small rocks. When the waves deposit these materials on the shoreline, a ​beach forms.
Beach​ materials can also be deposited by rivers carrying s​ ediment​ on the way to the ocean.