Download Features of the Ocean Floor

20.2
Continental Margins
 The line that divides the continental crust from the
oceanic crust is not always obvious.
 Shorelines are not the true boundaries between the
ocean floor and continents.
 The real boundary lies offshore beneath the ocean and
thick sediments.
Continental Shelf
 The gently (<1°) sloping platform at the edge of the
continent.
 The shelf is generally thought to be an extension of the
continent and not really a part of the ocean basin.
 The average water depth on the shelf is about 75
meters, varying from zero at the shoreline to about 150
meters near its edge.
 A typical continental shelf is 60 kilometers wide, but it
exceeds 100 kilometers off the Florida coast and is less
than a few kilometers wide in places along the West
Coast of South America.
 The rock underlying the thin veneer of sediments is
granite similar to the basement rock elsewhere
beneath the continents.
 The width of the continental shelf varies.
 On the west coast of S. America it is only a few
kilometers wide.
 The widest shelves extend out about 1,280 km from
Siberia and Alaska into the Arctic Ocean.
Continental Slope
 The continental slope marks the transition between
the shelf and deep ocean floor.
 It has an average slope of 3-6°.
 This may not sound like much, but over a distance of
100 kilometers water depth increases from 75 meters to
4000 meters.
 Typically, continental slopes are crisscrossed by a
series of deep submarine canyons the origin of which
is controversial.
 Some represent drowned stream valleys, but others
were clearly never above sea level and can not have
been cut downward by stream erosion.
Continental Rise
 Represents the accumulation of sediment at the base
of the continental slope.
 Result is a gentler slope and the buildup of "turbidite"
deposits.
 Uplifted turbidite deposits are common along the
coastline of southern California, particularly at Blacks
and Torrey Pines beaches north of San Diego.
 Southern California turbidites are thought to form
during major earthquakes which cause sediments to
slide off the edge of the shelf and accumulate on the
ocean floor as "fining upward" sequences of
sedimentary rocks.
Deep Ocean Basins
 Deep ocean basins have their own distinct features
such as: submerged volcanic mountains, deep
trenches, and plains.
Trenches
 The deepest feature on earth’s surface.
 The deepest place in the world is the Marianna Trench
– over 11,000 m deep.
 Located between the western Pacific Ocean near
Guam.
 Trenches are associated with earthquakes and located
along the Ring of Fire.
 At least 22 trenches have been identified although not
all are classified as major.
 Of this number, 18 are in the Pacific Ocean, three in
the Atlantic Ocean, and one in the Indian Ocean
 In order to better illustrate the actual depth of the
Mariana Trench, consider the following; if Mount
Everest, which is the tallest point on earth at 8,850
meters (29,035 feet), were set in the Mariana Trench,
there would still be 2,183 meters (7,166 feet) of water
left above it.
Abyssal plain
 The ocean floor (covers about 30% of the earth's
surface).
 The average water depth is around 5000 meters.
 Consists of a layer of unconsolidated sediment
underlain by sedimentary rock and pillow basalt
 Extremely vast flat areas that lie in the deep ocean
basins where ocean depth is often great than 4 km.
 Flattest regions on earth.
Mid-Ocean Ridges
 The most prominent feature of the Ocean basin are
the mid-ocean ridges.
 They are underwater mountain ranges.
 Form when plates pull away from each other.
 Have a narrow depression, or rift, where magma
reaches the sea floor and forms new crustal rock.
 Different parts of the ridge separate at different rates,
the new crust breaks into a series of faults called
fracture zones, that run perpendicular to the ridge.
Seamounts
 Submerged volcanic mountains at least 1,000 m high
are called seamounts.
 Abyssal hills are less than 1 km in height.
 Both are generally associated with hot spots.
 Seamounts rise above the surface and form islands
such as Hawaii and the Canary Islands.
 As plate tectonics carries seamounts away from mid-
ocean ridges, the crust sinks.
 The seamounts sink with it.
 Submerged seamounts are called guyots or
tablemounts.