Oceans
May Marie A. Gauna
IV-6 BEEd
Professor Sia
• Continental Margins
• Oceanic Trenches and Island arcs
• Seamounts Oceanic Island
• Mid ocean ridges
Continental Margins
The continental margin is the zone of the ocean floor that
separates the thin oceanic crust from thick continental
crust.
-Continental margins constitute about 28% of the
oceanic area.
-continental margin is the submerged outer edge of a
continent.
Divided into 2 sections:
Continental Shelf
Continental Slope
The continental shelf is the region that
extends seaward from the shoreline to a
sharp drop-off that marks the beginning of
the continental slope. That drop-off is
known as the continental shelf break.
The continental slope marks the transition
between continental crust and oceanic
crust. Continental crust is composed
mostly of granite, whereas oceanic crust is
mostly basalt.
Basic composition of continental margins, which include the continental shelf,
continental slope, and continental rise.
• A submarine canyon is a steep-sided valley on the sea
floor of the continental slope. Many submarine canyons are
found as extensions to large rivers.
A turbidity current is a current of rapidly moving,
sediment-laden water moving down a slope through water, or
another fluid. The current moves because it has a
higher density and turbidity than the fluid through which it
flows.
The driving force of turbidity current is obtained from the
sediment, which renders the turbid water heavier than the
clear water above.
2 Types of Continental Margin
1. Active continental margins, where plates are
converging, coincide with plate boundaries, where the
continental and oceanic crusts are separated by a
subduction zone.
These margins are active tectonically and have less
width and sediment input than passive margins. They are
also marked by the addition of blocks from distant
sources to the continental mass at the subduction zone.
2. Passive margins are within plates and are separated
from the oceanic ridge plate margin by an expanse of
oceanic crust that was generated after rifting. Oceanic and
continental crusts meet in a region of low tectonic activity
that does however experience a broad pattern of
subsidence.
Passive margins are generally wide and may receive a
large influx of erogenous sediments or intrabasin
carbonate sedimentation from local sources.
Abyssal plains are the vast, flat, sedimentcovered areas of the deep ocean floor. They
are the flattest, most featureless areas on
the Earth, and have a slope of less than one
foot of elevation difference for each thousand
feet of distance.
Passive margins define the region around the Atlantic Ocean, Arctic Ocean, and
western Indian Ocean, and define the entire coasts of
Africa, Greenland, India and Australia. They are also found on the east coast of North
America and South America, in Western Europe and most of Antarctica. East Asia also
contains some passive margins.
Passive
Margin:
Subtropical
North
Iberian
Oceanic Trenches
The oceanic trenches are hemispheric-scale
long but narrow topographic depressions of the
sea floor. They also are the deepest parts of the
ocean floor.
• Oceanic trenches typically extend 3 to 4 km (1.9 to 2.5
mi) below the level of the surrounding oceanic floor.
• The deepest ocean depth to be sounded is in the
Challenger Deep of the Mariana Trench at a depth of
10,911 m (35,798 ft) below sea level.
There are three types of lithospheric plate
boundaries: divergent (where lithosphere and
oceanic crust is created at mid-ocean ridges),
convergent (where one lithospheric plate sinks
beneath another and returns to the mantle), and
transform (where two lithospheric plates slide past
each other).
Island Arc
An island arc is a curving series of volcanic islands that are
created through the collision of tectonic plates in an ocean
setting. The particular type plate boundary that yields
island arcs is called a subduction zone.
Island arcs are usually accompanied by
rapid erosion and sedimentation into accompanying basins.
Seamount
• is a mountain rising from the ocean seafloor that does not
reach to the water's surface (sea level), and thus is not
an island. These are typically formed from extinct, that rise
abruptly and are usually found rising from a seafloor of
1,000–4,000 meters (3,281–13,123 ft) depth.
Seamounts come in all shapes and sizes, and
follow a distinctive pattern of growth, activity,
and death. In recent years, several active
seamounts have been observed, for
example Loihi in the Hawaiian Islands.
Atolls
Atolls are circular, oval, or horseshoe-shaped
arrays of coral reef islands that are perched around
an oceanic volcanic seamount and encircle a shallow
central lagoon. The small islands are separated from
each other by channels that lead from the sea into
the central lagoon.
Mid-Ocean ridges
The mid-ocean ridge represents an area where,
in accordance with plate
tectonic theory, lithospheric plates(also called
tectonic plates) move apart and new crust is created
by magma (molten rock) pushing up from
the mantle . The mid-ocean ridge system is an
example of a divergent (rather than a convergent or
transform) plate boundary.
There are two types of mid-ocean ridges:
• fast-spreading
• slow-spreading