Ocean Basin
Chapter 3
Global elevations
continental crust : Thick and Lighter
Average height: 840 m
oceanic crust : Thin and denser
Average depth: 3729 m
Two different types of crust floating in the
mantle (this is the concept of isostasy). Note
that the mantle is solid rock, but it flows and
deforms like a fluid on very long time scales.
Continental
Oceanic crust
Crust
Mantle
Nature of Coast Line
Active Margin
Western Coast of USA
Steep Continental Shelf
Narrow beach
Rocky cliffs
Passive Margin
East Coast
Wide, gentle beach, sandy offshore
islands
Broad Continental Shelf
Ocean Profile
Continental Margin
Provinces of a passive continental
margin
Figure 13.7
Major features of the seafloor
the seaward extension of the
A. Continental shelf continents
underlain by continental crust
water depths are less than 400
m and generally less than 100
m.
the seaward edge of the
continental shelf marks the point
of lowest sea level during the
last maximum ice age, -- it was
the beach!
Ranges in width from a few km
(such as off our eastern shore
here in Miami ) to 400 km (such
as in the Arctic Ocean).
Major features of the seafloor
Relatively steep slope
Continental slope separating the continental
shelf from the continental rise.
Averages 4 degrees but can
range up to nearly vertical.
The continental slope off
the western edge of Florida
is nearly vertical and would
make great rock climbing
material if the Gulf of
Mexico were drained.
Continental Rise
this is a province of very gentle slope between the continental slope and the
abyssal plains.
the continental rise is made up of piles of sediment that have been
delivered to the deep sea from the continental shelf.
Submarine canyons
The continental shelf and slope are cut in many places by magnificent
canyons large enough to dwarf the Grand Canyon. Many of these are
located off the mouths of major rivers, such as the Hudson Canyon. Their
origin remains mysterious but is likely due to the sediment washed into the
ocean by the rivers.
shelf
slope
rise
Abyssal Plains
These are the flattest places on earth where the slow
but steady rain of sediment has blanketed the ocean
crust and smoothed out the irregularities. Water depth
4-6 km
Atoll
Central circular lagoon fringed
by coral islands
Seamounts
Seamounts are isolated submarine volcanoes.
Occasionally these volcanoes reach the sea
surface and form volcanic islands. The western
Pacific Ocean has an unusual abundance of
seamounts. These islands often rise over 9.5 km
above the surrounding seafloor, dwarfing Mt.
Everest in total relief.
Flat top seamounts are also called tablemounts or
guyots
Seamounts less than 1000 meter in height are
called abyssal hills or sea-knolls
Seamount and Guyot
Mid-Oceanic Ridges
1.These are the most conspicuous features of the
ocean basins.
2.The mid-ocean ridge is a continuous chain of
mountains that runs from the central Arctic Ocean
down through the Atlantic, through the Indian Ocean
and northward through the eastern Pacific.
3.The mid-ocean ridge is approximately 1000 km
wide and rises 1000 to 3000 m above the adjacent
seafloor.
4.The mid-ocean ridge is 60,000 km long and covers
21% of Earth's surface.
5. Many portions of the ridge have valleys that run
along the crest of the ridge. These are called central
or axial rift valleys.
Distribution of the
oceanic ridge system
Figure 13.11
Ocean Profile
Atlantic Profile
Pacific Profile
Physiographic map of the world showing plate boundaries and major
Ocean-floor features like fracture zones, mid-oceanic ridges and trenches
Fracture Zones and Transform Faults
Semi-parallel fractures that cut across and offset
mid-ocean ridges.
Numerous shallow earthquakes occur along the
portion of fracture zones in between the ridge
axes. These active fractures are called Transform
faults
May reach lengths up to 3500 km.
Marginal Trenches
Narrow, steep-sided troughs parallel to continental
margins.
Remarkable for their length and continuity - the Peru-
Chile Trench is 5900 km long and 100 km wide.
Drop 2 to 4 km below the adjacent seafloor to form
the deepest parts of the ocean (water depths up to 11
km)!
Computer generated
Image of a Trench
Locate the following on the map
1. North Atlantic Ocean
2. South Atlantic Ocean
3. Indian Ocean
4. Mediterranean Sea
5. Caribbean Sea
6. Gulf of Mexico
7. Labrador Sea
8. Norwegian Sea
9. East Pacific Ocean
10. West Pacific Ocean
11. North Sea
12. Baltic Sea
13. Weddell Sea
14. Straits of Gibraltar
15. Drake Passage
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10
9
3
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