Dr. Tyler
OCE 2001 - Introduction to Oceanography
Chapter 2 – Ocean Basins
I. Earth's origins
A. Big Bang formed universe over 15 billion years ago
B. Earth formed from gases and dust ~ 4.6 bya
1. Cooled slowly  heaviest material sank to core
2. Volcanic activity source of oceans and atmosphere
C.Microbial and plant life slowly converted CO2 into O2
over last few billion years
II. Ocean Basins
A. Oceans cover 70.8% of Earth's surface
1. Average depth of ocean is 3800 m below sea level
2. Deepest is Mariana Trench - 11035 m
B. Three basins from Antarctica -- Atlantic, Pacific,
Indian Oceans
a. Atl.-Ind. boundary from South Africa-Antarctica
b. Atl.-Pac. boundary is Bering Strait - includes
Arctic Ocean
c. Pac.-Ind. boundary thru Indonesia, Australia to
Antarctica
C.Pacific Ocean
1. Deepest and largest -- holds 52% of ocean water
2. Continents with high mountains
3. Many islands
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D.Atlantic Ocean
1. Relatively narrow and shallow -- 25% of ocean
water
2. Many shallow, semi-isolated seas
3. Few islands (Greenland is largest)
4. Most large rivers
E. Indian Ocean
1. Smallest basin -- ~20% of water
2. Large rivers
3. Two sources of saline water - Red Sea & Arabian
Gulf
F. Southern Ocean
1. Connecting the three major basins
2. Absence of land allows unimpeded circumpolar
circulation
3. Coldest, densest water flows northward beneath
other oceans
4. Thick ice sheets (100-200 m thick)
G. Marginal Ocean Basins
1. Large, partially isolated seas (> 2 km deep) near
continents
2. Three types
a. Isolated by island arcs – Bering Sea
b. Long, narrow sea surrounded by continent – Red
Sea
c. Lying between continents -- Gulf of Mexico
3. Have very different character than open ocean smaller, rivers
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III. Continental Margins -- continent-basin boundary
A. Continental shelves
1. Submerged extensions of the continents
 7.4% of ocean area
2. Sediment up to 15 km (9 miles) thick - mostly from
land runoff
3. Passive margins (Atlantic-type)
a. More extensive shelf
b. Widest shelf is 1280 km (800 miles) wide - north
of Siberia
c. Florida (passive) is narrow due to Gulf Stream
4. Active margins (Pacific-type, seismic)
a. Narrow shelf
b. Andes Mts. (7 km high)  Peru-Chile Trench (8
km deep)
5. Shelf break
a. Relatively abrupt transition from shelf to slope
b. Consistent average depth 130 - 140 m (460 ft)
c. Antarctica & Greenland shelves  300-400 m
deep from ice
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6. Shelf width dependent on sea level
a. Narrower w/ low sea level  (coastal plain
exposed)
b. Sealevel was 300 m higher than present in
Cretaceous (120 mya)
c. Last Ice Age (18,000 ya), 125 m (410 ft) lower
than present
B.Continental slope
1. Extends to about 2-3 km depth w/ ~4O incline (70 m
per km)
2. Bottom of slope is 'true' continental margin
3. Submarine canyons cut into the shelf and slope
a. Often offshore of a river or stream
b. Can be up to 3000 m deep
c. Turbidity currents -- sediment/water mix from
avalanche scours out canyons
C.Continental rise
1. Reduced slope at bottom of cont. slope (~1/8 of
incline)
2. Apron of sediment, mostly from turbidity currents
3. Width varies from 100 - 1000 km
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IV. Topography of deep-ocean basins
A.Mid-ocean ridges
1. Mountainous chains mostly at center of basins
a. 1-3 km high off floor
b. 60-65,000 km long (37-40,000 miles)
c. Some break surface (Iceland, Azores, Easter I.)
2. Major ridges
a. Mid-Atlantic Ridge
b. East Pacific Rise
 off N. America at Gulf of California
c. Mid-Indian Ridge
 intersects Africa-Asia in Red Sea
d. Pacific-Antarctic Ridge (encircles Antarctica)
3. Rift valley
a. Steep-sided central valley of most ridges
b. Center of active sea-floor spreading
4. Transform faults
a. Seismically-active fractures that offset ridge
b. Lithospheric plates slide horizontally
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5. Fracture zones
a. Seismically-inactive zone extends out from
transform faults
b. Ridge is offset  crust on either side of zone is
different age
c. Older, cold, dense crust subsides below
younger, warm crust
6. Hydrothermal vents
a. Hot springs on oceanic ridges (350OC, 660OF)
b. Water seep contacts hot rocks
c. Expels super-heated water with dissolved
minerals, gases
d. Common along ridges with rapid spreading
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B.Deep-ocean floor
1. Abyssal plains – between continental rise & ridges
a. 3700-5500 m (12-18000 ft) deep
b. Flat expanse of sediment up to 5 km (3 mi) deep
1) Sediment is terrestrial or shallow-water origin
2) Masks topography of basalt basement rock
2. Seamounts (>1 km) and Abyssal hills (<1 km)
a. Submerged, inactive volcanoes formed at ridge
b. Transported outward and downward as seafloor
spreads
c. Guyots are flat-topped seamounts
3. Trenches
a. Narrow, long, arc-shaped depression
b. Deepest part of oceans – 3-4 km depression in
sea floor
c. Mariana Trench – 11,035 m (36,170 ft) deep
 Only few km wide, but 1000s km long
d. Some of most seismically active areas on Earth
1) V-shaped profile, steeper on one side
2) Island arcs are chains of volcanic islands on
steep side
4. Oceanic plateaus
a. Isolated part of ocean floor over 1 km above
adjacent floor
b. 'Micro-continents' - those underlain by
continental rock
c. Others are elongated volcanic ridges
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