I. Hypsometry

A. Two Levels

1. Mean Elevation of Continents - 800 m

2. Mean Depth of Oceans - 3800 m

3. Reflects basic difference between continental & oceanic crust

B. Extreme Elevation & Depth

1. Greatest Elevation = Mount Everest - Himalayas (8600 to 8700 m)

2. Greatest Depth = Challenger Deep - Marianas Trench (11000 m)

C. Edge of Continent = Continental Slope - NOT shoreline

II. Distribution of Oceans & Continents

A. Surface Areas

1. Ocean 360 x 106 km2 71%

2. Continent 150 x 106 km2 29%

3. Total Surface 510 x 106 km2 100%

B. Crustal Areas

1. Oceanic & continental crusts are tricky

a. ocean area includes continental shelf & slope, so oceanic crust area = ocean area less area of continental shelf + one half of continental slope

b. continental crust area = continental area + area of continental shelf + one half of continental slope

c. continental shelf = 27 x 106 km2

continental slope = 28 x 106 km2

2. thus, oceanic crust = [360 - (27 + 14)] x 106 km2 =

319 x 106 km2 63%

3. & continental crust = [150 + (27 + 14)] x 106 km2

191 x 106 km2 37%

C. Four Oceans (Three Major)

1. Pacific - biggest (50% of total ocean area, 53% of volume) & deepest (3940 m)

2. Atlantic - second biggest (26% of total ocean area, 25% of volume) & shallowest (3575 m)

3. Indian - smallest (20% of total ocean area, 21% of volume) & second deepest (3840 m)

4. Arctic - really a marginal sea (3% of total ocean area, 1% of volume) & very shallow (1117 m)

5. Marginal Seas - generally adjacent to continents & separated from major ocean by peninsulas or island chains

a. tend to be shallower & strongly influenced by adjacent continent

b. Pacific - Bering, Okhotsk, Sea of Japan, Yellow, East China, Philippine, South China, Gulf of Thailand, Sulu, Celebes, Java, Banda, Gulf of Carpentaria, Coral, Tasman, Ross, Bellinghausen, Gulf of California, Gulf of Alaska

c. Atlantic - Barents, Norwegian, North, Baltic, Bay of Biscay, Mediterranean, Gulf of Guinea, Weddell, Scotia, Caribbean, Gulf of Mexico, Labrador, Hudson Bay, Baffin Bay

d. Indian - Bay of Bengal, Andaman, Timor, Gulf of Aden, Red, Persian Gulf, Arabian

III. Morphology of the Ocean Floor

A. Three Major Morphologic Provinces

1. Continental Margin - Continental Shelf, Slope & Rise

2. Ocean Basin - Abyssal Floor, Seamounts & Oceanic Rises

3. Mid-Ocean Ridge (MOR) - Rift Valleys & Fracture Zones

B. Continental Margin

1. Continental Shelf

a. gradient - <1:1000 (<0.057_) [abrupt change to >1:40 at edge of shelf (shelf break)]

b. relief - low (<20 m)

c. width - a few km to >400 km (av. 78 km) - correlates with depth

i. wide = deepest generally

ii. narrow = shallowest generally

d. shelf break occurs at 130 m on average

i. deepest at high latitudes - 550 m max. (av. 350 m near Antarctica)

ii. shallowest at low latitudes (av. 50 m)

iii. deep shelf breaks from glacial erosion & shallow shelf breaks from upgrowth of reefs

e. Marginal Plateaus - drowned continental shelves that now interrupt the continental slope (e.g., Blake Plateau off southeast U.S.)

2. Continental Slope

a. gradient - >1:40 (>1.432_), av. 1:15 (4_) [can be very steep (30_-90_)]

b. relief - great for oceans (a few 10's of m to several 100's of m)

c. width - <200 km (av. 80 km)

d. base of slope

i. Atlantic-type margins - where gradient becomes <1:40 (generally between 1500 & 3500 m, although can be as deep as 5000 m)

ii. Pacific-type margins - at bottom of trench (generally between 6500 & 11000 m)

e. Submarine Canyons - erosional features incised into slope, but which often cut into shelf edge & extend onto continental rise

3. Continental Rise - found only on Atlantic-type margins

a. gradient - <1:40 (<1.432_), generally between 1:100 & 1:700 (0.082_-0.572_)

b. relief - low (<40 m)

c. width - 100 to 1000 km

d. base of rise - where gradient becomes less than 1:1000 (generally between 4000 & 6000 m)

e. submarine canyons may extend onto the continental rise as deep sea channels (generally <100 m deep)

i. deep sea fans are generally on the continental rise at the base of canyons, &, in fact, the rise usually consists of coalesced fans

ii. abyssal cones form off major rivers (Atlantic - Mississippi, Niger, Amazon, Congo, St. Lawrence, Hudson, Orange, & Nile & Rhone {Mediterranean}; Indian - Indus, Ganges; Pacific - Astoria)

f. Sediment Drifts (outer ridges)

i. continental rises detached from base of slope by abyssal currents)

ii. Blake-Bahama Outer Ridge off SE U.S. is a good example

g. Deep Sea Trench - replaces continental rise along Pacific-type margins

i. width - 40 to 120 km (av. 78 km)

ii. depth - 6500 to 11000 m (av. 9 km) [2-4 km below adjacent ocean basin]

iii. length - 800 to 6000 km (av. 2350 km)

iv. small abyssal plain often occurs at bottom of trench

C. Ocean Basin

1. Abyssal Floor

a. abyssal plains - dominate the Atlantic & Indian abyssal floors

i. gradient - <1:1000

ii. relief - low (<10 m)

iii. width/length - <200 to 2000 km (generally elongate)

iv. depth - 4000 to 6000 m (although some are perched on continental rises shallower than 4000 m & several occur in trenches deeper than 6000 m)

v. sediment has smoothed out rough morphology of underlying igneous oceanic crust

b. abyssal hills - dominate the Pacific abyssal floor (80-85%)

i. small, sharply defined hills rising <1000 m above adjacent lows

ii. size - generally 1 to 10 km (some >50 km in diameter)

iii. slopes are 1:4 to 1:60 (1_-15_)

iv. reflect morphology of underlying igneous oceanic crust

v. generally flank Mid-Ocean Ridge

2. Seamounts - particularly abundant in the Pacific

a. isolated features rising >1000 m above adjacent lows

i. can occur in long chains (Hawaiian-Emperor seamount chain)

b. size - generally 10 to 50 km (the largest are >200 km in diameter)

c. slopes are 1:4 to 1:10 (5_-15_)

d. extinct volcanoes formed over mantle hotspots

i. particularly large volcanoes rise above the sea surface, where erosion flattens its top

ii. subsidence produces a flat-topped seamount called a GUYOT

e. Moats - around large seamounts or groups of seamounts [due to weight of volcanic material]

f. Archipelagic Aprons - at bases of seamount groups (formed from erosional debris)

3. Oceanic Rises (Aseismic Ridges)

a. large areas 100's to 1000's of km in size standing 2 to 3 km above adjacent seafloor

b. some are long, linear features; others are more equidimensional

c. no earthquake activity now associated with them - hence aseismic

d. 2 types

i. volcanic ridges & plataeaus - formed over voluminous hotspots so that volcanoes are discrete features (Atlantic - Iceland, Rio Grande Plateau, Walvis Ridge; Indian - Ninetyeast Ridge, Mascarene Plateau, Chagos-Laccadive Ridge, Kerguelen Plateau; Pacific - Magellan Plateau, Shatsky Rise)

ii. microcontinents - small fragments of continental crust stranded by rifting (Atlantic - Orphan Knoll, Rockall Plateau, Jan Mayen ridge, Lomonosov ridge, Agulhas Plateau; Indian - Seychelles, Madagascar Plateau; Pacific - Ontong Java plateau)

D. Mid-Ocean Ridge

1. General Characteristics

a. 80,000 km long

b. a few 100 to 4000 km wide

c. crest averages 2500 m deep

d. 1000 to 3000 m above adjacent ocean basin

e. relief - 100 to 2000 m

i. greater in Atlantic & Indian, but lesser in Pacific

f. rift valley at crest

2. Rift Valleys

a. depth - >500 m (4000 m max) in Atlantic & Indian, but <500 m in Pacific

b. width - 30 to 50 km in Atlantic & Indian, but 5 km in Pacific

c. related to rate of spreading - slow spreading in Atlantic & Indian, fast spreading in Pacific

i. rift collapses in Atlantic & Indian, remains shallow in Pacific

ii. inner rift valley of Pacific similar to that of Atlantic & Indian

3. Fracture Zones

a. extensive linear zones of irregular topography marked by troughs, escarpments, seamounts & ridges

b. 10 to 100 km wide & up to 3500 km long

d. offset MOR crests

i. seafloor on either side show differences in elevation, relief or both

ii. most noticeable in Pacific, where Henry Menard first recognized fracture zones

iii. later mapped by Bruce Heezen along equatorial Atlantic where offset of MOR is greatest

e. generally aseismic except for the portions lying between the offset segments of the MOR

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