Ocean Basins

Earth's surface

World's oceans fill 3 large basins extending north from Antarctica where they connect.- basins are separated by land

    Pacific dominates the earth's surface - hold's 52% of the ocean waters

    Atlantic & Indian Ocean hold the rest except for -2% of water in ice

If we could drain the ocean basins, the most conspicuous features would be the mountain ridges in the middle of the Atlantic, eastern South Pacific and western Indian Ocean and circling Antarctica

    - global system of mountain ranges called the mid ocean ridges

Another feature of ocean basins is the deepening of the ocean bottom away from the mid-ocean ridges - trenches are the deepest part of the ocean around the margin of the Pacific

        Earth's surface has 2 distinctly different levels
            - land averages -840m above sea level
            - ocean bottom averages - 3800m below sea level

Hypsographic curve plots the amount of the earth's surface at each elevation or depth - shows the relationship betweem the height of the land and the depth of the
oceans
            20% of land area is above 2 km
            85% of the ocean area is below 2 km

Mt. Everest in Himalayas - highest place on earth - 8.8km (5.5 mi)

Marianas Trench - 11.04 km deep (6.64 mi)

Compared with the earth's radius - 6370 km - mountains and trenches are insignificant - earth is essentially a smooth sphere

4 continental land masses interrupt the ocean basins

- Eurasia-Africa - Antarctica

- N & S America - Australia

The principle boundary between continents and oceans is below sea level

Continental Margin - separates continents and ocean basins; consist of
 

1. Continental Shelf - submerged, upper parts of the margin; from shoreline, shelf slopes to shelf break, avg depth -130 m

- off Antarctica, shelf break is -500m deep because the weight of ice on land has depressed the continent

- around the Pacific, where there is active mountain building, the shelf is only few 10s of km wide

- wide shelves occur where margins are not active (no mountain building for millions of years)

- coastal plains are former continental shelves exposed above water


2. Continental Slope

- outer edges of continental blocks

- extends down to depths of 2 - 3 km

- most dramatic where coastal mountains parallel a trench

- e.g. west coast of S America where peaks of Andes (-7 km high) and the Peru-Chile trench (8 km deep) form a total of 15 km relief in a short distance.


Submarine Canyons - ripple marks observed on the floor of submerged canyons and sediments fanning out at the end suggest they were formed by moving sediments and water

- cut by turbidity currents

- caused by earthquakes or buildup of sediment on a steep slope

- fast moving avalanches of mud, sand and water that flow down slope, erode walls and pick up sediment

- as flow reaches bottom, it slows down, fans out and the sediment settles out

Submarine canyons cut through the shelves and slopes, look like river valleys on land, cut during periods of low sea level by turbidity currents,  some associated with major rivers, e.g. Hudson Canyon

Continental Rise - gentle slope at the base of the continental slope caused by:

- turbidity currents

- deposition of sediment by underwater landslides and other processes that carry mud, sand & silt down the slope

- occurs at the base of the slopes & gently slope seaward to the deep sea floor


Ocean Floor is seaward of the continental margin

- covers 30% of the earth's surface compared to 29% covered by the continents

- in most places, seafloor is a flat plain - Abyssal Plain

- covered by sediment deposits of turbidity currents covering an irregular seafloor plain interrupted by:

abyssal hills (< 1 km above the seafloor) - cover -80% of the Pacific and -50% of the Atlantic sea mounts (rise steeply, sometimes above the surface to form islands)

guyots (flat topped seamounts found most often in the Pacific)

table mounts, usually I - 1.7 km below the surface

- many have ancient coral reefs on top indicating that they were once at the surface
- flat tops due to wind & rain erosion
- subsided due to their own weight & crustal movement
Mid-Ocean Ridge
- continuous ocean mountain range -65,000 km long
- ridge systems are - 1 km wide and 1 - 2 km high
- along system's crest is a rift valley 15-50 km wide and 0.5 - 1.5 km deep
- the rift valley is volcanically active
Rift valley of mid-Atlantic ridge is bordered by mountains with peaks that reach within 2 km of the surface. E Pacific rise is broader, less rugged with mostly no rift valley

Fracture Zones cut the ocean floor and offset the mid-ocean ridges - series of parallel, narrow (10 - 100 km) long ridges and valleys

Trenches

- deepest part of the ocean floor, typically 3 - 4 km deeper than surrounding seafloor
- relatively narrow, few 10s of km wide and thousands of km long
- most occur in the Pacific, mostly western Pacific, but most of the Pacific is surrounded by trenches
- deepest spot in the oceans is the Challenger Deep in the Marianas Trench, 11,035 m
- trenches are associated with active volcanoes and earthquakes
- most are near chains of volcanic islands
- the narrow steep-sided trenches found in the Pacific are on the seaward side of volcanic island arcs e.g., Japan-Kuril Trench, Aleutian Trench, Phillipine Trench, Marianas Trench
Peru-Chile trench is the longest (5900 km, 3664 mi) down the west coast of South America

Middle Americas trench along Central America and the Peru-Chile Trench are not associated with island arcs but are bordered by continental volcanoes