Structure of Ocean Basins Chapter 4 Continental shelves

Structure of Ocean Basins Chapter 4

Continental shelves § § § Part of continents Exposed or not exposed depending on sea level Cut by submarine canyons Vary in width depending on type of margin Accumulate sediment from the continents (including rivers) § Fun fact: 25 % of global continental shelf is in the Bering Sea (very productive fisheries)

Continental Slope § Separated from the shelf at the shelf break (~ 130 m). § Continues to an average depth of about 4000 m (range is 3000 -8000 m) § Most prominent topographic feature on earth § Marks the end of continents and the beginning of ocean crust § Average width of 20 – 100 km § Average slope of 4 o (road grade) - 70 m/km or 370 feet/mile

Continental Rise § Sediments from the shelf move down the continental slope and accumulate on the continental rise. § Most are transported there by turbidity currents or through riverine-derived canyons. § Width of the rise varies and slope is gradual. § Deep sea currents shape the rises. § Slope is about 1/8 that of the Continental slope (about 0. 5 o or < 10 m/km or < 50 feet/mile) § Rise absent when slope terminates in a trench § Conspicuous features of passive (tectonically inactive) margins so rare in the Pacific

Trenches § Act as sediment traps! § Peru-Chile trench – oldest at the S end (shallower because its full of sediments) and vice versa § Feature of active margin/subduction zone

Deep Ocean Basins § § § Mainly, abyssal plains and ocean ridges Also, trenches, seamounts and guyots Include more than half the earth’s surface

Abyssal plain § True sea floor § Flat, sediment-covered ocean floor § Sediments often more than 1000 m thick (not so featureless underneath sediments) § Begins at the base of the continental rise § Very flat except for abyssal hills (which are low, < 1 km high, origin unclear), seamounts and guyots and islands § 25% of the earth’s surface and 42% of the ocean bottom is plain § Most common in the Atlantic and rare in the Pacific where trenches trap sediments § Average of about 4000 – 5500 m depth

Abyssal hills § Not very high and steep § Protruding ocean floor topography?

Abyssal plain sediments bury old mountains (image generated by Echo sounding)

Important features of the sea floor § Trenches – Extremely deep – Found in association with plate margins (converging); mainly in the Pacific because of plates there § Seamounts and guyots – Guyots are flat-topped seamounts (eroded) – Islands are seamounts that break the sea surface § Coral reefs – Most common in Pacific with range 40 o. N and 40 o. S – Require warm water, light (deepest living corals at 100 m), salinity (no freshwater forms) and low sediment load. – Fragile ecosystems

Trenches § § Convergence zones Cold at the bottom 3 to 6 km deeper than adjacent seafloor Trenches curved because of geometry of plate interactions on a sphere § Trench steeper on the island/continent side than the seaward side § Tectonically active § Big sediment traps

Island arcs § Parallel to trenches § Chain of islands behind a trench

Seamounts and guyots § § Circular or eliptical More than 1 km in relief Alone or in groups Old hot spots or extinct volcanoes formed at spreading centers § Guyots have flat tops

Coral reef formation § § Atolls – around volcanoes Fringe reefs Corals grow up to keep in the sunlight Evidence of old reefs at 1400 m

Oceanic Ridges 15 cm/yr at Iceland Includes diverging and converging boundaries.

Ocean ridge system § Describe plate boundaries § Largest and longest mountain range on earth § Made up of young, basaltic rock at active spreading centers § Ridges devoid of sediment § Oceanic ridges and associated structures account for 22% of the world’s surface (compare to all land which is 29%!) § < 60% of their length is at the center of basins § Youngest rocks at spreading centers § Steeper ridges at slower spreading centers

Lavas § Lava cooling underwater forms pillow lavas – Solidifies rapidly because of contact with cold seawater; this also slows flow – Water pressure keeps gases in magma “solution” during cooling § Lave extruded on land – Cools more slowly; flows longer – Releases gases (sometimes explosively)

Transform faults § Fractures in the lithosphere along which movement has occurred § Characterized by shallow earthquakes § A result of the earth being a sphere § Transform faults are the active parts of fracture zones § Fracture zones extend further away from ridge axis and are evidence of past transform faulting. § Lithospheric plate on either side of transform faults move in opposite directions § Outward sections of fracture zones move in the same direction

Structure of the mid-Atlantic Ridge between FL and W. Africa Depressed central rift valley is in blue

Further North and West Seismic profile of ridge being buried

Hydrothermal vents § Discovered only in 1977 along the East Pacific Rise (now found on mid-Atlantic Ridge, in the Sea of Cortez and on the Juan de Fuca Ridge) § Really hot (~ 350 o. C), mineral-rich water (black smokers). Can form chimneys, deposit minerals, affects ocean chemistry, etc. § Seawater descends through fissures to meet hot rock. Superheated seawater dissolves minerals and escapes upward. § Found in Lake Baikal – future ocean? § Average water temperature is 8 – 16 o. C (bottom ocean water is 4 o. C)

Broad continental shelves Trenches Ridges GBR Red Sea Galapagos Iceland

Take home points § Continental shelf, break, slope and rise (relative slopes) § Abyssal plains § Mid-ocean ridge system (describes plate boundaries) § Hydrothermal vents, coral reefs, seamounts and guyots

Opening since the Miocene (65 million years ago)