Ocean Floor The ocean basins are characterized by

Ocean Floor

• The ocean basins are characterized by recognizable geologic/topographic features. While the size of each feature may change within the various ocean basins they are always present:

Continental shelf • - the gently (<1°) sloping platform at the edge of the continent. • not part of the ocean basin. • average water depth 75 meters (0150 m) • avg. 60 kilometers wide

• Width of the slope is determined by proximity to a plate boundary • Passive margins have longer shelves than active margins • 18, 000 years ago we had 18% more land than we do now…slopes are greatly affected by sea level changes. Why?

Shelf Break • Marks the abrupt transition from shelf to slope

Continental slope • - the continental slope marks the transition between the shelf and deep ocean floor. • It has an average slope of 3 -6°. 100 kilometers water depth increases from 75 meters to 4000 meters.

Submarine canyons • continental slopes are crisscrossed by a series of deep submarine canyons the origin of which is controversial. • Some represent drowned stream valleys, but others were clearly never above sea level and can not have been cut downward by stream erosion.

Continental rise • - represents the accumulation of sediment at the base of the continental slope. Result is a gentler slope and the buildup of "turbidite" deposits.

• Abyssal plain - the ocean floor (covers about 30% of the earth's surface). • The average water depth is around 5000 meters. Consists of a layer of unconsolidated sediment underlain by sedimentary rock and pillow basalt.

Ocean ridges

• Does not run in a straight line because it is offset by transform faults • Hydrothermal vents are found here

Abyssal plain vs. Abyssal hills • Plains are flat • Hills are small sediment covered extinct volcanoes---less than 200 meters high • No abyssal plains in pacific…very little in Indian. Mostly Atlantic…why?

Seamounts • Volcanic projections that do not rise above the surface of the ocean • Usually circular or elliptical • • Higher than Ikm May be alone or in groups May form over hot spots More common in the pacific

Guyots • Flat topped seamounts that were once tall enough o break the surface of the ocean

Trenches • Arc shaped depression in deep ocean floor • Where converging plates are subducting • Most active areas

Island Arcs • Curving chains of volcanic islands • Seamounts also • Found parallel to concave edges of a trench • Aleutian islands • Marianna islands