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 (0-150 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 flat 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