Bathymetry of the Ocean Floor The ocean floor is mapped by SONAR. (Sound navigation and ranging) Depth = (time x 1500 m/sec)/2 (round trip) At 25 degrees Celsius

Relationship above and below the water

Ocean Floor Topography Major features: continental shelf, trenches, abyssal plains, mid-ocean ridges. After 1920’s, these features could be mapped in detail by sonar (bouncing sound waves off the ocean floor).

Continental Shelf Area of shallow water along passive continental margin (not an active plate boundary). Continental Margin – consists of continental crust (think shallow end of pool) Shelf drops off abruptly at outer edge. Much of shelf was above sea level during past glacial periods.

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

Submarine Canyons 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 - 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.

Trenches Occur along active margins (present-day plate boundaries) where subduction is taking place deepest part of the ocean floor, typically 3 - 4 km deeper than surrounding seafloor 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

Deep Ocean Basin - Abyssal Plane Flat areas making up much of the ocean floor apart from trenches and mid-ocean ridges

Deep Ocean Basin - seaward of the continental margin In most places, seafloor is a flat plain - Abyssal Plain covered by sediment deposits of turbidity currents covering an irregular seafloor plain interrupted by: seamounts - individual mountains made of volcanic material. Rise steeply, sometimes above the surface to form islands. guyots - (flat topped seamounts found most often in the Pacific) table mounts, - 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 Ridges Largest topographic features on earth. Broad ridges (where plates pull apart from one another) with a deep rift valley down the center. Rift Valley-forms between the ridges of mountains. Location where basaltic lava flows erupt. Age of the ocean floor Detailed studies in 1950’s showed that there are bands of similar aged rocks parallel to mid-ocean ridges. Bands get older with greater distance from the ridges.

Basins Abyssal plains Seamount Guyot Trenches Mid Ocean ridges Rift Valley