Bathymetry of the Ocean Floor

1. 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

2. Relationship above and below the water

3. 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).

4. Continental Shelf • Area of shallow water along passive continental margin (not an active plate boundary). • Shelf drops off abruptly at outer edge. • Much of shelf was above sea level during past glacial periods. • Important processes at a passive margin • Accumulation of sediment. • Subsidence (sinking) of crust as sediment is added. • Mass wasting at edge of continental shelf. • Turbidity currents (large rapid flows of dense, sediment-laden water).

5. 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

6. 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 • 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

7. Abyssal Plane • Flat areas making up much of the ocean floor apart from trenches and mid-ocean ridges

8. Mid-Ocean Ridges • Largest topographic features on earth. • Broad ridges with a deep rift valley down the center. • Offset by faults. • 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.

9. 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 • - 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

10. 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

12. Basins • Abyssal plains • Abyssal hill • Seamount • Guyot • Trenches • Mid Ocean ridges • Rift Valley