The Sea Floor And Its Sediments

Measuring the Depths Soundings – depth measurements Measured in fathoms ( 1 fathom = 6 feet) Early methods of taking soundings: o Rock attached to a long rope o Load weight attached to a hemp line o Cannonball on a piano wire All were good for taking bottom samples, but because of the extreme depth and weight of the line, oftentimes, the bottom was not easily determined. Most soundings measured deeper than actuality.

1920’s Acoustic Sounding Equipment Echo Sounder (Depth Recorder) – measures the time required for a sound pulse to leave the surface vessel, reflect off of the bottom, and return.

1920’s Acoustic Sounding Equipment 1925 – German vessel METEOR ~ 1 st large scale use of and echo sounder – detected the Mid-Atlantic Ridge for the first time.

1920’s Acoustic Sounding Equipment The ocean floor’s bathymetry was explored with more precision. Bathymetry – the study and mapping of sea floor elevations and the variations of water depth; topography of the sea floor.

Bathymetry of Today Methods of Today Obtain bathymetry at scales that range from centimeters (inches) to thousands of kilometers (miles). Small Scale, Direct Observations: ROV’s ~remotely operated vehicles Large Scale, Indirect Observations: Multi-beam Sonar Systems (Sound Navigation and Ranging) – tens of thousands of square kilometers; low cost, great accuracy.

Bathymetry of Today LADS – Laser Airbourne Depth Sounder System Flown in a small fixed-wing aircraft m ( ft) over the surface; the exact position is determined by GPS The laser measures the distance between aircraft and seafloor. 900 soundings per second (3.24 million/hr)

Bathymetry of Today Satellite Measurements of Sea Surface Elevation changes in elevation are caused by changes in the earth’s gravity field due to the seafloor makeup. These changes are detected by RADAR Altimeters The excess mass of features such as seamounts and ridges creates a gravitational attraction that draws water to them, resulting in an elevation of the sea surface… 5 meters over seamounts, 10 meters over ridges!

Bathymetry of the Sea Floor Undersea mountain ranges are longer, valley floors are wider and flatter and canyons are deeper than those on land. The erosion of ocean bathymetry is usually slow. Forms of Physical Erosion: 1. Waves 2. Currents Form of Chemical Erosion: 1. Dissolution of minerals in water

Bathymetry of the Sea Floor Agents of Physical Change: 1. Gradual burial by sediments 2. Volcanism from ridges, hot spots, island arcs, active seamounts, etc. Most of the features have stayed the same for the last 100 million years.

The Continental Margin Includes edges of land masses below the ocean surface and the steep slopes of these landmasses that extend to the sea floor. Contains: Continental shelf Shelf break Slope Rise Two Basic Types: 1) Passive (Atlantic) – divergence 2) Active (Pacific) - convergence

Continental Shelf Lies at the edge of continents Examples: a) Passive margin: Wave eroded b) Passive margin: Continental rock dam c) Active or Passive margin: Tilted rock dam (along the coast of North America ~ upturned rock d) Active Margin: Seamounts and island arcs e) Salt Domes (Gulf of Mexico) f) Coral Reefs

Continental Shelf Break Abrupt change in the slope and rapid increase in depth at the boundary of the continental shelf. Contains: Continental slope – steep slope extending to the ocean floor Continental rise – less steep slope extending at the end of the shelf break – accumulation of sediment Example: off the western coast of South America ~ Peru-Chile Trench

Features Along the Continental Slope Submarine Canyons A steep- sided, V-shaped cross-section with tributaries (similar to river cut canyons on land) Formed by: turbidity currents - Moving flows of sediments and water – caused by earthquakes or overloaded sediments

Features Along the Continental Slope Turbidity currents:

Features Along the Continental Slope Submarine Canyons

Features Along the Continental Slope Turbidites – graded deposits caused by turbidity currents ~ layers of course particles overlain by decreasing sizes

The Ocean Basin Floor 1. Abyssal Plain – flat plain extending seaward from the base of the continental slope ~ known as basins 2. Abyssal Hills – less than 1000 meters high; these are the most common feature (50% of the Atlantic, 80% of the Pacific are probably volcanic) 3. Seamounts – steep-sided volcanoes rising abruptly and sometimes breaking the surface (islands) 4. Guyots – submerged, flat-topped seamounts ~ most are 1000 to 1700 meters below the surface

The Ocean Basin Floor 5. Coral Reefs - in warm waters of the Atlantic, Pacific & Indian Oceans, coral reefs and coral islands have formed around seamounts. coral grows around an island in warm, shallow water ~ FRINGING REEF If seamount subsides slowly enough to let coral grow upward at a rate that is not exceeded by rising water ~ BARRIER REEF Once the seamount subsides completely ~ ATOLL REEF

Fringing reef – barrier reef – atoll reef

The Ocean Basin Floor 6. Ridges 7. Rises 8. Trenches - associated with island arcs: Japan – Kuril Trench, Aleutian Trench, Philippine Trench, Mariana Trench