The Ocean Floor
Origin of the Oceans
Features of the Ocean Floor
Origin of the Oceans
The atmosphere and ocean waters are thought to have been produced by outgassing
Volcanoes produce gases, the most abundant of which is water vapor
Others are carbon dioxide, hydrogen, and nitrogen
As the Earth cooled, water vapor condensed and fell as rain
An extensive ocean was present as much as 4 billion years ago
Seawater Composition
Salinity
3.5 % by weight of seawater
33 to 37 per mil (parts per thousand)
Mostly sodium and chlorine
99.9% of salinity due to chloride, sodium, sulfate, magnesium, calcium, potassium,
bicarbonate, and bromine
The Oceans
71% of the Earth’s surface is covered by an interconnected body of saltwater
Four main bodies are oceans
Pacific Ocean—The largest, with 53% of all water on Earth
Atlantic Ocean
Indian Ocean
Arctic Ocean
Smaller bodies are sea, which are usually marginal parts of a larger ocean
Ocean Data
The World’s Oceans
Ocean Water
Continental Margins
Transition from continent to ocean actually occurs under the continental slope
Includes continental shelf, continental slope, and continental rise
Continental margins are cut by submarine canyons
Ocean Topography
Continental Shelf
Between the shoreline and continental slope
Seafloor slopes seaward at much less than 1°, average slope 0.1°
Shelf-slope break
Seafloor inclination rapidly increases to several degrees
At an average depth of 135 meters
East coast has a wide shelf (several hundred km) west coast, narrow, a few km
Submarine Canyons
Steep-sided canyons occur mainly on the continental slope but many extend onto the shelf
Some submarine canyons can be traced to associated rivers on land, but others have no such
associations
The end in submarine fans on the continental rise
Submarine Canyons
Pleistocene sea level was as much as 130 meters lower than present sea level
1.6 million to 10,000 years ago
Exposed much of the continental shelf
Rivers flowed across the shelf and cut deep canyons which were subsequently flooded
The parts of submarine canyons that were never above sea level and those not associated
with rivers must have been eroded by other processes
Turbidity Currents
Dense, sediment-water mixtures
Flow downslope under the influence of gravity
Eroded submarine canyons and produced submarine fans
Existence demonstrated in 1929 Grand Banks earthquake where submarine cables were
successively broken
Slope and Rise
Continental Slope
Slopes seaward from 1 to 25°, but averages about 4°
On the Atlantic coast merges with the gentler continental rise
On the Pacific coast, slope extends to trench or ocean floor
Slope and rise are where most continentally-derived sediment is deposited
The Shelf-Slope Break
Landward of the break, sediments are affected by waves and tidal currents
Seaward of the break, bottom sediments are unaffected by surface processes
Seaward of the break sediments are moved simply by gravity
Types of Continental Margins
Active Continental Margin
Leading edge of a continental plate where oceanic lithosphere is being subducted
High seismicity
Young mountain range
Intermediate volcanism
Narrow continental shelf
Example—West coast of South America
Types of Continental Margins
Passive Continental Margin
Within a plate, rather than at a boundary
Broad continental shelves
Significant slope and rise
Seismically quiet
Lack volcanic activity
Example—East coast of North America
Active & Passive Continental Margins
Deep Ocean Basins
General Data
Average 3.8 km deep
This is below the ~100 meter depth to which sunlight generally penetrates
Ocean floor is completely dark
Temperature just a bit above 0° C
Pressures vary from 200 to over1000 atmospheres
Abyssal Plains
Flattest, most featureless areas on Earth
Generally flat
Interrupted by peaks more than 1 km high
Covered by fine-grained sediment
Derived from the continents
Deposited by turbidity currents
Because sediment transport is interrupted by trenches in the Pacific, abyssal plains fail to
develop
Ocean Trenches
Long narrow features
Sites of greatest oceanic depths
Lithospheric plates are consumed
Locations of shallow, intermediate, and deep focus earthquakes
Originally called Benioff Zones
Mark the location of subducting plates
Often flanked by a chain of volcanic islands
Ocean Ridges
Part of globe-encircling ocean ridge system
Ridges are a few thousand kilometers wide and rise a couple of kilometers above the adjacent
ocean floor
Many have a central rift 1-2 km deep and several kilometers wide
Places where new oceanic crust is added and seafloor spreading occurs
Fractures
Ridges appear to be offset on major fracture zones (we know that the ridge segments were not
ever connected)
Earthquakes occur on the fracture zone between ridge segments
These fractures are actually one kind of transform fault
Ocean Features
The Wilson Cycle
Seamounts, Guyots, Aseismc Ridges
Seamounts—Extinct volcanoes on the seafloor
Guyots—Former volcanoes
After the volcano went extinct waves eroded the top
Subsidence of the plate carrying the volcano dropped the eroded top below sea level
Aseismic Ridges—any ridge in the ocean floor not associated with earthquakes
Deep-Sea Sedimentation
Most sediments are fine grained
Larger grain sizes are not easily transported into the ocean
Ice is one of the few transport agents that could move larger grains into the ocean
Most sediments are windblown dust and volcanic ash and shells of microscopic organisms
Products of marine chemical reactions such as manganese nodules
Deep-Sea Sedimentation
Pelagic sediments
Fine grained material that settled from suspension far from the source
The bulk of oceanic sediment
Pelagic clay
Clay-sized particles from continents & islands
Covers the deeper parts of the ocean basin
Ooze
Shells of microscopic marine organisms
Calcareous—mostly calcium carbonate
Siliceous—silica from radiolaria or diatoms
Ocean Sediments
Reefs
Wave-resistant structures composed of skeletons of organisms
Three types
Fringing—solidly attached to the margins of an island or continent
Barrier—similar to fringing but separated from the mainland by a lagoon
Atoll—Circular to oval reef surround a lagoon
Reef Development
Ocean Currents
Surface currents are driven primarily by wind
Sailors were very familiar with these currents
Christopher Columbus used these currents in 1492 and 1493
Surface Currents
Ocean Currents
Deep ocean currents are very different
Deep ocean currents are driven by density differences, primarily caused by temperature and
salinity differences
Deeper Currents
Marine Resources
Dissolved compounds such as halite (sodium chloride or table salt) and magnesium can be
extracted from seawater
Sand and gravel
Oil and natural gas
Methane hydrates
Manganese nodules