Ocean Basins
Ocean Basins = about 71 percent (~two-thirds) of our planet. Only 3 percent of Earth's water is fresh, 97% is salty.
We study the sea floor
- directly: rock dredge, sea floor drilling, diving, submarines
- indirectly: remote sensing -- echos, seismic, magnetometer, radar
We learned the geography under the oceans by studying how much cable was played out for the Trans-Atlantic Cable in the 19th Century, by submarine studies beginning in the 20th Century and by SONAR, in common use after WWII. SONAR emits a signal which is reflected off the bottom and bounced back to the original emitter or a towed receiver. Depth is equal to 1/2[(1500 meter/second)(travel time in seconds)].
Some of the millions of things we've learned include
- Life on the ocean floor does NOT use sunlight for energy.
- Sea floor magnetism and the study of magnetic reversals support the idea that sea floor spreading is one of the driving forces of plate tectonics.
- Mid-ocean ridges produce lava continuously pushing the plates apart. Other features include rift valleys and transform faults.
- The base rock of the sea floor is ultramafic "mantle periodotite, over lain by mafic gabbro, then basalt, topped by pillows or vent basalts. The whole igneous base is covered by sea floor sediment which is classified by source: terrigeneous or pelagic or classified by type: clastic/biogeneous or chemical sediment
Continental margins can be either passive or active.
- Passive margins
- continental shelf
- carbonate platforms
- continental slope and rise
- submarine canyons
- abyssal fans
- turbidity currents (confined sorting due to pressure)
- Active margins
- oceanic trench
- submarine canyons
- abyssal fans
- fewer sediments preserved than in passive margins
- island arcs
Other features of the sea floor include
- Seamounts - undersea islands.
- Oceanic Islands - stick out through top of ocean, may be volcanos or continental chunks
- Atolls - eroded oceanic islands marked by coral reefs.
- Suspect ("accreted") Terranes - when any of the above is pasted onto a continent
Sea levels change over time. In general:
- Low relative sea level exposes continental shelves. Carbon dioxide escapes to the atmosphere and the temperature warms up.
- High relative sea level drowns shelves, CO2 is removed by organisms and temperature goes down.
The "Seven Seas" are really four basins: Atlantic, Pacific, Indian & Arctic. The Atlantic Ocean is spreading; New York and London are about 3 centimeters further apart every year. The oldest rocks in the oceans are about 180 million years old and are going down the Marianas Trench.
Sea Water
- Salinity + 35 parts per thousand or 3.5 percent.
- Temperature: warm at the Equator, cold at the poles
- Salt water does not freeze. Sea ice is fresh water.
Ocean Currents
- Since ancient times, people have known that prevailing currents usually flow in a particular direction. Surface currents flow away from Earth's spin at the equator and the poles, towards the spin in the midlatitudes, but are deflected eastward in the northern hemisphere and westward in the southern hemisphere by the so-called "Coriolis effect."
- Deep-sea currents are temperature/salinity dependent and rise to the surface in upwellings that bring underwater nutrients to the surface.
- Turbidity currents are underseas debris flows which produce a layered sedimentary deposit, sorted confined by pressure with the largest sediments on the bottom, trending up finer and finer to the top.
Tides
- Spring tides [German "springen" go up] -- big difference between low/high tide -- occur at full moon and new moon
- Neap tides -- small tides-- occur at first and last quarter moons
- Tide charts are available for U.S. coastlines. Do not go on unfamiliar beaches without a tide chart as well as consulting local people.
Waves are caused by:
- Wind pushing water.
- Earthquakes, landslides and bolide impact cause tsunami.
- powered vehicles or floating objects including large animals.
Waves have energy. Shorter wavelengths are more energetic than longer wavelengths. Features of waves include:
- Wavelength
- Crest
- Trough
- Wave Height
Waves rebound from solid objects.
- Reflection causes interference and is common in restricted areas.
- Refraction causes the wave to bounce off at an angle about 90 degrees to the angle at which it hit the shore. In light this is referred to as the "angle of incidence = angle of refraction."
- Shear occurs when a wave passes a solid object such as a rock or a shell.
- Undertow and rips can occur when the angles of the waves and the architecture of the shore combine to align the most force directly offshore.
- The shape of sediments on beaches show the wave energy at that beach.
Wave erosion is relentless and powerful because of
- Impact of wave and any debris or sediment the wave carried
- Abrasion, breaking big rocks into little rocks.
- Repetition
Sediment Transport
- Refraction.
- Long shore current (deposition above, erosion below)
- Tidal currents
Habitats at the water's edge include beaches, foreshore, backshore and intertidal zone and reefs.
Coastlines change over time
- Emergent Coastlines are part of a continent that was underwater and is now sticking out. This can cause wave-cut terraces as uplift continually raises land and the ocean cuts a new platform.
- Submergent Coastlines are part of a continent which is still underwater.
- Eustatic change sea level change results from:
- growth or melting of glaciers
- changes in temperature
- volume of the mid-ocean ridges
Topography and sediment control type of coast
- Sediment rich (sandy) coastline
- spits and bay-mouth bars
- barrier island
- lagoon
- Sediment poor (rocky) coastline
- wave-cut cliff
- wave-cut platform
- sea arch
- sea stack
- fjord (Hudson River, New York)
- estuary
Development and pollution affect shorelines and oceans.
- Methods to constrain sediment transport include
- groins and jetties to "control" deposition.
- sea walls and riprap to try to control refraction.
- tend to collect sediment above and erode below the object.
- Global Warming
- glaciers are smaller now than 10,000 years ago
- climate is warmer now than 10,000 years ago
- much coastline formerly emergent is now submerged.
- if the temperature keeps going up, the oceans will continue to rise.
Additional References
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