IB Oceans and their Coastal Margins

B1 – Introduction to Oceans: Morphology of Oceans & Ocean Water

MORPHOLOGY OF OCEANSB1 – Introduction to Oceans

Seamounts – extinct volcanic cones that lie below the surface.

Guyot - a flat topped volcano that once reached the surface but later subsided.

Abysall Plain - at the edge of the continental slope. These plains cover large areas of the sea floor at depths of between 4000m and 6000m. They are generally flat and featureless.

Abysall Plain - at the edge of the continental slope. These plains cover large areas of the sea floor at depths of between 4000m and 6000m. They are generally flat and featureless.

Continental Slope- the steeply sloping area of the seabed that stretches from the continental shelf to the abyssal plain.

Submarine Canyon – steep sided valley on the sea floor of a continental slope.

• Often (but not always) found as extensions to larger rivers.

• Submarine canyons are thought to be major conduits for sediment movement from the continents into the deep-sea.

Ocean Trenches

Ocean Trenches• The deepest part of the oceans. Arc-shaped

depressions, formed at subduction zones where one tectonic plate plunges under another one.

• Usually a dense oceanic plate going under a less dense continental one.

Mid-Ocean Ridges

• The largest feature of the ocean floor.

• Linear belt of submarine mountains.

• New magma forces its way up between two plates and pushes them apart.

• The rate of spreading at the mid Atlantic ridge is 5 cm a year.

Continental Shelf

• Relatively flat area of seabed, stretching from the land to the edge of the continental slope.

• The continental shelf is less than 250m deep and may be up to 70 km wide.

OCEANIC WATERB1 – Introduction to Oceans

Variations in Ocean Temperature

• Temperature in the oceans is not always constant.

• Ocean temperature not only varies through the seasons but also by latitude.

• Sea surface temperature usually recorded remotely by satellite.

f

Impact of currents can be seen:- Western side of continents shows cooler water

heading towards equator.

Sea Surface Temperature

----

• Ocean water also varies with depth:

– Temperature

– Salinity

• Ocean waters varies seasonally up to depths of 500 – 1000 metres.

-------• A boundary

usually occurs at a depth of between 200 and 800m called the thermocline; below that surface waters don’t mix.

• 90% of the total volume of ocean water is found below the thermocline.

Other Changes with Depth• Density of ocean water increases constantly

with decreasing temperature until the water freezes.

• Ocean water is saline therefore its normal freezing temperature is -1.94°C; significantly cooler than pure water.

• As sea water freezes it usually rejects some of the salt; therefore is only 1% saline; where as sea water is 3.5% saline.

• Salinity and density show the opposite trend to water as the depth of ocean water increases.

• Temperature declines with increasing depth but salinity and water density both increase.

• Salinity show a rapid increase at the same time temperature falls rapidly. This increase of salinity is called the halocine.

• At the same depth the water density also increases rapidly in the same zone which is referred to as the pycnocline.

Tropical Areas

• These changes are most noticeable at the tropical areas.

• The surface temperatures are warmer in the equatorial areas.

• At great depths the temperature of ocean water shows very little variation.

Changes with Latitude - Salinity

• Average salinity is 35 parts per thousand.

• Concentrations of salt are higher in warm seas, due to high rates of evaporation of water.

• In polar seas where there is an input of fresh water from rivers salinity is low.