I. Geography and Geomorphology of the Oceans
world ocean
-uneven distribution of water on earth
-divided for convenience (order of decreasing size):
-Pacific
-largest
-deep trenches
-Atlantic
-longer coastline
-many major river inputs
Amazon
Zaire
Mississippi
-Indian
-Brahmaputra/Ganges river
-Arctic
-shallowest and smallest ocean
-Southern Ocean
world ocean
continents
total area
362 X 106 km2
155 X 106 km2
average depth
3,729
m
840 m
max. depth/height
11,022 m (Marianas
trench)
8,848 m (Mt. Everest)
II. Cross section of the ocean basin
-continental shelf –
shallow areas at margins
of the continent
~7-8% of the total ocean
area
biologically very rich
-continental slope
sometimes followed by a continental
rise
-abyssal plain
-more flat, sediment covered
-4,000-5,000 m
-submarine ridges
-continuous chain
-found in all oceans (e.g.,
mid Atlantic ridge)
-trenches
-7,000-11,022 m deep
(Challenger
deep in the Marianas trench)
-seamounts
-isolated islands formed
by volcanic action
III. Plate Tectonics
-Alfred Wegener, 1912
-His ideas were rejected
because he lacked a mechanism
-Major plates of the world
-earth’s crust divided into
a number of large plates bounded by ridges and trenches
-7 major plates
-Difference between continental and oceanic crust
(earth surface)
| Continental crust granite 35 km thick on average density 2.7 g cm-3 oldest rocks are 3.8 billion years old |
Oceanic crust basalt 5-7 km thick on average density 3.0 g cm-3 oldest rocks are <200 million years old |
-When plates meet they may:
(1) Spread (diverging)
– oceanic ridges
seafloor spreading
(2) Collide
(subduction
or mountain building)
a. trenches –
ocean-ocean
ocean-continent
b. continent-continent -- mountains
(3) Slide –
transverse
faults (transform faulting); e.g., California fault
-‘ring of fire’ – volcanic and geothermal activity
-seismic activity
-Hotspots – weak points in crust, e.g., Hawaiian islands
-Coasts – parts of the continental margin may be
classified as
-active margin
-passive margin
IV. Physical Environment – wind, waves, tides, currents, temperature
A. Temperature and Vertical Stratification
-4 major biogeographical zones: polar, cold temperate, warm temperate (subtropical), tropical (equatorial)
-metabolic reactions of
different
organisms are often optimized for particular temperatures
-metabolism often increases by a factor of two for each 10 degree C
rise
in temperature
-most marine organims are poikilothermic (ectothermic)
-temperature varies with depth
-less dense water floats on more dense water
-thermocline – depth of most rapid temperature decline
-pycnocline – depth of rapid density change (caused by either
temperature
or salinity)
-what affects water density?
-density versus temperature
-for freshwater the most dense water is at 4 C
-ice is lighter and floats
-warmer water is lighter and
floats
-density versus salinity
-freezing point depression
-for seawater the colder the water gets, the
denser it gets
-if you increase salt content, then water is denser also
B. Water Masses
-Upper water mass – mixes
due to wind
-Deep water masses – often
several different layers
C. Circulation
-waves -
little
net horizontal motion
-wave height –distance between crest and trough
-wind
speed
-fetch – distance over which the wind blows
-length of time the wind blows
-wavelength – horizontal distance between the crests of waves
-period – time for wave to pass by (for two crests to pass a
fixed
point)
-wave energy mostly dissipated at a depth of ½ the wavelength
-when water depth is 1.3 times the wave height
-- breaking waves
-water molecules
remain in one
place horizontally, just move up and down
-internal waves on pycnocline - high
amplitude
-currents – water
movements that result in horizontal water movement
-Influenced by winds, uneven solar radiation, land masses, earth's
rotation
-Winds --
caused by sun
-Coriolis Effect (not a force)
-due to earth's rotation on its axis
-spin imparts a deflection to the water
-causes huge circular current patterns called gyres
-How do currents affect the deeper water column?
-Ekman spirals
-wind energy is passed through the water column
-friction dissipates energy
-Coriolis effect deflects each layer with respect to the layer above it
-limit of the spiral is generally ~100 m
-Currents may also cause
vertical motion when they interact with land masses
-If water is blown offshore (due to winds or Coriolis effects), then
you can
get
upwelling
-deep water replaces the surface waters that were pushed offshore
-consequences for organisms (nutrients, temperature)
-Upwelling at equator
-Water also can sink
if it increases in density:
(1) can get more salty –
-evaporation
-freezing of ice
(2) can get colder – at the poles
North Atlantic deep water
-causes
deep global water circulation patterns - thermohaline circulation
-refreshes oxygen in deep waters
-replenishes nutrients in surface waters
-hundreds of years are required for circulation of
these major water
masses
throughout the oceans