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Lecture 15

ENVIRSC 1G03 Lecture Notes - Lecture 15: Mid-Ocean Ridge, Longshore Drift, Abyssal Plain

Environmental Science
Course Code
Luc Bernier

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Chapter 15
December 3, 2014
10:15 AM
15.1 Introduction
Coast - region where land meets the sea
15.2 Landscapes beneath the seaa
Distinction between land and sea exists because continental and oceanic lithosphere differ
markedly from one another by composition,, thickness
Can produce clear ocean bathymetry
o Indicate different bathymetric provinces distingushed by water depth
Continental shelves, slopes, and rises
Continental shelf - relatively shallow portion of the ocean that fringes the continent
o 200-500 KM WIDE
o Water depth < 500m
o At eastern edge, the shelf merges with continental slope
o Continental rise - 4km down to ~4.5 km
Abyssal plain - vast nearly horizontal surface.
o 4.5 km deep
Broad continental shelves form along passive continental margins (margins that are not plate
boundaries thus lack seismicity)
o Originate after rift breaks a continent in 2
o When rifting stops, sea-floor spreading begins.
Submarine canyons - relatively narrow and deep valleys
o Downcut into continental shelves and slopes
o Some start off shore of major rivers and for good reason: rivers cut into continental shelf at
Bathymetry of oceanic plate boundaries
Mid ocean ridge -2km high submarine mountain belt
Axis of ridge may be bordered by escrapments.
Fracture zones can be traced into oceanic plate away from ridge axis where theya re not
seismically active but still form boundary between plates of different ages.
Trench - Deep elongate trough bordering a volcanic arc.
Abyssal plains and seamounts
Oceanic crust agesa nd moves away from axis of mid-ocean ridge and two changes take place:
1. Lithosphere cools and as it does so, surface sinks
2. Blanket of sediment gradually acumulates and covers basalt of oceanic crust.
i. Blanket consists mostly of plankton shells and fine flakes of clay
Seamounts - hot-spot volcano active or not , those that lie below sea level
15.3 Ocean water and currents
Composition and temperature
Ocean water has ~3,5% of dissolved salt = more dense
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Salinity - concentration of salt in water
o Reflects balance between addition of freshwater by rivers/rain and removeal of freshwater
by evaporation.
o Depends on water temp.
Water temp in ocean varies with depthj.
o Waters warned by sun = less dense and remain on surface.
Thermocline -below which water tems decrease sharply reachg near freezing at sea floor
o ~300m in tropics
o No pronounced thermocline in polar areas
Currents: River in the sea
Current - Water of ocean flows/circulates at velocities up to several km/h in fairly well-defined
Circulation in sea occurs at 2 levels:
o Surface currents - affect upper ~100 meters
o Deep currents - remainder of water column
Results from wind and sea surface friction
Coriolis effect - surface currents in nothern hemisphere to veer toward the right and surface
currents in southern heisphere to veer toward the left of avg wind direction.
o Generated by the earths rotation
o Causes surface currents to make a complete loop (GYRE)
o Surface water may be trapped at ecnter of gyre.
Surface water and deeper water in ocean exchange at # of locations:
o Downwelling zone - surface water sinks
o Upwelling zones - deeper water rises
o Occurs for # of reasons:
Winds blow surface water away from shore, deficit of water develops along cost,
deeper water must rise to fill gap.
Thermohaline circulation - Contrasts in density, caused by temp and salinity can also
drive this.
Denser water sinks, whereas water less dense rises.
Resulting in cold water in polar region sinking and flowing back along the
bottom of ocean toward equator.
15.4 Tides
Tide - generally twice daily rise and fall of sea level
High tide - habor's water is deep enough that ship can easily float over obstacles
Low tide, water may be may be so shallow that ship could run into obstacles
Tides develop response to tide-generatng force which is due to gravitational attraction of the sun
and moon. And in part to centrifugal force produced by revolution of the earth-mooon system.
o Tide-generating force results in 2 tidal bulges in oceans on opposite sides of planet.
Larger one occurs on side of earth closer to moon.
o Tidal reach - elevation difference between sea level at hightide and low tide which can
range between less than a meter to several meters.
15.5 Wave action
Wind driven waves make ocean surface always moving
o Developed when molecules of care in wind and water at the surface of sea.
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