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Ocean Water, Ocean Floors, and Ocean
Currents
Earth Science 11
World Oceans
• Cover more than 70% of
Earth’s surface
• In order of size:
– Pacific Ocean is the largest
– Atlantic Ocean is the 2nd largest
– Indian Ocean is the 3rd largest
– Arctic Ocean is the smallest
Salinity
• Is a measure of the dissolved solids
in sea water
• In 1 000 g of sea water:
– 965 g of water
– 35 g of salts
• In polar regions freezing of fresh
water can increase ocean salinity.
• In hotter areas, evaporation of
water can increase ocean salinity.
Ocean Layers
• Water has a high specific heat
capacity. This means that
oceans require huge amounts
of sunlight to heat their
surface layers by only a few
degrees.
• Most of ocean water is below
5°C even at the equator!
• Only the top km of water is, on
average, slightly warmer.
Ocean Layers
• A thermocline exists
as temperature
rapidly drops from
the bottom of the
surface layer to the
deep water zone.
• The surface layer is
also called the mixed
layer because winds
and waves stir it up.
Studying the Ocean Floor
Studying the Ocean Floor
• Is largely done using echo
sounders
– Sound waves are sent
downward
– They are reflected up from
the ocean floor
– Knowing the speed of sound
in water ( 1 500 m / s)
allows us to calculate depth
A profile of the
ocean floor can
then be made.
As a result:
• Three major areas are recognized:
– Continental margins
– Deep-ocean basins (includes the
abyssal plains)
– mid-ocean ridges
Not in notes.
A profile from New England to the coast of North Africa.
Continental Margins
• Consist of:
– The continental shelf,
– The continental slope, and
– The continental rise.
Continental Shelf
• Is a gently sloping, submerged
extension of the continents
• Can be almost nonexistent or can
extend outward as far as 1 500 km
• On average it is 80 km wide and
130 m deep (seaward edge).
• It drops about 2 m every km.
Continental Slope
• Marks the boundary between
continental crust and oceanic
crust.
• It is steep and relatively
narrow (20 km).
Continental Rise:
• Drops between 4 to 8 m per km.
• Can be hundreds of km wide.
• Has a thick accumulation of
sediment (moved from the
continental shelf).
• Contains the occasional submarine
canyon or submarine volcano.
Passive Continental Margins
• Such as along the Atlantic
Ocean:
– Are continuously being built from
the accumulation of eroded
continental sediment
Active Continental Margins:
• Such as along the Pacific Ocean:
– Have very little continental shelf.
– Have continental slopes that merge
with deep ocean trenches.
– Result from the subduction of one
plate underneath another.
Active and Passive Continental Margins
Formation of New Continental Margins
Sediment
the
erosion
of continental
inland,
As
Upwelling
the twofrom
land
of magma
masses
causes
separate,
cooling causes
continental
crust
the growth
of the
subsidence
crust to fracture.
of
the causes
continental
margins.
continental shelf.
Not in notes.
Submarine Canyons and Turbidity
Currents
Submarine Canyons
• Are deep, steep-sided valleys
that begin on the continental
slope.
• Can be as deep as 3 km.
• Are most likely formed by
turbidity currents.
Turbidity Currents:
• Are down-slope movements of
dense, sediment-laden water that
gouge the sea floor.
• Are created when sediment on the
continental shelf is dislodged
(earthquake) and thrown into
suspension.
• As it flows downward it gains in
sediment and speed.
In 1929 an earthquake off the coast of
Newfoundland occurred. A turbidity
current formed and, as a result, telephone
cables broke.
Turbidity current produced in a lab tank.
The times are too long after the
earthquake to have been caused by
the seismic waves.
Turbidity Currents:
• Usually originate along the
continental slope.
• Continue across the continental rise
still cutting channels.
• As the slope lessens, their speed
lessens and sediment begins to
settle out coarsest first and finest
last (graded bedding). These
deposits are called turbidites.
Not in notes.
Features of the Deep Ocean Basins
Deep Ocean Basin
• Lies between the continental
margin and the mid-ocean ridge.
• Located here are:
– Ocean Trenches
– Abyssal plains
– Seamounts
Ocean Trenches:
•
•
•
•
•
Are long and narrow.
Form the deepest part of the oceans.
Some exceed 10 km in depth!
Are located at subduction boundaries.
Trap sediment from turbidity currents
The continental
slope merges
with a trench.
A very narrow
continental
shelf is formed.
Why?
Ocean Trenches:
• In the open ocean are paralleled by
volcanic island arcs.
Ocean Trenches:
• Near the land are bordered by
volcanic mountains.
Abyssal Plains
• Are incredibly flat!
• Have accumulated lots of
sediment from turbidity
currents which has buried its
rugged features.
Seamounts
• Are submerged, steep-sided
volcanoes
• Form near mid-ocean ridges or
above hot spots.
• Can sometimes emerge as
islands.
• Can have coral reefs.
Coral Reefs
• Form in the warm, clear, sunlit
waters of the Pacific and Indian
Oceans
• Since light cannot penetrate very
deep, reefs only descend about
45 m
• Sudden temperature changes or
exposure to cold water can kill a
reef.
• Special reefs called atolls can form.
Coral Bleaching –
this reef is dead.
Atolls:
As the island gradually sinks, its reef grows
upward. Eventually the island will be completely
submerged and only the reef (called an atoll) will
touch the surface. These reefs, because they are
continually growing upward, can reach depths of
thousands of metres!
Mid-ocean Ridges
Mid-ocean Ridges
• Are like the seams on a baseball.
• Can be 500 to 5 000 km wide!
• Might occupy as much as one half of the
total ocean floor.
• Are divergent
boundaries.
• Become wider
from sea-floor
spreading.
•Are the longest
topographic feature on
Earth (65 000 km).
•Rise 2 – 3 km above
the ocean basin (but
are still submerged
under 2.5 km of
water!)
•Are made of basalt.
Recall: Rocks & Magnetism
• When rocks form in the presence of
Earth’s magnetic field, they are
influenced by the field.
• In cooling lava or magma, the iron
rich atoms are aligned with Earth’s
field, and then frozen into the rock.
Not in notes.
Sea Floor Spreading
• As the rock cooled, it inherited the
polarity of Earth’s magnetic field, and
then the sea floor spread apart like a
conveyor belt.
Not in notes.
Mid-ocean Ridges
• Are characterized by:
•
•
•
•
Hydrothermal systems
Deep sea vents (black smokers)
Pillow basalts
Living organisms !
Hydrothermal System
Cold seawater
enters the ridge, is
heated, and exits as
a plume of hot
water.
Deep Sea Vents (Smokers)
Deep Sea Vents (Smokers)
• The heated
saltwater can
dissolve
•One of the
minerals.
gases released
•isWhen
it gushes
hydrogen
from the
sulphide.
seafloor as
•Some
particle-filled
organisms
plumesby
of using
water
survive
it isgas
called
a
this
instead
“black
smoker”.
of
oxygen!
Pillow Basalts
• When lava directly encounters water it
is super-cooled into pillow basalt.
Pillow Basalt
Trinity Bay, Newfoundland
Life at Deep Sea Vents
Not in notes.
Ocean Layers
• Four main zones:
– Epipelagic
(0 – 120 m)
– Mesopelagic
(120 – 1 200 m)
– Bathypelagic
(1 200 – 3000 m)
– Abyssal
( 3 000 m to bottom
or 6 000 m)
Not in notes.
Obstacles to Life
in the Deep Ocean
•Pressure
•Varying Oxygen Levels
•Increasing Carbon Dioxide
•Diminishing Light
•Decreasing Plankton
Very Deep!
Very Deep!
Very, Very Deep!
Bottom
Bottom
Bottom
Ocean Currents
An Ocean Current:
• Is a continuous flow of water
along a broad path in the
ocean
• Flows at any depth or along
the surface
• Can move vertically or
horizontally
World Surface Ocean Currents
Notice that they turn clockwise in the northern hemisphere
and counterclockwise in the southern hemisphere.
An Ocean Current:
• That flows away from the
equator carries warmer water
• That flows toward the equator
carries colder water
• The direction a current circles
is caused by Earth’s rotation
Two Types of Ocean Currents
• Surface Currents
– Upper 400 m
– 10% of all ocean
water
• Deep Ocean
Currents
– 90% of all ocean
water
Forces Creating Ocean Currents:
• Solar Heating
• Winds
• Gravity
• Rotation of Earth
Solar Heating:
• Causes water to expand
– Water is actually higher (8 cm) at
the equator than anywhere else!
– This creates a slight slope and
water, under gravity’s influence,
flows down slope
Winds Help Drive Surface Ocean Currents
A mound of
water, called a
gyre, is
created. Within
a gyre large
circular
currents are
produced.
Not in notes.
Winds Help Drive Surface Ocean Currents
Two
Surface
Seasonal
setscurrents
of
changes
winds
are
in
caused
are
wind
responsible:
direction
by wind.
do not
Friction
•Trade
normally
winds
atcause
the ocean
surface
•Westerly
changescauses
inwinds
current
water
to
These
direction.
be dragged
winds occur
along.
in
belts
Monsoons
around
arethe
an
world.
exception to this
rule.
Monsoons
• Winds blow from one direction
in summer and a different
direction in winter
• Changing wind direction
changes current direction
Summer Monsoon
• Air over land heats and rises
drawing moist air in from tropical
oceans
Winter Monsoon
• Air over land cools and sinks drawing
dry air in over the tropical oceans
Density Currents
• Turbidity currents are an
example
– Because the water is laden with
sediment, it is more dense and
therefore sinks
• Changes in density cause water
to move vertically
Density Currents
• These changes are caused by:
– Salinity changes (higher salinity
means denser water)
• Evaporation increases salinity
• Freezing increases salinity
– Temperature changes
• Cooler water is more dense
Ocean Layers
• Because colder water is denser, water
from the polar regions will sink and
move beneath the other oceans
creating deep ocean currents.
• As it sinks, it carries with it the oxygen
added to it in the surface layer from
waves and plankton.
– This is the only source of oxygen for deep
ocean life!
Deep Water Masses
• Form from variations in temperature
and salinity. This causes layering of
deep ocean water.
Upwelling
• Occurs when cold, deep ocean
water comes to the surface
• Sometimes caused by winds along
the coast. This is called coastal
upwelling.
• Sometimes caused by winds in the
open ocean.
• Sometimes caused by density
currents.
Coastal Upwelling
• Occurs when:
– Winds blow parallel to the shore
– Surface water is pushed away from
the coast
– Deep water rises up to replace water
that has been moved out to sea
Coastal Upwelling
Not in notes.
Equatorial Upwelling
• Occurs as surface water spreads out causing
sea level to fall. This causes the surface layer
to thin.
• Cold water moves up to thicken the reduced
surface layer.
Upwelling is Important Because:
• Water rising brings with it
important plankton nutrients
• These nutrients increase
plankton numbers and thereby
increase fish stocks.
Ocean Water Circulates
• As a result of currents and
upwelling, deep ocean water is
recirculated around the globe
every 1 000 years.
• This means that contaminants
released into the ocean in one
spot can be found everywhere
given enough time.