Transcript Chapter 9/10 Oceans

The Ocean Basins
Ocean Basins
• Oceans are all connected, so it’s really just
1 big ocean!
• 5 ocean basins
– Atlantic
– Pacific
– Indian
– Antarctic (Southern)
– Arctic (northern extension of the Atlantic)
Ocean Basins
How old are the Earth’s Oceans?
• Oldest rock formation found on Earth dated at about 4
billion (4,000 million years old)
• These rocks were deposited in an ocean environment
• Recall Earth is about 4.5 billion years old)
The Earth’s Oceans
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cover 71% of the Earth’s surface
2-11 km (1-6 miles) in depth (~5 km or 3 miles average)
1.4 billion km3 (310 million cubic miles) water volume
Where did it all come from?
Origins of the Oceans
From the inside out: Volcanic Outgassing
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Water vapor and other
gases trapped within
Earth as it formed,
Volcanic eruptions,
which emit water vapor
and gas, more plentiful
in the geologic past,
Much of this lost to solar
wind. Accounts for
perhaps 10% of the
Earth’s atmosphere and
ocean material.
Origins of the Oceans From the outside in:
Cometary Water
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Volatiles lost from the Earth
and other inner planets
combined with debris from
formation of solar system to
form comets.
Comets are composed mainly
of rock and frozen gases,
including water.
Early solar system had many
more comets than today.
Current popular hypothesis is
that atmosphere and oceans
formed from impacts with
comets and other planetary
debris.
Oceanic crust and Continental crust
• Oceanic crust is
younger; oldest age
found is 200 million
years and most is
less than 60 my
(why is that?)
• Much continental
crust over one billion
years old!
• Oceanic crust is
mostly basaltic;
continental crust is
mostly granitic.
• Oceanic crust is
denser – due to the
iron content of
basaltic magma.
Oceanic crust and continental crust
• Ocean crust is thinner – 4-7 km versus 20-40 km
for continental crust.
• Continental crust sits much higher on the
surface of the Earth.
• And yes, oceanic crust is usually wetter. 
Profile View of Atlantic Ocean
Oceanic Crust and Continental Crust
Mid-Ocean Ridges
MidAtlantic
Ridge
East Pacific
Rise
Mid-Indian Ridge
Radar Image of Mid-Atlantic Ridge
Deep Ocean Trench at Oceanic-Oceanic
Boundary (e.g. Marianas Trench)
Radar Image of Deep
Ocean Trenches at
Boundary of Pacific
Plate
From North to South
•Aleutian Trench
•Kurile Trench
•Japan Trench
•MarianaTrench (curved)
•Philippine Trench
(between Philippine
Plate and Eurasian Plate)
The long purple line
south of the Aleutian
Trench is the Emperor
Seamounts
Studying the Sea Floor – Direct
Methods
• Rock dredge – openmouth steel net
dragged along the
ocean floor
• Coring – a pipe is
used to collect a mud
or sediment core
Studying the Sea Floor – Direct
Methods
• Sea-floor drilling – similar to equipment used for
off shore oil drilling
Studying the Sea Floor – Acoustical
and other Remote Methods
• Echo sounding –
used to map sea-floor
topography (SONAR)
• Submersibles – both
manned and robotic
Echo Sounder
Deep Sea Exploration Robot
Submersible
Studying the Sea Floor – Remote
Methods
• Seismic Profiler – like sonar, but higherenergy waves penetrate sea-floor and
image the subsurface
• Magnetometer – towed behind ships to
record magnetic fields of rock
• Microwave radar – used to map seasurface, surface mimics sea-floor
topography
Seismic Profiling
Magnetometer
Seawater Composition and Salinity
• The salt content or salinity is
around 3.5%, most being
NaCl, with lesser amounts
of KCl, bromides and
carbonates.
• The dissolved material in
sea water comes from
– Weathering of rock and
transport to sea by rivers
– Volcanic activity
– “Black smokers” at midocean ridges
Seawater Composition and Salinity
• Evaporation and freezing tend to concentrate
salt in sea water
– Arctic Ocean, Mediterranean Sea
• Salt is removed by biological organisms
• Sea water also receives an influx of fresh water
from precipitation and river flow
3 Major Density Zones in the Ocean (controlled by
temperature and salinity)
Current: continuous flow of water in a given
direction
Surface currents: caused
by winds
Winds blow in preferred
directions due to
differential heating of
the earth’s surface
Example of a surface
current: Gulf Stream
– 80km wide by
650m deep, up to
5km/hr. velocity
Currents influence the
climate (Labrador vs.
England)
Gyres - circuit of currents around the
periphery of an ocean basin
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Gulf Stream Current is part of the North
Atlantic Gyre)
Caused by Westerlies and trade winds
Trade winds blow from the northeast in
the or southeast in the tropics
Westerly winds (westerlies) blow from
the northwest or southwest in the midlatitudes
Gyres
• Gyres circulate
clockwise in the
northern hemisphere
• Gyres circulate
counterclockwise in
the southern
hemisphere
Surface Ocean Currents
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Currents on the west side of the ocean basin
(i.e. east side of continent) originate in the
tropics and are warm-water currents
Examples: Gulfstream, Brazil Current
Currents on the east side of the ocean basin
(i.e. west side of continent) originate at the
poles and are cold-water currents
Examples: California Current, Humbolt
Current (cause of El Nino)
Surface Ocean Currents
Deep-Ocean Currents- Thermohaline
Circulation
– caused by density differentials
– Temperature: cold water is more dense than
warm and will sink
– Salinity: saltier water is denser than fresh water
and will sink
– Deep-ocean currents are connected to the
surface current system
– Increased freshening of polar waters is
changing thermohaline circulation patterns.
What is the effect on surface currents?
Great Ocean Conveyor Belt
Near Greenland, water becomes very dense due
to salinity and sinks
This allows the North Atlantic Drift to continue
toward Northern Europe.
Global warming may shut this down
Disastrous consequences for Europe!
The diagram shows the South Pacific during a normal year.
• Strong S. Pacific Equatorial Current reinforces the northward-flowing
Humbolt Current
• This promotes upwelling of cold, nutrient-rich water, which supports the
S. American fishing industry
An El Nino Event (ENSO)
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Occurs every 3-7 years for a year
Weakens trade winds and S. Pacific Equatorial current
This in turn weakens the Humbolt Curent which reduces cold upwelling
This creates warmer surface water, depresses the fishing industry and
changes weather patterns in many places.
Consequences of El Nino
Anomalous warm
water off the coast of
South America
• Increased storm activity in many parts of the U.S and northwestern Europe
• Drought in Australia and S.E. Asia
If the Labrador Current (Cold) is too fresh to sink, it
may block passage of the N. Atlantic Drift (warm)
which moderates the climate of Great Britain and
Scandinavia
Global warming and sea-level rise
• Sea-level has risen and fallen in the past
– Past 40,000 yrs, 150m fluctuation
– More recently, sea-level is rising ~3mm/yr
– Warming temperatures raise sea level by
• Thermal expansion of water
• Release of glacial water that was on land
– Some low lying coasts and islands are
already feeling effects
Fig. 16-39, p.401
Rising Sea Level
Fig. 16.38a, p.423
Tides
• Both the Earth and the Moon revolve around a
point located in the interior of the earth, near
point A.
• The solid Earth is not affected, but the water
forms 2 permanent bulges.
Tides
• Bulge “A” caused directly by gravitational
attraction of moon.
• Bulge “B” caused by inertia of water as earth
swings inward toward the moon (“centrifugal
force”).
Tides
• As the earth rotates, shorelines pass through
each bulge approximately 12 hours apart.
• Other influences include:
– Gravitational effect of the Sun
– Shape of the coastline
Spring and Neap Tides
• Spring tide – highest tides, occur when
Earth, Sun & Moon are in alignment
• Neap tide – lower high tide, when Earth,
Sun & Moon form a 90o angle
Spring and Neap Tide
Beach Drifting and Longshore Currents
Fig. 16-22, p.390