An Introduction to Oceans

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An Introduction to Oceans
I would like to introduce you to an ocean.
Im the poor student who has to sit through this!
Im an ocean.
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What is the ocean made of?
Water!!
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OK, here is some water. Why doesnt the ocean
look like this (A great big ball of water
hangin out there above land)?
Hint Is the ocean a solid, a liquid, or gas?
How do we define a liquid?
A liquid maintains its volume but takes the
shape of its container.
WHY?
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Why does the ocean look like this instead?
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Gravity always pulls you (and water) towards the
center of the Earth.
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The surface of the ocean defines a gravity
equipotential (a surface on which the force of
gravity is equal everywhere)
This surface is (almost) a perfect sphere.
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So, we got the shape of the top surface of the
ocean covered. The top of the ocean is defined
by a surface of equal gravititational force.
What about the bottom (i.e., the container)?
What do you think is the shape of the ocean
bottom?
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To answer that question, we need to dig a bit
into the dusty archives of history and see how
others have answered it
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From the beginning, the ocean was used for trade.
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A drive for exploration led to the dispersal of
the Polynesian people.
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Studies of the ocean first began when humans
started sailing on the seas for trade and
exploration.
The Greeks noticed a strong current beyond the
Pillars of Hercules
(okeanos was the name of this current. Later,
Latin changed this to oceanus, from which the
name ocean comes)
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It was always nice to be able to return to a
place you went before (or to home!), so the
earliest traders and explorers had to make
charts
Eratosthenes, 3rd Cent. BC
a picky detail that will save you
embarrassmenta map is of land, a chart is of
the ocean. Never ask for a map of the ocean!
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However, early travelers needed a way to locate
themselves on the surface.
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Longitude (how far east or west you are from a
reference line (the Prime Meridian) was trickier
because the Earth is rotating and you need to
know not only the angle to the star but also
the time difference between the Prime Meridian
and your location.
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The Greeks not only figured out that the Earth
was round, but they also developed the system of
latitude and longitude
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Once people had charts, there was another minor
detail they needed to know
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The earliest widely made measurements of the
ocean were of its depth.
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Chart of Scotland shows how few soundings were
made
Edinburgh
units in fathoms (6 feet)
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Not until the 19th Century did people make
soundings deeper than a few hundred meters
Ross (of the Ross Ice Sheet fame) early 1800s
- S. Atlantic depth of 4433 m Maury - 1840s -
discovered mid Atlantic ridge (Father of
physical oceanography)
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but we really did not make much progress on the
shape of the ocean bottom until the advent of
submarines
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It was the need for submarine detection tools
that led to the major tools of modern oceanography
after WWII.
In addition to waves bouncing off the submarine,
they bounced off the bottom and bright folks
figured that they could use that information.
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Today, we use a multibeam array with
many receivers in order to eliminate the problem
of canyons and narrow features scattering the
sound waves. (Think CAT scan of ocean floor)
and in a mere 125 years, we will be able to
cover the ocean floor completely!
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but were Americans and instant gratification
takes too long1
1Meryl Streep in Postcards from the Edge
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Small variations (up to 200 m) of the oceans
height are created by gravitational attraction
of local topographic variations (not only does
the Earth suck, but rocks do also).
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From these variations,we can calculate the sea
bed topography
topex.ucsd.edu
and we discovered that the ocean was not
deepest in the middle (Sorry, Homer)!
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Mid- Atlantic Ridge
The ocean floor has greater relief than do the
continents!
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The deepest part of the ocean is NOT in the
middle but in some places at the edges by the
continents!
Japan
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The highest elevation Mt. Everest is NOT as high
as the ocean is deep.
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The Marianas Trench is REALLY deep!
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Anatomy of the ocean floor
Ocean basin
Continental margin - part of continent, not ocean
Ocean ridge - usually in center where oceanic
crust is created
Abyssal plain- broad, flat, bounded by
continental shelves, ridges, trenches
Trench - usually at margin where oceanic crust
returns to the mantle
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Lets summarize what we have learned about
shape 1. We know the top of the
ocean container is a gravitational
equipotential. 2. We know the bottom and sides
of the container are controlled by the Earths
crust and the shape is complicated with
continental shelves, abyssal plains, ridges, and
trenches
If we know the top, bottom, and sides of a
container holding a fluid, what can we calculate
about the fluid?
Its VOLUME!
1.37 billion cubic kilometers (0.13 of Earths
volume)
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Its AREA
361 million square kilometers
(70 of surface)
Its AVERAGE DEPTH
or higher than the San Gabriel mountains
AVERAGE TEMPERATURE
3.9C (about 40F)
as you will see, this has an enormous effect on
our climate.
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Does anyone wonder WHY the ocean botttom is
shaped the way it is?
Plate creation and destruction fundamentally
controls the shape of the ocean floor and
provides coupling between the hydrosphere
(water surface) and lithosphere (rock
surface).
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What ELSE might we ask about the ocean?
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Mutual gravitational attraction of
particles causes planet to grow
Gravitational attraction causes pressure
inside, melting rock and light stuff rises,
dense stuff sinks.
Core, mantle, crust forms.
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Outgassing of lightest fraction (water,
carbon dioxide, other gases)
Earth then looked like Venus now
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Alternative suggestion Icy comets hit Earth
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Regardless, we DO know that there was an ocean 4
By (Billion years) ago, only 0.6 By after the
formation of the Earth.
AGE OF OCEAN 4 By
How do you know?!!!
Types of sediments found today at the bottom of
oceans are found that are 3.9 By old, so the
ocean must have been there to deposit them.
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How do you know that there are sediments at the
bottom of the ocean and what they look like?
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We have also used deep diving submersibles to
look at the ocean bottom
Window for viewing
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We need specialized vehicles like these because
the pressure is so great at the bottom of the
ocean
Pressure is the weight (gravity pulling on the
mass) of the water above us.
10 m is equivalent to the weight of the
entire atmosphere on us (1 atm pressure)
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Most of our information about the ocean has come
from sampling of sediments, water, fish, . NOT
DIRECT OBSERVATION!
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Much oceanographic research is done
by governments and oceanographic institutes
1. Expensive equipment
2. Scientific research requires different
specialties working together.
What scientific disciplines have we discussed
today?
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What assumption are we making when we state that
the sediments in the 4 By old rocks were formed
at the ocean bottom because they look like
sediments we see at the ocean bottom today?
Uniformitarianism
The present is the key to the past.
Can we prove this?
NO!