If you look down at our planet from outer space, most of what you see is water; 71% of the planet's
surface is covered by ocean and it is because of this that the Earth is sometimes called "the water
planet". Only about three-tenths of our globe is covered with land.
The ocean wraps the globe and is divided into four major regions: the Atlantic Ocean, the Pacific
Ocean, the Indian Ocean and the Arctic Ocean. Some scientists consider the waters around
Antarctica to be a separate, fifth ocean as well. These oceans, although distinct in some ways, are all
interconnected; the same water is
circulated throughout them all.
If all the continents were crammed into one
corner of the Earth, the vast extent of the
world ocean could easily be seen. In
reality, of course, the continents are not
bunched together as shown in the figure to
the right, but instead are spread out over
the entire Earth's surface. Most
oceanographers, however, believe that a
long time ago in the Earth's geologic
history all of the continents were once
grouped closely together in much the same
manner.
Seventy-one percent of the Earth's surface is ocean.
Ocean Floor
Beneath the world's oceans lie rugged
mountains, active volcanoes, vast plateaus
and almost bottomless trenches. The deepest
ocean trenches could easily swallow up the
tallest mountains on land.
Around most continents are shallow seas
that cover gently sloping areas called
continental shelves. These reach depths of
about 650 feet (200 m). The continental
shelves end at the steeper continental slopes,
which lead down to the deepest parts of the
ocean.
Beyond the continental slope is the abyss.
The abyss contains plains, long mountains
ranges called ocean ridges, isolated
mountains called seamounts, and ocean
trenches which are the deepest parts of the
oceans. In the centers of some ocean ridges
are long rift valleys, where Earthquakes and
Facts about the Oceans
Area: about 140 million square miles (362
million sq km), ore nearly 71% of the
Earth's surface.
Average Depth: 12,200 feet (3,720 m).
Deepest point: 36,198 feet (11,033 m) in
the Mariana Trench in the western Pacific.
Mountains: The ocean ridges form a great
mountain range, almost 40,000 miles
(64,000 km) long, that weaves its way
through all the major oceans. It is the
largest single feature on Earth.
Highest Mountain: Mauna Kea, Hawaii,
rises 33,474 feet (10,203 m) from its base
on the ocean floor; only 13,680 feet
(4,170 m) are above sea level.
volcanic eruptions are common. Some
volcanoes that rise from the ridges appear
above the surface as islands.
Other mountain ranges are made up of
extinct volcanoes. Some seamounts, called
guyots, are extinct volcanoes with flat tops.
Scientists think that these underwater
mountains were once islands but their tops
were worn away by waves. The diagram
below shows the main features found on the
ocean floor.
Oceanographers know these features exist because much effort has been spent on
mapping the ocean bottom. In order to make maps of the ocean floor, the depth of the
ocean must be known in many places. In the early days of ocean exploration, sailors made
depth determinations called soundings by means of a lead line. This was simply a long
piece of rope, marked off in fathoms (six-foot intervals) and having a lead weight at one
end. The depth was measured by dropping the weight into the water and noting how much
line went out when the lead weight reached the bottom.
Measuring Water Depth
Today's
oceanographers
use sonar
instruments to
generate a
sound signal
that is bounced
or "echoed" off the sea floor
and then recorded on board
the ship. The speed of sound
in water is 1,500 m per
second, four times faster than
the speed of sound in air. By
carefully measuring the
Taking soundings this way is time-consuming,
especially in deep water. Today most depth
measurements are made using an echo sounder. Instead
of dropping a weight, a pulse of sound energy is
transmitted electronically toward the bottom. The time it
takes the pulse to travel to the bottom and be reflected
back up to the surface is measured. From this time
interval, the depth of the water can be calculated. When
pulses are sent out and received in quick succession, an
almost continuous recording of the ocean depth called a
bottom profile may be obtained.
The sedimentary rocks that exist on the ocean bottom
are much younger than any similar rocks found on the
continents. The cores of mud and rock brought back by
deep-sea drilling ships vary greatly in age, but no
round-trip time of the sound
waves and taking into account
the variables of temperature
and salinity, the depth of the
water and the distance to
another object can be
measured accurately.
deposits from the ocean floor seem to be more than
about 200 million years old. This makes oceanic crust
very young compared with the continents, which contain
rocks up to about 4 billion years old.
Continental Drift
The face of the Earth is always changing and throughout geologic history oceans have been created
and destroyed. Modern geologic evidence indicates that the ocean bottom is moving at a rate from
about one-half to six inches a year
through a process called plate tectonics.
Roughly 200 million years ago the Earth's
surface was very different from the
familiar pattern of land we know today.
All of the land masses were grouped
together into one vast supercontinent
called Pangaea. The rest of the globe was CONTINENTAL DRIFT
covered by a single great ocean known as Pangaea broke up with part of the continent drifting north and part
Panthalassa.
south. 1) The northern part split to form the North Atlantic Ocean
208-146 million years ago (mya). 2) The South Atlantic and Indian
oceans began to form 146-65 mya. 3) The continents continue to
drift. Today the oceans are still changing shape; the Atlantic Ocean
gets wider by a few inches each year.
Slowly, over millions of years, the great
land mass split apart. The pieces began to
move over the Earth's surface driven by
slowly churning currents in the molten rocks beneath the Earth's hard outer layers. The gigantic
plates on the Earth's crust move like a conveyor belt. As new areas of ocean floor form at mid-ocean
ridges, old areas are dragged down, or subducted, into the Earth's mantle, which explains why the
older rocks cannot be found.
By about 35 million years ago the pattern
of land and sea was very much like it is
today. But the continents are still moving
and as the Atlantic and Indian oceans
continue to get wider by a few inches
every year, the Pacific is slowly shrinking.
At the northeast corner of Africa we can
see the start of a new ocean. For the last 25
million years, the Red Sea has been widening. If it continues at the same rate, in 200 million years it
will be as wide as the Atlantic is today.
Water Cycle
Millions of years ago there were no oceans on the planet. The surface of the Earth was so hot that
water simply boiled away. But volcanoes poured huge amounts of steam into the atmosphere and as
the Earth cooled down the steam turned to water vapor that condensed as droplets and began to fall
as rain. This downpour lasted for many thousands of years filling great hollows in the land and thus
forming the world's first seas.
Today, the oceans are always losing and gaining water in a never-ending process called the water,
or hydrologic, cycle. They lose water when the sun and wind lift tiny particles of moisture from the
ocean surface. These invisible particles of water vapor mix with air. If the air cools, vapor particles
join up as water droplets that form clouds. Clouds shed rain or snow and most of it, in fact 77
percent of all precipitation, falls directly back over the sea. Rain water and melted snow that fall on
land run into rivers flowing back into the sea. Thus, the oceans never dry up.
Within the last few decades, scientists have developed a fairly clear understanding of the role the
ocean and the hydrologic cycle play in weather formation. One interesting result has been the
explanation of the effects of El Niño. The warm surface water temperatures associated with El Niño
lead to alterations in the movement of air masses and the development of irregular ocean currents.
Changes in the normal formation of weather conditions are the result, and the entire world is
eventually affected.
HOW SALTY IS THE OCEAN?...
How salty the ocean is, however, defies ordinary comprehension. Some scientists
estimate that the oceans contain as much as 50 quadrillion tons (50 million
billion tons) of dissolved solids.
If the salt in the sea could be removed and spread evenly over the Earth's land
surface it would form a layer more than 500 feet thick, about the height of a 40story office building. The saltiness of the ocean is more understandable when
compared with the salt content of a fresh-water lake. For example, when 1 cubic
foot of sea water evaporates it yields about 2.2 pounds of salt, but 1 cubic foot of
fresh water from Lake Michigan contains only one one-hundredth (0.01) of a
pound of salt, or about one sixth of an ounce. Thus, sea water is 220 times saltier
than the fresh lake water. What arouses the scientist's curiosity is not so much
why the ocean is salty, but why it isn't fresh like the rivers and streams that
empty into it. Further, what is the origin of the sea and of its "salts"? And how
does one explain ocean water's remarkably uniform chemical composition? To
these and related questions, scientists seek answers with full awareness that
little about the oceans is understood.
SALINITY AND ITS VARIABILITY...
Oceanographers report salinity (total salt content) and the concentrations of
individual chemical constituents in sea water -- chloride, sodium, or magnesium
for example -- in parts per thousand, for which the symbol o/oo is used. That is,
a salinity of 35 o/oo means 35 pounds of salt per 1,000 pounds of sea water.
Similarly, a sodium concentration of 10 o/oo means 10 pounds of sodium per
1,000 pounds of water.
The salinity of ocean water varies. It is affected by such factors as melting of ice,
inflow of river water, evaporation, rain, snowfall, wind, wave motion, and ocean
currents that cause horizontal and vertical mixing of the saltwater.
THE SALTIEST WATER...
The saltiest water (40 o/oo ) occurs in the Red Sea and the Persian Gulf, where
rates of evaporation are very high. Of the major oceans, the North Atlantic is the
saltiest; its salinity averages about 37.9 o/oo. Within the North Atlantic, the
saltiest part is the Sargasso Sea, an area of about 2 million square miles, located
about 2,000 miles west of the Canary Islands. The Sargasso Sea is set apart from
the open ocean by floating brown seaweed "sargassum" from which the sea gets
its name. The saltiness of this sea is due in part to the high water temperature
(up to 83º F) causing a high rate of evaporation and in part to its remoteness from
land; because it is so far from land, it receives no fresh-water inflow.
Low salinities occur in polar seas where the salt water is diluted by melting ice
and continued precipitation. Partly landlocked seas or coastal inlets that receive
substantial runoff from precipitation falling on the land also may have low
salinities. The Baltic Sea ranges in salinity from about 5 to 15 o/oo. The salinity
of the Black Sea is less than 20 o/oo. Water of the Puget Sound in the Tacoma,
Wash., area ranges in salt content from 21 to about 27 o/oo. This area is drained
by a number of fresh-water streams which discharge an average of about 4.1
billion gallons of water per day into Puget Sound. Salinity of sea water along the
coastal areas of the conterminous United States varies with the month of the year
as well as with geographic location. For example, the salinity of the ocean water
off Miami Beach, Fla., varies from about 34.8 o/oo in October to 36.4 o/oo in May
and June, while diagonally across the country, off the coast of Astoria, Oregon,
the salinity of sea water varies from 0.3 o/oo in April and May to 2.6 o/oo in
October. The water off the coast of Miami Beach has a high salt content because
it is undiluted sea water. Off the coast of Astoria, however, the sea water is less
saline because it is mixed with the fresh water of the mighty Columbia.
Water Movement – waves, tides, currents
Ocean Currents
The world's oceans travel in well-defined circular patterns called currents which flow like rivers.
When the atmosphere pushes over the surface of the ocean some of the energy goes to forming
waves while the rest goes to pushing the water in the direction of the wind. North of the equator
currents bend to the right; south of the equator, they bend to the left. This is called the Coriolis
effect. Winds, continents and the Coriolis effect make currents flow around the oceans in huge
loops called gyres.
Energy from the sun also causes currents to flow. Water near the equator is heated more than water
at middle latitudes causing a surface flow toward the poles. Where two currents meet, the colder
water sinks pushing warmer water up to the surface.
Each current has its own characteristic salinity, density and
temperature. The Gulf Stream, which was first mapped by Benjamin
Franklin, runs along the east coast of the United States and is one of
the strongest currents known. It is a warm, salty current up to 37
miles wide, 2600 feet deep and in some places it may travel more
than 60 miles in a day. The importance of currents can be seen when
they change. On the west coast of South America, the Humboldt
current normally brings cold water to the surface. With it come
minerals and other nutrients that feed huge schools of fish. During an
El Niño event, trade winds diminish and warm water flows down
western South America. This stops nutrient-rich cold water from
rising up and causes plankton and fisheries to fail.
Historical map of the Gulf
Stream.
GULF STREAM
The majority of the Gulf Stream is classified as a western boundary current. This means that it is a
current with behavior determined by the presence of a coastline - in this case the eastern United
States and Canada - and is found on the western edge of an oceanic basin. Western boundary
currents are normally very warm, deep, and narrow currents that carry water from the tropics to the
poles.
The Gulf Stream was first discovered in 1513 by the Spanish explorer Juan Ponce de Leon and was
then used extensively by Spanish ships as they travelled from the Caribbean to Spain. In 1786,
Benjamin Franklin mapped the current, further increasing its usage.
Path of the Gulf Stream
Today, it is understood that the waters feeding into the Gulf Stream begin
flowing off the west coast of Northern Africa (map). There, the Atlantic
North Equatorial Current flows from that continent across the Atlantic
Ocean. Once the current reaches eastern South America, it splits into two
currents, one of which is the Antilles Current. These currents are then
funneled through the islands of the Caribbean and through the Yucatan
Channel between Mexico and Cuba.
Because these areas are often very narrow, the current is able to compress and gather strength. As it
does so, it begins circulating in the Gulf of Mexico’s warm waters. It is here that the Gulf Stream
becomes officially visible on satellite images so it is said that the current originates in this area.
Once it gains enough strength after circulating in the Gulf of Mexico, the Gulf Stream then moves
east, rejoins the Antilles Current, and exits the area through the Straits of Florida. Here, the Gulf
Stream is a powerful underwater river that transports water at a rate of 30 million cubic meters per
second. It then flows parallel to the east coast of the United States and later flows into the open
ocean near Cape Hatteras but continues moving north. While flowing in this deeper ocean water, the
Gulf Stream is its most powerful, forms large meanders, and splits into several currents, the largest
of which is the North Atlantic Current.
The North Atlantic Current then flows further north and feeds the Norwegian Current and moves
the relatively warm water along the west coast of Europe. The rest of the Gulf Stream flows into the
Canary Current which moves along the eastern side of the Atlantic Ocean and back south to the
equator.
Causes of the Gulf Stream
The Gulf Stream, like all other ocean currents is mainly caused by wind as it creates friction when moving
over the water. This friction then forces the water to move in the same direction. Because it is a western
boundary current, the presence of land along the Gulf Stream’s edges also aids in its movement.
The northern branch of the Gulf Stream, the North Atlantic Current, is deeper and is caused by
temperature/salt circulation resulting from density differences in the water.
Impacts of the Gulf Stream
Because ocean currents circulate water of different temperatures all over the globe, they often have a
significant impact on the world’s climate and weather patterns. The Gulf Stream is one of the most
important currents in this regard since it gathers all of its water from the warm tropical waters of the
Caribbean and the Gulf of Mexico. As such, it keeps sea surface temperatures warm, causing the areas
around it to be warm and more hospitable. Florida and much of the Southeastern United States for
instance is mild all year round.
The greatest impact the Gulf Stream has on climate is found in Europe. Since it flows into the North
Atlantic Current, it too is warmed (though at this latitude the sea surface temperatures are cooled
considerably), and it is believed that it helps keep places like Ireland and England much warmer
than they would otherwise be at such a high latitude. For example, the average low in London in
December is 42°F (5°C) while in St. John’s, Newfoundland, the average is 27°F (-3°C). The Gulf
Stream and its warm winds are also responsible for keeping northern Norway’s coast free of ice and
snow.
As well as keeping many places mild, the Gulf Stream’s warm sea surface temperatures also aid in
the formation and strengthening of many of the hurricanes that move through the Gulf of Mexico.
Additionally, the Gulf Stream is important to the distribution of wildlife in the Atlantic. The waters
off of Nantucket, Massachusetts for example are incredibly biodiverse because the presence of the
Gulf Stream makes it the northern limit for southern species varieties and the southern limit for
northern species.
Waves
A wave is a way in which energy travels from one place to another. There are many kinds of waves,
such as water waves, sound waves, light waves, radio waves, microwaves and earthquake waves.
All waves have some things in common.
The highest point the wave reaches is called the crest. The lowest point is called the trough. The
distance from one crest to the next is the wave length. The number of waves that pass a given point
in one second is the wave's frequency.
When wind blows over the ocean's surface, it creates waves. Their size depends on how far, how
fast and how long the wind blows. A brief, gentle breeze forms patches of tiny ripples on the
surface called catspaws; strong, steady winds over long distances create large waves. But even
when you feel no wind at all, you may encounter large swells created by distant storms.
In the open sea, waves make floating boats bob up and down instead of pushing them along. This is
because the waves travel through water; they do not take the water with them. As a wave arrives it
lifts water particles. These travel forward, then down and back so that each particle completes a
circle. Circling movements of particles near the surface set off smaller circling movements below
them
Tides
Tide-generating forces are a result of the gravitational attraction between
the Earth, sun, and moon. It was not until Sir Isaac Newton (who lived
from 1642-1727) discovered the law of gravity that the effect of the sun
and the moon on the tides was fully understood. All surfaces of the Earth
are pulled toward the moon and sun. This force has little effect on land
masses, but it does have a very great and obvious effect on the water of
the Earth's oceans. Twice each month the tidal range reaches a maximum
and these large tides are called the spring tides. Halfway through the
monthly cycle the range is much smaller, and these weak tides are called
neap tides.
As the moon rotates around the Earth, it pulls the water on the nearest side
of the Earth outward into a bulge. A similar bulge on the opposite side of
the Earth is caused by the water being thrown outward by the planet's spin.
These two bulges travel around the globe, producing two high tides each
day. During time of the new moon and full moon, when the sun and moon
are in a straight line, their gravitational pulls combine and produce spring
tides; at this time the high tides are very high and the low tides are very
low. When sun and moon are at right angles from the Earth, during the
quarter phases of the moon, the gravitational pull on the oceans is less
producing a smaller difference between high and low tide known as a neap
tide.
Life in Oceans
The sea is home to billions of plants and animals. Many live only near the sunlit surface. Here you'll
find everything from microscopic plankton to the giant blue whale. Oceanographers classify marine
organisms by separating them into two primary groups: plants and animals.
Plants
There are two general types of plants found in the ocean, those having roots that are attached to the
ocean bottom and those not having roots which simply drift about with the water. The rooted plants
in the ocean are only found in shallow water because there is not enough sunlight to sustain
photosynthesis in deeper waters. Since sunlight does not penetrate more than a few hundred feet
into the ocean, most of the ocean is not capable of supporting rooted plants. Nevertheless, plants are
found throughout most of the oceanic surface waters.
The most abundant plants in the ocean are known as phytoplankton. These are usually single-celled,
minute floating plants that drift throughout the surface waters of the ocean. A bucket of sea water
might hold a million microscopic diatoms which are relatives of seaweed encased in glassy boxes.
To grow, phytoplankton need nutrients from the sea water and lots of sunlight. The most light
occurs in the tropics but nutrients there, especially nitrogen and phosphorus, are often in short
supply. When large quantities of diatoms and other phytoplankton are present they give a color to
the sea. Spectacular phytoplankton blooms are found in cooler waters where nutrients are brought
up from the sea floor during storms.
Animals of the Sea
Marine animals are divided into three groups: zooplankton, nekton,
and benthos. Zooplankton are drifting animals and are usually small,
however, they can grow to fairly large size. For example, the
jellyfish and the Portuguese man-of-war are examples of larger types
of zooplankton which are unable to propel themselves effectively
and are therefore at the mercy of either wind or current. The
zooplankton population also includes some temporary members such
as fish eggs or larval forms of organisms which may grow up and
leave the planktonic community to join the nekton or benthos.
Marine animals are divided into
three groups: zooplankton (those
that float), nekton (those that
actively swim) and benthos (those
that live on or in the ocean
bottom).
Nekton are the free swimmers and probably the largest portion of
familiar animals found in the ocean belong to this class. Common
fishes, the octopus, whales, eels and squid are all examples of
nekton. The nekton category includes a number of very diverse
creatures. The whale, dolphin and porpoise are certainly very
different from codfish or trout because whales represent sea
mammals whereas cod are true fishes.
The third type of sea animal spends its entire life on or in the ocean
bottom. This group of marine animals is called the benthos. It includes lobsters, starfish, various
worms, snails, oysters and many more. Some of these creatures, such as lobsters and snails, may be
able to move about on the bottom but their lifestyle is so bound up with the ocean floor that they are
unable to survive away from this environment.
Who eats what
All living organisms need food. The basic difference between
plants and animals is that plants make their own food, while
animals obtain food from their environment. Through
photosynthesis, plants manufacture organic materials (food) from
inorganic materials (water, carbon dioxide, and nutrients) using
sunlight as their source of energy. Because plants make their own
food, they are called producers. Animals are known as consumers.
They gather and consume organic material rather than making it themselves. Herbivores are animals
that eat plants; carnivores are animals that eat meat. Omnivores are animals that eat both plants and
other animals. Scavengers eat all the leftovers and other dead organisms. No matter what an animal
eats, all of its food can be traced back to the ability of plants to produce organic materials from the
sun.
As far as we know, nearly all life in the ocean is dependent on plants. Only plants have the ability to
manufacture food out of inorganic substances, such as energy from the sun. Thus all animals are
dependent on plants, since animals cannot derive nutrition from
inorganic substances. Plants produce; animals consume. Being
producers, plants form the first link in the food chain.
A food chain is a sequence of organisms in which each is food for the
next member in the sequence. The surface of the sea swarms with
billions of microscopic plants, called diatoms. With other plants such as
seaweed, diatoms form the first link in most marine food chains. All
subsequent links in the food chain are consumers, or the animals.
Orcas (killer whales) often eat cod and cod eat bivalves, especially
mussels and cockles. In turn, the bivalves feed on tiny zooplankton that they filter out of the water.
These zooplankton eat even tinier plants: the diatoms and other phytoplankton. Thus the diatoms are
the producers and the zooplankton, bivalves, cod and orca are all consumers.
In the ocean there are innumerable individual food chains overlapping and intersecting to form
complex food webs. Most marine creatures eat a variety of foods. If one link in a chain is depleted,
the other consumers in the chain have alternate food sources. Organisms generally belong to several
different food chains that are linked to form a food web.
The rich diversity of life in the sea forms a delicately balanced network of predators and prey and
all organisms are dependent on one another for survival.
What are Phytoplankton?
Derived from the Greek words phyto (plant) and plankton (made to wander or drift), phytoplankton
are microscopic organisms that live in watery environments, both salty and fresh.
Some phytoplankton are bacteria, some are protists, and most are single-celled plants.
Phytoplankton are extremely diverse, varying from photosynthesizing bacteria), to plant-like
diatoms, to armor-plated coccolithophores.
Like land plants, phytoplankton have chlorophyll to capture sunlight, and they use photosynthesis to
turn it into chemical energy. They consume carbon dioxide, and release oxygen. All phytoplankton
photosynthesize, but some get additional energy by consuming other organisms.
Phytoplankton are the bottom (or the beginning) of the ocean’s food chain.
Study guide questions:
1. How much of the earth’s surface is covered by water? __________________________
2. List the five parts of the ocean floor and draw a diagram in the box below.
3. Explain the water cycle and draw a diagram.
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4. Name the three types of water movement in the oceans and describe each.
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5. What factors affect the salinity of the oceans? _____________________________________
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6. Name the most famous current, tell why it is important, and draw a diagram.
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7. Tell what you know about life in the oceans and describe the food chain.
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Draw a diagram showing the ocean’s food chain.
8. What characteristics of the oceans affect the number of organisms that live there?
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