Science Notes---
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Ocean Basins
About 70% of the planet's surface is made up of ocean basins, which are the regions
that are below sea level. These areas hold the majority of the planet's water. In fact, it
will help you to recall this term if you remember that a 'basin' is a large bowl, much like
your kitchen sink. So, an ocean basin can be thought of as a large bowl that holds ocean
water. The floors of our world's oceans contain features that you might recognize as
being similar to some structures on land. In this lesson, you will learn about these
features, including undersea mountains and trenches and how they are formed.
Ocean Basin Types and Formation
Ocean basins can be either active, with a lot of new structures being created and shaped,
or they can be inactive, where their surface is slow to change and does little more than
collect sediment. The Gulf of Mexico is an example of an inactive ocean basin where the
main change that happens is the slow depositing of sand and sediment.
Active ocean basins undergo change mainly due to plate tectonics. Plate tectonics is the
theory used to explain the dynamics of the earth's surface resulting from the
interaction of the overlying rigid plates with the underlying mantle. According to the
theory, the earth has a rigid outer layer called the crust. This crust is somewhat fragile,
and like the shell of a hard-boiled egg, it can crack and break into plates. The earth's
crust breaks due to heat and pressure from the layer beneath the crust, called the
mantle.
These plates move very slowly and meet at their boundaries. These boundaries are
common areas of tectonic activity, which is the deformation of the earth's crust due to
movement of tectonic plates resulting in activity (such as earthquakes, volcanoes and
mountain building). So, an area of the world that is tectonically active, whether it is on
land or under the water, would be a mountainous area with earthquakes and volcanic
activity. This creates many of the ocean basins features.
Ocean Basin Features
Plates can spread apart by moving away from each other. This creates gaps where hot
molten rock, called magma, from the earth's mantle can rise up. When the magma seeps
through the gaps, it solidifies as it cools, creating a new layer of ocean crust. This
creates structures, such as oceanic ridges, which are continuous mountain chains located
under the surface of the sea.
You can think of their creation in much the same way as a scab gets created over a
wound. For example, when you cut your finger, it bleeds. The blood flowing out of the cut
relates to the magma flowing out of the gap between the spreading plates. The cut will
form a raised scab on your finger, and as the magma cools, it will form a raised layer of
crust on the sea floor.
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The Exploring our Fluid Earth Curriculum is guided by the OLP and associated fundamental concepts and scope and sequence.
An abyssal hill is another raised feature found within ocean basins. It is defined as a
small elevated landform that rises from the great depths of the ocean. It might help you
to recall this term by remembering that an 'abyss' is something that is very deep. So an
abyssal hill is a hill found on the deep ocean floor. An abyssal hill has sharply defined
edges but will remain relatively small, generally growing not much larger than 500 feet in
height and a few miles in width.
A seamount is a large submerged volcanic mountain rising from the ocean floor.
Seamounts can be very large, reaching heights of up to 10,000 feet, yet they remain
submerged under the surface of the water.
A guyot is similar to a seamount, but it is a submerged volcanic mountain with a flat top.
Guyots receive their flat top after years of erosion caused by waves and other erosive
processes.
Ocean Trenches
We have looked at some ocean basin features that rise up from the
ocean floor, but we also see depressions in the ocean floor, such as oceanic trenches.
These trenches extend down below the normal level of the ocean floor and are the
deepest parts of the ocean. There are 26 oceanic trenches in the world: 3 in the Atlantic
Ocean, 1 in the Indian Ocean, and 22 in the Pacific Ocean.
PROPERTIES OF THE OCEAN WATER Ocean waters are salty and contain minerals and
dissolved gases. They also have a high heat capacity and vary in density depending on
temperature and salinity.
Ocean Water Did you ever wonder why ocean water is so salty? Did you know that the
freezing point of ocean water is not 32 degrees Fahrenheit like ordinary freshwater? Did
you know that ocean water is actually layered, and that dense ocean water sinks to the
bottom while lighter water floats on top? Ocean water has some unique properties, and in
this lesson, you will learn about the properties of ocean water, including salinity,
temperature and density.
Salinity and Chemical Properties
If you look at earth from space, you will notice that much of the planet is covered in
water. The vast majority of this water, about 97%, is found in oceans, and we cannot use it
for drinking, cooking or watering our flowers. The reason this water is not useful in these
ways is because it has a high salinity, which is the amount of salt dissolved in water. In
other words, ocean water is salty. If you ever went swimming in the ocean and accidentally
took a big gulp of ocean water, this fact was unpleasantly obvious.
But why is ocean water salty?
Well, most of the salt that ends up in the oceans
originated on land. Rain, as well as moving water in rivers and streams, washes over rocks
containing the mineral sodium chloride, which you know as common table salt. The salt is
then carried into the oceans. Salt can also find its way into oceans through underwater
volcanoes that stir up salt and other minerals from deep layers of the earth.
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The Exploring our Fluid Earth Curriculum is guided by the OLP and associated fundamental concepts and scope and sequence.
The salt in oceans becomes more concentrated over time as water from the surface of
the ocean evaporates, leaving the salt behind. Salt is the main chemical ingredient in
ocean water, but it also contains other chemicals, such as magnesium, sulfate, calcium and
potassium. Many scientists believe that life began in the oceans, so it's not surprising
that chemicals found in seawater are important chemicals needed to sustain life.
There are also gases dissolved in ocean water, including nitrogen (N2), oxygen (O2) and
carbon dioxide (CO2). Nitrogen is found in fertilizer and used to enhance the growth of
plants in a farmer's field. If nitrogen-containing fertilizer runs off of farmer's fields
and makes its way into ocean waters, it can cause some ocean plants to grow like crazy.
These plants end up using more than their fair share of dissolved oxygen, which can
suffocate other ocean creatures such as crabs and fish. This illustrates one way human
activities impact the oceans.
Oceans act as carbon sinks, which means that they are natural environments that absorb
and store carbon dioxide. So oceans remove carbon dioxide, which is a greenhouse gas,
from the atmosphere, making them important players in the study of global climate
changes. Different bodies of water have different amounts of salt mixed in, or different
salinities. Salinity is expressed by the amount of salt found in 1,000 grams of water.
Therefore, if we have 1 gram of salt and 1,000 grams of water, the salinity is 1 part per
thousand, or 1 ppt.
The average ocean salinity is 35 ppt. This number varies between about 32 and 37 ppt.
Rainfall, evaporation, river runoff, and ice formation causes the variations. For example,
the Black Sea is so diluted by river run off; its average salinity is only 16 ppt. Freshwater
salinity is usually less than 0.5 ppt. Water between 0.5 ppt and 17 ppt is called brackish.
Estuaries (where fresh river water meets salty ocean water) are examples of brackish
waters.
Most marine creatures keep the salinity inside their bodies at about the same
concentration as the water outside their bodies because water likes a balance. If an animal
that usually lives in salt water were placed in fresh water, the fresh water would flow into
the animal through its skin. If a fresh water animal found itself in the salty ocean, the
water inside of it would rush out. The process by which water flows through a semipermeable membrane (a material that lets only some things pass through it) such as the
animal's skin from an area of high concentration (lots of water, little salt) to an area of
low concentration (little water, lots of salt) is called osmosis.
This is also why humans (and nearly all mammals) cannot drink salt water. When you take in
those extra salts, your body will need to expel them as quickly as possible. Your kidneys
will try to flush the salts out of your body in urine, and in the process pump out more
water than you are taking in. Soon you'll be dehydrated and your cells and organs will not
be able to function properly.
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The Exploring our Fluid Earth Curriculum is guided by the OLP and associated fundamental concepts and scope and sequence.
Ocean Water Temperature
Oceans have a wide range of temperatures. The
temperature is highest at the equator and grows
increasingly colder as the water approach the poles.
Compared to air, ocean water has a very high heat
capacity. This means that there is more energy required
to change the temperature of water. In other words, it
takes a long time for the sun's energy to heat the ocean
waters. And once the ocean is heated up, it takes a long
time for the water to fully release that heat.
Ocean Floor
Before the technology existed for
observing, measuring, mapping, and creating images of the
ocean floor, scientists had little idea of how it looked. A variety of technologies now
exist that help us visualize the seafloor. From submersibles that allow direct observation
to instruments that map the seafloor from satellites, technology is finally enabling us to
explore the 70 percent of our planet that is covered by oceans.
The ocean floor is irregular as to the land we can see. It has mountains and ridges and
plains and valleys and volcanoes. It even has underwater earthquakes, and just about
every other land feature you could name.
Ocean Literacy Principles
According to the OLP, every ocean literate person should understand these principles:
1.
2.
3.
4.
5.
6.
7.
Earth has one big ocean with many features.
The ocean and life in the ocean shape the features of Earth.
The ocean is a major influence on weather and climate.
The ocean makes Earth habitable.
The ocean supports a great diversity of life and ecosystems.
The ocean and humans are inextricably interconnected.
The ocean is largely unexplored.
The Ocean Literacy Principles are guided by the idea that understanding the ocean is
essential to comprehending and protecting the planet on which we live. Ocean basins are
composed of the seafloor and all of its geological features (such as islands, trenches, midocean ridges, and rift valleys) and vary in size, shape and features due to the movement of
Earth’s crust (lithosphere). Earth’s highest peaks, deepest valleys and flattest plains are
all in the ocean.
Why does the ocean get colder at depth?
Cold, salty water sinks to the bottom of the ocean. Cold water has a higher density than
warm water. Water gets colder with depth because cold, salty ocean water sinks to the
bottom of the ocean basins below the less dense warmer water near the surface. The
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The Exploring our Fluid Earth Curriculum is guided by the OLP and associated fundamental concepts and scope and sequence.
sinking and transport of cold, salty water at depth combined with the wind-driven flow of
warm water at the surface creates a complex pattern of ocean circulation called the
'global conveyor belt.' In contrast, the Earth gets hotter and hotter at depth primarily
because the energy of radioactive decay is leaking outwards from the core of the planet.
While this geothermal energy is transferred to ocean water along the sea floor, the
effect is so small that it's immeasurable by direct means.
Why? The actual
amount of heat
generated
per
square meter of
Earth
is
quite
small,
especially
compared to the
amount of heat
necessary to warm
the
ocean.
Geothermal energy
emanating from the
Earth averages only about one tenth of a watt per square meter. At that rate of heat flow
(without taking ocean currents into account), it would take well over a year just to heat
the bottom meter of the ocean by one degree Centigrade.
However, the ocean is not standing still. Complex deep ocean currents driven by density
variations in temperature and salinity are constantly replacing the bottom layer of ocean
water with colder water.
Exploring the Ocean
-"[T]he ocean remains one of the least explored and understood
environments on the planet—a frontier for discoveries that could provide important
benefits . . . . Ocean science and technology will play an increasingly central role in the
multidisciplinary study and management of the whole-Earth system." U.S. Commission on
Ocean Policy, An Ocean Blueprint for the 21st Century (2004)
Whether we know it or not, we are natural-born explorers. There are many reasons why
we explore. From birth we learn about life and how it works by exploring. No one can be
satisfied for very long without exploring. Whether you are talking to someone next to you
or looking around the room, you are exploring!
People are curious about everything. We learn something new every day. If you get bored,
you automatically look for something to do. That is the way we are. We like to learn new
things. We also like to understand things and how they work. From the time you were born,
you have been finding out how things work by exploring them. Curiosity makes us
Explorers. Exploration leads to knowledge and understanding, and that means you make the
world a better place as you explore. People have always tried to leave the world a better
place for future generations. Exploration is one way we can do that. It is a gift that people
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The Exploring our Fluid Earth Curriculum is guided by the OLP and associated fundamental concepts and scope and sequence.
of the past give people of the future. Working together on major projects in the deep
ocean environment will help make nations on Earth more peaceful. Anytime you have to
work with others, you learn about them and yourself. Working together with common goals
helps people understand each other. It is very hard to go to war with people you
understand. Working together also makes us more creative. If we are going to explore the
deep ocean, it can help our economy. Deep ocean exploration creates jobs and technology
that make our world better. The deep ocean might have products and materials that are
not available on land. We could also look for new; cleaner energy sources in the deep ocean
that might help protect our environment. Exploration helps us understand our place in the
universe.
Activity Questions:
1. Why do you think the world ocean has traditionally been divided into ocean basins?
2. Do you think dividing up the ocean this way is beneficial?
3. Why or why not?
4. Describe the general shape of each ocean basin. Record major land or water
features to the north, south, east, and west of each one. Refer to a globe or map.
5. Why do we want to explore the ocean?
6. The Atlantic and Pacific Ocean basins are often divided by the equator into the
North Atlantic and South Atlantic and North Pacific and South Pacific Ocean
basins. What other boundaries might further subdivide the ocean?
7. A globe is a three-dimensional model of the earth, and a map is a two-dimensional
representation of the earth. Explain the difference between two-dimensional and
three-dimensional views of objects. Draw an object to illustrate your answer.
8. Approximately what percentage of the earth's surface is covered by oceans?
9. Why is the ocean salty and where did the salt come from?
10. How can it be determined when an ocean basin is active or inactive?
11. What percentage of the Ocean Basin makes up the planet’s surface?
12. Which Ocean feature is a submerged volcanic mountain?
13. What does the word salinity means?
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The Exploring our Fluid Earth Curriculum is guided by the OLP and associated fundamental concepts and scope and sequence.