Introduction to Oceanography Unit II (Topic 1A-2) – page 1
Introduction to Oceanography Unit II:
Geology & Physics
Name:
Section:
Size and Shape of the Ocean
It somewhat ironic that we call our planet the “Earth” given that about 70% of the surface of the
Earth is covered by the ocean. The oceans are about 2.5 miles (4 kilometers) deep on average.
Most of the ocean bottom is pretty flat. We call the flat parts of the continents that have been
covered by water “continental shelves.” At the edge of a shelf, the bottom of the ocean plunges
downward. We call this region the “continental slope.” At the bottom of slope, the ocean meets
the abyssal plain, the flat bottom of the ocean that covers about 25% of the globe. (“Abyss”
means “deep”. Sometimes there is a trench between the continental slopes and the abyssal plains.)
Land,
Continent
Continental
Slope
Abyssal
Plain
Continental
Shelf
Continental
Shelf
Continental
Slope
Trench
Abyssal
Plain
Mid-Ocean Abyssal
Ridge
Plain
1. How much of the Earth’s surface is covered by the ocean?
2. How deep is the ocean?
3. What are the deep, flat parts of the ocean called?
4. What are the flat areas of the ocean that are close to the coast called?
5. What are the steep areas of the ocean between the continents and the deep-ocean floor
called?
Introduction to Oceanography Unit II (Topic 1A-2) – page 2
The mid-ocean ridge (MOR) is a chain of underwater volcanoes that runs all the way around the
Earth. It rises 1.5 miles above the ocean floor (on average). The ridge has a very broad slope, so
it also covers a large part of the Earth’s surface (about 23%). The ridge is not always in the
middle of the ocean, and runs into land in few places. Earthquakes and volcanoes are common
along the mid-ocean ridge.
The deepest parts of the ocean are the ocean
trenches. The deepest trench in the world is the
Mariana Trench which is over 6 miles deep.
Many people expect the deepest part of the ocean
to be in the middle of the ocean, but most ocean
trenches are found near land, either near the
continents or next to ocean islands. For example,
the Mariana Trench is found next to the Mariana
Islands. Like the mid-ocean ridge, earthquakes
and volcanoes are common near the trenches.
The Mariana Islands are volcanoes that grew tall
enough to poke above the surface of the ocean.
Most of the world’s trenches are near the edge of
the Pacific Ocean, so volcanoes are common
along the edge of the Pacific. This chain of
volcanoes is called the “Ring of Fire.”
6. What is the underwater chain of volcanoes that runs throughout the oceans called?
7. Does the mid-ocean ridge approach land anywhere,
or is it only located in the middle of the oceans?
8. What are the deepest parts of the ocean called?
9. Are ocean trenches typically located close to land (continents or islands)
or far away from kind of land?
10. Which ocean has the most ocean trenches?
11. Where do volcanoes commonly erupt in or near the ocean?
12. Where do earthquakes typically occur in or near the ocean?
Introduction to Oceanography Unit II (Topic 1A-2) – page 3
Sediments
Sediment are pieces or broken parts of various materials that cover the bottom of the ocean.
Sand, mud, soil, and dirt are good examples of sediments. There are 2 major kinds of sediments
on the ocean floor: broken pieces of rock and the remains of ocean life, especially their shells.
Bits of rock are typically washed off the land and into the ocean by rain, though sediments can
also be carried from the land into the ocean by winds. Larger rocks like boulders, cobbles, and
even gravel are too large and heavy to be carried out into the ocean, so smaller sediments like
sand, clay, and silt tend to cover the bottom of the ocean.
Shells make up most of the remains of ocean life on the sea floor, but fecal matter, mucous, and
other body parts are also found in ocean sediments. These kinds of ocean sediments can typically
be broken into 2 categories, calcareous ooze (made of calcium carbonate shells) and siliceous
ooze (made of silica shells).
13. Is a rock that you can hold in your hand a sediment?
14. What are the 2 major kinds (categories) of ocean sediments?
15. What body part are most of the remains of organisms in sediments mostly made of?
16. What are the 2 kinds of biological sediments?
Introduction to Oceanography Unit II (Topic 1A-2) – page 4
Winds and Ocean Currents
High Altitude Winds
The ocean is constantly in motion because of
winds blowing across its surface. The resulting
waves stir up the surface of the ocean, helping to
bring up nutrients and to keep phytoplankton
Surface Winds
(algae) from sinking too deep. Winds are powered
by the heat of the sun, which warms the surface of
Warm
Cold
Ocean
the Earth. If one place becomes warmer than
another, the air is warmed in this spot and warmer air rises. Cooler air sinks over the cold spot,
pushing air towards the warm spot to replace the air that rose upwards. Thus, the ocean causes
the atmosphere to move too.
17. Why does warm air rise? Why does cold air sink?
(Hint: Refer to your density lab materials.)
18. Do the surface winds move towards the warm spot where air is rising, or towards the cold
spot where air is sinking?
Introduction to Oceanography Unit II (Topic 1A-2) – page 5
Winds also create ocean currents. Ocean currents are like rivers in the ocean; they carry
water and everything in it (e.g., plankton) from place to place. The largest, most important
ocean currents flow in huge loops around the ocean basins called “gyres.” These currents
can have a big impact on climate by moving warm water towards the Poles and cold water
towards the Equator. For example, warm water evaporates more easily than cold water, so air
above warm water is more humid (contains more water vapor). Eventually, the air cannot
hold any more water, and the water falls out of the atmosphere as rain. Thus, places next to
warm currents, like the southeast coast of the United States, are more humid and get more
rain than places next to cold currents, like southern California (a desert). Most of our fresh
water comes from the ocean! The heat of the Sun causes water to evaporate from the ocean.
If the moist air cools, it will fall as rain. About 70% of our fresh water is used to grow food,
not for drinking, cleaning, etc.
Ocean currents are also important because they can bring up nutrients to the surface of the
ocean, fertilizing the phytoplankton and thus getting the entire ocean food chain started.
19. True or false? “Most of our drinking water was originally in the ocean.”
20. Does water evaporate more easily from warm water or cold water?
21. Where does more rain fall along coastlines next to warm currents or cold currents?
Introduction to Oceanography Unit II (Topic 1A-2) – page 6
Bacteria
Bacteria are small, single-celled organisms (even smaller than phytoplankton). Bacteria use a
huge number of chemical processes to extract the energy necessary to sustain their lives from
inorganic and organic matter, including dead, decaying material and fecal matter that animals
cannot use as food. Most bacteria decompose (“break down”) dead, decaying material into
carbon dioxide and nutrients like phosphates and nitrates, releasing them into ocean water and
making them available for phytoplankton to use for photosynthesis. This is important because
without bacteria “recycling” nutrients, there would be fewer phytoplankton in the ocean, and
therefore fewer animals as well.
22. What kinds of materials do bacteria decompose?
23. What do bacteria release into ocean water?
24. How do the materials released by bacteria help phytoplankton?