Download EXPLORE AN OCEAN`S FLOOR

M-12: GETTING THE LOW DOWN
MATERIALS LIST
4 ocean floor model boxes with 20 calibrated sounding sticks each,
of which 2 sticks are finely calibrated for making measurements
(silver dot on top)
Ocean Floor Profile Graph paper (enough for each student team, 2 students/team)
Photos of ocean floor, sounding ship
Pencils (enough for each student)
Laminated maps of Atlantic and Pacific floor
Maps “The Shaping of a Continent,” “Pacific and Indian Ocean,” Arctic and Atlantic
Ocean,” “Probing the Abyss”
47 sticks – green tip
27 sticks – red tips
8 calibrated sticks
Marine Environment
Getting The Low Down
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MAPPING THE OCEAN FLOOR
(GETTING THE LOW DOWN)
update: aug 2004
Source:
M – 12
Bobbie Callison & Karen Meisenheimer
Objective:
To understand the forces that shape the ocean floor and know different
between the Atlantic and Pacific floors.
Science Themes:
Scale and structure, patterns of change
Process Skills:
Observing, comparing, relating, inferring
characteristics
Ecological Concept Focus: Materials are recycled, energy is transferred in different forms
Grade Levels:
4–8
Focus Words:
Rift zone, continental margin, sediment, erosion, magma, mantle, seafloor spread,
subduction, convergent, divergent, plate tectonics, shelf, slope, rise, trench, submarine canyon, abyssal plain,
guyot, seamount
Background Information:
Information about the ocean floor has been obtained several ways. The oldest method, before the use of
sound waves, was to drop a measuring line from a ship moving across the water to determine the height
of structures on the ocean floor. Later, echo sounders replaced the measuring line. Today sound waves are
sent off between 5 and 20 seconds, depending on the speed of the ship. Ships navigate across the ocean,
using satellite beams to stay on course. In 1994 data was presented from three years of research that shows
mapping of the entire eastern and western continental margin of North America (200 miles out from the
continent). Cameras, heat sensors, core samples, magnetometers, and most recently, and more accurately,
satellites have added considerable information to earlier knowledge. The average ocean depth is 4,000
meters (13,000 feet). The Pacific Ocean is deeper than the Atlantic Ocean, with the Marianas Trench in the
South Pacific being the deepest area. An underwater vessel, the Trieste, went down 10,900 meters (35,800
feet) or about 11 kilometers (7 miles) to explore the bottom of the trench. If you could put Mount Everest
(8,847 meters) in the Marianas Trench, it would be swallowed up.
All of the ocean floor is covered with layers of sediment (mud, sand, minerals and the remains of marine
organisms). Some of the sediment is derived from land (erosion processes, weathering, earthquakes).
These sediments flow in rivers and streams each spring to be deposited on the continental shelf. From here
they may move down the continental slope to build up in a rise or accumulate in a deep-ocean trench or
move on to an abyssal plain. Other sediments may be the result of sea floor spread action. As new magma
is forced up through divergent plates, volcanoes and mountain ridges form. Basaltic material cools and
contracts slowly. Due to plate tectonics, sediments are continually moving away from the area of sea floor
spread zone only to subduct again in areas with convergent plates.
The Pacific and Atlantic continental margins and ocean floor have many similar structures along with
some differences. Let's look at the similar features first. The continental margin includes the continental
shelf,
Marine Environment
Getting The Low Down
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continental slope and continental rise. The continental shelf is a gently sloping, submerged surface
extending from the shoreline toward the deep ocean basin and is composed of sediments deposited by
streams. Most sea life and commercial fishing occurs on the shelf. At the seaward edge of the shelf the
continental slope begins. It has a steeper gradient than the shelf, allowing sediments to slide down to the
ocean floor. In areas where trenches do not occur the sediments pile up on the ocean floor into what is
called a continental rise. As sediments reach the ocean floor their rate of movement slows to form a long,
flat stretch of deep-ocean basin known as abyssal plains.
Scattered along the ocean floor are isolated volcanic peaks called seamounts originating along areas of sea
floor spread. Over millions of years, new material emerges at oceanic ridges, moving the older volcanic
peaks away from the site of the sea-floor spread zone. The moving volcanoes sink and lose their peaked
tops to become flat-topped structures called guyots. At the site of sea-floor spreading, new sea floor
emerging as magma is pushed upward to form mountainous areas referred to as rift zones. Rift or ocean
ridges are found in all major oceans although the Mid-Atlantic ocean ridge is the longest and most
extensively studied. This ridge forms a continuous mountain range, standing 2500 to 3000 meters above
the ocean floor. In Iceland the ridge has actually grown above sea level. Along the Mid-Atlantic ridge is a
deep linear valley (rift valley) that is deeper than the Grand Canyon of Colorado and two or three times as
wide. Gradually the ridge system levels out into the flat abyssal plains.
Plate tectonics is responsible for differences between the Pacific and Atlantic ocean floors. Trenches are
long, deep, narrow features that form mainly along the Pacific ocean floor at the base of the continental
slope. Trenches form as moving crustal plates subduct back into the mantle. Since there are no major
subducting plates on the Atlantic side, there are no trenches. The Pacific continental slope descends
abruptly into these deep ocean trenches, resulting in a narrow continental shelf and no continental rise.
How far can you walk out on the continental shelf at Santa Cruz or Half Moon Bay before the water
becomes too deep? Compare this continental shelf with the Atlantic shelf which is wide and consists of
thick sediments with no trenches. Along North Carolina or Florida you can walk on the continental shelf
for a hundred yards before the water becomes deep. An active sea-floor spread zone over millions of years
has also contributed to accumulation of this flat continental shelf.
The steep Pacific continental slope is also partly responsible for upwelling of cold, nutrient waters which is
a special condition found in the ocean off the California, Oregon and Washington coasts. Upwelling
results in the production of phytoplankton which, in turn, provide food for vast numbers of marine life.
Another feature along the Pacific continental margin is the presence of deep, steep sided valleys known as
submarine canyons. These are formed by a complex process of fault action and erosion by ancient rivers,
Monterey submarine canyon (one of the largest in the world) was formed by the seaward extension of a
river valley (Salinas river). Earthquakes caused additional movement of dense, sediment-laden water
called turbidity currents through this submarine valley, further eroding the canyon walls. Today this
canyon measures 2 miles (over 3 meters) at its deepest point; it extends from a half-mile to 90 miles
offshore. Submarine canyons do not exist along the Atlantic continental margin.
Activity:
1.
Spend about 5 minutes looking at the photos depicting various types of geological formations found
on ocean floors and going over the following:
Ask students why it is important to study the ocean floor? (It helps us to better understand the plate
tectonics theory, volcanoes, trenches, vents - to find mineral sources manganese nodules, copper - to study new
marine niches in hot deep water vents).
 Ask students how they think geologists know about what the ocean floor looks like. Did they dive
down miles to look, did a submarine go and take a picture? (Explain that the sound waves that bounce off
an object below water can tell a scientist the distance of an object from the sounding source. This is called
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Getting The Low Down
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SONAR – sound navigation ranging. As whales move through water, they use echolocation which allows them
to know if an object lies ahead. It’s a handy talent to have when visibility is reduced).
 Explain that other methods that have been used for looking at the ocean floor including the use of
robot deep diving devices equipped with cameras, satellite images, taking core samples, finding
magnetic fields and simply dropping a line to measure the distance from the water’s surface to the
ocean floor.
 Tell students they will pretend to be oceanographers, using the old method of dropping a line to the
bottom of the ocean floor. Show them the boxes with Glomar Challenger ship on the side. Half of the
boxes indicate structures on the Pacific Ocean while the other half represent structures on the Atlantic
ocean floor. Do not remove box tops at this point - let it be a surprise that students will discover
through measurement!
2. Pair students and have 4 students (2 teams) assigned to each sea floor model box. Each team of two
students will be given a set of ten sounding sticks. Each set is color coded. One stick is calibrated, it is
measured in 2000 meter increments with smaller marks representing increases of 500, 1000 and 1500,
etc.
(10,000 meters is 6.2 miles). Use this in each hole to determine the distance from the ocean floor to the
surface, then the team will match the measurement taken with the hole number on their piece of graph
paper. Mark with finger where the marker just touches the bottom (do not push down hard). Remove
the sound marker to read the depth (read from bottom of marker to where finger is located).
One student can call off the number of meters and the other team member can mark this on the
prepared graph paper.

When both teams have finished marking their graph and connecting the dots to get an ocean floor
profile, they can put all ten sticks in the holes to see the variation along the floor. A profile of the ocean
floor will be appear when looking at the tops of the dowels.
3. After both student teams have completed their ocean floor profile graphs, they can remove the box’s
lid and compare what they “sounded” with how the floor looks.
4. Ask each team to show the whole group what they discovered about their ocean floor. Do any of them
look the same? Why would there be so many differences? What are they differences between the
Atlantic and Pacific ocean floors? (Answers are in background information).
Ask students why the ocean is or is not flat. Where do the mountains or trenches come from?
(Answers are in background information).
5. If you have time, have students graph these: Tell them that the Marianas Trench is the deepest known
spot on the Earth’s surface, 11,000 meters. Mount Everest is almost 9,000 meters, Mount Mc Kinley is
just over 6,000 meters and California’s Mount Whitney is 4,400 meters. Have them mark a “V” at the
11,000 mark on the graph to show the Marianas Trench. If they were to turn the mountains upside
down, they can see that each one can easily fit inside the Marianas Trench.
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Depth In Thousands of Meters
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
5,000
5,500
6,000
6,500
7,000
7,500
8,000
8,500
9,000
9,500
10,000
10,500
11,000
11,500
12,000
12,500
13,000
13,500
14,000
1
2
Stick Number
3
4
5
6
7
8
9
10
OCEAN FLOOR PROFILE
Your Name
Partner’s Name
(check one below)
Atlantic Ocean profile _____
Marine Environment
or
Pacific Ocean profile
Getting The Low Down
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