Download IM_chapter9 Seafloor

Instructor’s Manual
GEOL
Chapter 9
The Seafloor
Chapter 9
The Seafloor
Table of Contents
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Chapter Outline
Learning Outcomes
Chapter Summary
Lecture Suggestions
Enrichment Topics
Common Misconceptions
Consider This
Key Terms
Internet Sites, Videos, Software, and Demonstration Aids
Chapter Outline
Introduction
LO1 Exploring the Oceans
LO2 Oceanic Crust: Its Structure and Composition
LO3 The Continental Margins
LO4 What Features Are Found in the Deep-Ocean Basins?
LO5 Sedimentation and Sediments on the Deep Seafloor
LO6 Reefs
LO7 Resources from the Oceans
Learning Outcomes
After reading this unit, the students should be able to do the following:
LO1 Examine the history and methods of oceanic exploration
LO2 Describe the structure and composition of the oceanic crust
LO3 Identify the continental margins
LO4 Discuss the features found in the deep-ocean basins
LO5 Discuss sedimentation and sediments on the deep seafloor
LO6 Explore coral reefs
LO7 Recognize the types of natural resources found in the oceans
Chapter Summary
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Scientific investigations of the oceans began more than 200 years ago, but much of our
knowledge comes from studies done during the last few decades.
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Present-day research vessels are equipped to investigate the seafloor by sampling and
drilling, echo sounding, and seismic profiling. Scientists also use submersibles in their
studies.
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Deep-sea drilling and observations on land and on the seafloor confirm that oceanic crust
is made up of, in descending order, pillow lava/sheet lava flows, sheeted dikes, and
gabbro.
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Continental margins consist of a gently sloping continental shelf, a more steeply inclined
continental slope, and, in some cases, a continental rise.
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The width of continental shelves varies considerably. They slope seaward to the shelfslope break at a depth averaging 135 m, where the seafloor slope increases abruptly.
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Submarine canyons, mostly on continental slopes, carry huge quantities of sediment by
turbidity currents into deeper water, where it is deposited as overlapping submarine fans
that make up a large part of the continental rise.
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Active continental margins at the leading edge of a tectonic plate have a narrow shelf and
a slope that descends directly into an oceanic trench. Volcanism and seismic activity also
characterize these margins.
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Passive continental margins lie within a tectonic plate and have wide continental shelves.
The slope merges with a continental rise that grades into an abyssal plain. These margins
show little seismic activity and no volcanism.
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Long, narrow oceanic trenches are found where oceanic lithosphere is subducted beneath
either oceanic lithosphere or continental lithosphere. The trenches are the sites of the
greatest oceanic depths and low heat flow.
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Oceanic ridges are composed of volcanic rocks, and many have a central rift caused by
tensional forces. Basaltic volcanism, hydrothermal vents, and shallow-focus earthquakes
occur at ridges, which are offset by fractures that cut across them.
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Seamounts, guyots, and abyssal hills rising from the seafloor are common features that
differ mostly in scale and shape. Many aseismic ridges on the seafloor consist of chains
of seamounts, guyots, or both.
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Submarine hydrothermal vents known as black smokers found at or near spreading ridges
support biologic communities and are potential sources of several resources.
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Moundlike, wave-resistant structures called reefs, consisting of animal skeletons, are
found in a variety of shapes, but most are classified as fringing reefs, barrier reefs, or
atolls.
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Sediments called pelagic clay and ooze cover vast areas of the seafloor.
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The United States claims rights to all resources within 200 nautical miles of its
shorelines. Resources, including sand and gravel as well as metals, are found within this
Exclusive Economic Zone (EEZ).
Lecture Suggestions
1. If a globe made before the mid-1960s is available, the fact that the seafloors have no
information plotted, like the “terra incognita” of earlier times, may serve to illustrate the
amount of knowledge that has been acquired about the seafloor and how important its
exploration was to geology and the formulation of plate tectonic theory.
2. Oceanographic studies have commonly been advanced by strategic and geopolitical
concerns. The echo sounder, used to determine ocean depths and ultimately to map the
oceans, was developed during World War II to provide better underwater information on
submarines. During the Cold War era, the submarine became a principle component of
offensive and defensive planning, and improved maps for all the ocean floors became
imperative. These improved maps formed the basis for other studies, and led to the
recognition of the relationships among such things as oceanic ridges, trenches, island
arcs, and the major fractures now known as transform faults.
3. The relationship between seafloor spreading and sea level may be illustrated with an
aquarium and two blocks of rock, one larger and representing the greater volume of hot
rock extruded at the mid-oceanic ridges during intervals of rapid spreading and the other
smaller one representing the cooler mid-oceanic ridges extruded during times of lower
spreading rates. By placing one block and then the other in the aquarium tank, you can
illustrate the different displacement of water.
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4. There has been a tremendous amount of work done lately on the environmental problems
facing the oceans. Bringing in some of these topics can add relevancy and immediacy to a
lecture.
Enrichment Topics
Topic 1. Our Dying Coral Reefs. Coral reefs are known as the “rainforests of the sea”
because they harbor such an incredible abundance and diversity of life. These
spectacular and beautiful ecosystems are home to more than one-fourth of all marine
plant and animal species. Reefs are built of tiny coral polyps that construct calcium
carbonate (CaCO3) shells around their bodies. The coral polyps enjoy a mutually
beneficial relationship with minute algae called zooxanthellae: The photosynthetic
algae supply oxygen and food to the corals, and the corals provide a home and
nutrients (their wastes) for the zooxanthellae. The algae give the coral their bright
colors of pink, yellow, blue, purple, and green. Coral are very temperature sensitive;
if the water gets too hot, the coral eject their zooxanthellae, giving up their means of
producing food. Without the algae, the coral turn white, a phenomenon called coral
bleaching. Sometimes zooxanthellae move back in when conditions improve, and
sometimes a heat-tolerant species of algae will take its place. However, if the reef
goes without algae for too long, the coral starves, and the reef dies. Coral reefs may
recover from one bleaching event, but multiple events can kill them. Dr. Clive
Wilkinson, coordinator of the Global Coral Reef Monitoring Network, blames the
current upsurge in coral bleaching on rising seawater temperatures due to global
warming. There are many other problems damaging or killing coral reefs, including
excess sedimentation, pollution, and insensitive tourists. According to Wilkinson’s
report, Status of Coral Reefs of the World: 2004, 20% of coral reefs are severely
damaged and unlikely to recover, and another 24% are at imminent risk of collapse.
Topic 2. Life at the Very Bottom of the Sea. The Challenger Deep is the deepest spot in the
ocean, a hole at the bottom of the Marianas Trench 10,896 meters below the sea surface.
Amazingly, Japanese researchers have found life in the sediments at these great depths. The
organisms are foraminifera, single-celled creatures that are common at the sea surface, but
rare in the deep sea. The Challenger Deep species are mostly new. Rather than being multichambered and having shell-like surface forms, they are soft walled. DNA analyses of these
critters suggest that they are a primitive form, dating from the Precambrian, and are the
ancestors of the more complex, more modern forms. The organisms probably evolved from
populations that were able to adapt to steady increases in pressure as the Challenger Deep
developed over the past 6 to 9 million years. National Geographic News, February 3, 2005,
http://news.nationalgeographic.com/news/2005/02/0203_050203_deepest.html
Topic 3. Hydrothermal Vent Life. Hydrothermal vents are incredibly harsh
environments for life. The fluids are very acidic (with a pH as low as 2.8). They emit
poisonous gases and toxic metals. Fluid temperatures can be up to 750oF (400oC), but
grade into frigid seawater. Hydrothermal vents are the only mid- or deep-sea
communities that produce their own food. Too deep and dark for photosynthesis, they
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are the only communities on the Earth in which chemosynthesis provides the bulk of
the food energy. Hydrothermal vent communities support a typical food web, in
which some animals, like snails, eat the bacteria and some, like fish, consume the
animals that eat the bacteria. Other animals live symbiotically with the bacteria. In
one symbiotic vent relationship, chemosynthetic bacteria live within the tissues of
giant tube worms (Riftia pachyptila); they provide the worms with a constant source
of food and, in return, are given shelter. Life forms differ between oceans and
between vents. In all, the vents are home to more than 300 species, nearly all of them
unique to vent sites, such as giant clams (Calyptogena magnifica) and eyeless shrimp
(Rimicaris exoculata). Exploring descriptions and photos of vent life is an interesting
way to draw students into this strange world.
Topic 4. Methane Hydrates: A Source of Energy or an Environmental Disaster?
Methane hydrates are found in offshore sediments in enormous quantities. These
compounds develop at depths of 660 to 1,650 feet (200 to 500 m) below sea level
when decomposed organic matter contacts cold water at the high pressures found
deep in layered sediments. Water molecules form an icy cage (a hydrate) that contains
a methane molecule. The hydrate structure is unstable: When pressure is removed, the
structure collapses, and the methane escapes. The methane-contained hydrates can be
used as fuel, and thousands of gigatons of methane are located in the oceans, equal to
the world’s total amount of coal. The technology for mining and harnessing this
abundant energy source has not yet been developed, but methane hydrates are
enormously interesting to fuel companies. However, according to many scientists, the
methane hydrates should remain where they are. Methane is a greenhouse gas, with
23 times the heat-trapping ability of CO2. Releasing all that methane into the
atmosphere would exacerbate global warming greatly. The paradox is that warming
temperatures may cause the hydrate structure to collapse anyway, as probably
happened in the Paleocene–Eocene Thermal Maximum. The ocean warmed, releasing
a load of methane, which warmed temperatures further, and brought about many
extinctions.
Topic 5. How Do Scientists Learn About the Seafloor? Learning about the seafloor is
difficult due to the great distances (horizontal and vertical) and tremendous amounts of water
that get in the way. Research ships travel out to sea for days to months, carrying equipment
that can sample sediment, rocks, and make maps. A bathymetric map shows the 3dimensional geographic features of the seafloor on a 2-dimensional map. A device towed
behind a ship sends out sound waves that strike the bottom then return. Since the speed of
sound waves is known, the amount of time it takes for a wave to make a round trip allows
scientists to calculate the distance to the object. When this information is compiled, a map of
the seafloor emerges. A gravity corer is a hollow tube deployed on a cable that accelerates
through the water. When it reaches the bottom, the tube slices into the sediment, so a nicely
layered sample is collected inside. Seafloor rocks are collected with a dredge, a giant
rectangular bucket that is dragged behind the ship. Submersibles are small diving craft that
are not attached to the mother ship. The battery-operated submersible Alvin can descend to
more than 13,000 feet (4,000 m), carrying a pilot and two passengers who collect samples,
take photos and video, and make accurate descriptions of what they see. Submersibles offer
scientists their best view of the deep ocean, but they are potentially dangerous and expensive.
Because remotely operated vehicles (ROVs) carry no human passengers, they can visit
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dangerous locations. The ROV Jason that photographed rooms inside the Titanic and Argo
has been dangerously close to hydrothermal vents. ROVs are attached to the mother ship by a
fiber-optic cable that returns data and video in real time. Scientists can interpret data and
make decisions on what to explore, while the vehicle is at its target. Drilling is the best way
to collect rocks and sediment from the seafloor because the integrity of the sediment and rock
layers is maintained. The Integrated Ocean Drilling Program (IODP), an international
consortium of marine research institutions, operates the drilling ship Joides Resolution. The
drill cuts through the layers of sediments and rocks and returns them to the ship via the drill
pipe.
Common Misconceptions
Misconception: The ocean floor is flat and featureless, except where islands protrude above
the surface.
Fact: The ocean floor has far more varied topography than the land areas, with higher and
longer mountain ranges and deeper canyons and trenches.
Consider This
1. Why do island arcs have an arch-like geometry?
2. Spreading ridges worldwide have elevations of 2500 m, +/– 200 m. Sea level is
controlled in part by the spreading rates as well as the constant rate of cooling of
spreading ridge basalts. Suggest a way to determine the depth of the Atlantic Ocean basin
at any time since the rifting of Pangaea.
Key Terms
abyssal plain
active continental margin
aseismic ridge
black smoker
continental margin
continental rise
continental shelf
continental slope
Exclusive Economic Zone (EEZ)
guyot
oceanic ridge
oceanic trench
ooze
ophiolite
passive continental margin
pelagic clay
reef
seamount
seismic profiling
submarine canyon
submarine fan
submarine hydrothermal vent
turbidity current
Internet Sites, Videos, Software, and Demonstration Aids
Internet Sites
1. NOAA Vents Program: http://www.pmel.noaa.gov/vents/
Research on underwater hydrothermal vent systems, including video of undersea
eruptions.
2. NOAA Oceans Home Page: http://www.noaa.gov/ocean.html
How the National Oceanic and Atmospheric Administration learns about the seafloor.
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3. Dive Into the Abyss. Nature, PBS: http://www.pbs.org/wnet/nature/abyss/
Website with videos and links for materials that accompany the Nature episode “Dive
into the Abyss.”
4. Oceanus: http://www.whoi.edu/oceanus/index.do
The magazine that “explores the oceans in depth,” by the Woods Hole Oceanographic
Institution.
5. Mystery of the Gakkel Ridge. NOVA scienceNOW, PBS, free video (2008, 8:01)
http://www.pbs.org/wgbh/nova/sciencenow/0406/03.html
Scientists go deep beneath the Arctic sea to find a spreading ridge.
Videos
1. Into the Abyss. NOVA Online, PBS, DVD
Dive in Alvin to the black smokers found off the Pacific Northwest coast to see strange
creatures and learn how scientists do their research 1.5 miles below the ocean surface.
2. Earth Revealed. Annenberg Media: http://www.learner.org/resources/series78.html
(1992, 30 min., free video):
 #4: The Sea Floor. The ocean floor, how it is studied, and what is found there.
3. Planet Earth. Annenberg Media: http://www.learner.org/resources/series49.html
(1986, 1 hour, free video)
 #2: The Blue Planet; A Study of the Oceans. The least known part of the planet is
described.
4. Ocean Wilds: Creatures of Coral. PBS Home Video (2001, 60 min., streaming video)
Coral reefs and other marine ecosystems are explored in this multi-part documentary.
5. Scientific American Frontiers: Going Deep. PBS (2002, free online):
http://www.pbs.org/saf/1305/index.html
The technologies that have opened up the farthest reaches of the ocean.
Slides
1. Educational Images Slide Sets: http://www.educationalimages.com/cg120001.htm
 Submarine Land Formations
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