DIVERGENT BOUNDARIES: ORIGIN AND 13
EVOLUTION OF THE OCEAN FLOOR
INTRODUCTION
Divergent Boundaries: Origin and Evolution of the Ocean Floor
CHAPTER OUTLINE
1.
Challenger
2.
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3.
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4.
5.
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6.
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7.
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8.
LEARNING OBJECTIVES/FOCUS ON CONCEPTS
13.1 Define explain
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13.4 Sketch and label Explain
13.6 List explain
13.8 Compare and contrast
TEACHING STRATEGIES
Clicker Questions:
Muddiest Point:
A. Echo Sounding
B. How Do Scientists Measure the Shape of the Seafloor?
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C. Other Ocean Educational Resources and Activities
TEACHER RESOURCES
Web Resources:
Bathymetry and General Ocean Science
o National Ocean Service Bathymetric Maps
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Rift Valleys
Supercontinent Cycle / Wilson Cycle
oSupercontinent hypothesis
Mid-Ocean Ridges, Vents, Seamounts, and Atolls
Ocean Floor
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Imagery, Videos, and Animations:
Images
Videos
Animations
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ANSWERS TO QUESTIONS IN THE CHAPTER:
CONCEPT CHECKS
13.1 AN EMERGING PICTURE OF THE SEAFLOOR
2. The equation to determine water depth from echo sounding is depth = ½ (1500m/sec x echo
3. Gravitational attractions cause subtle changes in the shape of the sea surface. Over seamounts
4. The three major topographic provinces of the seafloor are continental margins, deep-ocean
basins, and oceanic (mid-ocean) ridges.
13.2 CONTINENTAL MARGINS
1. The three features of passive continental margins are the continental shelf, continental slope,
2. Active continental margins are those located at convergent plate boundaries. These are
characterized by deep-sea trenches and accretionary wedges or subduction erosion. An
3. Active continental margins are those located at a plate boundary. Along the Pacific Rim, active
4. An accretionary wedge forms where sediments from the ocean floor and pieces of ocean crust
5. Subduction erosion occurs where sediment and rock are scraped off the bottom of the
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13.3 FEATURES OF DEEP-OCEAN BASINS
2. Abyssal plains are more extensive on the Atlantic seafloor because this ocean is bordered
primarily by passive margins and thus acts as a trap for sediments.
4. Oceanic plateaus are large flood basalts generated on the seafloor from vast outpourings of
fluid basaltic lavas.
13.4 ANATOMY OF THE OCEANIC RIDGE
1. The oceanic ridge system represents the divergent boundaries along the seafloor. These ridges
2. Mid-ocean ridges form from upwelling of mantle rock at divergent boundaries, not from
collisional plates such as those on continents. Oceanic ridges are made of many layers of
3. Rift valleys are down-faulted structures that form in the center of mid-ocean ridges. These
represent the location of diverging plates.
13.5 OCEANIC RIDGES AND SEAFLOOR SPREADING
1. The source of magma for seafloor spreading is magma created by decompression melting of
mantle rocks as upwelling occurs at active divergent margins.
3. Lithosphere thickens as it moves away from mid-ocean ridges because it cools and contracts
and thus becomes denser.
4. A slow-spreading center, like the Mid-Atlantic Ridge, is characterized by a rugged topography
from numerous volcanic cones and vertical displacement of crust along normal faults, well-
13.6 THE NATURE OF OCEANIC CRUST
1. The four layers of oceanic crust from bottom to top include layered gabbro and gabbro,
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2. Basaltic melt rises through conduits in oceanic crust near the ridge crest, feeding magma
3. As seawater percolates through fractures in oceanic crust, it is heated by magma chambers
below, creating very hot, chemically active water. This water alters olivine and pyroxene in
4. A black smoker is a metallic-rich solution that gushes from the seafloor, similar to geysers on
13.7 CONTINENTAL RIFTING: THE BIRTH OF A NEW OCEAN BASIN
2. Continental rifting is often completed in several stages. First, tensional forces and uplifting of
heated lithosphere cause the upper crust to break along normal faults; lower crust deforms by
3. Hot-spots and mantle plumes are thought to concentrate beneath supercontinents as a means
to dissipate heat trapped in the mantle by the large, insulating landmass. These plumes act to
13.8 DESTRUCTION OF OCEANIC LITHOSPHERE
1. Oceanic lithosphere that is very old has cooled and contracted enough to become denser than
2. In spontaneous subduction, old, cold, dense oceanic lithosphere will descend into the mantle.
Both lithospheric mantle and phase changes of minerals in the slab deep in the earth, make the
3. Mineral phase changes create more dense minerals at depth in a descending slab. These
4. About 180 million years ago the Farallon plate began subducting under the Americas faster
than it was being generated. This caused it to reduce in size. The westward migration of North
EYE ON EARTH
EOE #1 PALOS VERDES PENINSULA
2. This is likely a passive continental margin due to the absence of a trench, and a wide
continual shelf as well as a well-developed continental slope and rise.
EOE #2 OFFSHORE DRILLING
1. The site of the 2010 oil spill was the Deepwater Horizon offshore drilling rig.
2. Floating and semi-submersible oil rigs face risks by large storm winds and waves. Further,
EOE #2 GAS HYDRATES
1. At surface temperatures and pressures, methane is a gas. The issue with gas hydrate
2. Gas hydrates are only stable at very low temperatures and high pressures. Shallow
GIVE IT SOME THOUGHT
1.
a. (B) is the continental shelf, (D) is the shelf break, (A) is the continental slope, and (C) is
the continental rise.
2. The map shows about 35 atolls in the region of Mataiva. We can therefore conclude that the
3. A ridge with a slow spreading rate has a well-developed rift valley and steeply sloping flanks.
An example of a ridge with a slow spreading rate is the Mid-Atlantic Ridge of the Atlantic
4. As one travels from the ridge crest, the oceanic crust making up the seafloor gets older, and
thus cooler, and denser due to thermal contraction. The cooler oceanic lithosphere also
5.
a. This ridge crest is characteristic of a fast-spreading center because we see a central
6. At the location of the East African Rift, a new ocean basin may form in the future. The East
7. If an area of hot spot volcanism is not associated with tensional stresses that are strong
8. The Lesser Antilles subduction zone is located along the Puerto Rico Trench; Cascadia
subduction zone is located near the Juan De Fuca Ridge. Because the oceanic crust along the
9. The Juan de Fuca Ridge is relatively close to the Cascadia subduction zone. If the North
American plates movement is faster than the generation of new oceanic crust at the ridge, the
10. Oceanic crust is formed at divergent boundaries, and destroyed at subduction zones. While the
11. This basaltic lava flow that erupts underwater is called a pillow basalt. These structures form
as hot lava meets very cold seawater. The outer edge of the lava cools and hardens, while input
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