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Ocean basin floor, including volcanic island chains
MAGNETIC PROPERTIES OF THE SEA FLOOR
Confirmation of sea-floor spreading
Age of the sea floor
Ocean basins - description
Abyssal plains
Age and thickness of sediments
Volcanic features
Abyssal hills
Submarine seamounts
Guyots
Coral atolls
Chains of volcanic islands (e.g., Hawaiian islands)
Ocean basins - origin = Oceanic lithosphere that has cooled
and subsided as it spreads away from mid-ocean ridges
Subsidence explains guyots and atolls
Sediment distribution -- fits prediction of ocean-crust age
Volcanic islands and seamount chains -- "hot spot"
concept
Stationary plume of hot mantle material
Plate migrates over plume
Active volcanoes -- over plume
Extinct volcanoes -- cut-off from plume by plate motion
MAGNETIC PROPERTIES OF THE SEA FLOOR
Magnetic alignment of iron minerals in basalt
Evidence for "reversals" of Earth's magnetic field
Magnetic "anomaly" pattern of the sea floor:
Stripes parallel and symmetrical to mid-ocean ridges
Interpretation: Continuous creation of oceanic crust during magnetic
reversals.
Map of the sea-floor -- oldest crust is about 200 m.y.
Ocean basins are the province extending from continental margins to the mid-ocean ridge system. Typical depths are 4-6,000 m. Much of this province, particularly in the Atlantic, is a flat and feature region called the abyssal plains. Abyssal plains are flat because they are mantled by sediments, covering and obscuring and roughness of the basalt sea floor.
The thickness of sediments increases across ocean basins from mid-ocean ridges to continental margins. In addition, ocean drilling in the l960's and l970's showed that the age of basal (oldest) sediments increases with distance from mid-ocean ridges.
Although sediments can cover the existing topography of the
sea floor, there are numerous features of volcanic origin, especially
in the Pacific.
Abyssal hills: < 100 m high, very common.
Submarine seamounts: Higher, extinct volcanoes.
Guyots: Flat-topped seamounts whose tops have been eroded
by wave action;
guyots must have been at sea-level at one time, then subsided.
Coral atolls: As proposed by Darwin, atolls developed initially
around a volcanic
island, then evolved as the volcano subsided.
Chains of volcanic islands in the middle of a basin: Hawaiian
chain is the
best-known example. Hawaiian chain is associated with a chain
of seamounts extending to the northwest and north. Ages of these
islands and seamounts increase to the northwest away from the
active volcanoes on Hawaii.
Deep-ocean basins are simply oceanic lithosphere that has cooled and subsided as it spreads away from mid-ocean ridge spreading centers, where it was created. Cooling and subsidence explains the origin of features like guyots and coral atolls -- they were once volcanic islands that subsided as lithosphere cooled and spread. The distribution and age of ocean sediments are explained because the floor of the ocean should increase in age with distance from mid-ocean ridge spreading centers.
Volcanic island and seamount chains requires a different explanation. The origin of these features is attributed to the interaction between (a) a plume of hot mantle material that is fixed in position - mantle plume, or hot spot; and (b) an oceanic plate moving over the mantle plume. Active volcanoes exist immediately above the plume. Older, extinct volcanoes have been cut-off from the plume as the plate migrated. The age of an extinct volcano (island or seamount) is the time when that part of the plate was over the mantle plume. The trend of the chain marks the direction of plate motion.
TASA Chapter 17 - Hot Spots and Plate Tectonics:
From the beginning to the "age indicating time over plume"
- 14 frames.
When a basalt lava flow cools and solidifies, magnetic iron minerals align themselves with the direction of Earth's magnetic field at the location where they form. The minerals act like little compasses, showing us the past position of the Earth's magnetic poles. From sequences of age-dated lava flows from continents, scientists concluded that the magnetic field of Earth has reversed itself many times in the past. That is, north and south magnetic poles swapped positions. The duration of those intervals of "normal" polarity (like today) and "reverse" polarity range from 0.1 to about 10 million years.
Magnetic surveys of the sea floor (by research vessels towing a magnetometer) found an intriguing pattern of variations in the magnetic field strength. Those variations, which they termed "magnetic anomalies" were symmetrical on either side of mid-ocean ridges (and parallel to the ridges):
Slightly higher-than-average field strength: positive magnetic anomaly.
Slightly lower-than-average field strength: negative magnetic anomaly.
The most likely explanation is that the magnetic anomalies are due to the continuous creation of ocean crust during magnetic reversals:
Positive anomaly = basalt formed during time of normal polarity
(therefore it adds to the total magnetic field)
Negative anomaly = basalt formed during time of reverse polarity
(therefore it subtracts from the total magnetic field).
This interpretation gave strong and convincing support to the
concept of
sea-floor spreading:
New ocean crust (basalt) is produced at mid-ocean ridges, and records the magnetic field (polarity) of Earth at that time.
The basalt moves laterally away from the ridge like a conveyor belt, and the ocean basin becomes wider.
TASA Chapter 15 - Geomagnetic Reversals & Plate Tectonics:
Run through the frames, emphasizing the animations of magnetic
anomalies creation
The pattern of magnetization throughout the oceans (together with drilling sediments and ocean crust) have permitted us to make a geologic map of the sea floor showing absolute ages. Note that the oldest oceanic crust is only about 200 m.y. old. Although the oceans themselves are very ancient, oceanic crust and lithosphere is continuously being created (at mid-ocean ridges) and destroyed (at marginal trenches).
8-1. What is the general range of depth of deep-ocean basins?
How does depth change as we move away from mid-ocean ridges?
8-2. What is the name of the flattest, almost featureless region
of the deep-sea floor? Why is this region so flat and featureless?
8-3. How does the thickness of sediments and the age of the basal
sediments change with distance from mid-ocean ridges?
8-4. What are guyots? How did they form? Why are they important
in the theory of sea-floor spreading?
8-5. How do coral atolls with a central lagoon form? Who was the
first to propose this origin of atolls?
8-6. According to the theories of sea-floor spreading and plate
tectonics, what is the origin of deep-ocean basins? Does this
explanation account for the subsidence of sea floor as it moves
away from mid-ocean ridges? Explain.
8-7. What are "hot spots?'
8-8. Describe how the interaction between a hot spot and a moving
oceanic plate can create a chain of volcanic islands and seamounts
in the middle of an ocean. Where are active volcanoes located
in the chain? How does the age of the other islands and seamounts
change with distance from the active volcanoes?
8-9. What is meant by a "magnetic reversal?" How do
we know that magnetic reversals have occurred?
8-10. Describe the pattern of magnetic field-strength of the sea
floor. How were these "magnetic anomalies" interpreted
in terms of sea-floor spreading and magnetic reversals?
8-11. From a map of the age of the sea floor (e.g., Fig. 3.14
in the text), what is the maximum age of ocean crust (in millions
of years)? Compare that to the maximum age of continental crust.
Why are these ages different?
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