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Plate Tectonics
I. Tectonics
A. Tectonic Forces are forces generated from within Earth causing rock to become________________.
B.
1. The study of the origin and arrangement of Earth surface including mountain belts, continents, and
earthquake belts.
2. Plate Tectonics
a. This is the basic idea that Earths crust is divided into a few large, thick ___________________.
(1) Plates are part of the____________________ move slowly and change in size.
(2) Plates may be:
(a)
entirely _________________ rock
(b)
both _________________ and ___________________ rock
(c)
entirely ______________________rock
(3) Plate boundaries are geologically active with:
(a)
__________________________
(b)
__________________________
(c)
__________________________
b. Plate Tectonics combines two preexisting ideas
(1) __________________________ which is the idea that the continents move freely over
Earth’s surface, changing their positions relative to one another.
(2) __________________________ which is the hypothesis that sea floor forms a mid-oceanic
ridge crests and then moves horizontally away from the ridge towards oceanic trenches.
II. Sea Floor Spreading
A. This is the hypothesis that:
1. Was originally proposed by Harry Hess, a Princeton University geologist
2. Sea-floor forms at the __________________________
3. The sea-floor moves __________________________ from the ridge crest towards and oceanic
trench where it __________________________
4. The two sides of the ridge move in ___________________ directions.
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Plate Tectonics
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B. The Mid-Ocean Ridge
1. Hot mantle rock rises beneath the ridge as a result of convection.
2. This explains high heat flow and basaltic volcanic eruptions.
3. a. __________________________ at the ridge crest results in cracking open of oceanic crust to form
a __________________________
b. shallow focus earthquakes
c. __________________________
(1) Major lines of weakness in Earth’s crust
(2) Cross the mid-ocean ridge at nearly right angles.
(3) Extend for 1000's kilometers across the ocean floor.
(4) The mid-ocean ridge was once continuous across the fracture zones but is now offset.
d. __________________________ Faults
(1) Mid-ocean ridges are offset along fracture zones
(2) Transform motion of rocks on either side is not always in opposite directions.
(a)
(b)
Rocks move in opposite directions only in the section between two segments of ridge
crest.
This is the only section that experiences earthquakes instead of along the entire section
as would normally be expected.
4. Ocean
a. Sea floor moving away from the ridge cools
b. It becomes denser and __________________________ , perhaps sinking back into the mantle.
c. Trenches are explained by the downward plunge of cooler rock and explains negative gravity
anomalies.
5. Young Age of the Sea Floor
a. Less than __________________________ old.
b. New sea floor continually is formed by basalt eruptions at the __________________________.
(1) Basalt is carried horizontally away from the ridge crest where the ____________ rock is
found.
(2) Sea floor is continually destroyed by subduction into the mantle at the oceanic trenches.
c. Deep ocean sediment is thin or absent at the ridge crests.
ESC 101: Professor Vorw ald
Plate Tectonics
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III. Causes of Plate Motion.
A. Mantle
1. Heat transferred within a fluid due to density differences caused by temperature differences.
2. Source of heat:
a. Heat left over from Earth’s formation
b. The decay of radioactive isotopes.
3. But, rock isn’t a fluid so how can if convect?
a. Solid rock will creep when subjected to enough heat an pressure.
b. At 1,835o F it behaves like Silly Putty™. It behaves plastically like a fluid.
4. Rate of Convective Flow
a. On the human scale convection is slow. The hour hand of a clock moves 10,000 times faster
(about the rate at which fingernails grow)
b. However, this rate is geologically fast. It takes only 58 million year to move the bottom to the
top of the mantle.
5. The Pattern of Mantle Flow is not completely understood.
a. There are several models . . .
(1) Thermal Convection Model - Restricted to the upper mantle.
(2) Convection throughout the entire mantle.
b. In both models:
(1) Spreading ridges mark the rising limbs of neighboring convection cells.
(2) Trenches occur where the convection cells descend back into Earth’s interior.
c. Most recent model suggests that the lower mantle does not mix with the upper mantle.
d. Acting like a “lava lamp” heat flows across the core-mantle boundary and then variations in the
thickness controls mantle plumes.
B. Ridge-Push and Slab Pull: Contradict convection models which assume the plates are dragged along by
underlying mantle rock..
1. _________- Push
a. Spreading centers stand high on the sea floor.
b. As a plate moves away from a divergent boundary it cools and thickens causing the sea floor to
subside as it moves, forming the broad side slops of the ridge.
c. As the asthenospheric mantle cools it thickens creating a slope.
d. Lithosphere slides down.
e. Due to higher elevation at the ridge, a push is imparted to the tectonic plate.
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Plate Tectonics
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2. _______ -Pull
a. The dense, leading edge of a subducting plate pulls the rest of the plate along.
b. Subducting plate’s density increases due to cooling, loss of water, and phase (pressure)
transitions of minerals.
c. Motion is rapid along a steep slope.
C. Mantle Plumes and Hot Spots
1. _________________: Narrow columns of hot mantle rock that rise through the mantle.
2. _________________: Regions of active volcanism at Earth’s surface above plumes
3. Effects of Plumes
a. Flood basalts formed when the head of a plume contacts a continent, causing uplift and eruption
of basaltic lavas
b. Continental Rifting
(1) Causes a dome that breaks in a three-pronged pattern.
(2) The plume separates the crust along two of the three fractures
(3) The third fracture become inactive and eventually fills with sediment (called a failed rift or
aulacogen.
c. Hawaiian Islands and Emperor Seamount chain.
IV.
Types of Plate Boundaries
A. __________________________ Boundaries
1. Plates moving _________________ from each other.
2. Marked by rifting, basaltic volcanism, and uplift.
3. Tension causes shallow-focus earthquakes along normal faults along which the crust is stretched and
thinned.
4. In a continent a __________________________ forms as a central valley.
5. Found at:
a. __________________________ ridges
b. _______________________ rift valleys (East African Rift)
(1) After widening of the rift, eventually the plates separate and seawater floods into the linear
basin between the two divergent continents.
(2) Eventually opens into an ocean with a mid-ocean ridge in the center.
B. __________________________ Boundaries
1. One plate slides __________________________ past another.
2. Sites of shallow-focus earthquakes and less likely to have volcanic activity
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Plate Tectonics
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3. Strike-slip motion is common.
4. No new surface is formed or consumed.
5. Locations of transform motion.
a. __________________________ fault in California (between the Pacific Plate and the North
American Plate)
b. At mid-ocean ridge __________________________ (not plate boundaries).
C. __________________________ Plate Boundaries
1. Ocean-Ocean Convergence
a. Two plates capped by sea floor converge
b. One plate __________________________ beneath the other.
(1) The subducting plate bends downward forming the outer wall of an oceanic trench
(2) The trench forms a broad curve convex to the subducting plate due to Earth’s rounded
surface.
(3) A Benioff Zone Forms
(a)
Inclined seismic activity (angle of 30o to 60o) is created..
(b)
Above the Benioff zone magma works its way upward and erupts creating volcanic
islands of and island arc.
i)
String of volcanic islands
ii)
In a curved line parallel to the ocean trench
(4) __________________________ Wedge
(a)
Inner wall of a trench that is towards the arc
(b)
Thrust faulted and folded marine sediment along with pieces of ocean crust.
(c)
“Snowplowed” off the subducting plate by the overlying plate.
(5) ______________________ Basin
(a)
Lies between the accretionary wedge and the volcanic arc.
(b)
Relatively undeformed.
c. Examples:
(1) Pacific plate subducting under the western Aleutian Islands
(2) Japanese Islands formed by the Pacific Plate subducting beneath the Eurasian Plate.
2. Ocean-Continent Convergence
a. Oceanic crust is subducted under continental lithosphere resulting in a active continental margin.
b. A ________________ zone of earthquakes dips under the edge of the continent.
c. A new mountain belt is formed.
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Plate Tectonics
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d. Magma rises forming a magmatic arc with the continent (rather than a island arc).
e. Andesitic volcanism occurs
f. Beneath the volcanoes are large plutons in thickened crust, seen on land as batholiths when
exposed by extensive erosion.
g. The more buoyant continental plate experiences intense deformation, metamorphism, and
melting
(1) crust thickens
(2) also rises isostatically
(3) thrust faults , associated with folds, move slivers of mountain-belt rocks landward over the
continental interior.
h. Today, this is occurring where:
(1) the Juan de Fuca Plate is subducting under North America
(2) the Nazca Plate is subducting under South America.
i. ____________: Evidence of an Ancient Convergent Boundary
(1) Believed to be slivers representing ocean crust and upper mantle.
(2) Pieces of oceanic plate that have been thrusted (obducted) onto the edge of continental plates.
(3) Similar to the structure of oceanic crust, determined from seismic studies and drilling.
3. Continent-Continent Convergence
a. Two continents __________________________.
b. Continents become welded together along a dipping ____________ zone.
c. A mountain belt forms at the interior of the new continent. Examples:
(1) Himalayas between Eurasia and India
(2) Appalalchians - Formed when Pangaea collided with North America
d. Originally separated by an ocean floor that is being subducted under one of them, which, thus,
has a volcanic arc.
e. When the two continents collide, the lithosphere can’t subduct.
(1) It’s density is too low.
(2) One plate may partially slip below the other.
f. Crust thickens by:
(1) Original arc thickening the crust at site of impact
(2) Shallow underthrusting of one continent beneath the other
ESC 101: Professor Vorw ald
Plate Tectonics
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