Download Plate Tectonics and the Ocean Floor opens with a brief history of

Plate Tectonics and the Ocean Floor opens with a brief history of plate motions and the development of
theory of plate tectonics. Major features of Earth’s surface are associated with plate boundaries and tell
us about Earth’s internal structure. The evidence for plate tectonics is varied and convincing even though
we do not completely understand the driving forces.
Learning objectives:
Upon completion of this chapter, the student should be able to:
1. List the evidence used by Alfred Wegener to support his continental drift theory including:
A. similarities in rock sequences on different continents
B. comparison of continental glacial deposits
C. distribution of common fossils
2. Distinguish between Wegener’s explanation for continental drift and current thinking
regarding the forces that drive plate movement.
3. Discuss the supporting evidence for plate tectonics including:
A. paleomagnetism
B. apparent magnetic pole movement
C. reversal of magnetic polarity
D. ocean floor magnetic anomalies
E. seafloor spreading
F. age of the ocean floor
G. global earthquake distribution
4. Describe Earth’s structure as defined by the
different chemical compositions of the
A. core
B. mantle C. crust
9. Using what you have learned about plate
movement, predict what Earth’s surface
might look like in 50 million years.
5. Describe the structure of Earth as defined
by the different physical properties
including:
A. inner core B. outer core C.mesosphere
D. asthenosphere E. lithosphere
6. Differentiate between the following types of
plate boundaries including the type of
activity:
A. divergent boundaries
B.
convergent boundaries C. transform
boundaries
7. Compare and contrast hot spots and mantle
plumes and discuss the relationship of
these features to current tectonic theory.
8. Recount oceanographic changes in Earth’s
oceans from 540 million years ago to the
present.
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Chapter outline:
I.
Plate Tectonics or the new global geology
A. Thin, rigid plates make up surface of Earth
B. Plates move with respect to each other
C. Plate motions produce major features
II.
Evidence for Continental Drift
A. Alfred Wegener (1921) proposed continental drift using such evidence as
1. Puzzle-like fit of continents
a. Edward Bullard in 1960 fit continents together at continental slope
2. Matching rocks (structures and ages) and mountain belts
a. Rocks of similar ages, types, structures in one continent match rocks on other continents
b. Mountain ranges continue from one continent to another (e.g., Appalachians and
Caledonides)
3. Glaciated rocks and glacial deposits
a. Evidence of glacial ice in temperate regions 300 million years ago
4. Direction of glacier movement
5. Other climate evidence
a. Fossil plants/animals indicate different climates than today
B. Distribution of fossils
1. Land-living organisms found in separate continents without “land bridges”
C. Objections to Wegener’s model of continental drift
1. Continents cannot “plow” through ocean-basin rocks
2. Energy source unlikely to be gravity and tides
III.
Evidence for Plate Tectonics
A. Earth’s magnetic field and paleomagnetism
1. Magnetic minerals in igneous rocks recorded magnetic field at time they cooled below Curie
point (600oC)
2. Magnetic minerals in sedimentary rocks also recorded magnetic field at time they were
deposited
3. Paleomagnetism in rocks records magnetic dip (magnetic inclination) and therefore latitude
a. Continents did move relative to each other
b. Apparent polar wandering
4. Magnetic polarity reversals occurred at specific times in the past
a. Magnetic anomalies of seafloor
b. Symmetric about axis of mid-ocean ridge
c. Stripe patterns trend north-south
B. Sea floor spreading
1. Bathymetry of the seafloor reveals mid-ocean ridge and ocean trenches
2. Henry Hess (1960) proposed idea of seafloor spreading
a. Ocean crust created at mid-ocean ridge
b.
Older ocean floor destroyed at oceanic trench
(subduction zone)
3. Fredrick Vine and Drummond Matthews (1963) related magnetic anomalies on ocean floor to
reversals in polarity of Earth’s magnetic field
4. Radiometric age dating establishes that ocean floor is youngest at mid-ocean ridge and oldest
toward continents
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5. Heat flow is higher at mid-ocean ridge, lower at oceanic trench
6. Most earthquakes occur at edges of plates (plate boundaries)
C. Plate tectonics accepted by the late 1960s
1. Unequal heat distribution likely driving force
IV.
Earth structure
A. Earth is divided into three main layers on the basis of chemical composition
1. Crust, low density, mainly silicate minerals, thin, average depth from surface of Earth is 30 km
2. Mantle, higher density material, mainly Fe and Mg silicate minerals, extends from base of
crust to 2900 km
3. Core, highest density material, mainly Fe and Ni, extends from base to mantle to center of
Earth (to 6370 km)
B. Earth divided into five main layers on the basis of physical properties
1. Lithosphere—cooler temperature, rigid and brittle, outermost layer, surface to about 100 km
deep
2. Asthenosphere—hotter temperature, plastic, partially molten, from 100 km to 700 km
3. Mesosphere—plastic and rigid behavior, due to high pressure, from 700 km to 2900 km
4. Outer core—liquid; Inner core—rigid
C. Interior of Earth studied by
1. Gravity: density of layers
2. Seismology: earthquakes generate seismic waves that can travel through Earth; velocities
depend on density
a. P and S seismic waves
b. Seismic tomography—3 D maps of Earth (e.g., temperature differences)
D. Lithosphere
1. Oceanic crust composed of basalt, average thickness 8 km
2. Continental crust composed of granite, average thickness 35 km
E. Asthenosphere
1. Highly viscous material flows slowly
F. Isostatic adjustment vertical movement of crust caused by different densities
1. Less dense material is buoyant and “floats” on denser material
2. Oceanic crust and continental crust “float” on asthenosphere
3. Oceanic crust is thin (and more dense) so it “floats” lower
4. Continental crust is thick (and less dense) so it “floats” higher
5. Areas that were once loaded with additional weight (e.g., glacial ice) rebound when load is
removed (e.g., ice melted)—isostatic rebound
V.
Plate boundaries
A.
Divergent
1.
2.
3.
4.
Plates move away from each other at crest of mid-ocean ridge
Pull-apart faults create void (rift) that is filled in with upwelling magma from magma chambers
Ocean floor created at first as narrow, linear seas (e.g., Red Sea) then widened to large oceans
Faster spreading rates create broad, gentle mid-ocean ridge with less intense earthquakes (as
measured by seismic moment magnitude)
5. Slower spreading rates create narrow, steep mid-ocean ridge with more intense earthquakes
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B.
Convergent
1. Plates move toward each other and collide
2. Ocean floor destroyed at oceanic trench by subduction into mantle
3. Oceanic-continental convergence
a. Ocean plate subducted beneath continental plate
b. Continental arc made up of andesite volcanic rock created through explosive volcanic activity
4. Oceanic-oceanic convergence
a. Denser oceanic plate subducted beneath other oceanic plate
b. Island arc made up of basalt
5. Continental-continental convergence
a. Two continental plates meet; neither is subducted
b. Formation of high mountain ranges made up of deformed
rocks
6. Earthquakes extend from near surface down to 670 km (Wadati-Benioff seismic zone)
C.
Transform
1. Plates slide past each other between segments of mid-ocean ridge
2. Oceanic transform faults are perpendicular to mid-ocean ridge
3. Continental transform faults cut across continent (e.g., San Andreas Fault)
VI.
Some applications of plate tectonics
A.
Mantle plumes and hotspots
1. Intraplate volcanism centered on columnar areas of hot magma (mantle plumes) that create
volcanoes on surface of Earth (hotspots)
2. Mantle plumes have deep roots (to mantle-core boundary)
3. Few mantle plumes occur near divergent plate boundary
4. Island chains (nemataths) record motion of plate over mantle plume
B.
Seamounts and tablemounts
1. Seamounts are conical underwater volcanoes; tablemounts (guyots) are flat underwater volcanoes
2. Moved away from mid-ocean ridge
C.
Coral reef development
1.
2.
3.
4.
Charles Darwin proposed that coral reefs differ because of subsidence of volcanic islands
Coral animals build large structures of calcium carbonate in shallow, warm ocean water
First, fringing reef id located on margins of volcanic island or continent
Second, barrier reef is separated from land mass by lagoon; coral reef builds upward as land
subsides (e.g., Great Barrier Reef)
5. Third, atoll is reef with no land above sea level
D.
Using satellites to detect plate motion
1. Measurements over 20 years indicate that locations on Earth are moving apart
E.
Paleoceanography
1. Changes in shape, composition, and character of oceans
2. Continents, plate boundaries move through time
3. Ocean basins are created and destroyed through time
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4. Large landmasses (e.g., Pangaea) grow by continental accretion (e.g., Laurasia and
Gondwanaland)
5. Large landmasses (e.g., Pangaea) can be split by rifting and separated by creation of seafloor (e.g.,
Atlantic Ocean)
F.
Future positions of continents and ocean basins deduced through plate
tectonics
1. Red Sea widens; East African Rift Valley may become narrow ocean
2. Atlantic Ocean widens, Pacific Ocean narrows
3. Central America may no longer connect North and South America
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