Download chapter 3 – answers to questions in text

R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 3
PLATE TECTONICS: A UNIFYING THEORY
OUTLINE
INTRODUCTION
EARLY IDEAS ABOUT CONTINENTAL DRIFT
Alfred Wegener and the Continental Drift Hypothesis
Additional Support for Continental Drift
PALEOMAGNETISM AND POLAR WANDERING
MAGNETIC REVERSALS AND SEAFLOOR SPREADING
PLATE TECTONICS AND PLATE BOUNDARIES
PERSPECTIVE Tectonics of the Terrestrial Planets
Divergent Boundaries
An Example of Ancient Rifting
Convergent Boundaries
Recognizing Ancient Convergent Boundaries
Transform Boundaries
HOT SPOTS AND MANTLE PLUMES
HOW ARE PLATE MOVEMENT AND MOTION DETERMINED?
THE DRIVING MECHANISM OF PLATE TECTONICS
HOW ARE PLATE TECTONICS AND MOUNTAIN BUILDING RELATED?
Terrane Tectonics
PLATE TECTONICS AND THE DISTRIBUTION OF LIFE
PLATE TECTONICS AND THE DISTRIBUTION OF NATURAL RESOURCES
SUMMARY
CHAPTER OBJECTIVES
The following content objectives are presented in Chapter 3:
 Plate tectonics is the unifying theory of geology and has revolutionized geology.
 The hypothesis of continental drift was based on considerable geologic,
paleontologic, and climatologic evidence.
 The hypothesis of seafloor spreading accounts for continental movement and the idea
that thermal convection cells provide a mechanism for plate movement.
 The three types of plate boundaries are divergent, convergent, and transform. Along
these boundaries new plates are formed, consumed, or slide past one another.
 Interaction along plate boundaries accounts for most of Earth’s earthquake and
volcanic activity.
 The rate of movement and motion of plates can be calculated in several ways.
 Some type of convective heat system is involved in plate movement.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
 Plate movement affects the distribution of natural resources.
 Plate movement affects the distribution of the world’s biota and has influenced
evolution.
LEARNING OBJECTIVES
To exhibit mastery of this chapter, students should be able to demonstrate comprehension
of the following:
 the hypothesis of continental drift and the evidence supporting it
 the reasons continental drift was not readily accepted by the geologic community
 the importance of paleomagnetism and polar wandering in reigniting the hypothesis
of continental drift
 the theory of seafloor spreading, including the supporting evidence of
paleomagnetism and magnetic reversals
 the three types of plate boundaries, including modern examples of each as well as
criteria used to recognize ancient plate boundaries
 the concepts and importance of mantle plumes and hot spots in plate tectonic theory
 convection cells as the proposed mechanism that drives plate tectonics
 the role of plate tectonics in mountain building
 the influence of plate tectonics on biological evolution and the distribution of
organisms through time
 the influence of plate tectonics on the distribution of natural resources
CHAPTER SUMMARY
1. The concept of continental movement is not new. The earliest maps showing the
similarity between the east coast of South America and the west coast of Africa
provided people with the first evidence that continents might once have been united
and subsequently separated from each other.
Figure 3.1
Fossil Glossopteris Leaves
2. Alfred Wegener is generally credited with developing the hypothesis of continental
drift. He provided abundant geologic and paleontologic evidence to show that the
continents were once united into one supercontinent he named Pangaea.
Unfortunately, Wegener could not explain how the continents moved, and most
geologists ignored his ideas.
Figure 3.2 Alfred Wegener
Figure 3.3 Continental Fit (Active Figure)
Figure 3.4 Similarity of Rock Sequences on the Gondwana Continents
Figure 3.5 Glacial Evidence Indicating Continental Drift
Figure 3.6 Fossil Evidence Supporting Continental Drift
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
3. The hypothesis of continental drift was revived during the 1950s when
paleomagnetic studies of rocks indicated the presence of multiple magnetic north
poles instead of the one there is today. This paradox was resolved by constructing a
map in which the continents could be moved into different positions such that the
paleomagnetic data would then be consistent with a single magnetic north pole.
Figure 3.7 Earth’s Magnetic Field
Figure 3.8 Polar Wandering
4. Magnetic surveys of the oceanic crust revealed magnetic anomalies in the rocks,
indicating that Earth's magnetic field has reversed itself numerous times in the past.
Because the anomalies are parallel to and form belts adjacent to the oceanic ridges,
new oceanic crust must have formed as the sea floor was spreading. Harry Hess
proposed the theory of seafloor spreading to account for continental movement in
1962.
Figure 3.9 Magnetic Reversals
Figure 3.10 Topography of the Atlantic Ocean Basin
Figure 3.11 Magnetic Anomalies and Seafloor Spreading
5. Radiometric dating reveals the oldest oceanic crust is less than 180 million years
old, whereas the oldest continental crust is 3.96 billion years old. The ocean basins
are recent geologic features.
Figure 3.12 Age of the World’s Ocean Basins
6. Plate tectonic theory is based on the simple model in which rigid lithosphere, of
both oceanic and continental crust as well as the underlying upper mantle, is broken
into seven major and numerous smaller plates. Most geologists accept plate
tectonic theory because the evidence overwhelming supports it and because it
provides geologists with a powerful theory for explaining such phenomena as
volcanism, earthquake activity, mountain building, global climatic changes, the
distribution of the world’s biota, and the distribution of some mineral resources.
Figure 3.13 Earth’s Plates
7. Three types of plate boundaries are recognized: divergent boundaries, where plates
move away from each other; convergent boundaries, where two plates collide; and
transform boundaries, where two plates slide past each other.
Table 3.1
Types of Plate Boundaries
Enrichment Topic 1. Investigation into Plate Motions
The United States Geological Survey offers an interactive website, complete with
enlargeable photographs of the various plate motions. “Understanding Plate Motions” is
accessible through the website http://pubs.usgs.gov/gip/dynamic/understanding.html
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
8. In divergent plate boundaries or spreading ridges, plates are separating and new
oceanic lithosphere is forming. When continents break up, a rift valley forms, then a
linear sea fills the valley, and if rifting progresses, the spreading ridge is located in
the middle of an ocean basin. Ancient divergent boundaries are recognized through
rift valleys with thick sedimentary sequences and numerous dikes and sills.
Figure 3.14 Pillow Lavas
Figure 3.15 History of a Divergent Plate Boundary (Active Figure)
Figure 3.16 East African Rift Valley and the Red Sea—Present-Day Examples
of Divergent Plate Boundaries
Figure 3.17 Triassic Fault-Block Basins of Eastern North America—An
Example of Ancient Rifting
Enrichment Topic 2. Rift Zones and Aulacogens
Rifting often begins at a triple arm junction. As rifting proceeds, one of the segments or
arms of the rift may fail; these failed arms, once filled with sediments, are called
aulacogens. Investigate rifting in more detail through this UK website:
http://www.le.ac.uk/geology/art/gl209/lecture3/lecture3.html. An aulacogen is suspected
by some geologists to be the cause of the New Madrid, MO earthquakes of 1811 and
1812. Details of the earthquake are discussed at ShowMe-Net:
http://www.showme.net/~fkeller/quake/lib/enigma.htm .
9. Crust is destroyed and recycled at convergent plate boundaries. Because there are
two types of crust (oceanic and continental), three types of convergent boundaries
exist. Ancient convergent boundaries can be recognized through andesitic rocks,
mélanges, and ophiolites.
Figure 3.18 Oceanic-Oceanic Convergent Plate Boundary (Active Figure)
Figure 3.19 Oceanic-Continental Convergent Plate Boundary (Active Figure)
Figure 3.20 Continental-Continental Convergent Plate Boundary (Active
Figure)
Figure 3.21 Ophiolites
10.
In transform boundaries, plates slide past each other laterally, and crust is neither
created or destroyed. The majority of transform faults exist in the oceanic crust.
Transform faults generally do not leave any characteristic or diagnostic features in
the rock record.
Figure 3.22 Transform Plate Boundaries (Active Figure)
Figure 3.23 The San Andreas Fault—A Transform Plate Boundary
Enrichment Topic 3. Transform Boundaries and the San Andreas Fault
In transform zones, the associated faulting that occurs is strike-slip, in which movement
occurs horizontally, as opposed to vertically. Investigate transform zones on About.com
Geology (http://geology.about.com/library/bl/blnutshell_transform.htm), and focus on
one of the most famous continental transform zones, the San Andreas fault. One webpage
highlights various segments of the San Andreas, as well as its history:
http://geology.about.com/od/geology_ca/tp/aboutsaf.htm.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Discovery News also reported on fault cores recovered from the San Andreas, revealing
the mineral serpentine. (http://dsc.discovery.com/news/2008/05/30/san-andreasfault.html). Geologists suspect the serpentine acts as a lubricant in certain segments of the
fault.
11. A hot spot is the location on Earth’s surface where a stationary column of magma
originating from the mantle has risen to the surface. A hot spot will leave a trail of
extinct and progressively older volcanoes as a plate moves over it. Mantle plumes are
essentially fixed with respect to Earth’s rotational axis and can be used to determine
the direction and rate of plate movement.
Figure 3.24 Hot Spots
12. The average rate of movement and relative motion of plates can be calculated in
several ways. The results of these different methods all agree and indicate that the
plates move at different average velocities. Absolute motion of plates can be
determined by the movement of plates over hot spots.
Figure 3.25 Reconstructing Plate Positions Using Magnetic Anomalies
Enrichment Topic 4. Mantle Plumes: Fact or Fiction?
At mantle plumes, hot mantle wells up from deep within Earth and forms a hotspot,
producing volcanoes on the Earth’s surface. Although geologists have proposed that the
volcanoes at Hawaii, Yellowstone, and Iceland formed above hotspots, seismologists
reported no evidence for the upward flow of hot mantle under Yellowstone and Iceland,
which may mean that there are no plumes at all. Those who favor the plume model cite
the jump of volcanism from one plate to the other as a mid-ocean ridge passes; this is
believed to be evidence that the source of the plume must be deeper than that of the midocean ridge. The volume of magma produced at Hawaii must result from the upward
movement of a lot of very hot mantle, especially because the isotope ratios are
characteristic of the deep mantle. Hotspot volcanism is stable, long lived and can begin
with an enormous volume of magma. New seismology techniques may find better
evidence for mantle plumes. Ascribe Higher Education News Service, May 8, 2003.
13. Two models involving thermal convection cells have been proposed to explain
plate movement. In one model, the convection is restricted to the asthenosphere; in
the other, the entire mantle is involved. Slab-push and ridge-pull are gravity-driven
mechanisms that have been proposed for plate movement.
Figure 3.26 Convection in a Pot of Stew
Figure 3.27 Thermal Convection Cells as the Driving Force of Plate Movement
(Active Figure)
Figure 3.28 Plate Movement Resulting from Gravity-Driving Mechanisms
14.
Geologists now realize that plates can grow when terranes collide with the margins
of continents.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Enrichment Topic 5. Terranes.
When geologists recognized the importance that terranes can play in continental
accretion, some refinement of geologic maps was necessary. This USGS website
(http://esp.cr.usgs.gov/info/gmna/comparisons1.html#1) compares 1965 and 2005
geologic maps of northwestern Canada and Alaska. Some major changes in the 2005 map
are pointed out. Have students click on the 2005 map, and investigate the complex
geology of the region. How many different terranes can be identified? Very basic
geologic maps illustrating the overview of the accreted terranes of western North
American may be accessed at the Federation of American Scientists’ webpage:
http://www.fas.org/irp/imint/docs/rst/Sect2/FigS11-1.jpg and
http://www.fas.org/irp/imint/docs/rst/Sect2/accreteterranes.jpg
15.
The present distribution of plants and animals is controlled by climate and
geographic barriers, both of which are controlled in part by the distribution of
oceans and mountains over the surface of Earth.
Figure 3.29 Plate Tectonics and the Distribution of Organisms
16.
Plate movement affects the formation and distribution of some natural resources.
Figure 3.30 Copper Deposits and Convergent Plate Boundaries
LECTURE SUGGESTIONS
Reconstruction of Pangaea
1. Have students investigate part of Alfred Wegener’s original evidence that led to
the continental drift hypothesis. Students should photocopy a map, outline
continental shapes, and cut out the shapes. Without referencing an illustration of
Pangaea, students should try to arrange the continents into a best-fit jigsaw.
2. Ask students the following questions: Which continents fit together best? Which
continental shapes were difficult to place, or could have been placed in multiple
locations? Why would using the continental shelf outlines improve the
effectiveness of this exercise?
Oceanic Spreading Ridge Dynamics
1. Many students understand that the separation of plates at the mid-ocean ridge
creates new crust. However, the bilateral symmetry of the paleomagnetic reversal
pattern or the migration of the ridge axis can be confusing. This demonstration may
help to clarify these concepts.
Depending on the class size, all or part of the class may participate. Classrooms
that have a central aisle are ideal. The volunteers, about 20 to 30 students, gather in
the central aisle. They are referred to as the magma in the chamber underlying the
central ridge axis.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
They are told to shuffle slowly to the front of the class in pairs, and for each pair to
separate, one going left and the other to the right upon reaching the front of the
room. When they reach the front of the room, they are to hold their arms up if the
instructor's arms are up, or leave them down if the instructor's arms are down.
In this way, the plates grow at the front of the room, as the students diverge from
each other at the ridge axis and are subsequently replaced by other students
emerging from the magma chamber. Hands up are normal polarity rocks and hands
down are reverse polarity rocks. The bilateral symmetry of the paleomagnetic
pattern and progression of rock age from oldest (first to come out) to youngest (last
to come out) should now be obvious. Having the students take a small step toward
the audience for each minute that they are part of the new crust illustrates the
subsidence of the plate as it moves away from the ridge. By varying the rate at
which the students walk and separate, various paleomagnetic reversal pattern
widths and ridge topographies (East Pacific Rise versus Mid Atlantic Ridge) can be
generated.
Relative Rates of Plate Motion
1. This exercise will illustrate relative rates of motion between tectonic plates.
Students are designated as plates and plate margins that move with a set of tape
measures.
The setting is the western margin of North America 40 million years ago, involving
the Pacific, Farallon, and North American plates, a spreading ridge, and a
subduction zone. Five students are designated as follows:
Student 1 = Pacific plate.
Student 2 = West side of spreading ridge; reels out Pacific plate with tape measure.
Student 3 = East side of spreading ridge; reels out Farallon plate with tape measure.
Student 4 = Farallon plate.
Student 5 = North American plate and subduction zone; reels in Farallon plate.
Given: Rate of Pacific Plate with respect to North American plate = zero (therefore,
students 1 and 5 are stationary in the illustration). Both students 2 and 3 reel out
tape at a rate of 1 m/10 million years (1m = 500km).
Set-up: Two roll-up tape measures are used with a piece of bright electrical tape
attached at each 1 in interval.
Student 1 stands on one side of the room. Students 2, 3 and 4 start approximately 3
meters from student 1. Student 5 stands across the room, approximately 8 meters
from student 1. Students 2 and 3 move together throughout the demonstration, and
reel out tape at the same rate. Student 5 pulls in tape reeled out by student 3.
Student 4 travels along with the tape reeled out by student 3. The following table is
drawn on the blackboard:
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
RATES OF MOTION
Plate A relative to Plate B
Rate in km/million of years
Pacific plate to North America plate. (0)
North America plate to ridge.
(50)
Pacific plate to ridge.
(50)
Farallon plate to ridge.
(50)
Pacific plate to Farallon plate.
(100)
Farallon plate to North America plate. (100)
The illustration is set-up and run for each set of plates in the table. The difference
in rates between each pair of students is easily ascertained.
2 Point out that the process of plate tectonics is essentially a global scale process of
magma differentiation (or partial melting) in which mantle material is continually
brought to the surface. When subducted, and during some types of volcanism, a
small amount of material is differentiated from the basaltic oceanic lithosphere
and added to continental material, and sometimes basaltic crust is directly
incorporated into continental masses. Ultramafic rock is no longer extruded and
differentiated from mafic lavas as it was in the Archean Eon, because the outer
Earth's temperature gradient has declined below its melting point.
Hot Spot Volcanic Signatures
1. To illustrate the relationship between hot spots and surface volcanic chains, use
the analogy that the crust, either oceanic or continental, is like a piece of paper
and that the hot spot is like a lighted match. As you move the paper over the
match, a burn trace is left. Volcanic chains like the Hawaiian Islands-Emperor
Seamount chain, or the Columbia Plateau-Snake River Plain-Yellowstone
volcanic fields formed in a similar fashion. This also can be easily illustrated with
a piece of white paper (the plate) and a red marker (the plume/hot spot).
2. Lava lamps are always popular and can be used to show how hot material rises,
then sinks when it cools, as in a convection cell or at a mantle plume.
Modern Tectonic Settings
1. Students may enjoy visiting vivid images of active plate boundaries and tectonic
features on National Geographic’s Plate Tectonics Photo Gallery
(http://science.nationalgeographic.com/science/photos/plate-tectonicsgallery/virunga-mountains.html). Which each image, ask students to identify how
extinct plate boundaries and features would be identified in the rock record.
2. Using Google Earth, students can search for additional examples of convergent
and divergent plate volcanism, hot spot activity, and transform zones.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
Continental Drift Versus Plate Tectonics
1. Review the differences between continental drift and plate tectonics. Emphasize
that much of the evidence presented by Wegener and du Toit for the continental
drift theory strongly supported the theory of plate tectonics. Stress that the great
weakness in Wegener’s continental drift hypothesis was Wegener’s lack of a
believable mechanism for plate movement.
CONSIDER THIS
1. Why is plate tectonics given the status of theory while continental drift attained
only the status of hypothesis?
2. If plate movement has been directly measured, is plate tectonics a theory or an
assembly of facts?
3. Wegener had no mechanism for continental drift; however, his thesis failed for
other reasons as well. Investigate the other, nonscientific objections to his thesis
and work.
4. What is the origin of the term "transform" in transform fault?
5. Why are many transform faults associated with spreading ridges, at approximately
right angles to the ridge?
6. Why can spreading ridges never be directly connected to subduction zones, but
instead the two must be linked by a transform fault?
7. Which would come first on the Earth, the "slab-pull" or the "ridge-push" in plate
tectonic mechanisms? Explain.
IMPORTANT TERMS
continental-continental plate boundary
continental drift
convergent plate boundary
Curie point
divergent plate boundary
Glossopteris flora
Gondwana
hot spot
Laurasia
magnetic anomaly
magnetic reversal
oceanic-continental plate boundary
oceanic-oceanic plate boundary
Orogeny
paleomagnetism
Pangaea
plate
plate tectonic theory
seafloor spreading
Terrane
thermal convection cell
transform fault
transform plate boundary
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
SUGGESTED MEDIA
Videos
1. The Sea Floor, Earth Revealed #4, Annenberg/CPB
2. The Birth of a Theory, Earth Revealed #5, Annenberg/CPB
3. Plate Dynamics, Earth Revealed #6, Annenberg/CPB
4. Mountain Building, Earth Revealed #7, Annenberg/CPB
5. Earthquakes, Earth Revealed #9, Annenberg/CPB
6. Volcanism, Earth Revealed #13, Annenberg/CPB
7. Living With Earth: Part I, Earth Revealed #25, Annenberg/CPB
8. Earthquakes, EME Corporation
9. Volcanoes, EME Corporation
10. Plate Tectonics, EME Corporation
11. Sea Floor Spreading, EME Corporation
12. Raging Planet: Volcano, Discovery Channel
13. The Ultimate Guide: Volcanoes, Discovery Channel
14. Supervolcano, Discovery Channel
15. Volcano: Nature’s Inferno, National Geographic Society
16. Plate Tectonics, Cambridge Educational
17. Continental Drift: Legacy of Fire, BBC
Software
1. Plate Tectonics and How Earth Works, Tasa Graphics Arts, Inc.
2. Tasa Photo CD-ROM, Tectonics and Mountain Building, Tasa Graphics Arts, Inc.
3. The Theory of Plate Tectonics, Tasa Graphics Arts, Inc.
4. ERUPT, RockWare, Inc.
5. Explore Volcanoes, Geological Society of America
6. Explore Earthquakes, Geological Society of America
Demonstration Aids
1. Plate Tectonics Lab Program (Mid-Ocean Ridge, Subduction Zones, Transform
Faults, Plate Motions and Effects, Volcanoes), EME Corporation
2. Plate Tectonics and the Spreading Sea Floor, slide set, Educational Images, Ltd.
3. The Drifting of the Continents, slide set, Educational Images, Ltd.
4. Continental Drift, slide set, Educational Images, Ltd.
5. Satellite Imagery – Earth from Space, slide set, Educational Images, Ltd.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
CHAPTER 3 – ANSWERS TO QUESTIONS IN TEXT
Multiple Choice Review Questions
1. d
2. a
3. b
4. b
5. a
6. c
7. c
8. e
9. c
10. a
Short Answer Essay Review Questions
11. Organisms evolve to fit a specific set of ecological circumstances. Greater diversity
of organisms results with a greater number of separate ecosystems on Earth.
Separating continents or oceans isolate gene pools, producing evolutionarily
divergent communities. Continents clustered latitudinally have low diversity;
continents spread from pole to pole have greater variation.
12. Wegener noted that marine and non-marine rock sequences of the same age are
found on widely separated continents; mountain ranges and glacial deposits match
up when continents are united into a single landmass; the shorelines of continents
fit together to potentially form a supercontinent; and many extinct fossil organisms
are found today on widely separated continents.
13. Although Wegener had evidence to support his hypothesis, the mechanisms that he
used were inadequate and did not convince scientists. Until a suitable mechanism
could account for continental movement, many scientists remained unconvinced.
14. Seafloor spreading, supported by the magnetic reversal data from the ocean
seafloor, was a plausible mechanism by which continents could move. Hess
proposed that continents do not move through oceanic crust, but that continents and
oceanic crust move together as a single unit. Hess postulated that the seafloor
separates at oceanic ridges, where new crust is formed by upwelling magma. The
newly formed crust moves laterally away from the ridge. This was supported by the
magnetic reversal data. Hess revived the heat transfer system—or thermal
convection cells—within the mantle as the mechanism to move the plates. Because
ocean basins are newly formed crust, they are geologically young. This was
confirmed by radiometric dating.
15. Thermal convection is thought to be the major force driving plate movement
because most the models fit the data observed, and the models use the internal heat
of the Earth as the mechanism driving the movements of the plates. Spreading
ridges mark the ascending limbs of adjacent convection cells, and trenches are
present where convection cells descend back into Earth’s interior. Slab-pull and
ridge-push modified the total reliance on convection cells because this gravitydriven mechanism also depends on thermal differences. In slab-pull, the subducting
cold slab of lithosphere pulls the rest of the plate along as it descends into the
mantle. In ridge-push, gravity pushes the oceanic lithosphere from the higher ocean
ridges and toward the trenches.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
16. Hot spots allow geologists to determine absolute motion because they provide an
apparently fixed reference point from which the rate and direction of plate
movement can be measured. The mantle plume is stationary, so when plates move
over the mantle plume, the resulting hot spots leave a trail of extinct and
progressively older volcanoes that record the movement and direction of the plate.
17. At divergent plate boundaries, new lithosphere is created. Earthquakes result when
rocks are displaced along fractures, while volcanism results when rising magma
erupts at the spreading ridge. Convergent plate boundaries are also associated with
earthquakes and volcanoes. As an oceanic plate is subducted or two continental
plates collide, earthquakes are produced by the friction of the two plates against
each other. If an oceanic plate is subducted, partial melting of the plate occurs;
magma rises to the surface forming a chain of volcanoes.
18. Metallic ores are produced both at convergent and divergent plate boundaries in
conjunction with igneous processes. The magma generated by partial melting of a
subducting plate rises toward the surface, and as it cools, it precipitates and
concentrates various metallic ores. Along divergent boundaries, minerals adjacent
to hydrothermal vents can precipitate.
19.
Plate tectonics has undoubtedly affected the distribution of organisms on both land
and the ocean. The Isthmus of Panama separated organisms that once co-existed,
leading to separate species in the Atlantic and Pacific oceans. Similarly, mountain
ranges can separate continental populations, resulting in the development of new
species. Through the mechanism of natural selection, separated populations evolve
independently in response to their unique environments.
20.
Plate tectonics explains the existence and distribution of a variety of seemingly
unrelated features and events: andesitic volcanism, earthquake epicenters and foci,
orogenic belts (elongate mountain chains), matching coastlines across the Atlantic,
young oceanic rocks versus older continental rocks, and present occurrence of
fossil organisms.
Apply Your Knowledge
1. To solve this problem we assume that the Hawaiian hotspot is located beneath the
youngest volcano, Kilauea. We then determine the distance in kilometers of each
island from Kilauea from an atlas. The rate of motion for the spot on which each
volcano sits is calculated as age/distance. These numbers are roughly: Maui = 100
km/1.0m.y. = 10 cm/year, Molokai = 200 km/1.6m.y .= 12.5 cm/yr, Oahu = 340
km/2.8 m.y. = 12.1 cm/yr, and Kauai 510 km/4.7m.y. = 10.8 cm/yr. This averages to
a rate of motion for the Hawaiian Islands of 11.3 cm/yr, although this number is just
an estimate. The rate is different for each volcano, in part because volcanoes are
large and formed around the hotspot, as well as directly over it.
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R.M. Clary, Ph.D., F.G.S.
Department of Geosciences
Mississippi State University
The rate Pacific plate motion has increased recently, resulting in an average of the 8.0
cm/year over the entire Hawaiian Island Emperor seamount chain (Clague and
Dalrymple, 1987).
2. San Francisco is 530 kilometers northwest of Los Angeles. By dividing the distance
(km) by the rate of motion (cm/yr), we calculate the amount of time it will take for
Los Angeles to be opposite of San Francisco, or 9.6 million years.
3. Volcanic activity might indicate tectonic activity, especially if the volcanoes
represent an arc on the surface of Mars that would indicate subduction of one plate
beneath another, with magma resulting from partial melting of the descending slab.
Extensive reverse or thrust faulting might also indicate compressional tectonics, such
as that found in convergent boundaries. Volcanism and igneous activity – such as
lava flows—might also indicate rifting or tensional tectonics. Tensional tectonics
(found in divergent boundaries) also might be recognized through numerous normal
faults, and resulting fault-block basins.
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