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.
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 3.1Fossil 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 3.2Alfred Wegener
Figure 3.3Continental Fit (Active Figure)
Figure 3.4Similarity of Rock Sequences on the Gondwana Continents
Figure 3.5Glacial Evidence Indicating Continental Drift
Figure 3.6Fossil Evidence Supporting Continental Drift
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 3.7Earth’s Magnetic Field
Figure 3.8Polar 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 3.9Magnetic Reversals
Figure 3.10Topography of the Atlantic Ocean Basin
Figure 3.11Magnetic 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 3.13Earth’s Plates
- 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 3.1Types 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
- 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 3.14Pillow Lavas
Figure 3.15History of a Divergent Plate Boundary (Active Figure)
Figure 3.16East African Rift Valley and the Red Sea—Present-Day Examples
of Divergent Plate Boundaries
Figure 3.17Triassic 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: 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: .
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 3.18Oceanic-Oceanic Convergent Plate Boundary (Active Figure)
Figure 3.19Oceanic-Continental Convergent Plate Boundary (Active Figure)
Figure 3.20Continental-Continental Convergent Plate Boundary (Active
Figure)
Figure 3.21Ophiolites
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 3.22Transform Plate Boundaries (Active Figure)
Figure 3.23The 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 ( 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:
Discovery News also reported on fault cores recovered from the San Andreas, revealing the mineral serpentine. ( Geologists suspect the serpentine acts as a lubricant in certain segments of the fault.
- 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 3.24Hot Spots
- 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 3.25Reconstructing 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 mid-ocean 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 3.26Convection in a Pot of Stew
Figure 3.27 Thermal Convection Cells as the Driving Force of Plate Movement
(Active Figure)
Figure 3.28Plate Movement Resulting from Gravity-Driving Mechanisms
14.Geologists now realize that plates can grow when terranes collide with the margins of continents.
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 ( 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: and
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 3.29Plate Tectonics and the Distribution of Organisms
16.Plate movement affects the formation and distribution of some natural resources.
Figure 3.30Copper 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 ridgecreatesnew 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.
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:
RATES OF MOTION
Plate A relative to Plate BRate 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
- 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 fieldsformed 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).
- 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