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5.1: Introduction to Plate Tectonics - Geosciences


Learning Objectives

This chapter has several goals and objectives:

  • Compare and describe each of these Earth layers: lithosphere, oceanic crust, and continental crust.
  • Describe how convection takes place in the mantle and compare the two parts of the core and describe why they are different from each other.
  • Explain the concepts of the following hypothesis: continental drift hypothesis, seafloor spreading hypothesis, and the theory of plate tectonics.
  • Describe the three types of tectonic plate and how the processes lead to changes in Earth’s surface features.

Essential Questions

  • What is the driving force of plate tectonics and how does this impact earthquakes and volcanoes around the world?
  • How does the theory of plate tectonics help explain the different types of earthquakes and volcanoes around the planet?

1.5 Fundamentals of Plate Tectonics

Plate tectonics is the model or theory that has been used for the past 60 years to understand Earth’s development and structure — more specifically the origins of continents and oceans, of folded rocks and mountain ranges, of earthquakes and volcanoes, and of continental drift. It is explained in some detail in Chapter 10, but is introduced here because it includes concepts that are important to many of the topics covered in the next few chapters.

Key to understanding plate tectonics is an understanding of Earth’s internal structure, which is illustrated in Figure 1.6. Earth’s core consists mostly of iron. The outer core is hot enough for the iron to be liquid. The inner core, although even hotter, is under so much pressure that it is solid. The mantle is made up of iron and magnesium silicate minerals. The bulk of the mantle, surrounding the outer core, is solid rock, but is plastic enough to be able to flow slowly. Surrounding that part of the mantle is a partially molten layer (the asthenosphere), and the outermost part of the mantle is rigid. The crust — composed mostly of granite on the continents and mostly of basalt beneath the oceans — is also rigid. The crust and outermost rigid mantle together make up the lithosphere. The lithosphere is divided into about 20 tectonic plates that move in different directions on Earth’s surface. (For a more accurate depiction of the components of the Earth’s interior, please see Figure 9.2.)

An important property of Earth (and other planets) is that the temperature increases with depth, from close to 0°C at the surface to about 7000°C at the centre of the core. In the crust, the rate of temperature increase is about 30°C/km. This is known as the geothermal gradient.

Figure 1.6 The structure of Earth’s interior showing the inner and outer core, the different layers of the mantle, and the crust [Wikipedia]

Heat is continuously flowing outward from Earth’s interior, and the transfer of heat from the core to the mantle causes convection in the mantle (Figure 1.7). This convection is the primary driving force for the movement of tectonic plates. At places where convection currents in the mantle are moving upward, new lithosphere forms (at ocean ridges), and the plates move apart (diverge). Where two plates are converging (and the convective flow is downward), one plate will be subducted (pushed down) into the mantle beneath the other. Many of Earth’s major earthquakes and volcanoes are associated with convergent boundaries.

Figure 1.7 A model of convection within Earth’s mantle [http://upload.wikimedia.org/wikipedia/commons/thumb/2/27/Oceanic_spreading.svg/1280px-Oceanic_spreading.svg.png]

Earth’s major tectonic plates and the directions and rates at which they are diverging at sea-floor ridges, are shown in Figure 1.8.

Exercise 1.2 Plate Motion During Your Lifetime

Using either a map of the tectonic plates from the Internet or Figure 1.8, determine which tectonic plate you are on right now, approximately how fast it is moving, and in what direction. How far has that plate moved relative to Earth’s core since you were born?

Figure 1.8 Earth’s tectonic plates and tectonic features that have been active over the past 1 million years [http://commons.wikimedia.org/wiki/File:Plate_tectonics_map.gif]


Rubric

This criterion is linked to a Learning Outcome Requirements

Includes all of the required components, as specified in the assignment.

Includes most of the required components, as specified in the assignment.

Includes some of the required components, as specified in the assignment.

Includes few of the required components, as specified in the assignment.

This criterion is linked to a Learning Outcome Content

Demonstrates strong or adequate knowledge of the materials correctly represents knowledge from the readings and sources.

Some significant but not major errors or omissions in demonstration of knowledge.

Major errors or omissions in demonstration of knowledge.

Fails to demonstrate knowledge of the materials.

This criterion is linked to a Learning Outcome Critical Analysis

Provides a strong critical analysis and interpretation of the information given.

Some significant but not major errors or omissions in analysis and interpretation.

Major errors or omissions in analysis and interpretation.

Fails to provide critical analysis and interpretation of the information given.

This needs to be in essay format, without any of the questions included and none of the instruction text. Attached is my current paper with all the answers. It just needs to be put into essay format with an intro, references, and conclusion.


Introduction to Plate Tectonics This unit introduces Wegner's hypothesis about continental drift, Hess' theory of seafloor spreading, and the modern comprehensive theory of plate tectonics.

The presentation about plate tectonics is divided into 3 separate discussions: Continental Drift, Seafloor Spreading and Plate Tectonics. This approach is meant to highlight the historical development of the theory of plate tectonics. The development of the theory is an excellent example of the cumulative nature of science and the scientifc process.

Continental Drift this presentation reviews Wegner's original hypothesis about the apparent movement of the continents.

PowerPoint Click to download the MS Powerpoint file (70 Mbytes)

PDF Click to view or download the presentation in PDF (1.9 Mbytes)

HT ML Cl ick to view the presentation in html format.

Online Lecture. Click here to view a streaming lecture about the Wegner's eveidence for Continental Drift. (

This animation shows a reconstruction of the movement of the continents for the last 180 million years. This and other animations are available from the UCSB Educational Multimedia Visualization Center of the Department of Earth Science . This animation is available by clicking here .

S eafloor Spreading this presentation reviews the major features of the seafloor and Harold Hess' original theory of seafloor spreading.

PowerPoint Click to download the MS Powerpoint file (34 Mbytes)

PDF Click to view or download the presentation in PDF (2.1 Mbytes)

HT ML Cl ick to view the presentation in html format.

Online Lecture. Click here to view a streaming lecture discussing the rock cycle and igneous rocks. (

This animation depicts sea floor spreading on three spreading centers that are connected by transform faults. The inset in the amination shows magnetic polarity reversals and the formation of seafloor magnetic stripes. This and other animations are available from the UCSB Educational Multimedia Visualization Center of the Department of Earth Science . This animation is available by clicking here.

Plate Tectonics this presentation reviews the modern theory of plate tectonics, plate margins and the occurrence of volcanism and earthquakes.

PowerPoint Click to download the MS Powerpoint file (146 Mbytes)

PDF Click to view or download the presentation in PDF (5 Mbytes)

HT ML Cl ick to view the presentation in html format.

Online Lecture . Click here to view a streaming lecture discussing the rock cycle and igneous rocks. (

This animation shows subduction of oceanic crust under continental crust at a convergent margin. As the plate is subducted into the mantle, water in the slab is released and moves upward by buoyancy. The influx of water into the upper mantle lowers the melting point of rocks resulting in the formation of basaltic magma. As the magma rises buoyantly, its composition changes from basalt to andesite by a variety of processes. Hence, a line of volcanoes forms on the overriding plate that are parallel to the deep ocean trench. This and other animations are available from the UCSB Educational Multimedia Visualization Center of the Department of Earth Science . This animation is available by clicking here.

Classroom Activities

Wegner's Puzzle. Click to download a zipped folder of documents for this classic activity developed by the U.S. Geological Survey. This is an excellent starting activity for a unit on plate tectonics. In addition, the activity can demonstrates the fit of the continents to form Pangea and may be used as an introductory phenomenon. This activity permits students to use graphical displays (e.g., maps) to identify temporal and spatial relationships. (1.9 Mbytes). Zip Folder

Modeling Seafloor Spreading. Click to download this activity where students model seafloor spreading and the formation of magnetic "stripes" that provide primary evidence for the theory of plate tectonics. This activity permits students to use a model to test ideas about a phenomenaon at an unobservable scale (i.e., to understand the nature of the seafloor's magnetic stripes). In addition, it permits students to construct an explaation using the model.

  • Teacher Instructions PDFWord document
  • Student Data Sheet PDFWord document

Where Do Earthquakes and Volcanoes Occur? In this activity, students work in groups of four to plot the locations of 20 active volcanoes and 20 major earthquakes. Students compare their results with a plate tectonic map to investigate the relationship between plate boundaries and the locations of earthquakes and volcanoes. This activity permits students to use graphical displays (e.g., maps) of large data sets (earthquake and volcano locations) to identify spatial relationships with tectonic margins.

  • Student Data Sheet PDFWord Document
  • Map Key PDF

Where Did Pinnacles National Park Come From? In this activity, students examine movement of the Neenach volcanic rocks in Pinnacles National Park northward along the San Andreas fault. This activity permits students to apply mathematical concepts (e.g. rate) to a scientific question about the rate of motion along the San Andreas fault and predicting future movement.

  • Student Data Sheet PDFWord Document
  • Teacher Answer Key PDF

How Fast is the Pacific Plate Moving? PDF Word Document In this activity, students examine geochronological data for lava flows that form the Hawaiian Islands and use that data and the distances of the islands from the Hawaiian mantle hotspot to detemine the rate that the Pacific Plate is moving over. This activity permits students to apply mathematical concepts (e.g. rate) to a scientific question about the rate of motion of the Pacific plate over the Hawaiian hotspot . Teacher Key

Online Video and Media Resources

This Dynamic Planet Interactive Map This is a very cool interactive map that shows the locations of volcanoes, earthquakes, plate boundaries and impact craters. This website has a lot of graphic data and requires a fast internet connection (be patient while loading). Smithsonian Institution.

This Dynamic Planet The USGS has produced a spectacular printed map. This website is the companion to the printed map and includes links to downloadable maps. USGS

This Dynamic Earth: The Story of Plate Tectonics This is an online publication that provides detailed information about plate tectonics. USGS

Teachers on the Leading Edge There are many resources and activities developed by this professional development program for middle school science teachers.

Plate Tectonics: An Introduction This video introduces plate tectonics and how the Earth's surface has changed over geologic time. Click here to access the source website including additional resources. WGBH Educational Foundation

Plate Tectonics: The Scientist Behind the Theory This video introduces the three major types off plate boundaries. Click here to access the source website including additional resources. WGBH Educational Foundation

Plate Tectonics: Further Evidence This video introduces the three major types off plate boundaries.Click here to access the source website including additional resources. WGBH Educational Foundation

Plate Tectonics: Lake Mead, Nevada This short video examines the Lake Mead area of the Basin and Range province where extension of the North American crust is occurring. Click here to access the source website including additional resources. WGBH Educational Foundation

Continental Divide: The Breakup of Pangaea Interactive animation shows the breakup of Pangeae with evidence from rocks and fossils. WGBH Educational Foundation

Plate Tectonics: The Hawaiian Archipelago This video introduces the formation of Hawai'i from a mantle hot spot. Click here to access the source website including additional resources. WGBH Educational Foundation

Earthquake! When Plates Collide (WGBH Educational Foundation). Click here to access the source website including additional resources.

USGS Earthquake Hazards Program earthquake.usgs.gov/ This USGS site is the main entry point for information on earthquakes including realtime earthquake maps.

Realtime Global Earthquake Map http://earthquake.usgs.gov/earthquakes/map/ This is an interactive map showing the occurrence of earthquakes. Please note that you can zoom into any location for more detail and change the map options to display different magnitude earthquakes.

United States Geological Survey (USGS) www.usgs.gov/ The USGS is a federal agency within the U.S. Department of the Interior and has primary responsibility for geological (hazards, resources, etc.) and environmental issues of national and regional importance.

The Educational Multimedia Visualization Center emvc.geol.ucsb.edu/ This website contains terrific animations illustrating tectonic plate motion. UCSB

Teachers on the Leading Edge orgs.up.edu/totle/ There are many resources and activities developed by this professional development program for middle school science teachers.

NGSS Disciplinary Core Ideas

Grade 2
ESS1.C: The History of Planet Earth. Some events happen very quickly others occur very slowly, over a time period much longer than one can observe .

Grade 4
ESS1.C: The History of Planet Earth. Local, regional, and global patterns of rock formations reveal changes over time due to earh forces, such as earthquakes. The presence and location of certain fossil types indicate the order in which rock layers were formed.

ESS2.B: Plate Tectonics and Large-Scale System Interactions. The locations of mountain ranges, deep ocean trenches, ocean floor structures, earthquakes, and volcanoes occur in patterns. Most earthquakes and volcanoes occur in bands that are often along boundaries between continents and oceans. Major mountain chains form inside continents or near their edges. Maps can help locate the different land and water features areas of the Earth.

ESS3.B: Nature Hazards. A variety of hazards result from natural processes (e.g., earthquakes, tsunamis, volcanic eruptions). Humans cannot eliminate the hazards but can take steps to reduce their impacts.

Middle School
ESS2.B: Plate Tectonics and Large-Scale System Interactions Maps of ancient land and water patterns, based on investigations of rocks and fossils, make clear how Earth's plates have moved great distances, collided, and spread apart.

ESS3.B: Natural Hazards Mapping the history of natural hazards in a region, combined with an understanding of related geologic forces can help forecast the locations and likelhoods of future events.

High School
ESS3.B: Natural Hazards Natural hazards and other geologic events have shpated the course of human history [they] have significantly altered the sizes of human populations and have driven human migrations.

Common Scientific Misconceptions

Crust and Lithosphere (or plates) are synonymous terms

Asthenosphere is liquid (students are only familiar with liquid convection, not solid convection, many secondary education earth science films also specifically refer to a molten internal layer).

Lower Mantle is liquid (for reasons similar to above).

Earth's core is hollow, or that large hollow spaces occur deep within Earth (a relict of older cosmology and a mainstay of popular literature and Hollywood movies).

Only continents move (Wegener's original concept, along with the common use of 'Continental Drift' term in general texts, secondary education earth science films, etc.)

Most crust motions (especially those associated with processes of mountain building or deep sea trench formation) are due to vertical motions, not lateral (terms like 'mountain uplift' and earth science textbook terminology, as well as relict idea from old cosmologies).

Divergent ocean ridges are due to vertical uplift or convergence, rather than divergence (In students' experience, buckling is usually due to convergence or uplift, not heat/density differences, so illustrations of ridges do not readily fit with a pulling apart motion).

Present oceans only began as Pangea broke apart - tied to general idea that Pangea was the original continent at the Earth's start (few educational earth science films mention what came before Pangea & emphasis on Atlantic spreading leads to Pacific being overlooked).

Plate movement is imperceptible on a human timeframe (common use of fingernail growth analogy is only true for slowest plates and underestimates importance of motion).

Plate motion is rapid enough that continent collision can cause financial and political chaos, while rifting can divide families or separate a species from its food source.

Oceans are responsible for oceanic crust (rather than being closer to other way round).

Continental 'shelves' are similar to shelves in homes, extend out over edge of continent and can break and collapse to form tsunamis (so Boxing Day tsunami was due to shelf collapse)

The edge of a continent is the same thing as a plate boundary.

Over time there has been no significant change in ratio of oceanic to continental areas (idea of stasis is a common misconception, but this was also part of Lyell's original concept).

Apart from differences due to changes in ice volume, sea level has remained relatively constant through time (recognition of impact of plate speed on sea level not even recognized by geologists until relatively recently).

A plate boundary type is the same thing as a plate. For example, a plate has to be divergent or convergent.


Space Geodesy Programme

During the 19th and early 20th century, several geologists explored the idea that the continents may have moved across the Earth's surface. They were all inspired by the remarkable fit between the Atlantic coasts of Africa and South America. The hypothesis of continental drift was largely developed by the German Alfred L. Wegener, a lecturer in astronomy and meteorology, who suggested that the Earth's continents had at one time been joined in two supercontinents. In the year 1912, Wegener made the proposal that all the continents were previously one large continent, but then broke apart, and had drifted through the ocean floor to where they are now located. Apart from using the fit of the two continents already mentioned, Wegener also used fossil distribution and lithological similarity as evidence. Wegener was born in Berlin on November 1, 1880 the youngest child of an evangelical preacher. By the time he was in his teens, he had developed a strong interest in the earth sciences. He studied astronomy at the University of Berlin, where he received a doctoral degree in 1904. Wegener exhibited a talent for expounding complex subjects with great ease. Together with the force of his personality, the clarity of Wegener's vision inspired great enthusiasm and loyalty among his students.

Of course, the "Drift" theory was not immediately accepted by Wegener's peers, as it is difficult in the world of science to change accepted or established doctrines or views. Two other viewpoints prevailed at this time. Those who believed that the continents and basins were basically unchanged in their position and relative configuration since they were formed were called "Permanentists". Others believed that as a result of the gradual contraction of the solid earth, ocean floor became dry land, and dry land in turn became ocean floor these scientists were called "Contractionists".

Wegener studied the distribution of animals and fossil land plants to help him in his interpretations. Wegener found that the plant Glossopteris had left behind leaf remains which were relatively common in the Southern Hemisphere continents. This supported his hypothesis, as Wegener reasoned that in order for Glossopteris leaves to be found in the widely spaced continents of the Southern Hemisphere, the continents must once have been joined. Using this evidence, he joined all of the southern continents, together with India, into a supercontinent which he named Pangea.

The splitting of Pangaea

Wegener also studied the distribution of major geological bodies, such as crystalline basement (rocks and continental crust) complexes and mineral deposits. He found that the fit predicted by map estimates was confirmed by the alignment of geological complexes on either side of the Atlantic Ocean. For instance when he fitted Africa and South America together along their continental shelves, he found that large blocks of ancient rock called cratons formed contiguous patterns across the dividing line. The mountains that run from east to west across South Africa seemed to link with the range near Buenos Aires in Argentina. The distinctive rock strata of the Karoo system in South Africa, which consists of layers of sandstone, shale and clay laced with seams of coal, were identical to those of the Santa Catarina system in Brazil.

South Africa's contribution

Wegener's most compelling and enthusiastic support came from a South African geologist, Alexander Du Toit. South African scientists were far more favorably disposed to the idea of continenal drift for a simple reason: All around them they could see a plethora of geological phenomena that closely resembled those of the other continents in the Southern Hemisphere. Du Toit spent five months in Brazil, Uruguay and Argentina amassing evidence. He found it difficult to believe that he was on another continent as not only did he find the same plant and animal fossils he knew at home, but he found them in the same complex sequence, embedded layer by layer in the rock. Du Toit was confident he had found conclusive proof that the continents were once joined. In a book dedicated to Wegener and entitled Our Wandering Continents, Du Toit proposed a prior configuration for the continents that was different from Wegener's.

Instead of a simple supercontinent, Du Toit reconstructed the continents at the South Pole and grouped the northern continents near the Equator. He called his southern supercontinent Gondwanaland and the northern land mass Laurasia. He devoted most of his book to Gondwanaland and as evidence for its existence he produced an impressive mass of data far more detailed than anything Wegener had attempted.

Alexander Du Toit's map of two ancient supercontinents

Was this good enough for the academic critics ? No. Du Toit's flamboyant writing style pained the critics. For instance he would write "the dumbfounding spectacle of the present continental masses, firmly anchored to a plastic foundation yet remaining fixed in space set thousands of kilometers apart, it may be, yet behaving in almost identical fashion from epoch to epoch and stage to stage like soldiers at a drill widely stretched in some quarters at various times and astoundingly compressed in others, yet retaining their general shapes, positions and orientations remote from one another throughout history, yet showing in their fossil remains common or allied forms of terrestrial life possessed during certain epochs of climates that may have ranged from glacial to torrid or pluvial to arid, though contrary to meteorological principles when their existing geographic positions are considered - to mention but a few such paradoxes!" Du Toit's overdramatization succeeded only in decreasing the value of his substantial contributions to the evidence. "This," sniffed an academic critic, "is the colorful language of a pamphleteer."

How the critics were converted

The discovery of palaeomagnetism and the development of oceanography was a necessary step in the development of science which Wegener's and Du Toit's theories awaited.

Paleomagnetism

Paleomagnetism is based on the principle that in molten igneous rocks, or unlithified sediments, magnetic particles will align themselves with the Earths's magnetic field. This magnetic record is stored within the rocks when they cool and within the sediments when they become lithified. The deviations in the alignment of these paleomagnetic particles from the current direction of the Earth's magnetic filed shows that the continents have moved. A British physicist Patrick Blackett, who had won the Nobel Prize in 1948 for his work in nuclear physics and cosmic radioation, developed a sensitive device called the astatic magnetometer. Using this equipment, it was possible for the first time to detect the orientation of extremely weak magnetic fields. This enabled researchers to conduct paleomagnetic studies of types of rocks whose magnetism could not be discerned by earlier equipment.

Oceanography

During the 1960's two Cambridge scientists, Drummond Matthews and Fred Vine discovered that on either side of the Mid-Atlantic Ridge there were a series of linear magnetic anomalies. Strips of ocean crust had alternating magnetic orientations. These observations were explained in terms of a sea floor spreading model by which new oceanic crust forms along mid-ocean ridges as the two halves of an ocean move apart.

From these simple observations the theory of PLATE TECTONICS developed.

According to the plate tectonic model, the surface of the Earth consists of a series of relatively thin, but rigid, plates which are in constant motion. The surface layer of each plate is composed of oceanic crust, continental crust or a combination of both. The lower part consists of the rigid upper layer of the Earth's mantle. The rigid plates pass gradually downwards into the plastic (soft) layer of the mantle, the astenosphere. The plates may be up to 70 km thick if composed of oceanic crust or 150 km incorporating continental crust. Plates move at different velocities, The African plate moves about 25 mm per year, whereas the Australian plate moves about 60 mm per year.

Most of the Earth's tectonic, seismic and volcanic activity occurs at the boundaries of neighbouring plates. There are three type of plate boundaries: divergent, convergent and transform boundaries.

Divergent plate margins

At this type of boundary new oceanic crust is formed in the gap between two diverging plates. Plate area is increased as the plates move apart. Plate movement takes place laterally away from the plate boundary, which is normall marked by a rise or a ridge. The ridge or rise may be offset by a transform fault. Presently, most divergent margins occur along the central zone of the world's major ocean basins. The process by which the plates move apart is referred to as sea floor spreading. The Mid-Atlantic Ridge and East Pacific Rise provide good examples of this type of plate margin.

The Mid-Atlantic Ridge system

Convergent plate boundaries

At a convergent boundary two plates are in relative motion towards each other. One of the two plates slides down below the other at an angle of around 45 degrees and is incorporated into the Earth's mantle along a subduction zone. The path of this descending plate can be found from analysis of deep earthquakes and the initial point of descent is marked on the surface by a deep ocean trench . Plate area is reduced along the subduction zone. When two plates of oceanic crust collide a volcanic island arc may form. As one of the plates is subducted beneath the other it begins to melt at a depth of between 90 and 150 km and the resulting magma rises to the surface above the subduction zone to form a chain or arc of volcanoes. The edge of the plate which is not descending is therefore marked by a chain of volcanic islands.

Conservative or transform margins

The San Andreas fault system is the most famous example of this type of boundary. Here two plates move laterally past each other and oceanic crust is neither created nor destroyed.

    The rate at which each plate moves apart from a divergent margin varies from less than 50 mm per year to over 90 mm per year and can be determined from the pattern of magnetic anomalies either side of a spreading ridge. Either side of a spreading centre, weak magnetic anomalies 5-50 km wide and hundreds of kilometres long can be identified. molten rock cools between diverging plates the magnetic minerals present align themselves with the orientation of the Earth's magnetic field at that time. The polarity of the Earth has changed at regular intervals throughout geological time. Magnetic north has alternated between the Arctic (normal polarity) and the Antarctic (reversed polarity). As a result of this, sections of crust formed during a period of normal polarity have a paleomagnetic remnance which is oriented towards today's magnetic north, while a section of crust formed during a period of reversed polarity does not. These long linear strips of magnetic anomalies form a symmerical pattern either side of a spreading centre. A record of the changes in the Earth's magnetic polarity has been established and dated for the Cenozoic and is the basis for magnetostratigraphy. This record, in conjunction with the magnetic stripes found either side of a spreading ridge, allows the rate and pattern of sea floor spreading to be examined.

What causes plates to move ?

This question has yet to be fully resolved. Four main hypotheses have been put forward to explain this.

Convection currents

This hypothesis suggests that flow in the mantle is induced by convection currents which drag and move the lithospheric plates above the astenosphere. Convection currents rise and spread below divergent plate boundaries and converge and descend along convergent. Three sources of heat produce the convection currents:

(1) cooling of the Earth's core

(2) radioactivity within the mantle and crust

The convection hypothesis has been proposed in several different forms throughout the last 60 years. Convective models of plate evolution clearly show how important convective heat transport is to the modern Earth, over length scales as small as 100 km and times of 60 million years. Earth is a spendthrift, living on its inherited capital of primaeval heat, not on its radiogenic modern income.

Magma injection This hypothesis invokes the injection of magma at a spreading centre pushing plates apart and thereby causing plate movement.

Gravity Oceanic lithosphere thickens as it moves away from a spreading centre and cools, a configurationwhich might tend to induce plates to slide under the force of gravity, from a divergent margin towards a convergent margin.

Descending plates This hypothesis suggests that a cold dense plate descending into the mantle at a subduction zone may pull the rest of the plate with it and thus cause plate motion.

To summarize, the plate tectonic model provides a mechanism by which:

(1) continents can move across the surface of the globe

(2) patterns of volcanism can change and shift across the globe as plates and their boundaries evolve and move

(3) new oceans may grow and different sedimentary basins evolve

(4) oceans and sedimentary basins close and are deformed to produce mountains

Do measurements using VLBI, SLR and GPS support the findings from paleomagnetism ?

Yes, it does.

Geodetic data from VLBI, SLR and GPS indicate that plate velocities as measured over the last 15 years nearly equal those averaged over the past 3 million years.


Introduction to Plate Tectonics Continental Drift - PowerPoint PPT Presentation

May occur under oceanic or continental crust . Low density continental crust is not subducted, but may partially underlie the . &ndash PowerPoint PPT presentation

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Introduction to Plate Tectonics with Google Earth

Plate tectonics is a unifying framework for understanding the dynamic geology of the Earth. The theory posits that the outermost layers of the Earth (the crust and uppermost mantle) make up the brittle lithosphere of the Earth. The lithosphere is broken up into a number of thin plates, which move on top of the asthenosphere (middle mantle). The asthenosphere is solid, but flows plastically over geologic time scales. Plate interiors are relatively stable, and most of the tectonic action (earthquakes, volcanism) takes place where plates meet – where they collide at convergent boundaries, move away from one another at divergent boundaries, or slide past one another at transform boundaries.

Reconstructions of the Earth’s tectonic plate locations through time are available, for example, at:

http://www.scotese.com/newpage13.htm (Links to an external site.)
http://www.ucmp.berkeley.edu/geology/tectonics.html (Links to an external site.)

But how do we define plates and plate boundaries? On what are plate reconstructions and animations based? How do we know plates are moving, how can we track their positions in the past, and how can we predict their positions in the future?

To answer these questions, this assignment guides you through an examination of patterns on Earth – the topography of the earth’s surface above sea level, the bathymetry of the ocean floor below sea level, and the distribution of earthquakes and volcanic rock ages. These patterns reveal plate boundaries, just as they did for geologists first developing plate tectonic theory in the 1960s. You’ll then use geologic data to determine long-term average plate motions, to predict how our dynamic planet will change in the future.

To do this, you’ll use the program Google Earth, and Google Earth layers compiled from various sources.

A. Getting started with Google Earth

On your computer, install the latest version of Google Earth Pro from https://www.google.com/earth/versions/ (Links to an external site.)

Once installed, open Google Earth, under the Tools/Options/3D View/ menu on a PC, or under the Preferences/3D View menu on a Mac, choose the “Decimal Degrees” and “Meters Kilometers” options and makes sure the “Use High Quality Terrain” box is checked.

Open the View menu. Go ahead and experiment with the options, but in general you should just have the Tool Bar, Side Bar and Status Bar checked. Also, on the View menu, hover over Navigation and you will see several options for the compass arrow and slide bars in the upper right corner of the Google Earth screen.

“Automatically” is a good choice as it leaves a ghost of the image visible until you hover over it.

Load the DynamicEarth.kmz file from into Google Earth Pro. It is located at https://serc.carleton.edu/sp/library/google_earth/examples/49004.html (Links to an external site.) and is the top file in the “Description and Teaching Materials” list. You should be able to double-click on the filename and it will open. Or, you can download the file onto your computer first, and then open it in Google Earth Pro by using File/Open and navigating to the file.

Once the DynamicEarth.kmz is loaded, click and drag to move it from “Temporary Places” to “My Places.” Then save “My Places” by clicking File/Save/Save My Places. DynamicEarth.kmz will now be available every time you open Google Earth Pro on your computer.

When you exit, Google Earth Pro should save “My Places” for the next time.

But you should manually save “My Places” whenever you make significant changes to it, as Google Earth Pro does not autosave during a session.
You now have an interactive view of the Earth! Take some time to explore the Earth with Google Earth and figure out how the navigation works using the keyboard, your touch pad, your mouse. For example:

Zoom in and out, move N, S, E, W, grab and spin the globe, etc. The resolution will change as you zoom. Clicking on the “N” of the navigation compass reorients the view so north is “up.”

At top left, “search” (and fly to) any place of interest. Zoom in and click on the “street view” icon (orange stick figure under the compass at top right) to explore an area as if you were on foot

Zoom in to see individual buildings, roads, cars, etc.

Go 3D – zoom into a significant topographic feature (e.g. Mount Everest, the Grand Canyon, Niagara Falls). Hold the Shift key down and tilt the terrain using the Up/Down arrows to tilt the terrain, and spin the terrain using the Right/Left buttons. Do the same thing for topographic features on the ocean floor. Note that under Tools/Options/3D View you can increase the vertical exaggeration by up to 3x. This is useful to emphasize subtle features, but is pretty scary when you look at the Grand Canyon that way!

On the Google Earth tool bar, click the clock-with-an-arrow icon to explore historical imagery in an area of interest (views through time of your favorite city, for example)

By clicking and dragging, you can move things that you have found and want to save, from the “Search” menu into “My Places.” You can also re-organize “My Places” by adding and deleting items, changing the order of things, making subfolders, etc.

Explore the built-in items under the Layers menu at bottom left, and Dynamic Earth layers in your Places menu.

Expand and contract the folders and subfolders, turn various items on and off, etc. For example, with the Dynamic Earth/Volcanoes of the World layer displayed, right-clicking on a volcano (double-clicking with a Mac) brings up an information box about it.

B. Topographic Patterns
Uncheck all of the layers and focus on topographic features of the Earth.

Topography of the earth ABOVE sea level

Are mountains randomly distributed on the continents, or do they tend to occur in particular patterns (clusters, linear chains, arcs, etc.)?
Find Mt. Everest, the highest point on earth. Zoom in enough to see the summit, then pan your cursor around to locate the highest point (elevations shows up in the status bar at the bottom, as long as View/Status Bar is selected). The elevation of Mt. Everest is how many meters?
Topography of the earth BELOW sea level

We are all relatively familiar with the topography of the Earth’s surface above sea level, but less so with the bathymetry of the Earth below sea level. Before this was known, most people assumed that the seafloor was relatively flat and featureless, and personal experience with lakes and rivers suggested that the deepest part would be in the middle. Actual mapping of the sea floor, however, showed some surprises.

Such mapping began in the 1930’s but accelerated during World War II with the advent of submarine warfare. Princeton Geosciences Professor Harry Hess played a pivotal role as captain of the USS Cape Johnson he used the ship’s echo-sounder to “ping” the seafloor and measure depth as the ship traversed the Pacific Ocean between battles. After the war, this data led him to propose seafloor spreading, a process crucial to the development of the theory of plate tectonics.

Modern methods to measure bathymetry include multi-beam echo sounders that map a wide swath of seafloor, and satellite measurement of variations in sea level due to variations in gravitational pull over bathymetric features – sea level is slightly lower over low spots on the sea floor and slightly higher over high spots.

On Google Earth, the bathymetry is shown in shades of blue: the darker the blue, the greater the depth. You can get Google Earth Pro to draw topographic profiles by a) using the “Add Path” tool to draw a path across a region of interest b) saving that path to My Places and c) right-clicking on the path in My Places and choosing “Show Elevation Profile.”

In order to see a bathymetric profile of the sea floor, (as opposed to a topographic profile on land), there is one more important step to take. In the information box for the path you create, click on “Altitude”, and then choose “clamped to the sea floor” instead of “clamped to the ground”. Otherwise your profile will simply show you a flat line for the sea surface.

Examine the Atlantic Ocean between North/South America and Eurasia/Africa. Note that the deepest part is not the middle instead, an underwater mountain range runs down the middle of the ocean.
Shore of South America leads to the Atlantic Ocean which ends at the shore of Africa.

Features like this are called mid-ocean ridges or spreading ridges (more on the “spreading” later in this lab). Zoom in enough to see that although the ridge is a topographic high, it also has a valley (the “rift valley”) running along the middle of it. In the space below, complete the topographic profile of the Atlantic Ocean floor between South America and Africa. Take a digital photograph of your sketch to including in your lab report.

Scan around to see the ocean ridges in the Indian, Pacific and Southern Oceans.

Pacific Ocean leads up to the South American shoreline.
If the earth’s lowest spots aren’t in the middle of the ocean, where are they? Focus on the west coast of South America, and in the space below complete the topographic profile of the Pacific Ocean floor from South America westward about 600 miles (1000 km). Take a digital photograph of your sketch to including in your lab report.

The deep linear features, the lowest points on Earth, are called ocean trenches.
Using Google Earth, “fly to” Challenger Deep, the deepest place on Earth (once Google Earth gets you there, you may have to zoom out to see where you are). Where is it?

Challenger Deep reaches 11 km (11,000 meters, equivalent to 36,000 ft) below sea level. Which is greater, the elevation of Mt Everest above sea level (see Question 3), or the depth of Challenger Deep below sea level, and by how much?
In the space below, give the locations of three other ocean trenches on Earth.

C. Seismic Patterns
An earthquake is a vibration of Earth caused by the sudden release of energy, usually as an abrupt breaking of rock along planar fractures called faults.

Earthquakes originate at a point called the focus (or hypocenter) which is not at the surface of the earth, but instead at some depth within the earth. The epicenter of an earthquake is the point directly above the focus on either the land surface or seafloor the depth of an earthquake has nothing to do with water depth, but instead is the depth in the solid earth from epicenter to focus.

Only rocks that are cold and brittle (the earth’s lithosphere) can be broken in earthquakes. Rocks that are hot and ductile will stretch and deform slowly over time without breaking (the earth’s asthenosphere) – and thus do not produce earthquakes. So observing where earthquakes occur, both horizontally and with depth, tells us something about where stress is concentrated, and also about the material properties of the earth.
Representation of two faults shifting. Where the slip happens underground is the focus. Where the slip occurs on the surface is the epicenter.

(Source: https://www.windows2universe.org/earth/geology/quake_1.html (Links to an external site.))

Expand the Dynamic EarthSeismicity item and click “on” the “Twenty years of large earthquakes” layer to show the epicenters of large earthquakes (those with magnitudes = 6.0) during a 20-year period.

Describe any patterns you see in the distribution of earthquake epicenters over the Earth’s surface – do they form lines, arcs, circles or clusters? Are patterns connected or disconnected?

Look closely at and around the Earth’s ridges and trenches. The earthquake depth patterns associated with these features are different. Complete the chart below:
In the vicinity of ridges.
(Scan 1500km or so on either side) In the vicinity of trenches.
(Scan

1500 or so km on either side)

Describe the depth or range of depths of earthquakes, and the distribution (symmetric or asymmetric?)

Is there any pattern to the depth distribution?

Using earthquake depths as evidence, is the Earth’s lithosphere thicker in the vicinity of ridges or in the vicinity of trenches? Justify your answer.
D. Volcano Patterns
A volcano is an opening in the Earth’s surface through which melted rock (magma), volcanic ash and/or gases escape from the interior of the Earth.

Leaving the earthquake layer on, click on the Active Volcanoes layer. Describe the relationship between the locations of most active volcanoes and locations of earthquakes:
E. Plate Boundaries

The theory of plate tectonics holds that the Earth’s lithosphere is broken into a finite number of jigsaw puzzle-like pieces, or plates, which more relative to one another over a plastically-deforming (but still solid) asthenosphere. The boundaries between plates are marked by active tectonic features such as earthquakes, volcanoes, and mountain ranges and there is (relatively) little tectonic activity in the middle of plates.

Unclick all the layers, and then click on the “plate boundary model” layer (click the box to show it and then click the + or arrow to expand the legend). This shows plate boundaries and the names of major plates.

Find the boundary between the African and South American plates

Where is this plate boundary, relative to the coastlines of Africa and South America?

Now click the other layers on and off so that you can see relationships between plate boundaries and these features. If you did not have the “plate boundary layer” available to you, how could you determine where this plate boundary was? Be sure to consider topography/bathymetry as well as the earthquake and volcano layers. List several ways and be specific.
Travel westward across the South American plate to its boundary with the Nazca plate

Where is this plate boundary, relative to South America?
If you did not have the “plate boundary layer” available to you, how could you determine where this plate boundary was? List several ways and be specific.
F. Plate motion

Motion across the mid-Atlantic ridge: the South American plate vs. the African plate

Turn on the “Seafloor age” and the “Plate Boundary” Google Earth (GE) layers. The “Seafloor age” layer shows the ages of volcanic rocks that have erupted and cooled to form the ocean floor. Focus on the Atlantic Ocean. Note that the age bands generally run parallel to the spreading ridges. Seafloor age is a critical piece of evidence for plate tectonics these are used to reconstruct how ocean basins have developed over time and predict how they may evolve in the future.

How many million years (abbreviated Ma) does each colored band represent?

On average, continental crust is 2 billion years old the oldest rocks are 3.8 billion years old, and some of the grains in those rocks are even older.

What is the age of the oldest seafloor? _______________________________

On average, which is oldest – the continents or the ocean basins? _________________

Find the South American plate, the African plate, and the Mid-Atlantic Ridge that marks the boundary between them. What happens to the age of the seafloor as distance increases away from the Mid-Atlantic Ridge?

Is crust being created or destroyed at this plate boundary (and other spreading ridges)?

Is this plate boundary divergent, convergent, or transform? ________________

Focus on the northern Atlantic Ocean, near the east coast of the US and the northwest coast of Africa. How long ago did the northern Atlantic Ocean begin to open up or start spreading? Describe your reasoning.

Did the northern Atlantic Ocean basin start to open at the same time as the southern Atlantic Ocean basin? How much older or younger is the northern Atlantic basin than the southern Atlantic basin? Describe your reasoning.
G. Putting it all together:

Prepare a report documenting this lab activity. Your report should discuss how plate tectonic theory relates to earthquakes, volcanoes, and the bathymetry (sea floor topography) of oceans. Along the way, include answers to all of the questions in this lab. Your paper should be accompanied by the two drawings of your ocean floor profile sketches in questions 3 and 4. Your paper should be well organized and written in flowing paragraph form, instead of just a numbered list of questions and answers. Use APA format, according to the CSU Global Writing Center (Links to an external site.) including a title page, and citing and referencing any sources that you use to support your work, apart from this lab sheet.


Kiyoo Wadati

Kiyoo Wadati (1902-1995) was a Japanese seismologist who presented convincing evidence of deep earthquakes (>300km). He discovered what is today known as the Wadati-Benioff Zone, a region of intermediate and deep earthquake zones along oceanic trenches, which became the foundation for the plate tectonics hypothesis. Wadati graduated from the Institute of Physics, Imperial University of Tokyo. He subsequently worked at Central Meteorological Observatory during which time he discovered deep earthquakes.

Specific contributions to plate tectonic theory / solid Earth geophysics

Kiyoo Wadati hypothesized that earthquakes in Japan were a result of plate motion. His early research compared time curve data for the arrival of P and S waves of two earthquakes with close epicenters. The calculations demonstrated that one earthquake occurred at a depth of 30 km, while the other occurred at a depth of greater than 300 km. Wadati compiled evidence of more than a dozen earthquakes between 1924 and 1927 in the Honshu region that occurred at the same depth range. Wadati plotted data that demonstrated an inclined intermediate and deep earthquake zone near the oceanic trench dipping toward the Asiatic continent northwestward and the Philippine Sea westward. The intermediate and deep earthquake zone was named the Wadati-Bennioff zone after Hugo Benioff demonstrated that the zone existed in each area in the circumpacific region. His observations that deep earthquakes occured provided integral support for the theory of plate tectonics.

Later in his career, Wadati plotted data for ground motion at seismic stations. He demonstrated that curves were sharper according to the magnitude of the quake. Wadati attempted to use this method to estimate the magnitu de of destructiveness of earthquakes. Charles Richter later created the Richter scale based on Wadati’s methods.

Diagram of Wadati-Benioff zone from the U.S. Geological Survey (obtained from Wikipedia.com June 10th, 2011)

Other interesting scientific contributions

Wadati studied ground subsidence while director of Osaka District Meteorological Observatory. He discovered the relationship between ground water levels, consolidation of mudstone, and subsidence in the Osaka area. His research was valued in the recovery of Japan after World War II when industrial growth contributed to increases in ground subsidence.

Other cool stuff you should know

Wadati was placed on the retired list of the Central Meteorological Society for 1929-1931 because he was ill with tuberculosis. He was director general of the Japan Meteorological Society from 1956 until he retired due to an age limit in 1967.

Bibliography

Frolich, Cliff. “Kiyoo Wadati and Early Research on Deep Focus Earthquakes'

Introduction to Special Section on Deep and Intermediate Focus Earthquakes”. Journal of Geophysical Research, Vol. 92, No. B13, pp. 13,777-13,788, 1987


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