Content Benchmark E.12.C.2
home / earth science / earths composition and structure /

Students understand the concept of plate tectonics including the evidence that supports it (structural, geophysical and paleontological evidence). E/S

Why is the Earth so restless? What causes the ground to shake violently, volcanoes to erupt with explosive force, and great mountain ranges to rise to impressive heights? The answers to these questions were discovered as one of sciences’ most revolutionary and recent theories took shape. It began with Alfred Wegener…

Continental Drift
Alfred Wegener (German Astronomer and Meteorologist) proposed in 1914 that all landmasses were at one time connected as a supercontinent approximately 200 million years ago which he called Pangaea. In Wegener’s theory of continental drift, Pangaea progressively split up as the continents detached themselves and “drifted” away. Wegener provided physical, fossil, geological, and climate evidence to support this theory;

• Fit of the continents

Wegener noted that the shape of the continent’s coastlines appeared to match like pieces in a jigsaw puzzle.

• Fossil evidence (Mesosaurus, Lystrosaurus, Glossopteris)

Noted the occurrence of plant and animal fossils found on the match coastlines of South America and Africa. (Figure 1)

Figure 1: As noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined. (from http://earthobservatory.nasa.gov/Library/Giants/Wegener/wegener_4.html)

• Rock type and structural correlations
• Similar age, structure, and rock types on continents on opposite sides of the Atlantic Ocean. i.e. Appalachian Mountains (North America) and mountains in Scotland and Scandinavia

• Paleoclimatic evidence
• Found dramatic climate changes on some continents. Most notable was the discovery of coal deposits (made of tropical plants) in Antarctica which led Wegener to conclude that this frozen continent in an earlier time in Earth’s history must have been positioned closer to the equator – where the milder climate allowed lush, swampy vegetation to grow. (Figure 2)

Figure 2: Paleoclimatic evidence for continental drift. (from http://www.geology.ohio-state.edu/~vonfrese/gs100/lect25/index.html)

Detailed information regarding Wegener’s evidence can be found at;
http://pubs.usgs.gov/gip/dynamic/historical.html
http://www.geology.ohio-state.edu/~vonfrese/gs100/lect25/

The main reason Wegener’s hypothesis was not accepted was because he suggested no mechanism for moving the continents. His belief that the force of Earth’s spin (rotation) was enough to cause the continents to move was not shared by the geologists of the time who knew that rocks were too strong for this to be true.

For an article outlining the grand vision of drifting continents and widening seas to explain the evolution of Earth’s geography and his theory of continental drift, go to
http://earthobservatory.nasa.gov/Library/Giants/Wegener/

Sea Floor Spreading
Harry Hess (geologist and Navy submarine commander during WWII) studied newly published maps of the seafloor topography indicating the existence of a world-wide mid-ocean ridge system. He proposed, in the 1960’s, that ridges are where new seafloor is created from upwelling in the mantle. It was possible, he stated, that molten magma from beneath the earth’s crust could ooze up between plates and as this hot magma cooled, it would expand and push on either side of it. He also proposed subduction as a mechanism for recycling of the seafloor. His theory provided a mechanism for continental movement that Wegener’s model was lacking.

• Samples of the deep ocean floor show that basaltic oceanic crust become progressively older as one moves away from the mid-ocean ridge. (Figure 3)

Figure 3: Crustal Age of the Ocean Floor. (from http://www.ngdc.noaa.gov /mgg/ image/images/ AtlanticAge.jpg)

• The rock making up the ocean floor is considerably younger than the continents – no rock samples older than 200 million years are found in the ocean crust while ages in excess of 3 billion years can be found in continental rocks.

• Paleomagnetism studies of the ocean floor demonstrate that the orientation of the Earth’s magnetic field has changed over time. (Figure 4)

Figure 4: A theoretical model of the formation of magnetic striping. New oceanic crust forming continuously at the crest of the mid-ocean ridge, cools and becomes increasingly older as it moves away from the ridge crest with the spreading of the seafloor: a. the spreading ridge about 5 million years ago; b. about 2 to 3 million years ago; and c. present-day (from http://pubs.usgs.gov/gip/dynamic/developing.html).

For detailed background on ocean floor mapping, magnetic striping and polar reversals, go to http://pubs.usgs.gov/gip/dynamic/developing.html

Plate Tectonics
Since the early 1960s, the emergence of the theory of plate tectonics started a revolution in the Earth Sciences. The theory has revolutionized our understanding of the dynamic planet upon which we live. Unifying the study of Earth, the theory has drawn together the many branches of earth science, from paleontology (the study of fossils) to seismology (the study of earthquakes) to volcanism and mountain building. It provides explanations as to why earthquakes and volcanic eruptions occur in very specific areas around the world, and how and why great mountain ranges like the Alps and Himalayas formed.

The theory of plate tectonics states that the Earth’s rigid outermost layer (lithosphere) is fragmented into seven major plates and over a dozen smaller plates that are moving relative to one another as they ride atop the hotter, more mobile material of the asthenosphere (Figure 5 and 6). The primary force responsible for the movement of the plates is heat transfer which sets up convection currents within the upper mantle.

Figure 5: The layer of Earth we live on is broken into a dozen or so rigid plates (from http://pubs.usgs.gov/gip/dynamic/slabs.html)

The boundary between these lithospheric plates is where most of the action (earthquakes) takes place. Three primary plate boundaries exist (Figure 6);

• Divergent boundaries – where new crust is created as the plates pull away from each other (mid-ocean ridge)
• Convergent boundaries – where crust is recycled back into the mantle
• Transform boundary – where plates slide horizontally past one another

Figure 6: Artist's cross section illustrating the main types of plate boundaries (see text); East African Rift Zone is a good example of a continental rift zone. (Cross section by José F. Vigil from This Dynamic Planet -- a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.) (from http://pubs.usgs.gov/gip/dynamic/Vigil.html)

For simulations showing animated divergent (mid-ocean ridge) boundary, convection in the mantle and seafloor spreading, and convergent (subduction) boundary, go to http://scign.jpl.nasa.gov/learn/plate4.htm

Earthquakes
Hold a wooden pencil at its ends and push up with your thumbs in the middle. The pencil will bend with little stress placed upon it. However, apply too much stress and the pencil snaps – rapidly releasing its stored energy. The rocks of the lithosphere act in a similar manner to the pencil. Due to relative plate motion, rocks of the lithosphere are under considerable stress. An earthquake is a phenomenon that results from the sudden release of stored energy in the Earth’s crust that generates seismic waves. The boundaries between Earth’s plates are where earthquake (and volcano) occurrences are concentrated (Figure 7).

Figure 7: World seismicity map revealing earthquake occurrences are concentrated in zones around the world (from http://earthquake.usgs.gov/regional/world/seismicity/index.php).

Each and every earthquake generates Primary (P-wave) and Secondary (S-wave) seismic waves. P-waves are compression or longitudinal waves that travel the fastest of all seismic waves. P-waves travel through solids, liquids, and gases. S-waves are shear or transverse waves which travel slower and pass through solids only.

Figure 8: The different types of motion in P and S waves http://www.exploratorium.edu/faultline/basics/waves.html

For more information the science of earthquakes and characteristics and behavior of seismic waves, go to http://www.earthquake.gov/learning/kids/eqscience.php

Figure 9: Earth’s internal structure as inferred from the study of seismic waves http://www.seed.slb.com/en/scictr/watch/living_planet/beneath.htm

To learn more about seismic waves and Earth’s interior, go to http://www.solarviews.com/eng/earthint.htm

USGS facts about earthquakes can be found at, http://earthquake.usgs.gov/learning/kids/facts.php

back to top

Content Benchmark E.12.C.2

Students understand the concept of plate tectonics including the evidence that supports it (structural, geophysical and paleontological evidence). E/S

Common misconceptions associated with this benchmark:

1. Students incorrectly believe that the continents randomly drift about the Earth or that the continents are no longer moving.

Continental Drift, the supercontinent Pangaea, and plate tectonics are likely terms with which students are familiar, however the idea that continents are still on the move today offers a challenge to students and adults. Considering the two timescales involved - human timescale (say 10,000 years of civilization) of observation is far too limited compared to the processes of plate tectonic occurring on a geologic timescale (tens and hundreds of millions of years).

For more on this and other misconceptions related continental drift visit
http://departments.weber.edu/sciencecenter/earth%20science%20misconceptions.htm
http://k12s.phast.umass.edu/~nasa/misconceptions.html

For more on slow but continuous change, go to
http://www.project2061.org/publications/textbook/mgsci/report/Glencoe/GLEN_es2.htm

2. Students incorrectly believe that California will split apart from the rest of the United States and become an island (or fall into the Pacific Ocean), leaving parts of Southern Nevada with oceanfront property.

The San Andreas Fault is a transform plate boundary that exists between the North American Plate and the Pacific Plate. This means that the land west of the San Andreas Fault is sliding northwest past the rest of the United States, towards San Francisco. This sliding does not create any space between the two plates for water to fill in, in fact the two plates are actually being pushed together by the two plate’s relative motions as they slide horizontally by each other (VERY slowly).

For more on this and other misconceptions related to earthquakes go to Earthquake Country Southern California’s website, separating fact from fiction, at
http://www.earthquakecountry.info/10.5/MajorMovieMisconceptions/

3. Students incorrectly believe that Earth’s crust is several 100’s of kilometers thick.

Earth’s crust consists of two types; continental and oceanic. The less dense continental crust is the thickest, having an average thickness of approximately 30 km while the much thinner and more dense oceanic crust has an average thickness of approximately 5 km. In fact, Earth’s crust occupies less than 1% of Earth’s total volume and represents the extent to which the deepest wells drilled have not exceeded.

http://www.nagt.org/files/nagt/jge/abstracts/Steer_v53p415.pdf

For details about Earth’s Interior go to, http://pubs.usgs.gov/gip/dynamic/inside.html

back to top

Content Benchmark E.12.C.2

Students understand the concept of plate tectonics including the evidence that supports it (structural, geophysical and paleontological evidence). E/S

Sample Test Questions

1. What type(s) of evidence did Alfred Wegener use to support his theory of Continental Drift?

a. Earth’s magnetic field reversals discovered from ocean floor samples.
b. Molten material in the lithosphere pushed the continents in various directions.
c. Rocks of similar age, type, and structure found on widely-separated continents.
d. Core samples from the mantle of the Earth.

2. Why was Alfred Wegener’s theory of Continental Drift rejected by the scientific community in the 1920’s?

a. The earth was thought to be too young for such movements.
b. Lack of a mechanism for continents to plough through oceanic crust.
c. The discovery of underwater mountains made continental drift unlikely.
d. The continental crust would have sunk as it drifted into the oceans.

3. The diagram below is a portion of Earth’s interiorFigure reference:

Figure reference: http://www.nysedregents.org/testing/scire/sciarch/psestestja02.pdf

The arrows shown in the asthenosphere represent the inferred slow circulation of the plastic mantle believed to drive plate motion. This process is called

a. Convection
b. Conduction
c. Radiation
d. Insolation

4. The diagram below is a portion of Earth’s interior

Figure reference: http://www.nysedregents.org/testing/scire/sciarch/psestestja02.pdf

Which layer contains the deepest wells humans have drilled to directly observe Earth’s internal structure?

a. Crust
b. Mantle
c. Outer Core
d. Inner Core

5. The diagram below is a cross-section view of Earth which shows seismic waves traveling from the focus of an earthquake. Points A and B are located on Earth’s surface.

Figure reference: http://www.nysedregents.org/testing/scire/es106.pdf)

Which statement best explains why only one type of seismic wave was received at location B?

a. S-waves cannot travel through the liquid outer core
b. S-waves cannot travel through the liquid inner core
c. P-waves cannot travel through the solid outer core
d. P-waves cannot travel through the solid inner core

6. Compared to the speed of S-waves in a given Earth material, the speed of P-waves is

a. always slower
b. always faster
c. always identical
d. sometimes slower and sometimes faster

7. A seismic station received the P-waves generated by an earthquake but did not receive any S-waves. Which statement best explains the absence of S-waves?

a. The earthquake had a relatively small magnitude and therefore did not produce any S-waves..
b. The earthquakes epicenter and focus were at the same location.
c. The S-waves were absorbed by a fluid layer as they traveled toward the seismic station.
d. All of these are possible reasons.

8. Scientists have classified Earth’s internal structure into four zones based primarily on evidence gained by studying

a. Earthquake seismic waves
b. Gravity measurements
c. Deep drill cores
d. Volcanic eruptions

9. Which information indicates that new seafloor rock is forming along the mid-ocean ridge and then moving horizontally away from the ridge?

a. Most volcanoes are located under ocean water.
b. Paleomagnetic studies of the ocean floor demonstrate that the orientation of Earth’s magnetic field has remained constant
c. Fossils of marine organisms can be found at high elevations on continents.
d. The age of the seafloor rock increases as the distance from the mid-ocean ridge increases.

Students understand the concept of plate tectonics including the evidence that supports it (structural, geophysical and paleontological evidence). E/S

Answers to Sample Test Questions

1. (c)
2. (b)
3. (a)
4. (a)
5. (a)
6. (b)
7. (c)
8. (a)
9. (d)

back to top

Content Benchmark E.12.C.2

Students understand the concept of plate tectonics including the evidence that supports it (structural, geophysical and paleontological evidence). E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. This Dynamic Earth: The Story of Plate Tectonics
by W. Jacquelyn Kious and Robert I. Tilling. An outstanding resource covering the historical perspective, development the theory, understanding plate motion, and plate tectonics and people. It is available as an online edition and as a downloadable PDF edition (77 pages, 3.7MB) at
http://pubs.usgs.gov/gip/dynamic/dynamic.html

2. Plot that Quake!
The goal of this exercise is to motivate students to question why earthquakes occur where they do. Students plot earthquake data over time in order to discover that a pattern develops in the occurrence of earthquakes worldwide.
http://seismo.berkeley.edu/istat/ex_quake_plot/

3. Berkeley Seismological Laboratory
This site is a great starting place which contains a worldwide earthquake catalog, along with links to United States seismic data, and a map of California and Nevada earthquakes. http://seismo.berkeley.edu/faq/catalog_0.html

Link to education and outreach activities,
http://seismo.berkeley.edu/iup.overview.html

4. Earth Science Education
Professor Larry Braile of Purdue University has complied links with a great deal of activities, simulations, teaching modules, and investigations targeting earthquakes and plate tectonics. http://web.ics.purdue.edu/~braile/educindex/educindex.htm

5. IRIS (Incorporated Research Institutions for Seismology
The IRIS website contains a host of lesson plans and resources for educators along with earthquake maps, lists, and interactive software. To access IRIS
Education and Outreach, go to http://www.iris.washington.edu/about/ENO/index.htm

6. Southern California Integrated GPS Education Module
For simulations showing animated divergent (mid-ocean ridge) boundary, convection in the mantle and seafloor spreading, and convergent (subduction) boundary, go to
http://scign.jpl.nasa.gov/learn/plate4.htm
Associated activities,
http://scign.jpl.nasa.gov/learn/activity.htm

7. Earthquakes/tectonics
This site is a great educational resource, which provides students with numerous links for studying continental drift, plate tectonics, the structure of the Earth, earthquakes, and seismic waves. The following is a link to animations, simulations and additional teaching resources within the science of geology. http://www.scienceman.com/pgs/00_links_geology.html

back to top