The Moving Plates

By completing this lesson, the learner will:

• describe characteristics of lithospheric plates
• use latitude and longitude, negative integers, angle measure, decimals
• perform calculations using the computer
• determine absolute and relative plate motion rates

Concept Introduction - Part I

Long before animals began growing bony plates, the crust of the Earth itself took the form of gigantic rocky plates.
The Earth's crust is not frozen in place. The crust is composed of about nine plates that grind and crash into one another in titanic slow motion collisions. The term plate tectonics refers to the formation, lateral movement, and eventual destruction of crustal plates.

The theory of Plate tectonics has important consequences, including the explanation of earthquakes and volcanoes.

Long ago, there was a supercontinent that we now call Pangaea. All of today's continents were part of that giant continent. When Pangaea started to split apart 200 million years ago and its pieces began to drift away, today's continents were born. Examine a map or globe of the world and see if you can imagine how today's continents might once have fit together to form the ancient continent of Pangaea. Make a sketch of Pangaea as you imagine it might have been.

• The Absolute Plate Motion Calculator computes the velocity of a given point on a given plate relative to a hotspot reference system. An example is as follows:

• For Chicago, Illinois, Latitude 44N, Longitude 87W

Name of Plate = North America
Angular velocity = -11.10 degree/million years
Velocity = 2.55 cm
Direction = 242.72 degrees
• Interpretation of data

Velocity and direction of the plate at the given point is the only information needed by the students to mark their maps.

The Relative Plate Motion Calculator provides information about the movement of one plate with respect to an adjacent plate. Two adjacent plates are selected and the longitude and latitude for the point of contact at which the relative motion is desired. One plate is considered moving while the other is considered fixed. The results of the calculation show the velocity (in cm/year) and direction of the moving plate. Students may then draw a vector on a map showing the direction and speed of the moving plate. By reversing the fixed/moving plate data on the calculations, the student will see that the opposing plate's relative movement is at the same velocity in the opposite direction. An example is as follows:

• For Latitude 32S, Longitude100W ...

Relatively fixed plate = Pacific
Relatively moving plate = Nazca
Angular Velocity = 1.42 degree/million years
Velocity = 15.77 cm
Direction = 95.58 degree
• For the same latitude/longitude but reversing the plates ...

Relatively fixed plate = Nazca
Relatively moving plate = Pacific
Angular Velocity = 1.42 degree/million years
Velocity = 15.77 cm
Direction = 275.59 degrees
• Interpretation of data ...

The velocity and direction of the moving plate will provide the information needed by the student to mark their maps. The latitude and longitude entered confirms the given location. Note that the direction of each plates relative movement is the complement of the other, and the velocity is identical.

Scientists use vectors to represent plate motion on maps. A vector used in this situation is an arrow pointing in the direction of plate motion with a length proportional to the plate speed. Getting the arrow correct involves constructing an angle and a length.

• Since plates move a few centimeters per year, it is natural to make the arrow the same length as the plate movement. In the case of the absolute plate motion at Chicago, the arrow should be 2.55 cm in length.
• Because direction is given in degrees, a protractor is used to construct the angle corresponding to the plate's direction of movement. In the case of the absolute plate motion at Chicago, the arrow should point in a direction approximately 242 degrees (measured in a clockwise direction) from north.

• Macintosh computer with Internet access for every 4 students
• WWW Browser
• Metric rulers
• Protractors
• Atlas
• Student world maps showing plates, longitude, and latitude
• Student maps of the United States showing plates, longitude, and latitude
• Overhead transparency of map

• Explain Absolute Plate Motion to students as the motion of a specific location on a plate relative to fixed features in the Earth's underlying mantle.
• Review velocity and angle measurement/construction so that student maps may be properly marked with direction and movement of each position from calculations.
• Have students find the longitude and latitude for the following cities using an Atlas or the U.S. Gazetteer:
• Management note: Divide the class into groups and assign several cities to each group.

City                            Longitude                       Latitude
Boston MS
Washington DC
Atlanta GA
Miami FL
Detroit MI
St. Louis MO
Kansas City KS
Houston TX
Denver CO
Santa Fe NM
Salt Lake City UT
Seattle WA
Reno NV
Sacramento CA

Using the Absolute Plate Motion Calculator, find the direction and speed of plate motion at each city.

City                                    Direction                       Speed
Boston MS
Washington DC
Atlanta GA
Miami FL
Detroit MI
St. Louis MO
Kansas City KS
Houston TX
Denver CO
Santa Fe NM
Salt Lake City UT
Seattle WA
Reno NV
Sacramento CA

Have students draw a plate motion arrow for each city on their map of North America. The "tail" of the arrow should be positioned on the city and the "head" should point from the city in the direction of plate motion. The length of the arrow is the number obtained from the Absolute Plate Motion Calculator, measured in centimeters.

Using an overhead transparency, confirm the location, velocity and direction of each position for students to check their maps.

Teacher-Centered Discussion Questions

Which of these cities is moving the fastest?
Is the east or west coast of the U.S. moving faster?
Does plate motion get faster or slower as you go towards the leading edge of the plate?
Why is it possible for different parts of the U.S. to move at different speeds?

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Concept Expansion

Explain Relative Plate Motion to students as being the motion of one plate with respect to another. Take the map link below and select a point on the boundary between two plates, such as 30N latitude and 43W longitude. Enter these data in the Relative Plate Motion Calculator, using +30 for latitude and -43 for longitude and selecting the North America plate as Relatively Fixed and the Africa plate as Relatively Moving. The result, 2.28 cm/yr moving on a heading of 102.95 degrees, indicates that, from the perspective of an observer standing on the edge of the North America plate at that location, Africa is moving off to the southeast at 2.28 cm/yr. Indicate this on a trasparency using an arrow 2.28 cm in length having a reference angle of 103 degrees.

Have the students do similar calculations using the North America plate as the Relatively Fixed Plate. As they complete each calculation, add their arrows to the transparency.

Analyze data with discussion questions.

Which plate in contact with the North America plate is moving the fastest/slowest?
Which plates are moving towards/away from the North America plate?
Which type of boundary occurs at each point?
Which plate boundaries would represent a subduction zone?
How many years would it take for each plate to move 1 meter? One kilometer?