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What are Earthquakes?

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Abstract

Plate tectonics is a theory that describes the movement of the Earth's crust. The Earth's outer shell consists of individual plates, which interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself. Because each plate moves as a distinct unit, interaction among individual plates occurs along their boundaries. In this activity, students use graham crackers, frosting and Styrofoam to explore the different types of plate boundaries: Divergent, Convergent and Transform.

Developed as part of a workshop convened by Catherine French by
Leslie Bucar, 7-12 Science Teacher, Fond du Lac Ojibwe School, B.S. Biology, B.A.S. Teaching Biology, B.A.S. equiv Teaching Chemistry

Dan Johnson, 7-12 Math Teacher, Fond du Lac Ojibwe School, B.A.S. Teaching Math

 

 

 

 

 

 

 

Introduction

Introduction:

If you imagine the earth like an apple, there are three main parts: the core, the mantle (the "white" part of the apple), and the crust (the skin of the apple). Earth's rigid outer shell, the lithosphere, is made up of the crust and the top part of the mantle. The asthenosphere is the plastic-like layer below the lithosphere. According to the plate tectonic model, the lithosphere is broken into vast slabs called plates.

Plate tectonics is the idea that the Earth's outer shell consists of individual plates, which interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.

These plates are slowly and continually moving. This movement is caused by hot material moving up from deep within the Earth unevenly and spreading over the asthenosphere, setting the plates in motion. This is called a convection current.

When plates move, they can interact in several ways. Because each plate moves as a distinct unit, interaction among individual plates occurs along their boundariesDivergent boundaries occur where plates move apart, leaving a gap between them. The gap then fills with the asthenosphere from below, and cools, forming new crust. Convergent boundaries occur where plates move together, forcing one of the slabs of lithosphere to descend beneath the other or both get pushed up. Transform boundaries occur where plates grind past each other, scraping and deforming as they pass.

When plates move, it puts stress on the rocks near the plate edges. To relieve this stress, the rocks tend to bend, compress, or stretch. This stress often builds up over time, as the plates move relative to each other and the stress is built up along the edges. When the built-up stresses become large enough, the plates will break and move. An earthquake is the vibrations produced by the breaking of rock. The location where the plates break or move is called a fault. Faults interrupt rock layers by moving them out of place.

There are three kinds of faults. One kind pushes the rocks together, one pushes them apart, and in one, the rocks slide past each other. In a normal fault, there is a pull-apart force and rock above the fault surface moves downward in relation to rock below the fault surface. In a reverse fault, the rock gets squeezed and the rock is forced over and up. In a slip-strike fault, rocks on either side of the fault move past each other without much upward or downward movement. The San Andreas fault in California is a slip-strike fault.

Earthquake Engineering Component

Earthquake engineers must consider fault movement when designing a structure that spans a fault.  For example, a pipeline that crosses a fault must be able bend so that it doesn't fail when the fault moves during an earthquake.  

See how researchers at the NEES facility at Cornell University test a pipeline that crosses a fault to determine if it will survive an earthquake: http://nees.cornell.edu/gallery/videos.cfm?id=6

Learning Objectives and Standards

 

Links to the National Science Standards and to individual State Science Standards are available by using this link:

 

http://nees.org/education/for-teachers/k12-teachers#standards

 

Compare and contrast different types of plate boundaries.

Content Standards:

National:

Structure of the Earth System

Minnesota State:

History and Nature of Science 7-Scientific World View

History and Nature of Science 8- Scientific World View

History and Nature of Science 7-Scientific Inquiry

History and Nature of Science 8- Scientific Inquiry

Earth and Space Science 8-Earth Structure and Processes

Material List

  • 1 graham cracker per student
  • 1 piece of Styrofoam per student
  • 1 cup of water per student
  • Frosting
  • Wax paper
  • Plastic knife or spoon

Procedure

Procedure:

PART 1 DIVERGENT PLATE BOUNDARIES (Oceanic and Oceanic Plates) 1. Break a whole graham cracker in half following the perforations on the cracker. You should have two graham cracker squares. 2. Using a plastic knife/spoon, spread a thin (about the thickness of a playing card) layer of frosting on the center of the wax paper. 3. Lay the two pieces of graham cracker on the frosting and gently pressing down push them together. Now you are ready to begin modeling a divergent plate boundary. 4. To imitate the result of diverging oceanic plates, press down lightly on the crackers as you slowly push them apart. Do not push the crackers more than one centimeter. 5. Make observations and answer questions for Part 1.

PART 2 CONVERGENT PLATE BOUNDARIES (Continental and Oceanic Plates) 1. Remove one of the graham crackers from the frosting. 2. Lay a graham cracker and the styrofoam piece end to end (against each other) on top of the frosting. The graham cracker represents the thin but dense oceanic plate and the styrofoam represents the thicker but less dense continental plate. Now you are ready to model a convergent plate boundary. 3. Slowly push the graham cracker and styrofoam toward each other. Gently push the graham cracker under the styrofoam. This models what occurs on Earth’s surface as an oceanic plate is subducted under a continental plate. 4. Make observations and answer questions for part 2.

PART 3 CONVERGENT PLATE BLOUNDARIES (Continental and Continental Plates) 1. Reuse the two graham cracker squares used in Part 1 and Part 2. 2. Break the graham cracker into four rectangles following the perforations. Set aside two graham cracker pieces for Part 4. 3. Using two graham cracker rectangles, dip one end of both graham crackers about 2 centimeters into a cup of water. Immediately remove the crackers and lay them end to end on the frosting with the wet edges nearly touching. Now you are ready to model a convergent plate boundary. 4. Slowly push the two graham crackers together, modeling the convergence between two continental plates. The wet end of the graham crackers will curl and fold upwards as the graham crackers are pushed together. 5. Make observations and answer questions for part 3.

PART 4 TRANSFORM PLATE BOUNDARIES 1. Use the last two graham cracker pieces for this part of the lab. 2. Fit the two pieces together end to end on top of the frosting on the wax paper. Now you are ready to model a transform plate boundary. 3. Place one hand on each of the graham crackers and push them together by applying steady, moderate pressure. At the same time, also push one of the pieces away from you while pulling the other toward you. If you do this part correctly, the graham cracker should hold while you increase the push-pull pressure, but will finally break form the opposite forces. 4. Make observations and answer questions for part 4.

Links and Resources

Leslie Bucar, 7-12 Science Teacher, Fond du Lac Ojibwe School, B.S. Biology, B.A.S. Teaching Biology, B.A.S. equiv Teaching Chemistry

Dan Johnson, 7-12 Math Teacher, Fond du Lac Ojibwe School, B.A.S. Teaching Math

Feather, R., Snyder, S., and Zike, D. 2002. Earth Science. Glencoe McGraw-Hill, New York, NY.

Lutgens, F. and Tarbuck, E. 1998. Essentials of Geology. Prentice Hall, New Jersey.

Marshall, R. and Rosskopf, A. 1992. Earth Science. Media Materials Publishing,Maryland.

http://www.inghamisd.org/~goals2k/lessons/sciencelesson/tippin3.html

http://www.atropos.as.arizona.edu/aiz/teaching/nats102/images/faults.gif

Assessment

Collecting and Analyzing Data Key:

Part 1 Divergent Plate Boundaries 1. What happens to the frosting between the graham crackers, how does it look?

The frosting is forced up between the graham crackers, answers will vary

 

2. What do each of the graham crackers represent?

Crustal plates/lithosphere

 

3. What does the frosting represent?

asthenosphere

 

Part 2 Convergent Plate Boundaries 1. What type of plate does the styrofoam represent?

Continental plate

 

2. What type of plate does the graham cracker represent?

Oceanic plate

 

3. What happens when a piece of crust goes under another piece of crust (called subduction)?

It melts

 

Part 3 Convergent Plate Boundaries 1. What does the graham cracker represent?

Crustal plates/lithosphere

 

2. In what way are the wet graham crackers more like the real crustal plates than are the dry graham crackers?

They are able to bend

 

3. What feature is represented where curling and folding occurred at the ends of the wet graham cracker?

Mountains

 

Part 4 Transform Plate Boundaries 1. Why is this movement often described as "horizontal sliding?"

The plates move past each other horizontally with neither crustal plate moving up or down

 

2. What famous fault is associated with this type of movement? The San Andreas Fault

 

3. As you modeled this type of fault, nothing happened at the beginning, but as the pressure increased, the graham crackers finally broke. How is this similar to the situation in California?

The pressure is building up, until there is a release, resulting in an earthquake

Cite this work

Researchers should cite this work as follows:

  • Catherine Ellen French; NEES EOT (2011), "What are Earthquakes?," http://nees.org/resources/3828.

    BibTex | EndNote

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