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Earth Science > Subduction > Cookie Subduction
 

Site: http://www.exploratorium.edu/faultline/activezone/cookie.html

Grade Category: High School

Subject Category: Earth Science

Sub Category: Subduction

COOKIE SUBDUCTION

  • Eating an oreoThis is a quick demonstration/activity to show how large amounts of rock and sediment are added to the edge of continents.
  • What do I need?
    • Cream centered cookie (e.g. Oreo, Hydrox)
  • What do I do?
    1. Pull apart the cookie.
  • 2.Use the part with the most cream center still attached.
  • 3. With the cream side up, slowly slide the cookie into your mouth. While sliding, the upper front teeth should scrape off the creaming filling. The creamy filling should be plastered onto your front teeth when you're done.
  • What's going on?
    When an oceanic plate dives under continental plates, layers of the sea floor are often scraped off and plastered onto the edge of the continental plate next to it. This process of adding oceanic material to the edge of a continental plate is called accretion. It's an important process in the building of continents. Much of the west coast of the Americas is composed of accreted rocks or terranes.

    The diagram below shows how the edge of the continental crust bulldozes off the top layers of the subducting oceanic crust. In the cookie analogy, your teeth do the bulldozing, scraping off cream filling rather than sediments.

convergence-usgs

A diagram of subduction, showing how an oceanic plate dives under a continental plate. Lithosphere and asthenosphere are layers of the crust.

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Earthquake Model


  • This webpage details the model made by Ross Stein and demonstrated most recently at the May 2000 USGS Open House by the Earthquake Hazards Team. The website has been constructed due to the great public interest shown at the Open House, particularly by teachers.
  • This website includes a TV documentary film clip, animation and photographs of the model 'in action', a lengthy model description, and associated diagrams.  It also includes technical specifications (bottom of this page) to aid those wishing to build their own model.

link to video

See the earthquake machine in action

Click on image to see the earthquake machine in action.
Video from: Eye on the Bay, The Science of Predicting Earthquakes,
courtesy of Brian Hackney (CBS 5)


  • Earthquake hypotheses that can be explored with the model
     
  • Hypothesis 1: Earthquakes are periodic (in other words, all of the same slip, and all separated by the same amount of time). There is some evidence for this, particularly among very small earthquakes on creeping faults.

    Hypothesis 2: Earthquakes are 'time-predictable' (this means that the larger the slip in the last earthquake, the longer the wait until the next one.) This idea was formulated in the 1980's by Shimazaki and Nakata in Japan, and has been widely used.

    Hypothesis 3: Earthquakes occur randomly in time and and have randomly varying size. (This 'Poisson' hypothesis is also widely used, particularly when little information about a fault and its past earthquakes is available).

    These hypotheses are briefly explained in:
  • R. S. Stein,
  • Parkfield's unfulfilled promise (News & Views), Nature, 419, pp. 257-258, 19 September issue, doi:10.1038/419257, 2002.
  • [Printable article (300 kb)]
  • Model Description
     
  • The following is a description of Ross Stein's "spring and rider" (also known as the "brick and bungee") earthquake simulation machine.
  • The apparatus consists of a wooden board 3-4 feet in length with a winch on one end. You can buy a small trailer winch from an auto parts store, a trailer supply store, or perhaps at Home Depot. There is a pulley leading from the winch to the brick that is oriented so that the force acting on the brick has no vertical component; this is not required but is helpful. The board has a strip of adhesive-backed sandpaper down half of it to increase friction. Buy the nonskid at Home Depot type stores. Don't use the rubber-surfaced non-skid; instead it should have sand grains in it. The best material for the bungy cord is surgical rubber tubing, which you can buy from some pharmacies. If you can’t find this, you can use standard bungy cords.
  • Two types of material connect a brick to the winch. The first is a non-stretching cord, which is used to ensure that all of the accumulated stress is transferred into the second material - surgical rubber tubing.  This tubing is connected directly to the brick and is extremely elastic, which allows stress to build when the crank on the winch is turned.  The brick also has a strip of sandpaper on one side, for additional friction.  This is the basic set-up of the experiment.
  • With this, students can mark off "rupture length" during each "earthquake" by seeing how far the brick slips.  They likely will find that the lengths are not consistent.  They may also wish to time the "earthquakes" assuming a constant speed by the person turning the crank. Again, time is not always consistent either.  If they turn the crank slowly as the cord nears "failure" they may hear the sandpaper crackle a moment before the brick moves, thus simulating a foreshock.
  • There are some accessories to this model as well.  The first is talcum powder, which can be sprinkled on the board next to the sandpaper. When the brick is placed on the powder (the side of the brick without the sandpaper) students will observe an almost constant rate of motion.  This simulates creeping faults, such as is found on part of the San Andreas. An additional brick is included in the demonstration as well.  This may be stacked atop the first brick to produce larger "earthquakes."  It can also be removed from atop the first brick just at the moment of failure to show that slippage can occur even without additional stress, just by removing the additional brick.
  • Finally, the second brick is equipped with surgical tubing as well, so the two can be placed on the board in tandem.  In this way, the machine demonstrates how earthquakes "talk" to each other.  When stress is sufficient, the first brick moves forward, increasing stress on the second brick.  Eventually the second brick slips, reducing the backwards force on the first brick, and the first brick can slip again.
  • One additional accessory for the machine is a mass balance that can be used as a strain gauge.  Although it was previously shown that earthquakes are not consistent with time or in their rupture length, the brick tends to slip at the same reading on the gauge. With this set-up and the various accessories, this demonstration can show a variety of earthquake concepts.  If the students use the mass balance to weigh the bricks, they can calculate the coefficient of friction on the board, and predict what force is necessary to cause an "earthquake."
  • Despite being relatively simple and elegant, the machine is remarkably  true to the actual earth.  Thus students get a fun, hands-on look at stress and rupture  in the laboratory.


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  • Earthquake machine diagrams
  • Earthquake machine diagrams

  • Technical Specifications
  • Earthquake machine diagrams
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  • Close-up pictures

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