BRITTLE VS. DUCTILE ROCK DEFORMATION

Marcia Bjornerud
Geology Department
Miami University
Oxford, OH 45056


Level: Grades 3 - 6

Estimated time required: 30 minutes or less

Anticipated learning outcomes

Background

Rocks are forced to change shape at tectonic plate boundaries, where great slabs of Earth's tectonic plates collide, move apart, or scrape past each other. Depending on a variety of factors, this shape change (deformation) can be accomplished either by brittle behavior (slip along fractures or faults) or by ductile behavior ("flowing" of solid rock). Through time these processes can build great mountain ranges like the Rockies, the Appalachians and the Himalayas.

Two of the most important variables governing material behavior are temperature and the rate of deformation. High temperatures and slow deformation rates favor ductile behavior, while low temperature and fast deformation rates lead to brittle behavior. In most continental areas, the change from brittle to ductile behavior occurs at a depth of 15-20 km in response to the progressive increase in temperature with depth in the subsurface (the geothermal gradient). As a result, earthquakes, caused by sudden slip on faults, generally originate at depths of 20 km or less. An important exception to this are the very deep (up to 680 km) earthquakes that occur at subduction zones, where cold plates with oceanic crust plunges slowly back into the otherwise hot mantle.

Materials

Procedure

  1. Refrigerate overnight the wax, tootsie rolls, other material.
  2. Discuss how rocks may change shape either brittlely or ductilely; they can break or they can fold. Ask students how they think this happens. Temperature and rate of pressure being applied are important variables.
  3. Have students experiment with how the wax, or other material responds when pulled, pounded and flexed. As the material warms in their hands (or, in the case of candy, mouths?) they will note that its behavior changes.
  4. Once the material is softened, have students experiment with different rates of shape change. They will find that if they pull the material apart quickly, it will snap, but if they pull it slowly, it can be stretched much further before breaking. The same is true for rocks.
  5. After the students have finished with the wax or other material, let them examine the rock specimens or pictures and infer which were deformed brittlely in the cold upper crust and which deformed ductilely at the higher temperatures of the deep crust. To illustrate further the idea that solid rock can flow ductilely over long periods of time at slow deformation rates, students could roll some soft putty into a ball and set it aside overnight. The next day, the ball will have flattened under its own weight.

Results and Discussion

There are many possible directions for discussion following the students' investigations:

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