BRITTLE VS. DUCTILE
Oxford, OH 45056
Level: Grades 3 - 6
Estimated time required: 30 minutes
Anticipated learning outcomes
- Students explore the difference between brittle and ductile
deformation in everyday materials and how material behavior is influenced
by temperature and the rate of deformation
- Students discover that the same concepts apply to rocks
and are related to the occurrence of earthquakes and mountain ranges at
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.
- Silly putty, beeswax, candle wax and/or Tootsie Rolls.
- Specimens of brittlely and ductilely deformed rocks or
pictures of deformed rocks
- Refrigerate overnight the wax, tootsie rolls, other material.
- 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
- 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
- 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.
- 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:
- The cause and distribution of earthquakes and why earthquakes
occur (or do not occur) in the local area.
- How great mountain ranges are built by the bending, fracturing
and crumpling of rocks where tectonic plates collide.
- How solid materials like rock and glacial ice can flow
over long periods of time.