BACKGROUND:
Stress is an important geologic concept to understand.
It is important to distinguish the geologic meaning of this word from its
other meanings, such as anxiety and tension. In geology, a stress is a force
that is applied to a geological material, whether it is a tiny mineral
crystal or a gigantic plate.
Stresses that build from plate motion eventually
causes earthquakes. When the earthquake occurs, some of the stress is
relieved. If the stress accumulates a rupture or fault will occur. As the
plates continue to move, stress builds up in the rocks again. More
earthquakes will take place, but the ruptures this time will tend to happen
along the fault. It is an existing zone of weakness in the rocks.
There are three basic ways stress is applied to rocks
within the lithosphere. First, rocks can converge or be compressed. Second,
rocks can be pulled apart, or diverging. This is the opposite of converging.
Finally, rocks can be slip slide pass each other. Squeezing is most common
at converging plate boundaries, pulling apart at diverging plate boundaries,
and shearing at transform plate boundaries.
PROCEDURE:
- Define "plate" to the class. Explain that plates are large
areas of the Earth's outer portion (crust and upper mantle) that move
together.
- Explain the concept of stress in rocks to the class. Define the three
basic types of stress to the students. You can demonstrate these with
the wooden blocks from the Pre Lab, or use a sheet of paper. Converging
a piece of paper from both sides crumbles the paper toward the center.
Pulling on opposite sides of it is extension. Holding opposite sides and
moving one hand up and the other down is shear.
Explain that once a fault exists, it is a weak point in the crust. It
is likely to become the location of future crustal breakage.
- Here are answers and information for the lab exercise:
EXERCISE I. Have the students follow the directions on the lab sheet.
The glue ball represents a portion of the Earth. The glue ball is made
of a polymer used to insulate telecommunication cables. This material
"has memory". In other words, it will return to its original
shape after a stress is applied to it. Emphasize to the students that
the stresses applied to the glue balls will cause the "fault"
to reappear.
EXERCISE II. This exercise illustrates that under similar stresses,
different substances respond differently. For example, ask the students
if the rocks they observed in previous class would react differently if
subjected to similar stresses. The answer is no, with respect to the
stress a human can put on a rock, but yes in terms of geologic stresses.
EXERCISE III. It is important for the students to start hearing the
vocabulary words associated with stress and plate boundaries. Stress is
very important to understand but hard to explain. Letting them
experience stress by playing with the clay will help them to understand
the concept.
This illustrates the 3 major types of stress effecting the plates by
using hand motion and clay.
- Simulates "plates" moving past each other at a transform
plate boundary such as the San Andreas Fault.
- Simulates "plates" diverging from each other, such as
the Mid Atlantic Ridge.
- Simulates "plates" converging toward each other such as
the Himalaya Mountains, where the Indian and Asian plates collide.
