Exploring geological processes with a Deformational Sandbox
by Mary Ellsworth, Earth System Science Teacher, Laurent Clerc Center
[ after reading, return to the survey here ]
This photo shows a deformational sandbox, a design developed by the University
of Massachusetts Geoscience Department for students in K-12 classrooms.
The sandbox has a stationary backwall (to the left), a movable wall
(shown cranked nearly to the backwall), two side walls with Plexiglases
Students layer sand in the box and then crank the movable wall either
in (for contraction) or out (for extension) to learn how the Earth's
crust deforms under these conditions. When we run a contraction
experiment we attach a metal plate to the movable wall (not shown
here), which causes the faulting to begin more towards the middle of
the box instead of against the moving wall.
Concepts addressed with sandbox activities:
reverse or thrust fault
fault block mountains
writing and reporting
A Sandbox Experiment: Contraction
Contraction happens when the Earth's crustal plates collide.
Examples of questions for earth science students:
When the land is compressed....
What kind of faults will develop?
Where do the new faults start and what is their angle?
What happens to the older faults?
What will the surface of the Earth look like?
Students layer colored sand in the sandbox. A base of children's
play sand is poured in first, and then smoothed. Colored layers
at the edges along the viewing windows, mainly to conserve on the more
expensive sand, but the color cannot be seen away from the
windows anyway. After each colored 'layer' more play sand is layered in across the width of the box.
Here students have finished laying in the sand (six layers here) and
are now ready to run a contraction experiment. About 7 cm of sand
are used for a contraction experiment (as shown).
The sandbox set up with play sand reasonably models the Earth's
crust on a scale of 1 cm= 1 km, which means each grain of sand is about
the scale of one city block. It has been demonstrated that dry
quartz sand can reasonably approximate the mechanical properties of the
upper crust. [Cooke]
Students here are shown cranking the movable wall. After 20
turns of the crank they stop and observe the faults, measure the angles,
and draw what they see. Then they crank another 20 turns.
The photo on the right shows the results after about 60 turns, with a
with multiple faults. Six or seven reverse faults are clearly visible cutting across the colored layers. On the surface students can
observe fault block mountains and uplift.
Students make notes and drawings in their notebooks and label the
wedge, the back thrust fault, the toe of the wedge, the 1st, 2nd, 3rd,
etc faults to form. They also use a protractor to measure the
angles of the new faults and compare with the older faults. We
measure dip angles of new faults to be very close to 30 degrees, similar to
what is found in the Earth's crust. We observe that the new faults
are activated towards the front edge of the wedge, but after a certain
amount of compression the fault stops slipping and a new fault is
activated in front of it. Older faults tend to rotate but not
slip any further. These dynamics model well the actual dynamics
in the Earth's
crust, giving the students a three dimensional visual tool for
understanding how faults develop and interact, and how what is on the
surface (bird's eye view) relates to what is underground (cross sectional view).
A Sandbox Experiment: Extension
Extension happens in places where the Earth's crustal plates are pulling apart.
This photo shows the sandbox set up for an extension experiment. Two metal sheets
are attached, one each to the backwall and the movable wall. A rubber sheet is duct
taped between the two edges. The rubber helps to distribute the force pulling the
two plates apart under extension.
Here are examples of questions for students for an extension experiment.
Where do new faults form and what is the angle (dip), under extension?
(Are the angles the same as in compression?)
How does the width of the depression change as the land masses pull apart?
The students layer in colored sand, smoothing each layer
carefully. For an extension experiment the initial layers will
only need to be 4-5 cm deep.
On the right, the results after extension. A series of six normal faults are clearly visible from the side view. On the surface a basin and range
topography has developed. In the sandbox we find the dip angles of
these faults are regularly in the range of 70 degrees, similar to the
normal faults found in the Earth's crust.
Students use a protractor to line up the faults, and measure the angles. They tend to
like to use a ruler and marker to outline faults they see right on the window of the sandbox.
Again they then draw what they see in their notebooks and identify the faults, label
the angles, forces, etc.
Finally students will summarize what they learned about two kinds of faulting that
they have observed and draw comparisons between the two kinds of experiments.
More Sandbox experiments
The sandbox as a model of the Earth's crustal movements provides students the opportunity
to investigate many different questions. Students can vary the experiment and ask
different questions. These experiment can be run with various materials (flour, clay) or
with wet sand. Students can add a block of wood of various shapes in the middle of the
moving wall to study what different shapes of moving plates might do in the Earth's crust.
A short list of other questions that can be explored:
happens if part of the wedge is eroded while under contraction?
(simulates erosion of mountains)
- What if friction is different for one side of
the sandbox? (increase basal friction by adding a sand paper base to
part of the box)
- Suppose we run the experiment without the
rubber base sheet? (simulates extensional ridges within oceanic
crust at spreading centers)
- Where would be the safest place to build a nuclear power plant when known faults are in the area?
[ Return to the sandbox survey here ]