Pressure1 is an important factor in geological systems.
Under pressure, rocks can flow or fracture. It is the pressures within
the mantle that creates diamonds - at the surface, they are actually
unstable and slowly turn into graphite! This pressure is due simply to
the weight of the overlying rocks, called lithostatic
pressure by geologists. The pressure 1 km deep in the Earth is
27 MPa; the pressure of a blanket of only four meters of rock is
greater than one atmosphere! But just how great is one atmosphere of
The Magdeburg Hemisphere Experiment
In 1654, before an audience consisting of nobles, scientists and
Emperor Ferdinand III,
Otto von Guericke dramatically demonstrated the tremendous effects
of one atmosphere of pressure. Von Guericke placed two empty
copper hemispheres (called Magdeburg hemispheres) together and removed
the air from between them using an air pump (one of his numerous
inventions). Despite the fact that less than one atmosphere of pressure
was holding the bowls together, two teams of horses could not separate
We can replicate his experiment in the classroom by having students attempt to pull a standard plunger off a desk. (Commercial versions of the Magdeburg hemispheres are available, but we prefer our more prosaic substitute.)
To do this experiment, you will need:
1. Rub a little Vasolinetm on the rim of the plunger. Place it flat on the table and push it so that as much air is expelled as possible.
2. Invite a student (husky football players work well) to pull the plunger off of the desk. When the student fails, remove the plunger by tilting it to allow a little air into the bell.
What is the force holding the plunger to the table top? How might you measure this force? (HINT: What is the heavist thing that the plunger can lift?)
The effect of depth on pressure
While we have clearly shown the effects of just one atmosphere of
pressure, how does pressure change with depth? Does it increase?
Decrease? Stay the same? In the mid-1640's Blaise
Pascal performed a series of experiments that clearly showed the
increase in pressure with depth. One of his more elegent beautifully
illustrates this effect using water.
To replicate this experiment, you will need:
1. Remove the top from the bottle. Remove the strip of tape. Point out the distance that each stream of water travels in a horizontal distance.
Why does the water at the bottom travel farther out than that at the top? (HINT: Think about the force being exerted on the water.) Pressure at the bottom of a column of rock can be found from , where is the density of the rock, g is the acceleration due to gravity and y is the height of the colummn of rock. What would the pressures be at (a) the bottom of the continental crust (35 km), (b) the 670 km discontinuity, and (c) the bottom of the mantle? What might these extreme pressures do to rocks?
1. NOTE - Pressure is measured in a bewildering array of units. The most common are:
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This page designed by John DeLaughter email@example.com Update: Jan 02 1998