Demonstration goals:
- Show the effects of one atmosphere of pressure
- Show that pressure increases with depth
- Understand the implications of this for Earth systems
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 pressure?
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 them.
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:
- One toilet plunger (preferably unused)
- Vasolinetm or vacuum grease (Water will also work, though not as well)
- Table or other flat, hard surface
- Paper towels for clean-up
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.
For Discussion:
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:
- One empty bottle, 1 gal {4.4 l} or greater
- Water
- Basin to catch the water
- Nail
- Hammer
- Tape
Before the demonstration: Using the nail and hammer, punch three holes into the side of the bottle. One hole should be about 2.5 cm {1 inch} from the top, one an equal distance from the bottom and one in the center. Cover the holes with a strip of tape. Fill the bottle with water, and place over the bowl. Replacing the top on the bottle will help keep it from spilling. (You may want to sit the bowl on a table and raise the bottle with a book.)
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.
For Discussion:
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:
| Unit Name | Unit Abbreviation | Equivalent to | ||||
| Atmosphere | atm |
|
||||
| Bar | bar |
|
||||
| Pascal | Pa |
|
||||
| Torricelli | torr |
|