Page 85 - Laboratory Manual in Physical Geology

ACTIVITY 2.9 The Origin of Magma

Name: ______________________________________ Course/Section: ______________________ Date: ____________

A. Examine the pressure-temperature (P-T) diagram for PROCEDURES FOR MELTING EXPERIMENT
mantle peridotite in FIGURE 2.8 , and locate point X . IN ACTIVITY 2.9

This point represents a mass of peridotite buried 80 km
underground. 1. Turn the hot plate on a low setting (about 2 or 3 on

1. According to the continental geothermal gradient, most commercial hot plates) and allow it to heat up in
rocks buried 80 km beneath a continent would a safe location (be careful not to touch hot surfaces
directly).
normally be heated to what temperature?
2. Next, place two sugar cubes on a flat piece of
aluminum foil or in aluminum foil baking cups. Label

2. According to the oceanic geothermal gradient, rocks (on the foil) one sugar cube “dry.” Moisten the second
buried 80 km beneath an ocean basin would normally sugar cube with about 4 or 5 drops of water and label
be heated to what temperature? it “wet.”
Dropper

3. Is the peridotite at point X a mass of solid, a mixture
of solid and liquid, or a mass of liquid? How do you Aluminum foil
know?

DRY
WET
4. What would happen to the mass of peridotite at point

X if it were heated to 1750 °C?
3. Carefully place the aluminum foil with the labeled
sugar cubes onto the hot plate and observe what
happens. When one of the sugar cubes begins to

5. What would happen to the mass of peridotite at point melt, use crucible tongs and/or hot pads to remove
X if it were heated to 2250 °C? the foil and sugar cubes from the hot plate and
avoid burning the sugar.
Hot surface
Aluminum foil
B. At its current depth, the peridotite at point X in FIGURE 2.8

is under about 25,000 atm of pressure.
DRY

1. At what depth and pressure will this peridotite begin WET
to melt if it is uplifted closer to Earth’s surface and its
temperature remains the same?

Depth: Pressure:

2. What is the name applied to this kind of melting?

Low setting

3. Name a process that could uplift mantle peridotite to 4. Turn off and un-plug the hot plate as soon as you

start it melting in this way, and name a specific plate finish #3 above. Be careful not to touch the hot
tectonic setting where this may be happening now. surface!
( Hint: Study FIGURES 2.2 , 2.5 , and 2.6 .)
FIGURE 2.9 Procedures for melting experiment in

Activity 2.9. Be careful not to burn yourself on the hot surface or

in molten sugar. Do not create a hazard by burning the sugar until
it smokes or catches fire.
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