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6. Gelatin Volcanoes
Purpose
To understand how and why magma moves
inside volcanoes.
Background
Magma is molten rock, including crystals and dissolved gases, found
at depth in a planetary interior. When magma erupts onto the surface,
the volcanic products make distinctive landforms including lava plains
and volcanoes, depending on the details of the eruption. One of the
most interesting things to consider about magma is how it moves up from
underground reservoirs, called magma chambers, to erupt as lava on planetary
surfaces. Does it travel in natural tubes or pipes? Or along fractures?
This experiment strikingly reveals the answer.
Magma leaves underground reservoirs through
fractures in the surrounding rock. The fractures are either pre-existing
or are created by the erupting magma. An active dike is a body of magma
moving through a sheet-like, vertical or nearly vertical fracture. An
important aspect of magma flow not dealt with in the gelatin activity
is the heat lost during eruption. Magma, ascending as a dike begins
to cool and solidify and the flow may become localized in the dike.
Such localized eruption of magma over a long period of time produces
a volcano. Stresses in the planet affect the orientation of dikes. Dikes
open (widen) in the direction of least resistance. They propagate (grow
longer and taller) perpendicular to the direction of opening. Hawaiian
shield volcanoes are characterized by concentrated regions of dike injections,
called rift zones. A series of experiments using gelatin models was
conducted by researchers in 1972 to explain the growth
and orientation of Hawaiian rift zones.
The "Gelatin Volcanoes" classroom activity was inspired by this work.
This Activity
Gelatin, molded in bowls or bread pans, is used as transparent models
of volcanic landforms. Colored water is used as the dike-forming magma.
In this activity, dikes tend to propagate radially from the center of
bowl-shaped casts of gelatin because the resistance to opening is the
same in every direction. Dikes tend to parallel the long-axis of ridge-shaped
(bread pan) casts of gelatin because the narrow dimension provides less
resistance to opening than the long dimension. The dike opens in the
narrow dimension and we see propagation in the long dimemsion. With
a slow, steady injection rate, the colored water creates a dike and
generally erupts from the flanks or ends of the gelatin casts. Edge-on,
a dike appears as a line. When the gelatin cast is sliced through with
a knife, dikes appear as red lines in the vertical, cut edges.
Materials
Unflavored gelatin, 28 gm (one-ounce) box
containing four packages; Spoon; Bowls or bread pans, either one 2-liter
(or 2-quart) capacity, or smaller sizes; Red food coloring, to mix with
water in a glass to make "magma" ; Syringe for injecting magma, best
to use a plastic variety found at pet stores for feeding birds; Peg
board, 40 x 60 cm, with 5-mm-diameter holes spaced 2.5 cm apart; Two
bricks, 30 cm high; Large knife to cut through the gelatin model; Tray,
for collecting drips; Rubber gloves (optional) for protecting hands
from food coloring.
Preparation
Prepare magma by mixing water in a glass with enough red food coloring
to make a very dark liquid. Gelatin requires at least three hours of
refrigeration to set. Use a warm water bath to free the gelatin from
the bowl without getting water on the gelatin itself. Unflavored gelatin
is ideal for this experiment because of its transparency. Sweetened
gelatin desserts also work. If you prefer the dessert variety, then
use a flavor that is easy to see through, such as lemon. Another alternative
is agar. Agar hardens at room temperature, eliminating the need for
refrigeration, but it must be made so it is easy to see through. Two-liter
(or two-quart) capacity bowls work very well because the diameter allows
enough space for multiple dike injections. This size is large enough
for demonstration purposes. Smaller bowls, down to the size of margarine
containers, have also been used successfully.
6. Moon Archeology |
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8. Lava Layering
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