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 thegrowth 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.

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