7. Lava Layering

Purpose

To learn about the stratigraphy of lava flows produced by multiple eruptions.

Background

Dark, flat maria (layers of basaltic lava flows) cover about 16 percent of the Moon's total surface. They are easily seen on a full Moon with the naked eye on clear nights from most backyards. The maria, quite similar to Earth's basalts, generally flowed long distances utlimately flooding low-lying areas such as impact basins. Yet, the eruption sources for most of the lunar lava flows are difficult to identify. The difficulty in finding source areas results from burial by younger flows and/or erosion from meteoritic bombardment.

Generally, the overall slope of the surface, local topographic relief (small cliffs and depressions), and eruption direction influence the path of lava flows. Detailed maps of the geology of the Moon from photographs reveal areas of complicated lava layering. The study of rock layering is called stratigraphy. On the Moon, older flows become covered by younger flows and/or become more pocked with impact craters.

On Earth, older lava flows tend to be more weathered (broken) and may have more vegetation than younger flows. Field geologists use differences in roughness, color, and chemistry to further differentiate between lava flows. They also follow the flow margins, channels, and levees to try to trace lava flows back to the source area.

This Activity

The focus of this activity is on the patterns of lava flows produced by multiple eruptions. We use a short cup to hold the baking soda because we are looking at the flows and not at constructing a volcano model. Volcanoes, like those so familiar to us on Earth and Mars, are not present on the Moon. Three well-known areas on the Moon interpreted as important volcanic complexes are: Aristarchus plateau, and the Marius Hills and Rumker Hills (both located in Oceanus Procellarum). These areas are characterized by sinuous rilles (interpreted as former lava channels and/or collapsed lava tubes) and numerous domes.

Materials

Paper cups, 4 oz. size, some cut down to a height of 2.5 cm; Cafeteria tray or cookie sheet, 1 for each; Eruption source; Tape; Tablespoon; Baking soda; Measuring cup; Vinegar; Food coloring, 4 colors; for example, red, yellow, blue, green; Playdough or clay in the same 4 colors as the food coloring.

Baking soda-vinegar solutions and playdough are used to model the basaltic lavas. Different colors identify different eruption events; this activity calls for 4 colors. Students will be asked to observe where the flows traveled and to interpret the stratigraphy. Cover the work area and be prepared for spills.

Play Dough (stove-top recipe)
Best texture and lasts for months when refrigerated in an air tight container.

1. 2 cups flour
2. 1/3 cup oil, scant
3. 1 cup salt
4. 2 cups cold water
5. 4 teaspoons cream of tarter
6. food colorings (20 drops more or less)

Make this large batch one color or divide ingredients in half to make 2 colors. You will need 4 colors total. Combine ingredients and cook mixture in a large sauce pan, stirring constantly, until the dough forms a ball. Turn dough out onto a floured surface to cool. Then kneed until smooth and elastic. Cool completely; refrigerate in air tight containers.

Play Dough (no-cooking recipe)

1. 2 cups flour
2. 2 Tablespoons oil
3. 1 cup salt
4. 1 cup cold water
5. 6 teaspoons alum or cream of tartar
6. food colorings (as above)

Make this large batch one color or divide ingredients in half to make 2 colors. You will need 4 colors total. Mix ingredients and kneed until smooth and elastic. Store in air tight containers.

Wrap-up

Have students compare their layered lava patterns with their classmates' patterns. Did they recognize individual flows by color and outline? Point out how the oldest flow is on the bottom of the stack. Each succeeding flow covers older flows. The youngest flow is on top.

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