Demonstrating Stress, Strain, Rate of Strain, and Viscosity (Grades 4-6)

The interplay of several forces and factors influence the final form of lava. This activity describes several terms introduced in this lesson. Stress is the force applied to a body. For example, the weight of your body is applying a stress to the seat of your chair. Strain is how a body responds to an applied stress. For example, if the seat of your chair is rigid (made of wood or steel), the applied stress is not great enough to cause any strain (deformation). However, if the seat has a soft cushion, the weight of your body (an applied stress) is great enough to cause strain, and the cushion bows under your weight. Rate of strain would be how quickly the cushion bows as you sit down.

Stress on lava is exerted by the force of the eruption, the force of gravity, or forces exerted during cooling. Strain is reflected by fragmentation near the vent or during flow, the folding or breaking of the crust on a flow, and/or cracking during cooling. Rate of strain is how quickly the flow deforms. Rate of strain may be high on steep slopes and low on gentle slopes.

Viscosity is the resistance to flow. Viscosity can be demonstrated by pouring different liquids down a board. Water, pancake syrup, and cold molasses work nicely. The viscosity lava is greatly influenced by composition and temperature. High silica lava at relatively cooler temperatures (compared to basalt) behaves as a sticky, slow-moving fluid. Low-silica Hawaiian basalts are hot; thus, they behave as fast-moving fluids.



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