Passage V

Sorted circles (soil mounds 1-3 m wide bordered with rocks) are common in arctic regions (see Figure 1).

Two different processes combine to create sorted circles. Convection is the movement of water and soil due to temperature and density differences. When the water in the soil thaws and approaches 4°C, it sinks, forcing the 0°C water underneath upward. This lifts the soil surface and creates the soil mounds, as shown in Figure 2. Rocks that are on the surface roll down into the depressions and form the rock borders.

This convection is not capable of bringing rocks to the surface. Rocks are brought to the soil surface by frost heaving. Each time the temperature of the soil decreases to 0℃, the water around the top of rocks in the soil freezed first, pulling the rocks up with it. Fine clay particles fill in the space beneath so that the rocks end up in a slightly higher position than where they began.

Study 1

During spring thaw, a geologist moved rocks from the sorted circle's border to the center of the surface of the soil mound. By the next winter, the rocks had moved back to the border, and the frozen soil at the center of the surface of the mound was slightly higher than it had been during the thaw.

Study 2

A geologist observed the surface of a liquid that was being heated from below (see Figure 3). In the lighted areas, liquid was rising, and in the darker areas, liquid was sinking. It was noted that the liquid's surface resembled the group of sorted circles when viewed from above.

Study 3

A geologist buried a rock at a known depth in a container of damp soil. After the soil in the container had been frozen and thawed 4 times, the rock was 3 cm above its original position.

Figures adapted from Stefi Weisburd, "Halos of Stone." ©1985 by Science Service, Inc.