How do rock circles and soil polygons form in tundra regions?

In tundra regions, rock circles and soil polygons are types of patterned ground that form primarily due to the freeze-thaw cycle affecting the active layer of soil.12

Formation Processes

• Frost Sorting and Heaving: As the ground undergoes daily and seasonal temperature fluctuations, the constant expansion and contraction of water within the soil move coarser particles toward the surface. Further frost heaving—the upward displacement of ground caused by freezing moisture—arranges these rocks and stones according to their size.134
• Development of Patterns: When these sorted materials form circular arrangements, they are known as stone rings. As neighboring rings coalesce, they create polygonal stone nets. In these formations, coarser materials like pebbles and boulders typically form the perimeter, while finer materials are concentrated in the center.14
• Environmental Factors: These features are particularly common in areas with fine-grained sediments and poor drainage. While permafrost is not strictly essential for all patterned ground, it is frequently present in tundra regions and contributes to the saturated conditions that facilitate these movements.324

On steeper slopes, these circular or polygonal patterns may be stretched into stone stripes due to the slow, gravity-driven downhill motion of the waterlogged active layer, a process known as solifluction.124