How do stone rings form in permafrost regions?

Stone rings are a form of patterned ground created by the intense, repetitive freeze-thaw cycle that occurs within the "active layer" of soil in permafrost regions.123

The formation process involves several key mechanisms:

• Volume Changes: As water in the soil freezes and thaws, its constant change in volume stirs the ground.12
• Frost Heaving and Sorting: This repetitive action moves coarser particles—such as cobbles and boulders—toward the surface. Through further frost heaving, these rocks are sorted by size, with larger materials being pushed outward to form the perimeter of a ring, while finer sediments remain in the center.21
• Role of Permafrost: While these features can occur in any area with rigorous frost action, the underlying permafrost acts as an impermeable substratum. This prevents drainage, keeping the soil moisture necessary for these frost-driven processes to continue.143

When neighboring stone rings coalesce, they form larger stone nets, and on steeper slopes, these features may be stretched into stone stripes by the slow, gravity-driven movement of the saturated active layer.12