A study based on satellite images of African grasslands describes the circles’ life span, but the reasons for them are still unknown.
The circles, which range from about 6 to 30 feet in diameter, begin as bare spots on an otherwise continuous grass carpet; after a few years, taller grass starts to grow around the circle’s perimeter. This may be because the bare land “isn’t using the water, so the grass on the perimeter ends up taller,” Dr. Tschinkel said. Years of constant winds then blow soil from the circle, leaving a dish-shaped depression in the ground. Eventually, the circle is regrown with vegetation until it is no longer visible.
So while the reasons for the circles are still unknown, Dr. Tschinkel says his study has raised several crucial questions: “Why are they regularly distributed, rather than random or clumped; why do they appear with the grass dying suddenly; why is there taller grass at the perimeter; and why is there a difference in diameters?” Until then, he continued, “the mystery of the fairy circles, or at least what causes them, remains a mystery.”
Via The QI Elves