Arachnoids Arachnoids are smaller cousins of the coronas. Like coronas, they have a round ring of faults or ridges, but these rings lie inside a set of radial ridges. The rings range from about 50 km to 200 km in size, with the outer ridges running out another 200 to 400 km. Over 250 arachnoids have been mapped, and they tend to cluster near both coronas and other arachnoids. Also, like the coronas, arachnoids are rarely found in the lowest plains.
Arachnoids look like coronas and form near coronas.
Thus, they are thought to form in much the same way as coronas. They are smaller than most coronas, however, and they tend to show fewer lavas. Thus, they probably formed over smaller plumes. Since smaller plumes should have less magma and should cause less uplift, this model seems to fit the facts. However, the lack of lava flows also suggests that there are more intrusions in arachnoids than in coronas.