arachnoid: An unusual volcanic feature found only on Venus. An arachnoid is a roughly circular pattern of cracks. Some cracks are formed one inside the other. Some form radiating spokes from the center. Viewed from space, the cracks look a little like a spider web. The Greek word for spider is "arachne." The Greek word for something related to a spider web is "arachnoeides." Arachnoids were probably formed by magma that swelled upward. The magma created a bulge in the surface. Then the magma drained away. The bulge collapsed leaving a pattern of fractures. Arachnoids are similar to coronae, but smaller.
