Arachnoid (astrogeology)

In astrogeology, an arachnoid is a large geological structure resembling a spider web. They are of unknown origin, and have been found only on the surface of the planet Venus. They appear as concentric ovals surrounded by a complex network of fractures, and can span 200 kilometers. Over 90 arachnoids have been identified on Venus, so far. 
The arachnoid might be a strange relative to the volcano, however, it is also possible that different arachnoids are formed by other processes. One possible explanation is that an upwelling of magma from the interior of the planet pushed up on the surface, causing cracks. An alternate theory concerning their origin is that they are a precursor to coronae formation.
Much of what is known about arachnoids is the result of studies performed by C.B. Dawson and L.S. Crumpler.
External links.
"This article incorporates text from a public-domain NASA website."