The interiors of most coronae contain numerous, smaller volcanic features, such as lava flows , calderas , shield volcanoes , domes , and lava channels . The genesis of coronae appears to be associated with the arrival large mantle plumes beneath the Venusian lithosphere. The progressive evolution of these features involves both uplift and collapse, either concurrently, in sequence, or in cycles. Ring fractures develop as the surface bulges outward above the rising mantle plume, but subsequent relaxation of the uplift often results in collapse of the corona's interior along the concentric fractures. The abundant volcanic features found in the interior reflect melting of the plume head and subsequent extrusion of these melts at the surface.
Arachnoids are smaller than most coronae, but have a similar genesis.
They features differ from coronae in that they contain a network of both radial and concentric structures that gives the appearance of a spider sitting on interconnected webs of bright lines, hence the name arachnoid, which is Greek for spider . The radial structures appear to be dike intrusions that stand out as ridges. Although it is thought that both coronae and arachnoids form in association with rising plumes of mantle, the magmatism associated with arachnoids may be largely intrusive, whereas magmatism associated with coronae is largely extrusive. There have been 265 arachnoids thus far identified on Venus.