National Geographic : 1993 Feb
VOLCANISM More than 80 percent of Venus is covered with lava flows and plains and other volcanic features. It is easy to see why: The planet has 430 volcanoes 12 miles or more in diameter and tens of thou sands of smaller ones. Magellan saw arachnoids (left), which are surrounded by spiderweb-like fractures. Similar to other circu lar features called coronae, arachnoids most likely form when magma rises to just below the surface, causing the ground to bulge and fracture. Cooling and retreat of the magma result in collapse of the center. Meandering north of Aphro dite Terra is a channel (below) a mile wide and 4,225 miles long. Probably etched into the rocky crust by molten lava, it would reach from New York to Rome.