PASADENA, CALIF., NOV. 7 -- The Magellan spacecraft's pictures of Venus suggest immense volcanic eruptions once spewed gargantuan floods of lava that may have deluged more than half the planet, a NASA scientist said today.

"It seems to have happened all at once in the past, maybe 400 million years ago" as molten rock erupted from numerous cracks called vents on the Venusian surface, said geologist Steve Saunders, Magellan's chief scientist at Jet Propulsion Laboratory.

The other possibility is that a series of somewhat smaller lava floods, each covering hundreds of thousands of square miles, inundated various parts of Venus during different times, he added.

Saunders said the possible global-scale flows of molten rock were similar to, but much larger than, the volcanic cataclysm on earth that formed the Columbia River basalts 12 million to 20 million years ago. The basalts are huge solidified lava flows that cover much of the Pacific Northwest east of the Cascade Range.

The lava flows covering Venus's lowland plains also resemble the Deccan Traps, lava beds that cover much of west-central India, Saunders said. The Deccan flows were formed when Earth's crust split open as India drifted away from Africa about 66 million years ago, allowing huge volumes of molten rock to pour onto the landscape.

Saunders said Venus's oceans of lava covered "maybe as much as 60 percent of the planet," all of its lowland plains.

"There's no question that the vast majority of the surface of Venus has been formed by lava flows and volcanic activity," said Brown University geologist Jim Head, a Magellan scientist. "But the question is did it all happen catastrophically?"

Head added, "At this point, I don't think we have enough information to say these are global outpourings" during a single event.

Saunders said he suspected Venus was subject to massive volcanic activity because the planet has fewer large meteorite impact craters than the moon, and because "the ones we find are all relatively fresh." That suggests "there's a process of very widespread volcanism that occurs from time to time that destroys all the craters -- and everything else," Saunders said.

Magellan uses radar to make pictures of cloud-covered Venus. The spacecraft was launched from space shuttle Atlantis in May 1989, went into orbit around Venus on Aug. 10 and formally started making maps and pictures of the planet on Sept. 15.