The most violent of the eight active volcanoes discovered last March on Io, a moon of Jupiter, was found today to be dormant.

Curiously, the inactive Ionian volcano was the only one of four photographed by the approaching Voyager 2 spacecraft today that was not pouring a long plume of black ash into space. If anything, the three other volcanoes photographed were erupting with more force than they were in March when they were found by Voyager 1.

"We don't know why the largest one has turned itself off and the others have not," Dr. Laurance Soderblom of the U.S. Geological Survey said today at the Jet Propulsion Laboratory where the Voyager flight is directed. "Io's volcanoes could be like geysers here on earth that shut down when they exhaust their reservoirs and don't start up again until they refill them."

Soderblom said Voyager 2 photographs reveal that the dark lava lake surrounding the now-dormant volcano on Io has changed shape since March. In March, It was heart shaped. Today, it is more circular, suggesting the volcano was still erupting and laying down lava long after Voyager 1 left Jupiter Four months ago.

In March, the now-silent volcano was photographed sending a black plume of ash 220 miles into space above Io. At the time, its plume was almost 100 miles higher than any of the other seven active volcanoes photographed by Voyager 1.

Now moving at 37,000 miles an hour and less than 1.5 million miles from Jupiter, the 1.800-pound Voyager 2 will swing by Jupiter on Monday at 7:21 p.m. EDT when the spacecraft will be 448,000 miles from the giant planet. Voyager deputy project manager Esker Davis said today that the spacecraft would not be more than 10 miles from the spot it was targeted for when it was launced from Cape Canaveral almost two years ago.

"We're aiming for a bull's eye of an ellipse that's about 70 by 80 miles," Davis said. "The way it looks today, Voyager will be a little bit high and a little bit off the center of that ellipse.'

All through the day on Monday, Voyager 2 will photograph the four large Jovian moons Io, Europa, Ganymede and Callisto, seeing the opposite side of each moon than Voyager 1 saw four months ago. This is because Voyager 2 will swing by the moons as it heads in toward the planet. Voyager 1 flew by the four moons as it was outward bound from the planet.

As Io was the prime target for Voyager 1, Europa will be the principal focus of Voyager 2. Criss-crossed by dozens of huge cracks scientists believe are canyons caused by violent upheavals in its crust, the yellow-colored Europa began to look today like an over-ripe grapefruit.

Photographs taken today by Voyager 2 showed that Jupiter's Great Red Spot had altered its shape and color since March. Looking smaller and distinctly more orange this time, the red spot was nonetheless accompanied by a large white oval just south of it in the top of the planet's atmosphere.

"Strangely enough," Soderblom said, "This is a different oval than we saw in March in just about the same place. There are three white ovals in this zone, and that one that was below the red spot in March, today is on the other side of the planet." CAPTION: Picture, Callisto, a Jupiter moon, photographed yesterday, with meteorite pockmarks. AP