Uncertainty Pervades Montserrat as Volcano Threatens : Caribbean: Worried scientists can’t predict if, or when, eruption will occur. Thousands flee island.

The limits of science can be seen on tiny Montserrat, where uncertainty about if or when a volcano may explode has led authorities to evacuate two-thirds of the Caribbean island’s 12,000 people to its north end or off the isle altogether.

Since mid-July, experts from the United States, France and Trinidad and Tobago have been in the 11-by-7-mile colony, a British possession 250 miles southeast of Puerto Rico, monitoring the volcano.

Experts are worried that it may blow soon.

Although the volcano has belched ash and steam, and a new vent opened this week pointing directly at the island capital of Plymouth--forcing even the scientists to move their station to a safer place--it remains uncertain what might happen next.

There could be a major eruption. On Friday, volcanologists warned that magma, or molten rock, has risen to within half a mile of the crater surface and may have given its “last warning” before exploding. Some put the chance of such an event at 70% to 80%.

There has not been a major eruption on the island in 16,000 years, and there have been three episodes of volcanic unrest in the past century that turned out to be false alarms.

The scientists--who have agreed that only local authorities will make public statements on the situation--have speculated at other times this week that the uncertainty could persist for months.

As in other volcanic locales, this puts the authorities under great political pressure. Local officials were quoted Friday as saying the evacuation order for residents was being re-evaluated daily. They suggested it could be lifted in two days if no eruption occurs.

Such a short timetable for possibly letting everyone go home makes the scientists even more uneasy, because there have been periods of quiet just before other catastrophic eruptions this century.

This happened at Mt. Pelee on the Caribbean island of Martinique, where tens of thousands of people died in a sudden blast May 8, 1902.

Five American scientists are on Montserrat representing the Volcano Crisis Assistance Team, based at the Cascades Volcano Observatory in Vancouver, Wash., a facility of the U.S. Geological Survey. The head of the team is C. Dan Miller, who has been on Montserrat for more than a month.

Daniel Dzurisin, director of the Cascades Volcano Observatory, said this week that the team’s experience has demonstrated the limits of volcanic monitoring. “We can say there’s danger, but we can’t say at what precise moment an eruption would occur, or how big it would be,” he said.

The volcano wasn’t the only threat facing Montserrat. Tropical Storm Iris, packing 65-m.p.h. sustained winds and hurricane-force gusts, was expected to hit Montserrat and other islands in the Lesser Antilles on Friday night.