At the Norris Geyser Basin last summer, parts of unpaved trails were dissolved by acidic groundwater.

YELLOWSTONE NATIONAL PARK, Wyo. — The rolling pine forests, snowcapped mountains and crisp fall evenings here tend to make people forget the fact that the park sits atop a huge simmering underground volcano. But new geologic events have served up reminders.

In a few days in July, acidic ground water dissolved parts of the unpaved trails in the Norris Geyser Basin, and the ground temperature of the trails shot up to 200 degrees from the usual maximum of 80. Park officials closed nearly half of the basin's trails, and they remain shut.

On Aug. 21, a magnitude 4.4 earthquake shook the southern boundary of the park and startled residents. Yellowstone is one of the most seismically active places on the planet, with hundreds of shakes and shimmers throughout the year. They reach magnitude 4 usually only every other year.

In the park, such events are no great surprise. "Change is what we expect in Yellowstone," said the park geologist, Hank Heasler.

Although there is no indication that any of the changes suggest an impending eruption, even that would not be so surprising.

Over last 630,000 years, Yellowstone has experienced 29 eruptions the size of the one on Mount Pinatubo in the Philippines in 1991. The average interval here has been 20,000 years, and 70,000 have passed since its last eruption.

But the volcano, with a caldera 45 by 28 miles, has the potential for far more catastrophic explosions. The last major eruption, estimated at a magnitude 1,000 times as great as the Mount St. Helens explosion of 1980, was 627,000 years ago. The ancient blast blew up miles of mountain range, and ash from it has been uncovered in 22 Western states. It was so thick 1,000 miles away in Kansas that it was mined in the 1930's and used to make a cleanser.

Whether the caldera erupts or not, the stew of partly molten rock 5 to 10 miles below the park exerts a powerful and constant influence.

"The whole of the Yellowstone Plateau is going up and down from the magma," averaging one and a half centimeters a year, said Dr. Robert B. Smith, a professor of geophysics at the University of Utah and a member of the Yellowstone Volcano Observatory. "It's like a living, breathing thing."

In light of the new activity, safety is a growing concern, and officials are writing a hazard plan in case the region grows more active. The ground warming could mean that heat is increasing water pressure, a possible cause of eruptions.

In 1989, Porkchop Geyser in Norris Basin became clogged with silica. It exploded and created a 12-foot-wide crater now called Porkchop Hot Spring.

A hydrothermal explosion at Mary Bay in Yellowstone Lake some 13,000 years ago blew out a crater more than three miles across.

Serious earthquakes are always a possibility. Even though the temblor on Aug. 21 caused no damage, it was widely felt.

The largest quake recorded in the West, 7.5 on the old Richter scale, was centered just outside the park in 1959. It dislodged a huge slice of a mountain west of the park, buried 25 campers as they slept in a national forest campground and dammed the Madison River to create Quake Lake.

One question that occupies geologists is how the caldera affects fault lines and vice versa. Five major faults terminate in the molten caldera, and even far-flung events can shake the earth here. In November 2002, a magnitude 7.9 quake in Denali National Park in Alaska rippled through the region, leading to more than 500 other quakes that Dr. Smith watched simultaneously on a computer in Utah.

"The whole of Yellowstone lit up like a Christmas tree," he said. "It was exciting. I had a ranger call me and say, `I've called you before about earthquakes, but these are coming at us from all directions.' "

Except for the quake two months ago, Yellowstone has had far fewer quakes in recent years. "Seismically, its been deathly quiet," Dr. Smith said. "We average a half-dozen to 20 quakes" a day. "The last two years, we see a couple a day."