Without equipment to detect the eruption, airplane passengers just might find themselves living a high-altitude nightmare. In 1989, a Boeing 747 flew through an undetected ash cloud in Alaska. All four engines shut down and the airplane went into a nose-dive. It descended 13,000 feet before the pilots were able to restart the engines. Hundreds of thousands of people fly across the West Coast and above active volcanoes every day.

Eruptions in Alaska and California would also be felt across the nation. Anchorage is a major cargo hub, meaning that many FedEx or U.P.S. packages travel through Alaska. But an eruption might bring that to an alarming halt. And because California produces a large portion of the nation’s food, an eruption might limit the fruits and vegetables found at supermarkets as far as the East Coast.

“We’re not just doing this for academic purposes. This is so we can give good information to emergency managers,” Dr. Driedger said. “That’s the end in all of this.”

Hoping for slumber

Despite the permit’s recent approval, Dr. Driedger notes that there are still a number of steps before any instruments can be placed on Mount Hood. They will now have to choreograph the assembly of instruments, hire personnel and schedule helicopter trips around weather and other potential obstacles.

Moreover, the Forest Service and the observatory could still face a legal challenge from Wilderness Watch or other groups that adds years to the installation, if not blocking it altogether.

“This is more proof that the Forest Service has abandoned any pretense of administering wilderness as per the letter or spirit of the Wilderness Act,” said Mr. Macfarlane, whose group is discussing litigation with an attorney but has not yet decided whether to file suit.

And then there is more work to be done monitoring other hazardous volcanoes beyond Mount Hood.

Volcanologists across the nation were pleased this March when Congress passed the National Volcano Early Warning and Monitoring System Act, which seeks to ensure that volcanoes nationwide are adequately monitored.