Mount Rainier is an active stratovolcano 45 miles southeast of Tacoma and 60 miles south-southeast of Seattle. Its last eruption was "a light dusting of ash" in 1894, the USGS said in a report. Large eruptions haven't occurred for more than 1,000 years.

The U.S. Geological Survey said it and other agencies were monitoring the situation and would issue additional alerts "as needed."

A swarm of hundreds of small earthquakes have been detected rumbling deep below Mt. Rainier in Washington State early Tuesday morning, though geologists stress there is no cause for concern.