The Alaska Volcano Observatory on Tuesday elevated the alert status for Cleveland Volcano.
Officials say the new lava dome was spotted in the summit crater. The observatory says as of Monday, the dome was about 130 feet in diameter.
There have been no eruptions since Dec. 25 and Dec. 29, which destroyed the earlier lava dome built up over the fall.
Cleveland is a 5,675-foot peak on an uninhabited island 940 miles southwest of Anchorage.
Authorities say sudden eruptions could occur at any time, and ash clouds 20,000 feet above sea level are possible.