Story highlights Observatory: Pavlof volcano "continues to erupt," with "fire fountaining" at its summit

A "continuous" cloud of ash, steam and gas extends 30 to 60 miles downwind

Alerts are in effect for that volcano and another on the Aleutian range named Cleveland

Alaskans and air travelers remained on alert Thursday due to the rumblings of a more than 8,000-foot volcano emitting a "continuous ash, steam and gas cloud" that already extends up to 60 miles away.

The Alaska Volcano Observatory noted Thursday that the Pavlof volcano "continues to erupt," with "fire fountaining" at its summit 8,261 feet above sea level.

A resulting cloud that extends downwind 30 to 60 miles (50 to 100 kilometers) moved southeast on Thursday morning over the Gulf of Alaska. It reaches an altitude of about 20,000 feet above sea level.

"Satellite images show persistent elevated surface temperatures at the summit and on the northwest flank, commensurate with the summit fire fountaining and resulting lava flow," the observatory said in its Thursday update.

A lava fountain, the U.S. Geological Survey explains , is "a jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in molten rock." It typically spews lava 10 to 100 meters up, though it sometimes goes as high as 500 meters.

Remote but still connected to the Alaskan mainland, Pavlof is one of two volcanoes -- the other is named Cleveland -- that are on "watch" status due to heightened activity. They're also both under an orange code that relates to how their rumblings might affect planes flying over their summits. Both these alert levels are the second most serious out of four options, according to the Alaska Volcano Observatory.

Less was known Thursday about Cleveland, which is on an island and not monitored with ground instruments like Pavlof.

Two days ago, the observatory reported a 100-meter-wide swath of lava reaching about 1 mile down its southeastern flank. "Cloudy conditions" have made it difficult for satellites, and thus scientists, to assess its status over the last 24 hours.

Even so, the observatory continues to warn that "sudden explosions of blocks and ash are possible with little or no warning."

A SIGMET, short for significant meteorological information, advisory from the National Weather Service remained in effect Thursday around the volcanoes.

After "eruptive activity" last year at the Cleveland volcano, University of Alaska Fairbanks scientist Steve McNutt said 90% of air freight from Asia to Europe and North America flies over Alaska's airspace, and hundreds of flights fly through Anchorage's air space daily.

"We think of the Aleutian Islands as being remote and desolate," USGS scientist John Power told CNN earlier this week, "but when you come up to 30,000 feet, we are talking about 20 to 30,000 people there every single day."

Power described Pavlof as "one of the most historically active volcanoes in the Northern Hemisphere." Cleveland is also "very active," with its last large eruption in 2001.