Scientists have recorded a series of small explosions at the Cleveland Volcano on Alaska's Aleutian islands, causing the Alaska Volcano Observatory to issue an "orange" aviation color code — although no flights have been restricted.

According to the observatory, satellite infrasound detected the explosions starting at 5 a.m. Saturday.

"Satellite and webcam data suggest continuous low-level emissions of gas, steam, and minor amounts of ash over the past several hours with a faint plume extending eastward below 15,000 feet," according to a bulletin issued by the observatory. "Satellite data also show highly elevated surface temperatures at the summit.

"Sudden explosions of blocks and ash are possible with little or no warning. Ash clouds, if produced, could exceed 20,000 feet above sea level."

The Cleveland Volcano is on the remote Aleutian Islands chain and is 940 miles from the Alaska's largest city of Anchorage. It's about 45 miles from the community of Nikolski. Because there are no seismic monitors on the volcano, there can be a delay in detected any new activity. Distant seismic and infrasound instruments are used.

The perfectly conical volcano is considered to be exceptionally active. It is 5,675 feet tall.

Two months ago, the ever-simmering volcano let out an ash plume, although Chris Waythomas, a geologist with the observatory, said that ash burp was nothing alarming -- or even out of the ordinary -- for Cleveland.

The last real activity from Cleveland came in November of last year, when it spat an ash cloud to between 18,000 and 22,000 feet in a single event. It was downgraded again a little more than a week later. Cleveland's last big eruption came in February 2001, producing ash clouds that rose a whopping 39,000 feet above sea level.

Volcanic ash is a big problem for aviation: the 2001 Cleveland eruption led to a number of potentially dangerous encounters with aircraft, according to an American Meterological Society paper on that event.

"Onboard radars can only occasionally detect concentrated ash within or near eruption plumes," noted the paper authors, explaining why aviation experts and researchers must keep a close eye on volcanic eruptions. "Only total avoidance of the ash ensures ?ight safety."