Alert after eruption of Alaskan volcano Mount Pavlof

Mount Pavlof, one of Alaska's most restless volcanoes has shot an ash cloud 15,000 feet into the air in an eruption that is visible for miles.

An air traffic controller in the region said small planes have flown around the ash plumes.

The ash would have to rise to tens of thousands of feet to threaten larger planes.

The eruption began on Monday, and photographs show lava spraying out from the summit of the volcano, located 625 miles from Anchorage.

The Alaska Volcano Observatory said clouds of ash, steam and gas have occasionally reached the 20,000-foot level and have been visible from the nearby communities of Cold Bay and Sand Point.

Onsite seismic instruments have detected an increase in the force of tremors from the 8,262-foot volcano.

Typically, Pavlof eruptions are gas-rich fountains of lava that can shoot up to a few thousand feet, but its ash clouds are usually less dense than the plumes of more explosive volcanoes that pose a greater hazard to aircraft, scientists say.

This image shows a pyroclastic flow (superheated fluid mix of gas, lava and melted snow) racing down the slopes of Alaska's Pavlof volcano after it erupted May 13, 2013. 

CREDIT: NASA Earth Observatory

Pavlof is a 5,675-foot peak and among the most active volcanoes in the Aleutian arc, with nearly 40 known eruptions, according to the observatory.

The Aleutian Islands are about 625 miles (1,000 kilometers) southwest of Anchorage, Alaska

The volcano last erupted in 2007 and during the 29-day eruption, Pavlof emitted mud flows and erupting lava, as well as ash clouds up to 18,000 feet.

In early May, Cleveland Volcano, on an uninhabited island in the Aleutian Islands, experienced a low-level eruption.

Satellite imagery shows the volcano has continued to discharge steam, gas and heat in the past week.

New analysis of earlier images showed a small lava flow going over the south east rim of the summit crater, the observatory said.

Cleveland's most recent significant eruption began in February 2001 and sent ash clouds up to 39,000 feet above sea level. It also produced a rocky lava flow and hot debris that reached the sea.

The Aleutian Islands have long been  a source of volcanic activity as the Nasa image below shows.

Cleveland Volcano, Aleutian Islands

Image credit: NASA

At 3:00 p.m. Alaska Daylight Time on May 23, 2006, Flight Engineer Jeff Williams from International Space Station (ISS) Expedition 13 contacted the Alaska Volcano Observatory (AVO) to report that the Cleveland Volcano had produced a plume of ash.

Shortly after the activity began, he took this photograph.

This picture shows the ash plume moving west-southwest from the volcano’s summit. A bank of fog (upper right) is a common feature around the Aleutian Islands.

The event proved to be short-lived; two hours later, the plume had completely detached from the volcano.

The AVO reported that the ash cloud height could have been as high as 6,000 meters (20,000 feet) above sea level.

Cleveland Volcano, situated on the western half of Chuginadak Island, is one of the most active of the volcanoes in the Aleutian Islands, which extend west-southwest from the Alaska mainland.

It is a stratovolcano, composed of alternating layers of hardened lava, compacted volcanic ash, and volcanic rocks.