Mount Cleveland viewed from the air (May 31, 2012). / Photography by Cyrus Read. Courtesy the Alaska Volcano Observatory and U.S. Geological Survey 
(Editor's Note: At the time of posting, the Anchorage Daily News image directory is malfunctioning. Volcano images can be viewed at the original article on FrontierScientists.)
Laura Nielsen for Frontier Scientists
On Saturday May 4th the Alaska Volcano Observatory detected a series of low-level explosions at Cleveland volcano. Three discrete explosions occurred at 5:00 am, 9:17 am, and 11:44 am Saturday, while subsequent less powerful rumbles on Sunday denoted an ongoing low-level eruption. The sequence of eruptions emitted ash, gas, and steam into local airspace. Cleveland is a 5,676 foot tall conical stratovolcano, a restless volcano prone to rumbles, small explosions, and lava flows. It forms the western half of uninhabited Chuginadak Island in the Pacific Ocean, 940 miles away from Anchorage. Chuginadak is the name of the Native Aleut goddess of fire, who has been said to inhabit the volcano.
The Aleutian Island chain that dots the Pacific ocean southwest of Alaska is home to many volcanoes. The Alaska Volcano Observatory, a joint program of the United States Geological Survey (USGS), the University of Alaska Fairbanks Geophysical Institute (UAFGI), and the State of Alaska Division of Geological and Geophysical Surveys (ADGGS), is responsible for monitoring all 90 of Alaska’s active volcanoes.
 Mount Cleveland erupting ash (November 16, 2010), viewed from Kagamil Island, Aleutian Islands, Alaska. / Photography by Mandy Lindeberg. Courtesy National Oceanic and Atmospheric Administration (NOAA), the National Marine Fisheries Service 
Despite Mount Cleveland’s isolation, monitoring the volatile volcano’s activity is important because Cleveland sits below a major air-thoroughfare for commercial planes travelling between North America and Asia. Ash poses a significant threat to planes: particles of volcanic ash ingested into airline engines can cause engine turbines to stop functioning. Happily the series of small volcanic explosions beginning on May 4th, 2013 expelled an ash plume into the atmosphere at or below 15,000 feet. In contrast, commercial jets flying the trans-Pacific oceanic route usually cruise at an altitude of 35,000 feet. If the ash clouds gain height, the Alaska Volcano Observatory and the National Weather Service will work with the Federal Aviation Administration to safeguard aircraft by diverting routes away from the hazardous area.
The eruption underway poses limited hazards, and so Cleveland volcano is currently under Watch and has an Aviation color code of Orange, signifying minor volcanic-ash emissions. The Alaska Volcano Observatory website states ”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. If a large ash-producing event occurs, nearby seismic, infrasound, or volcanic lightning networks should alert AVO staff quickly. However, for some events, a delay of several hours is possible. Cleveland Volcano does not have a local seismic network and is monitored using only distant seismic and infrasound instruments and satellite data. AVO will continue to monitor the volcano and issue additional information as available.” Notifications for the volcano can be found at the Cleveland volcano page.
When volcanoes spew ash and other atmospheric particulates into the sky, the airborn pollutants can promote cooling by causing less sunlight to successfully pierce the atmosphere. Volcanoes can have a potentially significant impact on global climate. They can also impact human health by altering air quality.
 Airwaves resulting from the (May 5, 2013) Cleveland explosion as seen on AVO seismometers and an AVO infrasound sensor. The data streams are sorted by distance to Cleveland with the uppermost panel being the closest and the lowermost panel the furthest. The high amplitude pulses are carried by airwaves and arrive later at the further stations. The most distant station in this figure is on Great Sitkin Volcano, approximately 500 km (300 mi) away from the volcano. / Created by Matthew Haney. Courtesy the Alaska Volcano Observatory and U.S. Geological Survey 
Forecasting eruptions and managing the effects of eruptions is a big job, and the AVO uses a diverse set of monitoring tools to accomplish the tasks. While some volcanoes have local seismic networks in place that notify scientists of earthquakes and surface deformation caused by magma bulging under mountain flanks, Cleveland has no local seismic equipment. A webcam points its way, but cloudy or severe weather often obscures the mount. The scientists make use of distant seismic networks located at other volcanoes to monitor Cleveland’s explosions. They also engage the increasingly comprehensive coverage available from satellites with sensors for visible light as well as thermal cameras that measure the heightened temperatures common to eruptions. At other times, researchers must rely on eyewitness reports from mariners or airplane pilots.
Seismic stations measure shaking in the earth’s crust. Where the Pacific and North American tectonic plates meet, one plate is subducted under the other. Their grinding superheats and melts rock, forming magma and applying intense pressure to the earth’s crust. That pressure can cause tremors. Magma forced upward by the pressure can erupt through volcanoes. That motion is measured by seismometers.
Meanwhile, infrasound detectors detect airwave signals which indicate volcanic activity. These instruments pick up the low-frequency sounds emitted by eruptions, the pressure, explosions, and motion of underground magma. Emitted at a low frequency of less than 20 hertz, the noises are much too deep to be picked up by human ears. However, the noises can be picked up by instruments at stations very far away from where they originate, including stations at the University of Alaska Fairbanks’ Geophysical Institute.
 A true color Terra MODIS satellite image of Cleveland volcano’s eruption plume (May 4, 2013). / Rapid Response satellite imagery from the Land Atmosphere Near-real time Capability for EOS (LANCE) system. Courtesy NASA, Goddard Space Flight Center (GSFC), Earth Science Data and Information System (ESDIS) 
"It’s a difficult volcano for us to monitor... we only get these reports every now and then, or satellite data. The weather out there is horrible. It’s the central Aleutians, the weather is always bad. [Cleveland] could be erupting little puffs, tiny puffs of ash, on a weekly basis and we would just have no clue. It’s kind of a low-level, almost constantly active volcano. I mentioned that different volcanoes have different personalities -I mean some volcanoes are quiet quiet quiet, have a really big eruption, and go back to sleep- Cleveland is always sort of restless, and giving little belches and things like that. It rarely puts ash up very high or for very long periods of time, so in that sense it’s not as hazardous as some of the volcanoes that have larger eruptions, but it’s a bit of a nuisance for us." ~ Michelle Coombs, research geologist, Alaska Volcano Observatory
 Cleveland volcano expelling a plume of ash (May 23, 2006), viewed from space. / Photography by astronaut Jeff Williams (ISS Expedition 13). Courtesy NASA’s Johnson Space Center & the International Space Station Program 
Watch Volcano From Space: An astronaut aboard the International Space Station spots an eruption and contacts the Alaska Volcano Observatory
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
Alaska Volcano Observatory, a joint program of the United States Geological Survey (USGS), the University of Alaska Fairbanks Geophysical Institute (UAFGI), and the State of Alaska Division of Geological and Geophysical Surveys (ADGGS)
FrontierScientists “Alaska’s Cook Inlet Volcanoes” featuring volcanologists with the Alaska Volcano Observatory
Global Volcanism Program “Cleveland” Smithsonian Institution, National Museum of Natural History
National Aeronautics and Space Administration (NASA) Earth Observatory “Activity at Cleveland Volcano, Aleutian Islands, Alaska”
Wired Science Blogs “Small Explosive Eruptions at Cleveland in Alaska” by Erik Klemetti, assistant professor of Geosciences at Denison University