Scientists are on high alert after threat levels rose for three US volcanoes Thursday, suggesting eruptions may be imminent. 

The US Geological Survey (USGS) placed Alaska's Great Sitkin on 'Watch' because it is 'exhibiting heightened or escalating unrest' and the state's Mount Spur is under 'Advisory' after showing signs of unrest.

Lava began erupting slowly in the summit crater of Great Sitkin Volcano this week and small earthquakes were detected over the past day.

Mount Spurr, located in the Tordrillo Mountains about 80 miles from Anchorage, has also experienced an uptick in seismic activity over the last 10 months. Scientists warned it has a 50-50 chance of erupting soon.  

Hawaii's Kilauea is also under 'Watch' after erupting earlier this week for its ninth episode of activity since December 23.

The volcano released a massive stream of lava on Tuesday, shooting it more than 300 feet into the air. 

While Kilauea is located inside the secluded Hawaii Volcanoes National Park on the Big Island, its latest eruption produced gas, ash and windblown shards of volcanic glass that can be hazardous to human health.

As of Friday morning, the volcano's activity had settled. But experts say it is likely to erupt again within the next three to six days. 

The USGS uses a nationwide volcano alert-level system for characterizing conditions (quiet, unrest, eruption) at US volcanoes.

The codes include 'Normal,' 'Advisory,' 'Watch' and 'Warning,' which are also shown in colors: Green, Yellow, Orange and Red.

The US is home to 169 active volcanoes spread out through Alaska, Hawaii, the Pacific Northwest, Arizona, California, Colorado, Idaho, Nevada, New Mexico, Oregon, Utah, Washington and Wyoming.

Each volcano is monitored by the USGS that looks at seismic activity, ground deformation, volcanic gases, thermal emissions, and changes in water levels and chemistry.

And those factors were enough to place three volcanoes under elevated alerts.  

While Mount Spurr is exhibiting signs of heightened unrest, it is not actively erupting yet.

Since the earthquakes first began in April 2024, their rate has increased from an average of 30 per week to an average of 125 per week, ramping up in early October. 

Experts at the Alaska Volcano Observatory (AVO) believe this seismic activity is being driven by new magma churning beneath the volcano, a sign that an eruption could be imminent. 

If Mount Spurr does blow, it could produce explosive plumes of ash, destructive mudslides called lahars and avalanches of hot gas, ash and rock called pyroclastic flows, which would race down the volcano's side at over 200 mph.

Fortunately, there are not any communities within the potential path of pyroclastic flows or lahars from Mount Spurr. 

But the amount of ash that would spew forth from this volcano could pose a threat to human health, causing breathing difficulties and releasing toxins linked to cancer. 

Great Sitkin, which sits in the western Aleutian Islands, was elevated to a higher risk following small earthquakes and steaming from its summit crater. 

This volcano explosively erupted in May 2021 following a series of earthquakes beneath it. 

The eruption sent a plume of ash and gas 15,000 feet into the air, and this ash cloud prompted the USGS to upgrade Great Sitkin's threat level to a 'Warning' and code its aviation risk as red, indicating an urgent warning to planes flying overhead. 

Volcanic ash particles are angular and sharp enough to cause jet engines to shut down, which is why experts warn pilots when a volcano poses an active threat.

Two months after this eruption, Great Sitkin began effusing lava, which is when molten rock steadily flows out of a volcano unaccompanied by an explosive event.

This volcano has been leaking lava ever since, but the recent earthquakes detected beneath it suggest another violent eruption could be looming.

Fortunately, this island volcano is surrounded by water and sits in a remote location, so its lava flows are unlikely to threaten human lives.

But just like Mount Spurr and Kilauea, ash and gas produced by Great Sitkin can be hazardous to human health when it blows into communities. 

Kilauea is the only one of the three that has been truly active this week, releasing over 22 hours of lava fountains on February 12.

'Last night, several lava flows continued to slowly flow on the floor of Halemaʻumaʻu and other incandescent areas remained visible on the crater floor as recently erupted lava continues to cool,' the USGS said Thursday.

'The north vent continues to degas and glow faintly at night, indicating magma remains at relatively shallow levels in the vent.