Picture a volcanic eruption: fiery lava and smoke billowing skyward as a towering mountain empties its over-pressurized belly of a hot meal. At least, that’s how most of us think it works. So you can imagine volcanologists’ surprise when they discovered that Mount St. Helens, which was responsible for the deadliest eruption in US history, is actually cold inside. 
 
 Apparently, it’s stealing its fire from somewhere else. 
 
 
 
 Mount St. Helens is one of the most active volcanoes of the Cascade Arc, a string of eruptive mountains that runs parallel to the Cascadia subduction zone from northern California to British Columbia. It’s also one of the strangest. Most major volcanoes of the Cascade Arc sit neatly along a north-south line, where the wedging of the Juan de Fuca tectonic plate beneath the North American plate forces hot mantle material to rise. Mount St. Helens, however, lies to the west, in a geologically quiescent region called the forearc wedge. 
 
 Advertisement 
 
 “We don’t have a good explanation for why that’s the case,” said Steve Hansen, a geoscientist at the University of New Mexico in Albuquerque. 
 
 Seeking answers, Hansen recently led a seismic mapping survey of Mount St. Helens. In the summer of 2014, his team deployed thousands of sensors to measure motion in the ground around the volcano. Then, they drilled nearly two dozen holes, packed the holes full of explosives, triggered a handful of minor quakes, and watched as seismic waves bounced around beneath the mountain. “We’re looking at what seismic energy propagates off in the subsurface,” Hansen explained. “It’s a bit like a CAT scan.” 
 
 
 
 Advertisement 
 
 Their analysis, which is published today in Nature Communications, appears to have created more questions than it answered. From seismic reflections, Hansen and his colleagues learned that the types of minerals present at the boundary between Earth’s crust and mantle are markedly different to the east and west of Mount St. Helens, confirming that this area is geologically special. But instead of finding a hot mantle directly beneath the volcano, seismic data indicates a relatively cool wedge of serpentine rock. 
 
 Not only is Mount St. Helens out of place, but it also lacks the magma reserves we’d expect given its violent history. So, where on Earth is Mount St. Helens getting its fuel? 
 
 Hansen suspects the volcano’s magma source lies to the east, closer to the rest of the Cascade Arc, where material in the upper mantle is hotter. But that still leaves the question of why gooey rock being forced westward, through the crust or upper mantle, to erupt in this one off-kilter location. Earthquakes in the deep crust may be partially responsible, but more data is needed to confirm such a link. 
 
 Advertisement 
 
 Fortunately, more data is exactly what Hansen, and other scientists associated with the Imaging Magma Under St Helens (iMUSH) project, are now collecting. What geologists learn about this weird volcano—how its magmas form, how they move around, when and why they erupt—could improve our understanding of volcanic arc systems around the world. 
 
 “Mount St. Helens is pretty unusual,” Hansen said. “It’s telling us something about how the arc system is behaving, and we don’t yet know what that something is.” 
 
 
 
 [Nature Communications] ||||| Below most volcanoes, Earth packs some serious deep heat. Mount St. Helens is a standout exception, suggests a new study. Cold rock lurks under this active Washington volcano. 
 
 Using data from a seismic survey (that included setting off 23 explosions around the volcano), Steven Hansen, a geophysicist at the University of New Mexico, peeked 40 kilometers under Mount St. Helens. That’s where the Juan de Fuca tectonic plate releases fluids due to intense heat and pressure as it descends beneath the North American plate. Those fluids rise and trigger melting in the rock above, fueling an arc of volcanoes that line up like lights on a runway. All except for Mount St. Helens, which stands apart about 50 kilometers to the west. Still, Hansen and colleagues expected to see a heat source under Mount St. Helens, as seen at other volcanoes. 
 
 Instead, thermal modeling revealed a wedge of a rock called serpentinite that’s ||||| This map plots the detectable earthquakes detected on Mount St. Helens from March 14, 2016 to May 4, 2016. 
 
 Tiny temblors are shaking Mount St. Helens, indicating that the magma beneath the volcano is on the move, the U.S. Geological Survey (USGS) reports. 
 
 These mini earthquakes, along with the fact that the ground around the volcano is moving ever so slightly away from it, suggest that Mount St. Helens will one day erupt again, said Seth Moran, the scientist in charge at the USGS Cascades Volcano Observatory in Vancouver, Washington. 
 
 However, this upcoming eruption is likely "years to decades down the road,"Moran told Live Science. 
 
 Mount St. Helens, located in the Cascade Mountain range in southern Washington, is known for its enormous eruption on May 18, 1980. The eruption, which was preceded by more than 10,000 earthquakes, killed 57 people, according to USGS. [In Photos: The Incredible 1980 Eruption of Mount St. Helens] 
 
 Mount St. Helens, located in southern Washington, is experiencing some low-magnitude tremblers. Credit: Victoria Ditkovsky Shutterstock.com 
 
 These earthquakes started small, but grew into the 4-magnitude range as the volcano neared its eruption, finally reaching a magnitude 5.1 the morning of May 18, USGS reported. 
 
 "What we're looking at [now] is way smaller," Moran said. 
 
 Mount St. Helens had a much smaller eruption lasting from 2004 to 2008, USGS said. 
 
 The latest earthquakes — most registering a magnitude of 0.5 or less, with the largest reaching a magnitude 1.3 — started March 14, 2016, at a depth between 1.2 miles and 4 miles (2 and 7 kilometers), USGS said. 
 
 During the past eight weeks, researchers have recorded more than 130 of these tiny earthquakes. The rate of the temblors has been increasing since March, reaching almost 40 earthquakes a week, the USGS said. 
 
 Moreover, global positioning system (GPS) instruments placed around the volcano show a slight ground movement. Over the past eight years, the ground has moved between 0.4 inches to 0.8 inches (1 to 2 centimeters) away from the volcano, Moran said. 
 
 Both of these signs — the earthquakes and the movement — are volcano-tectonic in nature, the USGS said. These signs are probably happening because of a slip on a small fault, the agency said. 
 
 "Such events are commonly seen in active hydrothermal and magmatic systems," the USGS said. "The magma chamber is likely imparting its own stresses on the crust around and above it, as the system slowly recharges. The stress drives fluids through cracks, producing the small quakes." 
 
 Similar events happened in 2013 and 2014, and in the 1990s a swarm of earthquakes were even more frequent and released more energy, USGS said. 
 
 However, the USGS hasn't detected any anomalous gases coming from the volcano, which would suggest magma is pushing up toward the surface. "As was observed at Mount St. Helens between 1987 [and] 2004, recharge can continue for many years beneath a volcano without an eruption," USGS said. 
 
 What's more, Mount St. Helens is hardly the only active volcano in the United States. For instance, the Pavlof volcano in Alaska spewed ash 20,000 feet (6,000 meters) into the air in late March, and Hawaii's Kilauea volcano has been continuously erupting since 1983, Moran said. 
 
 Follow Laura Geggel on Twitter @LauraGeggel. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.
– Mount St. Helens already stands out as one of the most active volcanoes in the Cascade Arc and the deadliest in the US, since its 1980 eruption claimed nearly 60 lives. It's also an outlier in a literal sense, sitting 30 miles west of the volcanoes that neatly line the Cascade Arc from north to south. Now scientists are reporting in the journal Nature Communications that they've discovered another oddity: The volcano appears to be perched atop what Gizmodo calls "a cool wedge of serpentine rock"—dramatically unlike the fiery cauldrons of hot magma beneath other volcanoes. "We don’t have a good explanation for why that’s the case," Steve Hansen, a geoscientist at the University of New Mexico in Albuquerque, tells Gizmodo. His team drilled a couple dozen holes, filled them with explosives, and triggered minor earthquakes to watch seismic wave activity beneath Mount St. Helens, "a bit like a CAT scan," Hansen says. But their findings leave them with more questions, namely: What's the volcano's heat source, if it's not right below the volcano itself? Hansen surmises that it's coming from further east, but until his team does more research, it's what Science News is calling "a cold case." (Earlier this year, there were dozens of small quakes on the mountain every week.)