Magma levels at Mount St. Helens are rising, but geologists are not expecting an eruption in the near future.

Mount St. Helens unexpectedly erupted on 18 May 1980, killing 57 people. The explosive force from the event spread debris over 230 square miles. Days later, rain in parts of the northeast tasted like ash.

Before the eruption 34 years ago this month, the mountain experienced an earthquake, measuring 5.2 on the Richter scale. Seismic activity has recently become more common on the volcano, which geologists believe is caused by an uplifting of the ground due to rising levels of magma.

"Analysis of current behavior at Mount St. Helens indicates that the volcano remains active and is showing signs of long-term uplift and earthquake activity, but there are no signs of impending eruption," researchers from the U.S. Geological Survey (USGS) stated in a press release about the magma buildup.

Seth Moran, a seismologist with the Cascades Volcano Observatory in Washington state studied recent activity of the volcano, located 96 miles south of Seattle. He and his team made careful measurements of the relative height of the surface over time and first noticed the ground swelling.

So far, the ground has only risen by about half an inch, a change that started back in 2008. That year, magma began collection in a pool nearly three miles under the surface of the mountain.

"This doesn't mean it's getting ready to erupt. The balloon has inflated, and it could stay inflated for decades. What we can say, is when it is ready to erupt, we will know," Moran said to Live Science.

The GPS system in use on the mountain is accurate within 1/16th of an inch, and uses less energy than a light bulb.

The volcano at Mount St. Helens last erupted during the years 2004 to 2008, although that event was not nearly as dramatic as the 1980 eruption.

"The 2004-2008 eruption produced many earthquakes, with well over one million occurring in association with the construction of a new lava dome complex," the USGS explained on their Web site.

Researchers believe the rise in the surface levels is due to a "re-charging" of the magma chamber. This process is also seen in other volcanoes, and can continue for years before an eruption occurs.

The 1980 eruption was the deadliest and costliest volcanic explosion in the history of the United States. The eruption blasted the top 1,300 feet of the mountain into the atmosphere.

Results of the geological study were presented at the annual meeting of the Seismological Society of America.