Mount St. Helens is showing signals of long-term magma rising and increased earthquake activity, but there are no signs of an impending eruption, the US Geological Survey said Wednesday.

The 8,363-foot volcano in Washington state famously erupted in 1980, killing 57 and destroying a large swath of forest.

Scientists with the USGS Cascades Volcano Observatory and the Pacific Northwest Seismic Network have been monitoring seismic activity and ground surface inflation since a steam eruption that lasted between 2004-2008.

It has long been theorized that fresh molten rock is recharging inside the volcano since its last eruption, but it has only now been proven.

"Careful analysis of these two lines of evidence now gives us confidence to say that the magma reservoir beneath Mount St. Helens has been slowly re-pressurizing since 2008," the USGS said in a statement.

The USGS reported that it's likely that re-pressurization is triggered by the arrival of a small amount of magma between 2.5 miles and 5 miles beneath the surface. Re-pressurization of a volcano magma reservoir is common after an eruption and can continue for many years without leading to an eruption.

"This is to be expected while Mount St. Helens is in an active period, as it has been since 1980, and it does not indicate that the volcano is likely to erupt anytime soon," the USGS said.

"The magma could be at current levels for another 100 years before any eruption," Carolyn Driedger, spokeswoman for the USGS Cascades Volcano Observatory in Vancouver, told NBC News.