It's the water.

UO geologists studied water and other elements they pulled from surface rocks on cinder cones surrounding Mount Lassen. From their analyses, they discovered that water being released from rocks sinking into the Earth's mantle play a role in forming lava that rises into fiery volcanic eruptions in the Cascade Range. Mount Lassen in at the southern edge of the Northwest mountain range.

The discovery helps solve a puzzle about plate tectonics and the Earth’s deep water cycle beneath the Pacific Ring of Fire, says Paul J. Wallace, a professor in the UO's Department of Geological Sciences. He was the principal investigator of a paper, led by his doctoral student Kristina J. Walowski, that is in the May issue of the journal Nature Geoscience.

Read more in the UO news release "Explosive volcanoes fueled by water, say Oregon researchers."
 
The study also received coverage in Ars Technica after it went live online late last month.