Volcano Review 2008

After the May 18, 1980 eruption, thick pasty lava crept onto the crater floor, building a lava dome. Pressure from steam and volcanic gas caused this dome to explode, sending hot rocks across the crater floor.

Over the next six years this cycle of dome building eruptions and destructive explosions occured several times. Geologists monitored earthquakes, gas emissions, and surface deformation to help forecast impending eruptive activity.

When the last dome building eruption ended in October of 1986, the dome was 900-feet tall and 3000-feet wide. Today, the crater and lava dome are often cloaked in a haze of dust resulting from rocks falling from the crater walls and occasional wisps of steam from the lava dome.

In the crater, much more snow falls each winter than melts during the summer. The almost perfect north facing amphitheater, formed by the crater walls, shades this snow. Rocks that tumble to the crater floor insulate this growing mass of snow and ice. Time and pressure from over-lying snow slowly change this snow to ice.

Since 1982, scientists estimate that the thickness of the ice mass has increased by nearly 50 feet each year. Surface cracks suggest that the ice mass is beginning to move. As the size of the ice mass increases it may begin to erode the lava dome it surrounds. Today, the snow and ice in the crater is equal in volume to all of the pre-eruption glaciers on Mount St. Helens combined.

At Mount St. Helens, geologists are carefully monitoring the growth of North America’s youngest glacier. The glacier has geologist’s attention because rapid melting by a future eruption could produce a massive mudflow and potentially threaten downstream areas.