Growth of the new lava dome inside the crater of Mount St. Helens continued during 16-20 February, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. Gas measurements made on 15 February suggested that the volcanic-gas flux remained unchanged from recent measurements. Observations made on 17 February revealed that the northeastern active part of the new lava dome was developing a steeply inclined jagged spine. At its top, temperatures as high as 580 degrees Celsius were measured using a thermal sensor. St Helens remained at Volcano Advisory (Alert Level 2); aviation color code Orange.

Geological Summary. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers.

Source: US Geological Survey Cascades Volcano Observatory (CVO)