Lava extrusion continued through early July. The rate of advance of the new lobe was about 4-5 m/day in June, but the net increase in length was substantially reduced by rockfalls from its front, which remained 75-100 m above the base of the dome in early July. Since extrusion began, the thickness of the lobe just downslope from the vent has increased from a few meters to at least 50 m.

Increasingly rapid deformation of the lava dome had preceded previous extrusion episodes but ended as lava reached the surface. However, deformation did not stop when extrusion began in February (SEAN 08:01), and deformation measurements showed that intrusive activity has continued since then. Deformation remained strongest in the NE part of the dome, which moved outward an average of 1 m/day and downward about 30 cm/day in June. Both of these rates gradually decreased in June. In contrast, outward movement of the much less active SE sector of the dome increased from 4-5 cm/day through 28 June to 8 cm/day between then and the following measurement 5 July. Acceleration of the N side of the dome was also noted.

The SO₂ emission rate averaged about 65 t/d 3-16 June, increased by a factor of three to roughly 200 t/d 20-28 June, then dropped to about 75 t/d 4-6 July.

The number of earthquakes and the rate of seismic energy release in June were both similar to May values. Low-frequency shocks continued to dominate the seismic records as they have since extrusion began in February. Surface events, primarily rockfalls from the dome and walls of the crater, were more frequent in June. Much of the change can probably be attributed to warmer weather, which typically increases the number of rockfalls from the crater walls.

Geological Summary. Prior to 1980, Mount St. Helens was a conical 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 breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it 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. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: D. Swanson, T. Casadevall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.