Images

Mount Washington peak in central Oregon.

Mount St. Helens four years after the May 18, 1980 eruption—lava dome in the crater and drainage channels development on flanks; view from Johnston Ridge.

Lava fragments ejected by lava fountains are called tephra, a general term for all fragments, regardless of size, that are blasted into the air by explosive activity. A variety of terms are also used to describe specific types of fragments, including Pele's hair, Pele's tears, scoria, spatter, bombs, and reticulite.

Low fountains in foreground erupting from a fissure that opened just before the main vent began to erupt.

Lava dome within Mount St. Helens crater grew between 1980 and 1986 as dacitic lava erupted from the vent. This photograph, taken in September 1984.

Vegetation began reappearing as early as the summer of 1980 as many small trees and plants were protected by the snowpack on May 18. Seeds, carried by the wind or by animals, also entered the area and grew. By 1985, the ridges surrounding the volcano were covered with new growth.

The strength of the magnetic field increased as the dome cooled and magnetic minerals formed. During the eruptions the strength usually changed rapidly as magma heated and deformed the dome.

The U.S. Geological Survey established both periodic and continuous 24-hour monitoring programs at Mount St. Helens to study and predict eruptions. In this slide, geologists used a steel tape to measure the distance across a crack on the crater floor. Widening of cracks was an indication that magma was rising and deforming the area, leading to an eruption.

An increase in deformation rates is an indication that magma is slowly entering the dome. In the early 1980s deformation rates often reached 30 feet per hour (10 meters/hour) as magma rose and the dome expanded before extrusion started. During the winter months, the instrument stations often had to be dug out of the snow before measurements could be made.

Lava production from these "2,900-m vents" decreased from less than 100,000 m3 per hour on April 9 to about 20,000 m3 on April 13, which was the last full day of eruption. Active lava flows extended less than 2 km from the vents. Note two geologists for scale in lower right (in green flight suits). Mauna Loa summit in upper left.

Lava production from these "2,900-m vents" began to decrease in late March but declined most rapidly between April 7 and 9 from about 300,000 m3 per hour to less than 100,000 m3 per hour. Photo taken at 9:09 a.m.

Blockage and overflow of main ‘A‘ā lava channel leading from the 2,900-m vents; note helicopter below the blockage. Large, floating debris of solidified lava (called lava boats) and debris from surface crusts sometime congregate at constrictions along the channel, causing blockages of the main channel.

After repeated blockages and overflows of the main lava channel between April 5 and 8, the steady supply of lava to well-developed flow fronts below the 1,850-m level ceased. As the lowest flow fronts stagnated and the rate of eruption from the vents slowed, hazard concerns for Hilo diminished.

Lava flows from the 1984 eruption of Mauna Loa loom above the town of Hilo. Photograph taken near the Hilo airport on April 4.

These are 19 km east of the original outbreak point that began within Moku‘āweoweo caldera about 36 hours earlier.

Aa lava flows erupting from Mauna Loa.

Pohaku Hanalei cinder-spatter cone (upper left) is located about 3.2 km (2 mi) NE from the north edge of the caldera rim. Eruption rates were as high as 2.9 million m3 per hour during the first 6 hours of the eruption, then diminished to about 0.5 million m3 per hour for the next 12 days.

The eruption began at 1:25 a.m., and the glow was visible from much of the island. After about 4:00 a.m., the activity migrated into the upper northeast rift zone (to the right of the glow) and diminished in the summit area.

Plume of steam and volcanic gas rises from fissures erupting on the upper northeast rift zone of Manua Loa Volcano. At about 4:30 p.m., lava erupted from a new fissure about 5 to 7 km further down the rift (to the right). This new fissure became the site of all lava production for the remainder of the eruption.

Harry's Ridge monitoring station, 8 km (5 mi) north of Mount St. Helens' crater.

The May 18, 1980 debris avalanche from Mount St. Helens covered over 24 square miles (62 square kilometers) of the upper Toutle River valley and blocked tributaries of the North Fork Toutle River. New lakes such as Castle Lake (pictured here) and Coldwater Lake were created.