Nature Notes

Vol. II October-November, 1984 No. 6


Mt. St. Helens -- an active volcano -- major geological events happening within our lifetimes -- right in front of our eyes -- and right in our own backyard! What a thrill it was to partake in the second USFS-NPS trip this summer into the restricted zone around Mt. St. Helens and to explore first-hand the processes and results of ongoing volcanic activity. Harry Glicken and Steve Brantley, of the USGS, met us at Windy Ridge. After sharing an overview of the mountain and its recent escapades, they led us into the restricted zone to take a closer look at the results of the May 18, 1980 eruption. Travelling down the dry dusty road toward Spirit Lake, we stopped in several places to examine the rocks and ash, both old and new, that form this unusual and desolate landscape. By climbing to the top of an unconsolidated ridge, jumping down into a recently eroded gulley out through thick layers of ash, and tossing large, but surprisingly light, chunks of pumice into the air, we got -- literally -- a taste of and a feeling for the geology of this volcano.

At 8:30 a.m. on May 18, an earthquake measuring 5.1 on the Richter Scale shook Mt. St. Helens. An enormous rockslide ensued, carrying off a large portion of the mountain. The removal of this overlying mass uncorked the gas-rich magma that had been rising into Mt. St. Helens from deep in the earth. Suddenly relieved of pressure, it blasted through the north side of the mountain. Additional blocks of the mountain slid down while vertical explosions continued to vent the unleashed pressure.

The rockslide, or rockslide debris avalanche, as it is called, spread shattered pieces of the mountain's northern slope across 15 miles. The hummocky terrain that now lies north of the crater is typical landslide topography. Part of the rockslide careened down the lower slopes of the mountain, and quickly arrived at Spirit Lake, about four miles away. It pushed through the lake, continued up the far ridge, and also dammed the lake's outlet. When the turmoil settled, Spirit Lake's surface level was nearly 200 feet higher than it had been but a few minutes earlier because so much debris accumulated on the lake bottom. Changes in the lake are nothing new. Spirit Lake has existed for thousands of years in one guise or another, altering and shifting with each new episode of active volcanism.

The blast also caused a second type of rock deposit, referred to as the blast deposit. It is composed of 50% old andesites and basalt; from the slopes of the mountain and pieces of previously erupted pumice and 50% blast dacite, newly formed volcanic rock. While the dacite is steely grey-blue in color, the blast deposit as a whole has a greenish tinge to it.

Lense-shaped pieces of clay lie near the top of the blast deposit. During the morning of the May 18 eruption, these came raining out of the clouds as "mud balls." Because the mud fell across a large area within a short time, these clay deposits serve as a time marker, helping geologists to determine the sequence of events.

At midday, observers noticed the column spewing from the volcano had become lighter in color indicating a significant change in activity. Instead of the older andesites and basalt;, new material was now being blasted into the air. Ashflows of this material, called pyroclastic flows or ignimbrites, formed a third type of deposit. Basal flows of relatively low energy draped lobes of pumice into stream valleys, while above them, ash clouds of gases and hot particles flashed by, overriding the topography and leaving behind thick layers of buff-colored ash. Water running down from the mountain and from Spirit Lake accumulated in scattered pockets. Hot ash flows would encounter these pools, causing explosions. Small craters remain as evidence of these phreatic explosions.

Now more than four years after the eruption, changes continue at Mt. St. Helens. The level of Spirit Lake has risen another 60 feet as rainwater and snowmelt collect in this basin with no outlet. The rockslide dam blocking the lake's outlet is still 65 feet above the present water level. The dam is very unstable and contains a substantial amount of ash: This ash is susceptible to "melting" when combined with water; thus if the lake level continues to rise, the dam could fail unleashing catastrophic flooding on communities downstream. To offset this potential flooding, the level of Spirit Lake has been maintained by pumping water over the dam. Each summer, an impressive cache of some 16-20 bulk fuel tanks are filled to keep the pumps running through the winter when the area is snowed in. Now a different, less labor intensive, approach to this problem is being pursued. Huge tunnels are being drilled through the dam to allow gravity flow to maintain the water 20 feet lower than its current level.

The dome, too, continues to change, as lava thrusts its way to the surface and oozes down its sides. Internal growth also occurs as magma rises without surfacing and expands the dome. This growth occurred in intervals for three years, followed by a year of continuous growth. Rockfalls occur sometimes when the growth is rapid. Occassionally, hot magma comes into contact with groundwater or gases accumulate in the dome, causing explosions.

Needless to say, Mt. St. Helens is a living classroom for geologists. By studying the rock material blasted out of the mountain and the newly formed topography, Harry and his colleagues have been working to "put the mountain back together again" and re-construct the events. Other scientists from around the world have come to Mt. St. Helens, looking for clues which might unlock the secrets of other volcanoes now lying dormant as well as those of ancient times which have left their marks on the land. While studies focus on the May 18 eruption, the many subsequent, but smaller events have provided additional important keys to this search for understanding. The mountain remains under careful surveillance -- to collect data and, hopefully, to predict upcoming events, but we'll just have to stay tuned to discover what this living volcano will do next.

Barbara Gross

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