Empirical evidence of catastrop. geology: Case 2

Mount Saint Helens is an active volcano in Washington State.  A series of eruptions began in 1980 when a large landslide and powerful explosive eruption created a large crater, and ended 6 years later after more than a dozen extrusions of lava built a dome in the crater. Larger, longer lasting eruptions appear to have occurred in the past and are likely to occur in the future. Although the volcano seems to have returned to a period of quiet, scientists closely monitor Mount St. Helens for signs of renewed activity.

The first sign of activity at Mount St. Helens was a series of small earthquakes that began on March 16 1980. After hundreds of additional earthquakes, a steam explosion on March 27 blasted a crater through the volcano's ice capped summit. Within a week the crater had grown to about 1,300 feet in diameter and two giant crack systems crossed the entire summit area. By May 17, more than 10,000 earthquakes had shaken the volcano and the north flank had grown outward at least 450 feet to form a noticeable bulge. Such dramatic deformation of the volcano was strong evidence that molten rock (magma) had risen high into the volcano.

On May 18, 1980 at 8:32 a.m., a magnitude 5.1 earthquake caused the volcano's bulge and summit to break away in a huge landslide - the largest in recorded history. The landslide depressurized the volcano's magma system, triggering powerful explosions that ripped through the sliding debris. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north. This lateral blast of hot material accelerated to at least 300 miles per hour, then slowed as the rocks and ash fell to the ground and spread away from the volcano; several people escaping the blast on its western edge were able to keep ahead of the advancing cloud by driving 65 to 100 miles an hour! The blast cloud traveled as far as 17 miles northward from the volcano and the landslide traveled about 14 miles west, down the North Fork Toutle River.

The lateral blast produced a column of ash and gas (eruption column) that rose more than 15 miles into the atmosphere in only 15 minutes. Less than an hour later, a second eruption column formed as magma erupted explosively from the new crater. Then, beginning just after noon, swift avalanches of hot ash, pumice, and gas (pyroclastic flows) poured out of the crater at 50 to 80 miles per hour and spread as far as 5 miles to the north. Based on the eruption rate of these pyroclastic flows, scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m. Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano.

During the first few minutes of this eruption, parts of the blast cloud surged over the newly formed crater rim and down the west, south, and east sides of the volcano. The hot rocks and gas quickly melted some of the snow and ice capping the volcano, creating surges of water that eroded and mixed with loose rock debris to form volcanic mudflows (lahars). Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying roads and bridges. The largest and most destructive lahar was formed by water seeping from inside the huge landslide deposit through most of the day. This sustained flow of water eroded material from both the landslide deposit and channel of the North Fork Toutle River. The lahar increased in size as it traveled downstream, destroying bridges and homes and eventually flowing into the Cowlitz River. It reached its maximum size at about midnight in the Cowlitz River about 50 miles downstream from the volcano.

A 320 ft thick deposit was laid at the bottom of Spirit Lake due to the eruption and the flooding caused by the main rockslide debris that displaced the water, creating huge waves that scoured the basin and created the thick deposit (Meyer, W. and P. J. Carpenter. 1983. Filling of Spirit Lake, Washington. U.S. Geological Survey Open File Report 82-771).

Stratified layers up to 400 feet thick formed as a result of landslides, pyroclastic flows, , etc., during the Mt. St. Helens eruption. Fine laminae from only a millimeter thick to layers more than a meter high formed in just a few seconds each. A deposit more than 25  feet in thickness, and containing upwards of 100 thin layers accumulated in just one day on June 12, 1980.

Rapid erosion was caused by mudflows, landslides, and waves of water. On March 19, 1982, a small eruption melted the snow that had accumulated in the crater over the winter, and a resulting hot mud flow carved a system of canyons up to 140 feet deep and 17 miles long in a single day. The deepest of the canyons has affectionately been called the "Little Grand Canyon" of the Toutle River. The small creek that now flows through the bottom would appear to have carved this canyon over a great length of time, but this unique event has demonstrated that rapid catastrophic processes were instead responsible. The picture to the right above shows a 100-ft deep canyon carved into the pumice plain by August 1984.  I think the photo is by Steven A. Austin (geologist).


One prominent fast erosion feature is Loowit Falls. Loowit Falls is one of two major waterfalls that occurs along the the north flank of Mount St. Helens. When the mountain erupted in 1980, it created a catchment basin in the resulting crater. Beginning in the winter of 1980-1981 the Crater Glacier began to form and is currently the largest glacier on Mount St. Helens, as well as the youngest and fastest growing glacier in the United States. Once the glacier began growing, it also began melting and its meltwater has channeled largely into what is now Loowit Creek. Mt Saint Helens receives 183 inches of rain and snow each year (wet climate). As Loowit Creek intersects the blasted out north slope of the mountain it carved out the rugged Loowit Canyon, with Loowit Falls found at its head. National Geographic in May 2000 stated "Spilling from the crater, Loowit Falls reshapes the north slope of the volcano. You would expect a hardrock canyon to be thousands even 100's of thousands of years old, but this was cut in less than a decade.  Loowit Canyon formed in a matter of about 15 years and over that time Loowit Falls has constantly changed in appearance. When surveyed in 2011 the waterfall cut stood 186 feet tall. Imagery available in Google Earth shows the brink of Loowit Falls has retreated upstream by 40 feet since 1994. Some time between 2006 and 2009 a smaller but significant lower tier of the falls was buried almost entirely by a landslide and over the years there has been an increase in rockfall debris collecting at the base of Loowit Falls. Loowit Canyon is incredibly unstable and the height and appearance of the falls will continue to change in the coming decades. The falls have existed only since 1981 and with the rapid advance of the Crater Glacier there is a possibility that the stream may either be diverted or the glacier may even cover the falls and spill into Loowit Canyon at some point. 

Loowit Canyon is over 100 feet deep and was gouged in part through old hard volcanic rock (andesite).  This rock was not eroded over millions of years, mudflows eroded the canyon in just a few months in 1980.

Pumice (relatively porous, low density, lightweight rock formed from volcanic flow) is relatively more erodible than other rock types, but is not soil, so the erosion rate from recent water flow and the catastrophic erosion from the mudflows (lahars) is still impressive.