The geological legacy of Mt St Helens

Andrew Snelling
01 May, 2005 5 min read

The 1980 eruption of Mt St Helens in Washington, north-west USA, was not the worst in recorded history. The 1902 eruption of Mont Pelée on the West Indian island of Martinique annihilated the 30,000 inhabitants of the capital city St Pierre within a few minutes.

The 1815 eruption of Tambora on the Indonesian island of Sumbawa ejected 100 cubic kilometres of ash and killed at least 117,000 people. The stupendous eruptions in 1883 of Krakatoa, in the Sunda Straits between Sumatra and Java, were heard 4,600km away in Sydney, Australia. A vast cloud of ash rose 80km into the atmosphere, while catastrophic sea waves (tsunamis) up to 37m high killed more than 36,000 people as they swept across adjacent coastlines.

Mt St Helens

On 18 May 1980, following a magnitude 5+ earthquake, the northern flank of Mt St Helens gave way, creating a two cubic km avalanche of rock debris. As the summit and northern flank of the volcano slid off, pressure was relieved on a vast internal reservoir of super-heated liquid water.

Within seconds the water turned to steam and a northward-directed blast, equivalent to the power of 1,000 Hiroshima-size atomic bombs, devastated 600 square km of surrounding forest within six minutes.

Ash rising from this directed blast formed a cloud that rose more than 30km into the atmosphere. Half a cubic km of rock debris was dumped into Spirit Lake, north of the volcano, initiating an enormous water wave that stripped trees from the surrounding slopes 260m above the pre-eruption water level.

Then, for nine hours, a column of steam and ash rose 19km into the atmosphere before gradually subsiding.

The significance of Mt St Helens (one of the best documented volcanic eruptions ever) lies in the geological legacy it produced. During the 18 May and subsequent eruptions, critical energy thresholds were exceeded by potent processes — which were thus able to accomplish significant geological work over very short timescales.

Most modern geologists believe that gradual processes occurring over tens of millions of years created the earth’s rock strata and shaped its surface. By contrast, the Mt St Helens area has become a natural laboratory for the documentation of

catastrophicgeological change — demonstrating that major geological features can be formed in minutes or hours.

We now review some of these processes.

Rapid deposition of sediments

The eruptions produced a sequence of ash layers up to 183m thick — deposited by the primary air blast, landslides, waves on Spirit Lake, ash flows and mudflows, as well as from air-falls and condensing steam.

The most dramatic depositions resulted from ‘pyroclastic flows’ — ground-hugging, fluid-like, turbulent slurries of fine volcanic debris which moved at high speeds off the flank of the volcano.

The resulting deposits consist of

laminae(thin sheets) of fine ash and beds from a millimetre to more than a metre thick — each representing just a few seconds to several minutes of accumulation (see photo).

Geologists had always thought that such

laminaeand beds represent the very slow accumulation of sediment layers due to seasonal variations or annual changes (in rainfall, etc). However, the Mt St Helens sediment layers formed rapidly by flow processes.

On 12 June 1980, a layered deposit 7.6m thick (containing alternating

laminaeof coarse and fine particles, and what is known as ‘cross-bedding’) accumulated in less than five hours (see Question Box 1). It was deposited by pyroclastic flows moving at 160km/h, resulting from the collapse of the volcano’s eruption column.

Rapid erosion of landscape

During the eruptions, erosion of existing landscape occurred by the scouring action of steam blasts, landslides, water waves, hot ash flows, and mudflows. After the eruptions, the dominant erosion processes were sheet flooding and channeled water flows, with occasional mudflows.

About 60 square km of the North Fork of the Toutle River valley (to the north of the volcano) became blocked by almost 3 cubic km of rock debris and ash. This deposit has been rapidly eroded since 1980 in various ways.

Water and ice buried under hot ash boiled, producing steam jets that carved out huge explosion craters with walls up to 30m high. Within days, associated erosion of the crater walls produced a dendritic (tree-branch) pattern of rills and gullies more than 38m deep and resembling a ‘badlands’ topography. Geologists usually assume that such topographies take thousands of years to form.


Mudflows, pyroclastic flows and water also scoured the northern flank of the volcano, deeply eroding solid pre-1980 volcanic deposits and lavas. Two canyons with 30m high cliffs were produced within a few months.

Such erosion of solid rock is usually thought to require tens of thousands of years. But here, fast-moving catastrophic mud and ash flows carved rapidly through solid rock on a colossal scale.

An explosive eruption on 19 March 1982 melted the thick snowpack in the volcano’s crater, generating a destructive sheet-like flood which became a devastating mudflow. This mudflow breached the rock and ash debris filling the North Fork of the Toutle River valley, surmounting all obstacles.

Within hours it eroded a whole new dendritic drainage pattern of deep canyons with cliffs 30m or more high (see Question Box 2). This canyon system is a one-fortieth scaleversion of the famous Grand Canyon!

It would usually be assumed that the creek now flowing through this canyon system had slowly eroded it over tens of thousands of years. However, the creek is there today only because the canyon system was carved out catastrophically in a single day by a completely different agency.

Buried logs and peat

The landslide-generated waves from Spirit Lake stripped the forests from the surrounding slopes, leaving millions of logs floating as an enormous ‘mat’ over about 8 square km of the lake’s surface.

Many of the logs floated in an upright position, with their root-balls submerged. Over a period of years, many of these upright logs became waterlogged and sank to the lake floor, where they have been buried by sediments.

This has been confirmed by scuba divers. A sonar survey also confirmed that many upright logs are buried in the lake floor sediments, giving the false impression of a forest that then had been slowly buried and fossilised.

These observations challenge the popular buried

in situforest interpretation for petrified upright trees in the geologic record, such as those fossilised in volcanic ash in Yellowstone National Park.

The prone floating logs in the enormous log mat also lost their bark and branches. The scuba investigations found water-saturated sheets of bark on the lake floor, where a peat layer 8-12cm thick had also accumulated. This peat resembles, both in composition and texture, some coal beds of the eastern United States.

Coal beds are normally thought to have formed by the accumulation of peat in swamps — a process requiring 500 years to form each centimetre of coal. But the peat layer in Spirit Lake demonstrates that coal-forming peat can accumulate rapidly (burial and slight heating would transform this peat into coal).

Invalid dating

The new lava dome, formed inside the Mt St Helens crater since October 1980, stopped growing in 1986. In June 1992 a sample of this lava was collected and sent for potassium-argon radiometric dating — a method commonly used by geologists to assign ages of millions of years to old volcanic rocks.

Because potassium decays into argon very slowly, this new rock should have dated ‘too young to measure’. However, the lava sample yielded a false age of 350,000 years, and one of the minerals in it a false age of 2.8 million years (see the following article).


The May 1980 eruption of Mt St Helens may not have been the most destructive in recorded history, but it has left an unsurpassed legacy that challenges some firmly-held geological dogmas.

Processes that were thought to require thousands of years — deposition of strata, erosion of landscapes, burial and fossilisation of upright trees, and the production of peat layers — were found to have occurred catastrophically in days and weeks.

If this happened on the relatively small scale of Mt St Helens, it is easy to imagine what would have happened during the global catastrophe of the Genesis Flood.

Clearly, the millions of years usually assigned to the formation of geological strata are highly questionable. The Bible’s description of a young creation and a recent global Flood can thus be totally trusted.

Andrew Snelling BSc (Hons), PhD (Geology) was for many years Geologist, Senior Research Scientist and Editor of the

CEN Technical Journalat the Creation Science Foundation (now Answers in Genesis), Australia. He is now Associate Professor of Geology at the Institute of Creation Research, San Diego, USA.

Further Reading: John Morris and Steven A. Austin,

Footprints in the ash: The explosive story of Mount St Helens, Master Books, Green Forest, Arkansas, 2003.

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