1980 Eruption of Mount St. Helens

In the Plinian phase of the infamous eruption of Mount St. Helens, 520 million tons of ash were hurled – upwards of 80,000 ft – into the air. That’s the mass of over 85 million African elephants (the big ones)! Imagine throwing 85 million elephants 80,000 feet into the air (with parachutes, of course). That would take a lot of energy (remember, one way to consider energy is the acceleration of a mass over a distance, which is known as work, a form of energy). In my first blog post, we compared the energy released from this eruption to that of a staggering 24 megatons of TNT. This week, before we get into the nuts and bolts of how volcanoes work, we’ll have a look at this case study – a specific event (i.e. the eruption of Mount St. Helens) that is used to inform a more general inquiry (i.e. how volcanoes work).

Image of the May 18 eruption of St. Helens. Source: USGS.

Fortunately, the eruption, as with most volcanic eruptions, came with warning. Earthquake swarms, the development of a bulge (that grew 5 ft per day), steam explosions, and high surface heat flow all hinted that St. Helens had awoken after a 123 year period of repose.

Then on May 18, after two months of precursory activity, St. Helens lived up to its status as the most active volcano in the Cascades. The eruption began when a magnitude 5+ earthquake triggered a landslide on the north flank where the bulge had been building (Image B). The removal of the large mass of land allowed hot water within the system to expand and explode through the landslide scar (Image C). The massive blast, moving at >300 mi/hr, removed the top ~1000 ft of volcano and felled a dense forest of trees in a 6 mi radius arc. The magma (molten rock) below the volcano was stored at very high pressure, and the removal of the upper cone reduced the confining pressure, unleashing a torrent of rapidly ascending magma (similar to the way a soda bottle that’s been shaken “erupts” after removing the cap). Rapid expansion of hot gases fragmented the ascending magma, causing a massive plume of shattered magma (now, cooled to a glass) and hot gases to be hurtled into the air. Within minutes, the eruptive column reached heights of 15 mi and searing avalanches of hot gas and rocks – known as pyroclastic flows – poured out of the eruptive vent at speeds up to 80 m/hr.

Initiation of the eruption of the May 18 eruption. Source: modified after USGS images.

The erupted ash traversed the US in 3 days, while the eruption waned, and in the 15 days it took the ash cloud to travel around the world, the eruption ended. The effects of the eruption were devastating. The lives of 57 people were swept away. Over 330 million cubic feet of timber was damaged or destroyed. And the economic impact was severe.

This series of images show the aftermath of the eruption on the volcanic edifice. Most of the pre-1980 volcanic edifice was built in the last 3000 years, meaning that it was younger than the great pyramids of Egypt! Source: USGS.

I’d like to continue reviewing eruptions that have ushered in profound changes in Earth’s history, such as that of St. Helens. So, look forward to monthly posts on Revolutionary Eruptions. We’ll travel to Mexico in the ’40s, where a volcano emerged in the middle of a cornfield one day. Another eruption we’ll consider gave the northern hemisphere a “year without a summer”. We’ll see volcanic eruptions in distant worlds. And more!