Exploring Mount St. Helens blast zone using Google Earth

May 18th marked the 44th anniversary of the 1980 eruption of Mount St. Helens. Over on Threads, someone started an account that posted pseudo-realtime updates leading up to the eruption and its aftermath. It’s been really fascinating to follow and it stoked my interest in learning more about the eruption (no surprise, given my past geology background, eh?).

Like most things that I start digging into, I ended up finding  a book!

Eruption: The Untold Story of Mount St. Helens by Steve Olson. It details events surrounding the eruption and explores how a number of victims ended up around the mountain on the fateful Sunday morning. Reading it sent me down a rabbit hole of Wikipedia entries, USGS reports and Google Earth sleuthing…

In the summer of 2009, I visited Johnston Ridge Observatory and was able to see the volcano first hand (see image below). Johnston Ridge Observatory is located on the site of the Coldwater II observation post — where volcanologist David Johnston famously radioed his last words before the lateral blast swept over the ridge, destroying his encampment (Johnston’s body was never found): “Vancouver, Vancouver! This is it!”

Source: Me

The lateral blast, the result of a M5.1 earthquake that triggered the largest landslide in recorded history (sheering 1,300 feet off the top of the mountain), sent a violent pyroclastic blast northward, scouring the landscape for miles. You can still see the results of the blast to this day.

When we visited in 2009 — 29 years after the blast, evidence of the lateral blast was evident in obvious signs of tree fall (below image) — gigantic trees snapped over in the direction of the blast as if they were toothpicks.

Source: Me

Johnston Ridge (and the site of the Coldwater II Observation Post) sit about 5 miles from the Mount St. Helens. Looking out over this grand vista, your sense of scale is completely messed up. The mountain is so huge that it looks like you can reach out and touch it — you swear to yourself that it’s just right there, a short hop and skip away.

“I’m going to go on a quick hike to the volcano. I’ll be back by lunchtime,” you say.

Everyone else: “lol”

The shockwave and pyroclastic blast that resulted from the lateral blast were estimated to have reached upwards of 670 miles per hour. At that speed, it would have taken 30 seconds to travel from the volcano to overtopping the ridge.

Looking at my own photos from the observation post, you can’t help but wonder what David Johnston was thinking as he saw the shockwave and pyroclastic blast rapidly spread across the valley below, approaching his location. It was probably an awesome sight to see, quickly followed by “Oh. Shit.”

Thanks to the wonders of modern technology, we have some fantastic exploration tools. I loaded up the Google Earth web app and set about exploring the area.

One of the first things I notice is how huge the mountain is (err… was?) and how small and insignificant Johnston Ridge seems, especially in the face of the resulting landslide and pyroclastic blast.

Via Google Earth

Zooming in on the Spirit Lake area, you can still see floating tree trunks grouped together, covering the northern part of the lake (I assume due to prevailing southerly winds in the area).

Via Google Earth

If we turn toward the west and look at Johnston Ridge, you can see deposits left over as the pyroclastic blast topped the ridge. They are the lighter grey outcrops you see around the map. (I’ve attempted to poorly outline them below).

Via Google Earth

Let’s pop over the the valley just to the north of Johnston Ridge (where Spirit Lake Highway runs). We can zoom in and see a mess of tangled tree trunks along the banks of South Coldwater Creek.

Via Google Earth

At the top of that valley, we can see more evidence of pyroclastic blast deposits. Like the image of Johnston Ridge above, look for the light grey outcrops and exposures.

Via Google Earth

Alright, let’s check out how far the effects of the lateral blast were felt. If we zoom out a bit and go to the top of the ridge (the next ridge north of Johnston Ridge — I am unsure of the name), we see more evidence of blast zone tree fall. At this point, we’re about 6.5 miles from the volcano.

Via Google Earth

If we skip north across the next valley that contains Coldwater Lake, we get to the third ridge we’re going to look at. Again, at the top, we see evidence of blast zone tree fall. This is 8 miles from the volcano.

Via Google Earth

Now that we’re getting a sense of the scale of the blast, we can zoom out and start putting things together. Wherever this sort of tree fall exists, it almost looks like the landscape was scoured (it was!).

Let’s see if we can find anything else interesting. We zoom out and see some scour marks on ridges way off to the north.

Via Google Earth

The area I circled looks interesting. It’s called Goat Mountain and it’s nearly 12 miles from the volcano. Let’s zoom in… ah, yes. There is the distinct “hash mark” pattern we keep seeing, that represents the blast zone tree fall.

Via Google Earth

From our computer screen, it’s hard to get a proper sense of scale. If we use Google Earth to measure the length of one of these “match sticks” (a big dead tree!), we get about 33 feet!

Via Google Earth

A USGS report on the lateral blast showed evidence of 100 foot tall trees knocked over that were located 19 miles from the volcano! Try as I might, I am unable to find evidence of this via Google Earth, as the margins of the blast zone seem to merge with areas where loggers have clear cut the forest.

Below is an example of a clear cut logging area about 30 miles away from the volcano (this was not affected by the blast zone).

Via Google Earth

“But Dave,” I hear you say, “how do you know some of those are from the blast and some are from logging?”

You’re right! In a way, I don’t.  However, one potentially easy way to tell is by the presence of logging roads. In my example from Goat Mountain above (12 miles from the volcano), the tree fall was located on a ridge, away from any sort of easily accessible logging road.

There was one section of Steve Olson’s book that I found particularly fascinating, especially because I hadn’t heard about it before. At the exact time the mountain erupted, a small plane was flying overhead with two geologists as passengers — Keith and Dorothy Stoffel.

They were on their fourth pass over the north rim of the crater, flying west to east, when Keith noticed something moving. “Look,” he said, “the crater.” Judson tipped the Cessna’s right wing so they could get a better view. Some of the snow on the south-facing side of the crater had started to move. Then, as they looked out the plane’s windows, an incredible thing happened. A gigantic east-west crack appeared across the top of the mountain, splitting the volcano in two. The ground on the northern half of the crack began to ripple and churn, like a pan of milk just beginning to boil. Suddenly, without a sound, the northern portion of the mountain began to slide downward, toward the north fork of the Toutle River and Spirit Lake. The landslide included the bulge but was much larger. The whole northern portion of the mountain was collapsing. The Stoffels were seeing something that no other geologist had ever seen.

A few seconds later, an angry gray cloud emerged from the middle of the landslide, and a similar, darker cloud leapt from near the top of the mountain. They were strange clouds, gnarled and bulbous; they looked more biological than geophysical. The two clouds rapidly expanded and coalesced, growing so large that they covered the ongoing landslide. “Let’s get out of here,” shouted Keith as the roiling cloud reached toward their plane.

Excerpt From Eruption by Steve Olson

Now, wait a minute! You’re telling me that at the exact time the volcano erupted, there were people flying overhead? I know this happened in 1980, but there just has to be photos of this, right?

Yes, there are photos!

Via Dorothy Stoffel

Via Dorothy Stoffel

Via Dorothy Stoffel

Via Dorothy Stoffel

The photos correlate well to a famous series of images captured by Gary Rosenquist as the initial moments of the landslide and eruption unfolded.

Via USGS / Gary Rosenquist

Here’s a fun aside (if you can call something related to an epic natural disaster “fun“). A YouTuber took the series captured by Rosenquist and ran some magical AI frame interpolation on them (essentially — an AI tries to generate content to fill in missing information between frames of a video). The result is a near real-time simulation of what those initial moments of the blast may have looked like.

After taking the photos, Rosenquist and his fellow friends correctly decided it was time to leave. Immediately.

He took one last photo (this is another one I don’t remember seeing before).

Via Gary Rosenquist

Do you like geology? Want more? Here’s a post I wrote in 2010 that took a deep dive into earthquake frequency.