Kara here; we’re going to talk volcanoes today. I’m taking over from Gunther ‘cuz when ya’ll need to be edumacated then he just ain’t up to it…we all know I’m the smart one in the family.
Anyhoo; Wednesday we headed off to visit the National Volcanic Memorial down at Mount St. Helens about 120 miles away down towards Portland a bit. We were originally going to go Tuesday but it rained all day. Wednesday didn’t look much better…it was only forecast to get up to ‘partly cloudy’ down near the volcano but we headed out anyway since Connie will be sore for a couple of days after Thursday.
By the time we got down there; it might have been partly cloudy by WA weather guesser standards but it was pretty much overcast and foggy by any normal person’s standards. Hence we didn’t get any really decent pictures of the volcano itself; we’ll see if we can grab a few more next week when we head down to the Lewis and Clark National Historic Site if the weather cooperates a little better. However; Neil snagged a few copyright free photos from the web that were taken in better visibility because they are better than his shots in the fog and so that the vulcanology lesson will make sense. He was able to get a few halfway decent shots at an overlook just in case partway up and it turned out to be a good thing he did.
Ok then…on with the lesson. First up is a map of the area around the mountain along with some description of the topography to orient you so the rest of the discussion will make sense. Neil cribbed this photo from one of the exhibits at the Johnston Ridge Observatory that they visited.
The Johnston Ridge Observatory is at the black You are here star in the center of the map. Before the event, starting from the observatory at an height of 4,200 feet 5.5 miles almost due north of the summit of the mountain…heading towards the mountain into the South Fort Toutle River valley 1,200 feet below the ridge at an height of 3,000 feet…then rising about 6,500 feet to the 9677 foot summit of the volcano. Before the blast the summit cone was a more typical round cone and not the U-shape that the above post eruption map shows. Spirit Lake to the northeast of the summit is at an elevation of 3,400 feet…about 400 feet higher than the valley floor. The general slope of the valleys in this area is downhill from east to west.
The red circle labeled Scymansky is the approximate location where Neil got his photos of the mountain…good thing we stopped by the overlook on the way in as by the time we got to the observatory it was completely hidden in the clouds from a front rolling in.
Ok, let’s go back to spring of 1980 and talk about the eruption of the volcano. As everybody knows…the mountain blew up on May 18 at about 0832 in the morning, right? That’s what everybody thinks but in actuality that’s not precisely what happened. There was a volcanic eruption but that was a secondary event and was only a minor contributor to the damage caused by the event.
So, if it didn’t blow up what really happened. Stand by because here comes the vulcanology lesson.
Mount St Helens is a basalt rock volcano and (because it’s in the Pacific Northwest where it tends to rain a lot) is covered by snow most of the year which means that the rock forming the mountain is waterlogged most of the time. Since it’s a volcano it periodically over the centuries has magma rising into the mountain below the cone…this magma contains a lot of dissolved sulfur dioxide gas. As the magma comes into contact with the wet rock the gas mixes with the water and forms sulfuric acid…which tends to dissolve and weaken the basalt rock and turn a lot of the rock into clay. Now we all know what happens to clay when it gets wet, right? It’s gets very, very slippery.
So…over the centuries the mountain has been transformed from solid basalt rock to having lots of veins of gooey clay running through the mountain. The volcano had been mostly dormant overall for several centuries before the 1980 event but starting in March the watching vulcanologists started observing lots of earthquakes in and under the mountain as serious amounts of magma started to flow into the interior of the cone. These earth quakes increased in number until there were about 10,000 per day by the middle of May. Here is a shot taken from very near the site of the observatory looking almost due south towards the summit in early March 1980 before the earthquake activity.
So, the magma started pushing up from below and caused the small earthquakes under and in the mountain as it came up. The magma was stopped from coming out by the summit cone which was plugged at this time…and since it was being pushed from below it looked for someplace to go. After a little looking around it found the acid weakened, clay vein filled north slope of the peak and started pushing horizontally out towards the north. The vulcanologists called this phenomenon the Bulge and it grew by 5 to 80 feet per day as it bulged out from the north face. By May 16 when the picture below was taken the Bulge had grown in size to 450 feet out from the mountain and was a mile tall and a mile and a quarter wide.
Finally, at 0832 on May 18 gravity and the weight of the Bulge overcame the surface tension holding the weakened side of the mountain together and the north side of the mountain collapsed. The precipitating event was a 5.1 magnitude quake only a mile underground immediately below the north face of the volcano. This resulted in the largest known landslide in history…about 3/4 of a cubic mile of rock, snow, and mud. The landslide was the first phase of the event and headed north across the valley at a speed of 150 miles an hour. It hit the 1,200 foot high Johnston Ridge and a little of it sloshed over the top but the majority of the landslide turned either left and headed west or right and headed to Spirit Lake 8 miles away. Some of the landslide ended up in Spirit Lake which raised the elevation of the surface by 200 feet and doubled the area of the lake. The valley between the mountain and Johnston Ridge was filled with as much as 600 feet of debris with the majority of the debris flowing west and inundating the entire North Fork of the Toutle River. The river was covered for a length of 13 miles at an average of 150 feet deep with a maximum of 600 feet deep. Here is the crater post event; this and the previous two photos were taken from almost the same spot near the location of the present day observatory.
When the side of the mountain fell down…naturally the compressed magma in the center decided to leave next. So, starting about 90 seconds after the landslide the magma core of the volcano came out sideways in a pyroclastic flow. St Helens doesn’t erupt with relatively thin, glowing orange lava like you see from the volcanoes in Hawaii…it has pyroclastic flows instead which is a much thicker and much cooler than glowing orange type of lava…think of it as 600 degree concrete. This flow was essentially comfined into the area directly between the mountain and Johnston Ridge due to it’s viscosity.
The third part of the eruption was the gray ash cloud that we all saw on the news back then…rising to a height of 60,000 feet and producing a mushroom cloud about 25 miles in diameter. This ash cloud eventually spread worldwide via the jet stream.
About 90 percent of the material ejected was in the landslide and most of the rest was in the pyroclastic flow. The total energy released was about 25 Megatons…or about the size of both atomic bombs used in World War II combined.
Trees were shredded out to a range of about 8 miles, knocked down out to a range of about 13 miles, and killed out another couple miles past that. Over 230 square miles of forest were obliterated…and even today we were astounded by the few numbers of trees and the large areas of essentially bare rock that remain.
Here is two superimposed shots of the before and after profile of the mountain…it’s a pretty dramatic change.
Finally here are the few decent shots that Neil was able to get. The first two were taken from east on highway 504 from the Scymansky location on the first map…about at the bottom of the first dip to the south heading east from there…and show the crater from about 10 o’clock (the opening of the U is almost exactly at 12 o’clock) from about 15 miles away. The last shot is of the corner of Spirit Lake…taken from the observatory about 7 miles from the lake. The mountain itself is about 90 degrees to the right from the view toward Spirit lake and about 5.5 miles away…but unfortunately it was completely hidden by the clouds rolling in…in fact the view to Spirit Lake was obscured completely less than 10 minutes after this shot was taken. I wish he had been smart enough to rotate the camera 90 degrees to the right after the lake shot and take one of the obscured peak…the difference in visibility was extremely pronounced as you could see nothing but clouds/fog looking toward the peak and nothing of the peak or valley between the peak and the observatory on the ridge.
Shortly after the photo of Spirit Lake was taken the ranger came out for a talk about the event…he has been at the observatory for 27 years (essentially since it opened about 3 years after the event). He had lots of before and after pictures and talked through the entire sequence of the event with a lot of hand waving and pointing at various topology features around the area to give listeners a great understanding of how the topology of the area, the tectonic actions prior to the event, and the exact effect of the various things in the sequence of the event caused the effects we see today. Like many of these presentations we’ve seen over the years but especially since we hit the road full time…actually looking at the ground where it happened along with a discussion by somebody who knows the facts and can relate what those facts actually mean in regards to being on the ground proved really fascinating.
We also watched the Park Service movie inside the observatory before the talk…which was sort of cool but actually was relatively non-informative compared to the ranger talk…and then headed home as the weather was getting pretty socked in by this time due to a front coming in from the southwest. There’s been a circulating storm out off of the Aleutian Islands the past week or so and the counter clockwise flow from it keeps bringing in rain clouds from the southwest causing it to be really rainy the past week or so. Connie drove down the mountain pretty much in the cloud bank for the first 10 miles or so then it was just overcast the rest of the ride home. We did stop by a Home Depot and get some measurements on a couple of Weber gas BBQ grills that allowed us to decide what to order to replace our charcoal grill which is about rusted out.
We had dinner and watched TV but went to bed early about 2200 since we had a 0330 wakeup call/alarm to get to Connie’s scheduled medical procedure on Thursday morning.
I already posted about the results of Connie’s test. It’s supposed to rain from now through Sunday but we’ll head out to see the Lewis and Clark National Historic Site whatever day between now and next Thursday that it’s nice weather.