Interlude: When Vehicles Become Part of the Geologic Record

The conversation might have gone something like this:

Geologists: ?Hey, boss person, we need to order vehicle parts and then destroy them. For science!?

Boss person: ?Ummm? okay.?

The thing is, things happen to vehicles when they?re caught up in a directed blast. What the volcano did to them can tell us a lot about what was taking place inside that blast cloud. Vehicles in the blast zone at Mount St. Helens sustained all sorts of damage. See if you can figure out what this is without looking at the caption, which I will cleverly not put directly under the photo:

Vehicle damage! Lots of it! And now you know why I filch their captions. Image courtesy USGS.

I?d say the eruption severely depreciated the value of this vehicle. If Captain Jack Sparrow were here, he?d be asking, ?But why is the back seat gone?? That?s what the USGS caption says we?re looking at: ?Back seat of vehicle heavily damaged by May 18 eruption, near Meta Lake 13 km northeast of Mount St. Helens.? That is an ex-back seat. Still, I suppose it fared better than this poor thing, which nobody appears to be able to identify:

Interior(?) of vehicle damaged by May 18 eruption of Mount St. Helens. Cowlitz or Skamania County, Washington. June 1980. Image and caption courtesy USGS.

Note the caked-on volcanic deposits. Yum!

What happened to those vehicles tells geologists a lot about what conditions were like within that directed blast. Granted, we had eyewitnesses ? stressed, terrified, traumatized, fallible human witnesses. That fallibility is why scientists turn to inanimate objects to verify and further study the effects of the eruption. These inanimate witnesses testify, too: they become part of the geologic record, and while you can?t grab a perfectly healthy person off the street and be all like, ?Hey, we?re going to expose you to different levels of heat and stuff until we?re able to replicate the burns these other people suffered! Won?t that be fun?! For science!? you can certainly order some auto parts and proceed to be as volcanic as you like.

First thing before you reach for the auto parts catalog, though, is field observations. What do we see when we inspect these vehicles in the field?

Vehicle heavily damaged by May 18 eruption, near Meta Lake 13 km northeast of Mount St. Helens. Skamania County, Washington. June 18, 1980. Image and caption courtesy USGS.

I don?t think we?re going to be able to trade that one in?

The directed blast was powerful, people. So powerful that even heavy logging equipment was sometimes moved, even flipped over, by it. Geologists saw battered and scorched vehicles up to 13 kilometers (8 miles) away from St. Helens: farther out, they were ?only sandblasted.? Think about this for a minute, and look at the car above, and realize that?s eight miles from the source of the explosion and there was still enough power to batter that car almost beyond recognition.

How, you might ask, did it do that? Well. Have a look at what it was hurling.

Damaged bed of a pickup truck located on the north side of the South Fork Toutle River, about 6 km west of Mount St. Helens. The large rock (partially in shadow) is a piece of dacite hurled into the pickup's bed during the eruption of May 18, 1980. Cowlitz County, Washington. June 30, 1980. Image and caption courtesy USGS.

And a double-take:

Different view of damaged bed of pickup truck located on north side of South Fork Toutle River, about 6 km west of Mount St. Helens. The large rock (partially in shadow) is a piece of dacite hurled into the pickup's bed during the eruption of May 18, 1980. Cowlitz County, Washington. June 30, 1980. Image and caption courtesy USGS.

Imagine the force of an explosion that can take a chunk of solid rock ? and that sort of massive dacite is compact and heavy ? and hurl it 6 km (over 3 miles) with enough force to deform a 1980s pickup truck. In fact, Mount St. Helens heaved boulders up to 30 cm (about 12 inches) into vehicles. The blast wasn?t quite as powerful to the west: rocks that size only got thrown 5 km (3 miles), whereas to the north, the blast cloud was able to carry them up to 15 km (9 miles). Fifteen.? Not rolling and bouncing them, either: they were carried at least 1.5 meters (almost 5 feet) above the ground (although a few may have been tossed up by falling trees). And if you think that?s mind-blowing, look at this:

Aerial view of scattered remnants of logging operation north of Mount St. Helens. This clear-cut area was buried in several feet of ash from the May 18 eruption. Photo by J. DeVine. Skamania County, Washington. June 1980. Image and caption courtesy USGS; cropped from original.

What is this I don?t even.

Damaged logging equipment on Coldwater Ridge, north of Mount St. Helens. Skamania County, Washington. July 2, 1980. Image and caption courtesy USGS; cropped from original.

I don?t know about the first photo, but judging from the fact the logging equipment is twisted into an unrecognizable mess, I?m going to hazard a guess that we?re well within 13 kilometers. The second photo was within 8-10 km (about 4-6 miles). Amazing what a directed blast can do to heavy equipment.

So let?s see what these vehicles are testifying to, as they are now part of the geologic record.

Firstly, you wouldn?t want to be within 13 km of the volcano during this eruption. In fact, I couldn?t track down any survivors inside that zone.

Secondly, we know that vehicles (not to mention trees, but we won?t because we?re saving those for a future installment) were overturned early. How? Because stratigraphy. Geologists saw that within 15 km (9 miles), vehicles that hadn?t been flipped over had the same order of deposits they were finding on the ground. Vehicles that have been overturned also have the same stratigraphy as the ground beside them, showing that the blast cloud knocked them about in the first erosive moments, then dropped material atop them.

Sometimes lots of material. Look at Reid Blackburn?s car at Coldwater I:

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Reid Blackburn's car, located at Coldwater I observation station, approximately 10 miles (sic) from Mount St. Helens. Reid was a photographer for National Geographic as well as the Vancouver's Columbian newspaper. U.S. Geological Survey image taken on May 22, 1980 by Pete Lipman. Image and caption courtesy USGS.

Coldwater I was actually about 13 km (8 miles) away ? within that zone where havoc was wreaked in earnest. The whole sequence is here, and you can see how deep it is. Geologists found that the deposits entered laterally, blown in through the windows, which had been shattered in the initial phase. There was still enough force even that far away to hurl things sideways.

(And this is where I consider the fact that Reid didn?t stand a chance, and pause for a moment to wish fiercely he?d been just a little farther away.)

That?s some pretty serious volcanic action, and we haven?t even got to the heat, and why geologists were flipping pages in automotive catalogs looking for things to destroy. We?ll hit that up in our next installment, wherein I will show you how surprisingly informative melted plastic is. Especially when scientists are set loose with some nice, fresh auto parts and a heat source or two?

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References:

Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.

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