There's a fundamental fact one learns about trees when growing up in dry country forests: they're flammable. Folks in Flagstaff, Arizona can tell what part of summer it is by the smell. If it's all piney-fresh, it's May or early June, and everything's still safely damp from the spring snowmelt; if it smells like warm turpentine and dust, it's mid-June; and if it smells like winter with all of the fireplaces cozily burning logs, its the late-June-early-July dry-lightning season, and you're hoping the monsoon rains come before the whole county burns. I've seen smoke that looks like a volcanic eruption billowing from fierce fires. I've felt like someone caught in the middle of the apocalypse. I've choked on wood ash on hot summer nights. Our forests gets so dry you find yourself avoiding heated language in them. Our trees ceased being lovely green oxygen-producers with sweetly-scented wooden trunks and become tiki torches, just waiting for one stray spark to light the place up.
You can imagine my relief when I moved to the Pacific Northwest and discovered that the trees on the western side of the Cascades are usually too wet to burn. But they're still made of wood. Apply adequate heat, and they'll at least char. Raise the temperature enough, and you can even persuade them to burst into flames.
The geologists who studied the cataclysmic May 18th eruption can tell you precisely how much heat you need to barbecue a west-side PNW tree in May: they experimented. Their
adventures in pyromania sober science revealed you need temperatures of around 350°C (662°F), give or take 50°C (122°F), to achieve a nice, deep char. If you want just a thin crust of char with a nice unburnt center, turn the heat down to about 300°C (572°F), plus or minus 50°C (122°F). And if you just want a nice seared tree that's perfectly raw beneath, yet still very dead, you'll probably want to keep it between 50-200°C (122-392°F).
And yes, Mount St. Helens was perfectly capable of those temperatures. It could also serve up trees en flambé in certain sectors.
You might expect due north to be the hottest part of the blast, but it was actually the west-northwest and northeast sectors that endured the worst. Also, it was very not good to be the side of the tree facing Mount St. Helens. The closest trees, of course, were pulverized and incorporated into the blast deposits. Where the blast cloud left the trunks standing, it stripped the bark from the near sides, then charred the wood black to thicknesses between .1 mm up to .5 mm, which may not sound like much until you consider the fact these trees were several kilometers away, and all the bits of ash and rock riding the blast sandblasted off some of the char. Out to as far as 15 km (9 miles) north, the trees got scorched on their near sides – and that wasn't even the hottest part of the blast.
In areas where trees were afforded a bit of protection by a ridge or a goodly amount of distance, the bark stayed on, but suffered if it faced volcano: it was "dried, cracked, blistered, and partly detached from the underlying wood." But if you walked round to the far side, the bark was intact and not cooked. Sap even continued running under it for several months, until, like a headless chicken whose nervous system finally gets the memo that the brain is now in a bag, the remains of the trees finally finished dying.
So what are these severely singed trees saying about the blast? Well, the fallen ones testified that the burst of searing-hot gas that scorched and bent their roots either outlasted the portion of the blast that knocked them flat, or followed that unhappy event. Other evidence, which we'll explore further when we talk about the blast deposits, indicate that the hot gas wave followed the leading edge of the blast cloud.
The burst of burning-hot gas didn't distribute its temperature evenly. The hottest gas from the cryptodome escaped to the northeast, with another, extra-hot lobe roared off to the northwest. We know this because the surge of gas left trees smoldering and burning throughout the area. Eleven days after the eruption, geologists had a look through night-vision goggles, and discovered fires still burning out to 15 km (9 miles). The west-northwest and northeast sectors were orders of magnitude more fiery than the north: two orders to the west-northwest and a whopping three to the northeast. Wood fragments in the deposits in those sectors were baked into vitreous charcoal up to a centimeter thick; some of the smaller trees are very well done, deeply charred on the outside and brown and brittle on the inside. Some of these trees kept burning for weeks. Weeks. In the PNW. In May. The eruption was powerful enough to defeat a soggy west side spring, which is a pretty amazing feat.
Trees and tree fragments and other sectors fared relatively better. The bits weren't as thoroughly baked in the northwest-northeast sector, so that seems to have been somewhat cooler. (Not that you would've wanted to dabble your toes in the resulting blast deposit – it was certainly too toasty for toeses.) The southern part of the devastated area on the volcanoes east flank contained completely unburnt twigs, and the heat only ever got intense enough to yellow the incorporated needles, so that part of the blast wasn't very hot at all. The west flank is a different story – the fragments and trees there are scorched and rather wide zone, showing that part of the blast was searing-hot. If you wanted your trees done medium-rare, this sector was probably your best bet.
Now, here's the really wild part: all that searing, scorching, en flambéing hot blast madness doesn't gradually fade out: it ends abruptly. The standing trees in the scorch zone show a sharp demarcation between singed and unsinged. Geologists saw the base of the scorch pattern climb "through the trees from the inner edge to the outer edge of the scorch zone." Those pattern showed that the blast cloud rapidly lost energy and density, and when it did, it lifted like the lid being yanked off the pièce de résistance by a particularly theatrical chef.
So there you are: if you want to cook a whole forest of soggy PNW trees in the spring, all you need is a volcano with a hot cryptodome bulging out its side, an earthquake, and a nice toasty lateral blast. Shame about the uneven temperatures, but that's one of the hazards of cooking volcanically.
Lipman, Peter W., and Mullineaux, Donal R., Editors (1981): The 1980 Eruptions of Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1250.