Why have logs been floating on Spirit Lake after the Mount St Helens eruption in 1980? I was there a few years ago and they were still there. They call them tree mats. Pics are in "Gallery" section. Spirit Lake (Washington) - Wikipedia
Nope. I'll have to see if I can dig up the article I read this in. According to the author, rot only progrsses during a very small window of ideal conditions. It is when wood moves back and forth across that window frequently that it tends to rot quickly. Think about a post in the ground. The bottom of the post is likely to be at a high moisture content, the top out in the air low. Somewhere along it's length, the mc will be ideal for rot. That spot might move up and down slightly with the weather, but eventually it will go where the conditions have been right the longest.
The general would have 3 lights. It would go all the way up to red if it was past 17. I believe that it has more use to test kiln dried or lumber style wood. But noting that wood gone through that process, 17% is still considered “wet”
My guess would be after reading the wiki post is similar to what dingbat said, the lake is sub arctic. Mixed with the idea that o2 levels were so low for so many years. I'd venture a guess that the Ash and volcanic gasses in the water helped petrify the wood.
Oh I was thinking you knew but here’s the gist of how a pyroclastic explosion works. It HURLS super heated rock and dust. This killed the trees immediately, forces like this are intense and remember this was in 1980, nearly 40years later and still nothing grown back...This rock underneath us here is loaded with Silica, its very light when it cools and filled with air. When it coated the trees, it likely cooled and then preserved the trees well. Volcanic ash has its pros and cons. It’ll allow good growth in a garden but at this proximity, the ash was more than suffocating. Rendering everything dead and it just becomes a fallout zone, a cesspool that isn’t really living. It will be a long time until much is able to grow there. Here’s the descript in Wiki: After the eruption, Spirit Lake contained highly toxic water with volcanic gasesseeping up from the lake bed. A month after the eruption, the bacteria-carrying water was devoid of oxygen. Scientists predicted that the lake would not recover quickly, but the reemergence of phytoplankton starting in 1983 began to restore oxygen levels. Amphibians such as frogs and salamanders recolonized the lake, and fish (reintroduced by fishermen) thrived.[8][9]