Volcanic ash layers

[wayreth]

Always wondered how you figure the compaction of ash layers on , trilobites ? if ash layer over the trilobites is about 4 inches , just how deep was the ash after the volcanic activity stopped , I've dug trilobites in Utah and spend most of my time digging gold in Colorado , while digging for gold spotted a ash layer while cutting into the hillside. sparked a interest in the ash, and how it covered the planet at one time or another , any ideas where to look for the info?

[Joe Botting]

The thing is with ash, that virtually every bed is different. Most are made of siliceous glasses to some extent, which are unstable and transform to clay minerals over time. In the process they become much denser, and can compress dramatically. On the other hand, ashes are also often cemented very early on, making them resist any later deformation... and which one happens will depend on the composition, the texture, and many other factors.
The best way to assess compaction is actually fossils - you know what shape they were when they were buried, so you can see how much they've been compacted since then, and that gives you a marker for the whole bed. If they had robust skeletons, though, it's only a minimum estimate. Also look for nodules that preserve original lamination - they will protect it relative to the rest of the bed, and give you a compaction ratio that way. In many cases, though, there's just no easy way of telling...