ps418

I realize that you and Boro Nut are being facetious, but let me point out that both the Laetoli and Italian footprints were actually impressed not into lava, but into fine volcanic ash, which, once solidified, is called tuff.

Patrick

EDIT: Arvel beat me to it.

David M. Payne

I never said they were walking in hot lava BN, I just relayed the story. I would assume that they were walking in something soft and it was later fossilized.

David

SULPHUR

How many fossils have been found in tuff.?Or impressions?

ps418

Fossils, particularly well-preserved fossils, are fairly rare in tuff. More common are molds of trees and things like that there were buried initially, but then decayed/disintergrated leaving a mold behind. If you're too close to the ash source, the ash can be hot enough to burn anything above ground. Sometimes carbonized or silificied trees are found in tuffs.

There are some spectacular examples though. For a great little book on the subject, see Lockley, M., (ed). Volcanism and Fossil Biotas (Geological Society America Special Paper, No 244).

One of my favorite examples is Pompeii, which was buried by about 6 meters of ash from nearby Mt Vesuvius in 79CE. Though the plants in the gardens of Pompeii were not preserved, molds of their roots were preserved, which tells us what they were growing. And because they built cities in stone, virtually the whole city was preserved.



Another great example is Ashfall Beds State Park in Nebraska, which contains many exquisitely preserved rhinos, camels, horse, turtles and other vertebrates preserved in Eocene-age tuff (10 million years old).






Impressions are fairly common in tuff (dinosaurs tracks, for instance). But I wouldnt venture a guess about what percentage of tuff horizons contain such impressions.

Patrick

SULPHUR

Iam sorry that I didn't make my question quite clear. Obviously the the first ejected material,usually ash, will mould and preserve the landscape and with the continued deposition the coarser material known as tuff will form tens of metres of dense material which in most cases is not going to have a living thing wandering around in usually toxic conditions.Iam talking about alarge scale volcanic eruption. To go to the extreme case such as a Pelean? type eruption which hit St Martinique I doubt anything would be walking around.

SULPHUR

We may have a problem of definition here.To me ash is very fine powder.the coarser material mixed with ash is tuff and the larger angular material from about 2mm is breccia .This may differ from your idea of the deposits. I hope we can come to an agreement

ps418

The accepted definition of tuff includes any indurated pyroclastic rocks with grain sizes finer than 4 mm. Tuff is simply consolidated ash. You are thinking maybe of lapilli.

Patrick

ps418

Here's an illustration to help classify pyroclastic rocks according to grain size.

Patrick

SULPHUR

THank you for your reply.I noticed you used the word stone Professor Carey would have thrown anybody out if they used that word in a report. ON a more serious note I thought this would be a problem. I don't accept the arrangement of the the diagram.An example is the grading of diamonds where a 2mm fragment is said to be found in breccia using south afrrican terms.It comes down to say seperating a mudstone from a siltstone. The parameters are arbitrary and depend on who is teaching the subject and who wrote the textbook.I can't give a definite answer at the moment as all my text books are in storage.Maybe I'll have to haunt a library.

My respects tom

ps418

Diamonds and the kimberlite pipes they are found in are intrusive igneous rocks, not pyroclastic or extrusive igneous rocks, which is what we are discussing here. A different set of terms is used to describe intrusive igneous rocks.

And yet another set of terms are used to describe sedimentary rocks. Silstones are are lithified sediments with grain sizes 0.06 to 0.004 mm, while mudstones are <0.004mm.

Patrick