Volcanism and fossilisation
As a well-known fact, volcanism provides favourable conditions for the
fossilisation of terrestrial habitats. The cloud from an eruption may
cover everything with a layer of volcanic "ash". This layer contains
fine-disperse silica which is leached by rainwater and precipitated on
the decaying organic matter trapped below (Fig.1). Most of the petrified wood
had been formed in this way.
The leaching of silica from volcanic deposits continues for a long
time. Swamps with new vegetation are fed with silica-rich water which
may turn into silica gel as it mixes with the low-pH swamp water. While
more silica enters into the gel by diffusion and replaces the water
there, the whole swamp matter, including water, mud, microbes, swamp
plants
and trees, and creatures, too, may finally turn into chert
(Fig.2).
Samples silicified in volcanic deposit turned
into tuff and those
silicified in swamp matter turned into chert may occasionally be found
at the same
site, as the ones shown here had been.
Fig.1
(left): Lower Permian tuff found as a loose block, slightly rounded,
with embedded
coniferous-type wood with shrinkage cracks filled with some white
mineral. Length of the wood fragment 18cm.
Fig.2: Lower Permian coniferous-type
wood in white chalcedony, apparently torn
asunder while lying in water in a soft degraded state.
Width of the picture 4.5mm.
As another way of silicification in the aftermath of volcanic activity,
hot
springs can provide mineral-rich water for millions of years, which may
result in numerous ponds and swamps in ever changing positions
[1]. Such scenario had created the stack of dozens of
fossil-rich chert layers
between sandstones, shales, mudstones, and siltstones at the famous
Rhynie
location [2].
Volcanic ash falls, other than lava flows, are caused by
explosive eruptions of magma containing large amounts of bound gases
which are released as soon as the pressure decreases as the magma
ascends. The explosion due to escaping gases can be so violent that the
magma is scattered
into a big hot cloud of very small droplets. What comes next depends
on the
average density or weight per volume of the cloud.
With the low-density hot air and water vapour between the droplets, the
density of the cloud can be less than the
density of the surrounding air. In this case, the cloud ascends
and slightly cools by expansion until an equilibrium is reached where
it keeps
floating while the droplets fall out like raindrops from a rain cloud
and arrive on the ground as volcanic ash.
If the cloud is heavier
than air, it plunges down, flows down the slopes and keeps flowing at
high speed, driven by
its momentum, over level land and even uphill, flattening everything in
its way. The strong turbulence of the flow
prevents most of the
droplets from falling down. As soon as the flow has
become too slow to sustain the turbulence, the droplets simply
settle down so that the avalanche is reduced to hot air. Such
phenomena, known as pyroclastic flows, can tear off and carry away
branches, upset tree trunks, and blow away whole swampy habitats. This
may be
one reason why big petrified tree trunks are often found lying by
themselves.
A few additional considerations may be appropriate here. In
some publications
the damage caused by the avalanche-like flow is ascribed to the
explosion pressure wave * of the eruption. The
power of the bang from an eruption spreads mainly upward and decreases
rapidly with distance while the power of the flow keeps concentrated
and thus reaches farther. As a simple argument, a pressure wave spreads
with the velocity of sound, which is about 1200km/h, but the
velocity of the spreading damage had been reported as varying between
about 400 and 1000km/h, hence
the phenomenon cannot be called a wave. An avalanche carrying along
with it a pressure distribution is not a pressure wave, and also the
moving pressure field of the avalanche taken by itself is not a
pressure wave. It appears that some volcanologists use
the term "wave" in a colloquial sense also for flow phenomena without a
real wave, which impedes deeper comprehension. Some geologists refer to
those volcanologists when they do the same [3].
A more sophisticated but likewise inadequate construct is the sidewards
directed explosion pressure wave,
occasionally offered as the cause by which the wood on one
side of Mt. St.Helens was flattened,
and even as the cause of the asymmetry of the
whole mountain after the big eruption in 1980. In
fact, first came the
mountain slide which brought about an asymmetry, then the asymmetric
eruption and subsequent pyroclastic flow.
Another questionable item is the simplistic
explanation of the explosiveness of volcanos
as being caused by groundwater. Such causal connection seems plausible
at first but not so
after a second thought. It is tacitly assumed by some volcanologists
that contact between
magma and water makes an explosion. This idea seems to be based on only
one observation: A drop of water explodes when it falls on a red-hot
plate. By doing it the other way round, or simply by imagining how it
would work out, one comes closer to the magma problem:
Dropping the red-hot plate into the pool does not make an
explosion but only boiling water, the same as magma does when flowing
into the sea. Also one can easily imagine how groundwater behaves
when
in contact with magma: It recedes, driven back by vapour through the
porous
aquifer, away from the magma and kept there at a distance. Eventually
fluids released by the magma will enter
into the porous layer where the water had been and deposit minerals
there.
There are drawings by volcanologists illustrating
how the water allegedly flows
from an aquifer right into the magma of an active volcano, where it is
supposed to increase
its explosiveness. (This view is held up at the Steinmann Institute
Bonn and elsewhere [3].) In reality, explosive magma results from
molten sediments whose minerals contain crystal water. This water
content keeps dissolved in the magma under pressure but is released as
vapour as
soon as the hydrostatic pressure reduces as the magma
ascends, which may result in a pyroclastic flow
as described above.
Samples: found near the upper end of a field which is now the
golf course at Wilmsdorf in the Lower Permian Döhlen Basin
near Dresden.
The tuff
with embedded wood was found by Sieglinde
Weiss.
H.-J.
Weiss
2013, 2014
Annotation 2014: To those who find the views of physics
difficult,"explosion pressure wave" is still a suggestive term suitable
for an explanation of the destructive power of
an eruption.
Example from Freie Presse (Chemnitz), 22.1.2014, translated: "The
explosion
pressure wave broke trees like matches" said Ronny Rössler, head of the
Naturkunde-Museum. (For the original newspaper clipping see German
version.)
[1] C.M. Rice
et al.: A Devonian auriferous hot spring system, Rhynie, Scotland.
J. Geol. Soc. London
152(1995), 229-250.
[2] N.H.
Trewin, C.M. Rice:
Stratigraphy and sedimentology of the Devonian Rhynie chert
locality.
Scott. J. Geol. 28(1992),
37-47.
[3] R.
Rössler: Der versteinerte Wald von Chemnitz. Museum
für Naturkunde Chemnitz, 2001, 182-191.
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