Pyroclastic Flow &
Olivine Bombs
Pyroclastic Flows
A
pyroclastic flow, or pyroclastic density current, is an extremely
dangerous geophysical phenomenon that typically accompanies explosive
volcanic eruptions, which occur at volcanoes in the Caribbean - such as
the devastating eruption of Mount Pelée in Martinique in
1902,
which claimed approximately 29.000 lives due to a pyroclastic flow
(incidentally, this was the most destructive volcanic eruption of the
20th century). It is an avalanche of hot gases, ash, rock fragments,
and volcanic material that moves downslope at very high speeds (often
over 100 km/h, up to 400 km/h), burning and destroying everything in
its path with temperatures ranging from 200 to ove 1000 degrees
Celsius. Pyroclastic flows can alter landscapes, uproot trees, destroy
buildings, and bury everything in their path. Well-known examples of
pyroclastic flows include the destruction of Pompeji and Herculaneum
near Naples in 79 AD, which was triggered by an eruption of Mount
Vesuvius, or the effects of the eruption of Mout St. Helens in
Washington/US 1980.
Pyroclastic flows form when large chunks of rock break off from a lava
dome - often the entire lava dome collapses and explodes within the
volcano. This releases the gas dissolved in the lava, fragmenting the
lava. The steeper the slope of the volcano, the greater the likelihood
of a pyroclastic flow forming.
The driving force behind a pyroclastic flow is always gravity. A hot
cushion of gas reduces friction with the subsurface, and a pyroclastic
flow glides like a hovercraft a few centimeters above the ground -
hence its high speed.
In the solidified state, pyroclastic flows can be recognized in lava
rock by the lack of stratification (as in volcanic tuff), and the rock
is a mixture of volcanic tuff, volcanic ash, and various lava boulders,
ranging in diameter from a few centimeters to rocks weighing several
tons. Often, as here, volcanic bombs are embedded in the deposits,
which were subsequently ejected by the volcano and impacted the
deposits of the pyroclastic flow.
Pyroclasts
Volcanoes erupt more than just lava. They also erupt a
variety
of fragmentary material and volcanic gases (water vapor is usually most
common), especially during explosive events. The rapid exsolution of
gases from magma is the main drive of explosive eruptions, flinging
clots of lava into the air in some eruptions, and shattering it into
tiny bits (ash) in columns that may rise tens of thousands of feet into
the atmosphere. Collectively, the fragments ejected during expolsive
are termed pyroclasts, meaning "fire-broken" from the ancient Greek.
Pyroclasts may be ejected while still molten or partially molten, or
may consist of solidified magma or of other rock fragments.
Pyroclasts are classified by size and shape as follows:
- Ash: A pyroclast grain with a diameter less than 2 mm
- Lapilli: A pyroclast with a mean diameter between 2 and 64 mm
- Block: A pyroclast with a diameter greater than 64 mm and that has an
angular shape. Blocks are solid during eruption and transport
- Bomb: A pyroclast with a diameter more than 64 mm and that has
rounded or ellipsoidal shape that indicates that it was wholly or
partially molten during eruption and flight
Olivine Bombs
Olivine bombs are xenoliths from the upper mantle that
reach the
Earth's surface through volcanic and are hurled through the air by
explosive eruptions before being deposited, i.e., hitting or impacting
the subsurface. A xenolith is an inclusion of foreign rock within a
volcanic rock or plutonite. As it rises, the magma carries fragments of
the surrounding rock with it, enclosing them and overprinting them. If
these usually viscous, ejected fragments remain intact even after
solidification, they are called xenoliths. Olivine xenoliths are not
uncommon in the deposits of volcanoes with explosive eruptions (such as
tuff or pyroclastic rock) and are usually recognizable due to their
olive-colored, crystalline structure.
Your task to log
the
Earthcache:
Answer the following questions via message in English or
German via my geocaching profile:
1.) In the vicinity of the coordinates, you can clearly see several
olivine bombs embedded in the rock with the naked eye. Describe their
shape and size!
2.) Based on their shape and size, what type of pyroclasts are they?
3.) Describe the differences between pyroclastic flow deposits and
olivine inclusions in your own words!
4.) Were the olivine bombs molten or solid when ejected? Explain your
opinion!
5.) Optional: Post a photo with your log, showing you and/or your
GPS/or something identifying you
near the location!
After you've sent me the
message with your answers, feel free to log! If there's
something wrong,
I'll contact you!
Sources:
en.wikipedia.org
Rietze, M. (2010): Vulkane. Einführung in die Welt der
Vulkane. Darmstadt
Schmincke, H.-U. (1988): Pyroklastische Gesteine. In:
Füchtbauer, H. (Hrsg.): Sediment-Petrologie, Teil 2. Stuttgart
www.mineralienatlas.de
www.nps.gov
www.spektrum.de
www.vulkane.net
The pictures were taken by the author.
Enjoy the location and the great
view!
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