Download Mount Vesuvius and the Nature of Volcanic Eruptions

FEBRUARY 10, 2006
President Jim Fairchild called the meeting to order and welcomed everyone. He
mentioned the Burpee Museum PaleoFest coming up February 18 and 19. He also
reminded everyone that the Mazon Creek Open House would be at the Lizzadro
Museum November 11, 2006 this year. Dr. Wendy Taylor would be one of the speakers.
On February 18 Frank Pranschke will be having the Northeastern University Teachers
Open House for the fifth year. Our own ESCONI March Show will be March 18 and 19
and John Good said he needs help for many aspects of the show and has sign up
sheets here for help. There will be warehouse help days. Mineralogy will meet
tomorrow on Nova Scotia minerals. Jim then introduced our program for the evening
which followed an excellent and exciting Junior’s Program run by Dr. Wendy Taylor of
the Field Museum on ‘Daily Life in Pompeii, the Story of Vesuvius’ . The juniors made
some very creative mosaic tiles and volcanoes and learned about life there in 79 AD.
Her husband Dr. Phil Janney from the Department of Geology at the Field Museum was
our speaker for the evening.
Mount Vesuvius and the Nature of Volcanic Eruptions
Mt. Vesuvius blew at the height of Roman power when their kingdom was at its widest in
79 AD surrounding the Mediterranean basin. It involved about a quarter of the Earth’s
population at that time. This volcano and its neighboring volcanoes are the reason for
the rich soil that has made the area so desirable for growing grapes and olives. Pompeii
was an industrial and commercial center. But on August 24, 79 AD it was a different
story. Actually 17 years before that date the area had suffered serious earthquakes
related to the volcano but without any volcanic activity from it. About half of the buildings
in the city suffered some damage. So in 79 AD, when the earthquakes came again, the
fear was there, but not enough fear to cause them to flee what was to come. The clouds
that poured out of Vesuvius were not particularly threatening at first at about noon and
the ash that started to fall was light at first. By 6 pm the ash was falling at the rate of
inches per hour. Most of the residents by then chose to flee. The reason that we know
what happened at that time is because it was witnessed from a distance by Pliny the
Younger, the nephew of Pliny the Elder, from Mycenum some 30 km away.
At 8 am the next morning those who had not left suffered the end as the pyroclastic
surge full of gas and ash flew through the city at 100 mph killing all who were left at
temperatures of 1,000 degrees F. It burned flesh away and carbonized the bone. After
this eruption it was desolate and a slice of Roman life was preserved and buried. And
buried deeply as it was, not after destruction by war and looting. Many of the frescoes
and mosaics, usually lost were preserved, as well as many of the every day things of life.
An early fast food shop where hot food was cooked in pots for people to eat on the way
to or from work was found on one of the main streets.
Looking at a volcano like Vesuvius you have a vent rising up from the Earth’s crust from
which molten lava, gas and pyroclastic materials issue. Most are conical but they do not
have to be. It is the ash that forms the steep conical cones. Molten rock is called
magma when it is in the Earth and lava when it is out side the Earth. The Earth has
three main layers; the continental or oceanic crust which is 4 to 50 miles thick and is
made of plates; the mantle which is a rocky solid layer of iron, silicates and it does flow
but is not molten, and the core. The Earth has 15 main tectonic plates that move and
the major volcanic activity is where they meet. An example is at the mid-Atlantic ridge
where the North American plate is separating from the Eurasian plate. There are also
‘hot spots’ around the world, like Hawaii and Yellowstone, where melts a short distance
below the crust form flood basalts. The third type is where two plates are coming
together. It is usually at a convergent plate boundary where one is oceanic. An
example is the ring of fire around the Pacific Ocean. An example is the earthquake that
caused the December 24 tsunami last year in Asia.
Water is the key. Water laden sediment and water laden crust is subducted and is
heated as it is subducted. As some of the water gets deeper it lowers the melting
temperature of the mantle and causes it to rise and make its way to the surface. As it
nears the surface the water comes out of solution and helps to make the eruptions more
violent. The African plate is pushing under the Eurasian plate and Italy is the result of
the crust on the African plate uprising. The Roman Volcanic Province has many
volcanoes along the belt with Mt. Vesuvius being one of them. Mt. Vesuvius is actually a
volcano within a volcano. There is a ridge on the north side that is from a pre-existing
larger volcano that blew out 25,000 years ago. It blew itself almost all away and Mt.
Vesuvius grew up in its place.
A pelean eruption is the most violent type – it is an explosive fragmentation of a viscous
gas rich magma. These are the ones that cause the most deaths and destruction. It is
difficult to find shelter from the surges. Its characteristics include:
1. large eruption columns up to 7 to 12 miles high that level off and spread up to
200 to 300 miles. This causes fallout of tephra (ash and volcanic debris).
2. Huge pyroclastic surges or flows up to 100 mph and 1,000 degrees F.
3. Releases of large amounts of volcanic gases.
Volcanoes are classified by the extent of their gas content and the magma viscosity
which is determined by temperature and silica content as shown below.
Gas
content
Pelean
Vulcanian
Strombolian
Hawaiian
Magma viscosity
Silicon dioxide concentration increase
Temperature increase
As gas content increases, magma viscosity increases, the intensity of the eruption
increases. It is like shaking a bottle of champagne. The increased pressure is forced
out of the narrow neck of the bottle like the pressure of the magma is forced out the
narrow channel through the rock as the gas bubbles come out of solution. If the vent is
blocked there will be a huge explosion. This was the case with the explosion at Mt. St.
Helen’s.
Tephra is defined as ash and volcanic debris.
Volcanic ash is defined as ash less than 2 mm in size.
Lapilli is defined as ash that is 2-64 mm in size (little stones).
Pumice (textural term) is defined as ash of any size. It is up to 90% of gas bubbles.
Much of it will float.
Pyroclastic surges can accompany the initial surge or the collapse of an eruption
column. On August 25, 79 AD the cloud went 12 miles into the atmosphere and in about
one hour ash began to fall in Pompeii. In about two hours the cloud collapsed and this
began the series of 6 pyroclastic surges. The largest one was at 8 am and reached
Pompeii. Herculaneum was the first city found when they were digging a well. It was
buried under 4 meters of ash. It was 3 times the eruption of Mount St. Helens. There
were no lava flows - only pyroclastic flows. At Herculaneum, there were no body casts
because they got the first pyroclastic flows before they were buried in ash. Most were in
caves hiding. The bones were carbonized at that high temperature. At Pompeii the
bodies were buried by ash before the surge. Mt. Vesuvius has erupted 30 times since
79 AD. In 1631 there was a Pelean one. Now there are one million people living within
10 km of the volcano. They have earthquake monitors, geodetic monitors to study the
shape of the Earth, and gas sampling going on regularly. Let’s hope they can save
some of those lives next time.
Phil and Wendy had brought with them many books on Pompeii and artifacts from
Pompeii. The meeting was adjourned for further discussion.
Respectfully submitted, Karen Nordquist, Recording Secretary