Transcript Hazards Chapter 3a

CHAPTER 3A
VOLCANOES
-
We must overcome a number of
stereotypes about volcanoes and volcanic
activity
--- “the Fuji Complex” – snowcapped and
mystical; dwelling place of the
gods; climbing it is a supernatural
experience
--- catastrophic event – molten rivers of
lava; poisonous gases; villages
buried under mountains of ash
ASIDE
If you think about it, it makes sense…
everything about volcanoes seems larger
than life. They are thus attention grabbing
and memorable
… and yes, sometimes large numbers of
people die or are displaced by them
But overall, volcanoes are not the death and
destruction machines that we often make
them out to be:
(1) volcanoes frequently give us warning of
their actions
(2) many volcanoes are located in rural
uninhabited places
(3) if the eruption produces lava flows rather
than poisonous gas or flaming
particulates, it is more possible to
evacuate and avoid
Formation of Volcanoes
- The most basic requisite for volcanoes to
form is the presence of a molten rock
reservoir – magma chamber – which is
under sufficiently high pressure to force
various forms of volcanic materials to
the surface
--- This volcanic material (ejecta) may be
ejected to the surface through a
central conduit (volcanic vent) or
through fissures
--- The source of magma is generally within
60 mi of the surface
[in vicinity of undersea ridges magma may be
as little as 15 mi beneath the surface]
- The composition of magma can vary widely,
and is a primary factor in the ultimate:
(1) shape of the volcano formed
(2) the nature of the volcanic activity
[presence of silica or single magma vs multiple
magma sources]
- Elements such as gas content
(sulfur vs CO2 or H2O) and pressure are
determinants of ejection violence
Aside: that’s how you can get the quiet
outflows of Kilaueau and Mauna Loa
and the violent explosions of Vesuvius
and Krakatoa
Interesting: H2O is the leading element in volcanic
explosions (about 70%)
… other compounds include carbon monoxide;
carbon dioxide; hydrogen chloride;
sulfur trioxide and give rise to VOG
Text tells us that the origin of the “heat”
driving volcanism and plate tectonics –
Earth heat – is debated. But, among that
accepted explanations are:
(1) large amounts of residual heat from the molten
period of the Earth
(2) decomposition of radioactive material in magma
producing zones
(3) tidal friction slowing of the Earth’s rotation may
contribute
(4) pressure and friction in the zones of plate
subduction melts rock and alters mineral
structure
Geographic Distribution of Volcanoes
- The volcanoes of the Earth must number in
the tens-of-thousands
… obviously not all of this estimate are
“active” – most are dormant or extinct
… common estimate of “actives” number
500
-
We can make some generalizations about
the spatial distribution of volcanoes:
--- because of the correlation of volcanic
activity with active tectonics: volcanoes
tend to cluster in regions of subduction;
mid-ocean spreading and ridges; rifting;
etc
… observation of the Circum-Pacific Ring
of Fire; the mid-Atlantic Ridge; the
East African Rift Valley; and the
Atlas-Alpine-Caucasus – mentioned
previously is valid here
Aside
Interesting: there appears to be a distinct
latitudinal distribution of volcanic activity –
“Two-thirds of the world’s volcanoes are
located in the northern hemisphere and
only 18 percent are found between 10o S
and the South Pole”
[Ebert, p.21]
Volcano Types and Activities
- Can be classifies in several forms:
(1) shape – cone volcano (Mt. Rainier / Mt.
Shasta) and shield volcano
(Hawaiian Islands)
(2) type of ejecta – a major factor in the
type of volcano formed
ex: basaltic lava low viscosity;
remain fluid at relatively low
temperature; relatively tranquil
eruptions; shield volcanoes
high silica content (SiO2) ash is acidic;
tending to solidify quickly; has high
structural strength; in conjunction with
lava ejection can build high
stratovolcanoes
… classifications are not necessarily distinct,
there are intermediate versions / mixed
characteristics
ex: lava with pyroclastic lava blocks as
in Mt. Etna – or
cones of fine tephra and lapilli
Spectrum of ejecta
- laval flows
pahoehoe
Aa
- volcanic mudflows
- glowing avalanche
- volcanic bombs and tephra
- toxic volcanic gases
(and VOG)
(3) eruptive activities – volcanoes may be
classed by their activity or phases
… some are in fairly constant eruptive
state… ex: Stromboli, Lapri Islands,
Tyrrhenian Sea erupts up to 4x an hour
or
Kilauea, Hawaiian Islands has been in
more-or-less constant eruption since
1983
… conversely, some may be inactive and
erupt violently and unexpectedly
Volcanic Hazards
… Historically, volcanic eruptions have
instilled us with a curious mix of
excitement and fear (almost hypnotic)
--- for the tourist they are innocuous entertainment
“once in a lifetime thrill”
--- for the local resident a daily potential threat
[but still exciting – we will pay to be scared]
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Interesting to me is the fact that despite
the potential for catastrophic destruction,
populations continue to inhabit and utilize
areas of known activity
-
For example: White, reporting on the
study, Human Adjustment to Volcanic
Hazard in Puna District, Hawaii
[populated area, east and downslope of Mauna Loa
and Kilauea, it is part of their rift zone]
-
90+% of the populatopn contended that
there were greater advantages to living
there than elsewhere
… 82% of this group felt that their
quality-of-life would be as good elsewhere
-
57% of the population saw a threat from
the volcanoes, but they saw it as a threat to
property not to human life
[that they are above the threat / can avoid it]
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They were asked questions about “adjusting” to the
perceived threat
(1) greater attention and monitoring of the
volcanoes
(2) if emergency situation occurs plans were in
place to activate emergency personnel, National
Guard, and interestingly enough the Boy Scouts
… a personal adjustment seems to be preparing
to be away from property (previous
eruptions up to a month living in local
schools)
… praying to ancestors for intercession
… prayer to Pele, and food offerings (food,
candy, liquor, flowers, ??? Boy Scouts??)