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 characteristicsex: lava with pyroclastic lava blocks

as in Mt. Etna – or cones of fine tephra and lapilli

Spectrum of ejecta - laval flows

pahoehoeAa

- 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

orKilauea, 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]

- 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]

- 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??)