Volcanoes

Physical Dynamics On average, about 50 volcanoes out of the ~650 believed active erupt each year

• Active means we KNOW they're active

• Dormant and extinct could mean extinct or just that we don't know about an active volcano's long cycle

Volcanic eruptions seem to follow the magnitude-frequency pattern we've seen with earthquakes and floods:

• Most eruptions are small potatoes

• Cataclysmic ones happen perhaps once every thousand years or so

Even so, at least 200,000 people have been killed by volcanoes in the last 500 years

• Directly by the eruptions' various effects

• Indirectly by starvation

• The worst one in our century took place in 1902, when Mount Pelée (very aptly and accidentally named, given that the Hawai'ian goddess of volcanoes is Pele) on the island of Martinique sent a nuée ardente into the town of St. Pierre and killed 28,000 people, including all but two of the town's 25,792 inhabitants (one in a jail awaiting execution)

Geography of volcanism

• Very similar to that of earthquakes, because the plate tectonic forces that produce earthquakes also involve magma formation and release

• Pacific Ring of Fire

• The east-west band of mountains running across northern Africa, central and southern Europe, the Middle East, the Himalayas and related ranges, down to Southeast Asia

• Also, intraplate vulcanism, or hot spots

  • Hawai'i

  • Tibesti Massif in North Africa (northwest Chad)

• Very common but mostly liquid, runny eruptions ("effusive") in constructive zones

• Very common and sometimes very violent in destructive zones, especially where continental crust is involved

• Runny in hot spot volcanoes in oceanic plates

Eruption types

• Explosive

  • Result of high viscosity magmas (sticky) with a lot of gas in them, which can build up tremendous pressure, so, when it's released, it's explosive as the gas suddenly expands at the top of the vent

  • Examples:

    • Vesuvian: extremely violent, gassy explosion

    • Plinian: even more violent, sometimes involving caldera collapse and a huge jet of gas that carries clouds high up in a vertical column

    • Peléan: a viscous and gassy magma is blocked by a very resistant plug, pressure sometimes creates flank vents and nuées ardentes

• Effusive or runny, low viscosity, liquid eruptions

  • Magmas are often very hot, very mafic, and have little gas incorporated into them, making the magma very low in viscosity

  • Examples:

    • Hawai'ian: very runny for the most part, emanating from fissures, calderas, and pits; sometimes some gas will create lava spray fountains; very little ash

    • Icelandic: fissure eruptions quietly release lots of free- flowing basaltic magmas often as sheets

Lava types:

• Very mafic magmas are basaltic

  • Solidify at high temperatures (Bowen reaction series)

  • Very fluid, low viscosity

  • Generally very dark

    • If it solidifies in a ropy, smooth rock: pahoehoe

    • If it develops crusts and is brecciated: a'a

    • Frequently the source of obsidian (instant cooling)

• Very felsic magmas are rhyolitic

  • Solidify at cooler temperatures

  • Very viscous

  • Often contain a lot of acidic gases

  • Generally light in color

  • Pumices common (glasses incorporating gases)

  • Commonly produces tephra (ejected solids)

    • Huge volcanic bombs

    • Volcanic blocks (> 32 mm)

    • Lapilli (4-32 mm)

    • Ash (< 4 mm, the size of sand and gravel)

    • Dust and ash may be solidified together as

      • Tuff (ash ejected from a vent during an eruption, settled around vent, and then lithified)

      • Ignimbrite (layer of pumice carried in a pyroclastic flows and surges and then welded by heat and pressure)

      • Breccia (large broken chunks welded)

    • Intermediate lavas are called andesitic

Volcanic landforms:

• Cinder cones: symmetrical products of explosive, smaller volcanoes

• Shield volcanoes: gently sloping products of effusive eruptions

• Stratocones: alternating effusive and explosive (scenic, tall)

• Calderas: Huge craters

  • Often the result of a magma chamber's collapse (subsidence)

  • Volcanic plains and plateaux

• Gigantic outpourings of very low viscosity basalt

  • Columbia-Snake River Plateau

  • Deccan Plateau of south central India (between the Ghats)

  • Southern Cascades (Table Mountain near Oroville)

  • From great fissures rather than point craters

  • Interesting theory: extraterrestrial impactorsmay trigger trap eruptions on the opposite side of the planet. There's been speculation that Chicxulub might have triggered or accelerated the Deccan Traps, which were nearly antipodal to Yucatán at the time: https://www.livescience.com/50737-deccan-traps-chicxulub-impact-linked.html.

• Supervolcanoes:

  • Have produced just gigantic eruptions on the order of at least 1,000³ km in a single catastrophic event

  • This entails a large magma chamber filled with silicic, acidic, and gassy magma that, when the pressure on it is released, explodes with such force that the caldera collapses, which amplifies the original eruption and creates vast pyroclastic and ash deposits that can rival trap basalts in scale.

  • These can cover continental scale areas to some depth with these deposits and create global climate cooling for years due to the massive amount of sulfur species ejected into the atmosphere (this raises albedo) and fine dust (which creates shading/cooling).

    • Yellowstone is an example, creating three massive caldera eruptions 2.1, 1.3, and 0.67 million years ago.

    • Toba in Sumatra, Indonesdia, produced a massive eruption around 74,000 years ago, the largest volcanic eruption in some 25 million years. This altered world climates to such an extent for so long (perhaps 1,000 years) that it may have killed most human beings alive at the time, creating a genetic bottleneck that significantly reduced human genetic diversity at that time: We really are all "cousins" no matter our more recent differences! The magnitude and spatial reach of this disaster has been questioned and, though humans show signs of at least one genetic bottleneck around 70,000 years ago, no smoking gun directly links the two events.

• Lava tubes: cooling from ground and sky -> flow within

• Undersea volcanoes: actually pretty common, given that ~70% of the earth's surface is oceanic and the basaltic ocean floor is the product of volcanic activity. The most recent such event is Hunga Tonga-Hunga-Ha'apai in the island Kingdom of Tonga north of New Zealand and east of Fiji. Check out the eruption: https://www.earthobservatory.nasa.gov/images/149347/hunga-tonga-hunga-haapai-erupts

• Pillow basalts are mounded basaltic flows when lava reaches water, either from a terrestrial volcano that produces a flow all the way to the sea or a deep lake or from a submarine volcano that erupts underwater. The water instantly quenches the hot lava and it forms a crust with a pillow-like shape. This surface is then ruptured by the ongoing lava movement, which, again, quenches into another mound and so on. The deeper the water and the greater the water pressure, the smoother the pillow structures are; if the lava quenches in shallow water, it may have a frothy upper surface as gas is being released in the low pressure environment. Here is an image from the USGS: https://www.usgs.gov/media/images/a-photo-basalt-pillows.

• Tuyas are weird, flat-topped but steep-sided volcanic edifices that form under a polar ice sheet or a thick glacier. The lava is quenched by the ice and, confined by the weight of the ice, can't flow very much under that crust, so it just masses up, trapped by the overlying ice. If the eruption is massive enough, it may melt or sublimate its way through the glacier and then form a more ordinary volcanic structure (and explosive volcanic hazards) once it escapes.

Volcano eruption prediction

• Earthquake swarms up to ~ M_w 5 and 700/day

  • Type A: 1-10 km below a crater

  • Type B: > 10 km below

  • Caused by magma stressing rocks it moves through

• Harmonic tremor:

  • Nearly continuous vibration

  • Dominated by a single frequency ("harmonic")

  • Caused by the magma itself or heated fluids

• Fumaroles (volcano-related steaming springs): gas changes (especially more sulphur)

• Thermal anomalies in lakes, hot springs, and fumaroles

• Heating raises agitation of rock to where any iron-bearing mineral in it it loses magnetism

  • Hard to relate to time of eruption:

  • Days or a year or more

• Sometimes the magma just intrudes and never makes it to the surface (as happened in Long Valley Caldera in the eastern Sierra around Mammoth Lakes back in 1980 -- http://pubs.usgs.gov/fs/fs108-96/)

• Prediction is difficult, then, and the legal ramifications complicated

  • If the USGS were, on the basis of all signs, to issue a volcano eruption warning ... and made a Type I error, people and businesses in the area would suffer stress and loss of business and might sue (as happened in 1980 in Long Valley)

  • If, out of concern for the costs of a false positive, they were to hold off on a warning, and made a Type II error, people would be killed and there would certainly be lawsuits over that or political unrest (as seen after the Nevado Del Ruiz eruption and lahar in Colombia in 1985).

Social Dynamics

How human and other animal deaths are produced

• Lava flows minor, given their typical speeds of 1 m/day - 3 m/sec

  • It can happen, though

  • An example: 1977 Nyiragongo in Zaïre: a lava lake spilled over, reached speeds of up to 60 km/hr and overran 400 homes and killed 72 people)

• Doming of a rhyolitic or andesitic mass, which collapses and triggers avalanches

• Ashfall

  • Suffocates people and animals

  • Can bury them (as in Pompeii: ~16,000 people died)

  • Can also bury farmland and destroy equipment -> crop failure

  • Can kill farm animals through poison or ruining their feed

  • Can produce a winter effect globally

  • Nuées ardentes

    • Glowing clouds or avalanches of ash, dust, and gas ~ 600°C

    • Can travel up to 100 km/hr up to 10 km out

    • This is what happened at St. Pierre, Martinique

  • Pyroclastic flows

    • Like nuées ardentes: denser rock froth and debris flow suspended in gas

    • These can get up to 200 km/hr and get out 25-35 km

  • Air pollution by poisonous gases

    • What probably actually killed Pliny the Elder on the beach waiting for a boat to escape Vesuvius during the Pompeii eruption

    • Sometimes there's no actual eruption

    • Gases from a quiet volcano build up at the bottom of a lake and then suddenly are released: at Lake Nyos in 1986 and Lake Manoun in 1984 in Cameroon

    • The Nyos incident happened at night and CO₂ collected in low parts of the land, like a ground fog might, and suffocated 1,887 people. Here is a link to a collection of BBC stories on the event: https://www.bbc.co.uk/search?q=Lake+Nyos

    • Sulfur gas damaged 130 sq. km. of coffee crops near Masaya

    • Volcano in Nicaragua in 1946

  • Lahars: volcanic mudflows

    • Volcanoes might release a lake in a crater or snowcap

    • Nuées ardentes might hit a stream

    • Instant mud, flowing up to 50 km/hr on gentle slopes and twice that on steep slopes

    • Very big killers: thousands of people

So, what brings people into contact with volcanic forces?

• Volcanoes often produce excellent agricultural soils, especially in the humid tropics. Humid tropical climates produce so much rain that fertilizer is drained out of the topsoil, leaving a poor agricultural soil behind. The slopes of a volcano have relatively fresh soil fertility due to lava deposition, so it becomes attractive for people to settle.

• Couple that positive attribute with the poor cultural memory of eruptions and you have the ingredients for disaster.

• Eruptions may last have happened long before the written word or the transmission of oral histories: No-one remembers.

  • The Long Valley Caldera was formed during a supervolcano eruption about 760,000 years ago, which blasted 600 cubic km (150 cubic miles) of material into the air, deposited as "Bishop tuffs" all over Southern California, Nevada, Utah, Arizona, New Mexico, Colorado, Wyoming, and parts of Idaho. There have been smaller eruptions in and around the caldera over the last 500,000 years

  • Yellowstone produced massive supervolcano eruptions 2.1 million years ago (Ma), 1.3 Ma, and 640,000 years ago. The most recent one blanketed more than half of the 48 lower states with about 1,000 cubic kilometers (240 cubic miles) of ash!

  • Mt. Mazama erupted some 30 cubic kilometers (more than 7 cubic miles) in a caldera-forming eruption nearly 7,000 years ago, creating Crater Lake, Oregon.

  • Mt. Vesuvius famously erupted in 79 C.E., burying Pompeii and Herculaneum, but there was an even bigger eruption nearly 3,800 years ago, called the Avellino eruption. A comparable event would destroy Naples! People in Pompeii would have had no cultural memory of the Avellino event and, though they often experienced small eruptions of the type Vesuvius gives in the present day, they had no way to link those to the massive eruption to come.

    • Here is a link to a viewgraphs presentation on Pompeii: https://home.csulb.edu/~rodrigue/geog558/lectures/vesuvius.odp

    • Here is a link to a letter by Pliny the Younger describing the death of his uncle, Pliny the Elder, for the Roman historian Tacitus: https://home.csulb.edu/~rodrigue/geog458558/Pompeii.doc. The first letter is remarkable for how Pliny the Younger intuits the mechanism creating the "cloud" over Vesuvius, the cloud that would collapse as the eruption eased and removed its support and create the pyroclastic surges that wiped out Pompeii and Herculaneum. The second letter has a striking description of tsunami drawback (there was a minor tsunami in the Bay of Naples).
  

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  Last revision: 11/28/22