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The Tephra Fall Zone

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The Tephra Fall Zone Chapter 1-Tephra CHAPTER 1 The Tephra Fall Zone Silver firs near the edge of the blast zone; needles were coated by ash that fell as mud during the first eruption (July 26, 1980). Introduction deposits can enrich the soil, but often desert-like condi- tions can develop, especially when the tephra was part of Magma blasts from a volcano inject vast quantities of new a pyroclastic flow (see Ch. 5). Deep tephra layers eventu- material into the atmosphere during what are called Plinian ally become compacted to form a soft, porous rock called eruptions. Global effects, including significant temporary tufa that was favored by Romans for several centuries cooling often follow. This ejected rock eventually falls to starting in 200 B.C. to cover brick and concrete surfaces. earth and spreads widely to form a gradually thinning mantle downwind. Coarse, heavy material falls to earth Ash may circle the globe before settling to earth and first leaving fine dust to settle well away from the cone. during this time, it can have significant effects on the These air-fall deposits are often called ash, but they are weather. Eruptions 200 million years ago may have caused correctly termed tephra. Only fine-textured, powdery de- massive extinctions that allowed dinosaurs to develop and posits are properly called ash. Tephra usually is rocky, dominate the earth during the Jurassic and Cretaceous ge- frothy pumice, but it can also be formed by denser rock. ological periods. Pleistocene eruptions probably altered It can be transported long distances, ultimately being de- the course of human evolution, and historical eruptions posited where dictated by the height of the plume, wind often cooled the planet for several years and may have direction and velocity during the eruption. Young tephra triggered the European Dark Ages and the Little Ice Age. 6 Chapter 1-Tephra Past important tephra impacts populations suffered tremendously from the combined effects. Lakes were clogged with tephra, wildlife was dec- The climatic effects of tephra-fall have been studied in- imated and the vegetation withered. While the climate ef- tensively, while tephra deposits can be used to date soil fects of the Mazama eruption are conjectural, they are layers and lake sediments. However, its broad impact on supported in the pollen and limnological records that sug- terrestrial ecosystems has been largely neglected by ecol- gest profound effects lasted for decades. ogists, at least until Mount St. Helens came under scrutiny and the importance of tephra to human history has only European interest in volcanic eruptions seems to recently become widely appreciated. have begun in August 79 A.D. It was then that Pliny the Elder, Roman admiral and deeply inquisitive naturalist, Thick ancient ash layers periodically covered entire was the first to describe tephra-falls up close. Unfortu- landscapes throughout the Cretaceous. Sometimes, they nately, his dedication led to his death when he was smoth- “froze” a community in time, preserving a vivid picture of ered by pumice falling from Vesuvius. We commemorate what had been growing there. A remarkable case of this his valor by calling similar eruptions Plinian. The 1980 preservation comes from the late Cretaceous in Wyoming, eruption of Mount St. Helens is but one of many major when the first flowering plants were becoming common eruptions of this type to occur in the last 2000 years. in the fossil record. Thick tuffs (geologically compressed fine tephra ash) preserved vegetation in place, allowing We now believe that in A. D. 535 a huge eruption paleobotanist Caroline Strőmberg and others to recon- caused catastrophic global cooling that caused the Dark struct the vegetation, not just the flora (Wing et al. 2012). Ages in Europe and certainly altered human history. Mon- gol tribes fled horrible weather and invaded south-central Huge eruptions have repeatedly injected so much Europe, becoming today’s Serbs and Croats. Crops failed ash, carbon dioxide and sulfur dioxide into the atmos- from China to Ireland, famine was rampant and the Black phere as to affect climate, not just weather. Just when hu- Death raged from southern Asia to the Baltic. The dark- mans were becoming distinctly modern, our ancestors had ened sun was recorded in Constantinople and by Chinese to cope with global-scale effects of volcanism, particularly court reporters. Droughts severely affected the Peruvian of tephra, in addition to a highly variable climate. Around Moche people. Ice cores in Greenland and Antarctica pro- 70,000 years ago, in what is today Sumatra (Indonesia), a vide evidence for acidic dust and suggest an equatorial volcano called Toba produced a mega-colossal eruption, volcano. The guilty one has yet to be identified with cer- probably the largest on the planet in 25 million years. Py- tainty, although Rabaul (Papua New Guinea), Krakatau roclastic flows and the associated tephra covered over (Indonesia) and the Ilopanco caldera (El Salvador) are 20,000 km2 (roughly the size of New Jersey) to depths ex- candidates. The eruption was at least 40% larger than that ceeding 500 m. About 800 km3 of material became tephra of Tambora, the reigning champion for documented his- (while Mount St. Helens contributed only 1.2 km3 of toric eruptions. tephra, making Toba about 700 times bigger by this meas- ure). This event set off global cooling for at least a millen- Northern Europe recovered somewhat after 800 and nium and began with several very cold years with temper- even had a warm spell called the Medieval Warm Period atures depressed by about 3°C. This climatic catastrophe that allowed Vikings to colonize Iceland, Greenland and is believed to have reduced humans to very small num- Vineland. These halcyon days of Nordic colonization bers, causing a “bottleneck” in human evolution, followed soon unraveled when a distinct cooling trend occurred. by rapid population differentiation. Our species remains This is related to a huge eruption now dated to 1257 and genetically relatively homogeneous. associated with the Indonesian volcano Rinjani. Horrible winters and widespread famine recurred throughout Until Mount St. Helens erupted, the best-known northern latitudes from 1315 to 1320. This volcanic event eruption in the Pacific Northwest occurred about 7,700 sparked the start of the European Little Ice Age that lasted years ago when Mount Mazama produced an epic erup- until about 1850 and certainly altered the course of Euro- tion. In the aftermath, Crater Lake was formed. The huge pean history. tephra cloud covered nearly all of what would become Or- egon and much of the surrounding states and Canada. The To grasp what might be involved when volcanoes al- global climate cooled slightly and then continued to re- ter the climate, we can look at similar more recent events. bound from glacial times. The resulting deposits from Iceland’s Laki fissure erupted in 1783 - 1784 to become Mazama form an important tool in dating post-glacial the first definitely recorded weather-altering volcanic events in this region. Associated pyroclastic flows covered eruption. The local populations suffered most from the landscape as far as 60 km away. It is likely that human 7 Chapter 1-Tephra tephra. About half of the livestock and 25% of the popu- red sunsets. lation perished in the ensuing famine. Solid sod-roofed, In modern times, tephra eruptions have directly af- sunken dwellings called turf houses, that had survived fected commerce and well-being. In 1980, first eastern centuries of smaller eruptions, were crushed or buried, Washington and then Portland were cloaked with fine killing both the human and animal inhabitants. Europe tephra from eruptions of Mount St. Helens. The first began to suffer appalling weather, a result of the sulfur eruption caused major problems for transportation and cloud that covered northern latitudes. Nobody could even for sewage and water treatment throughout the inland em- imagine that a distant eruption was the cause, although pire of Washington. Just removing the ash was problem- Ben Franklin (living in Paris at the time) did suggest that atic. it might be due to an Icelandic volcano. The climatic con- sequences in Europe were staggering because the ensuing Deep deposits of coarse tephra also have significant haze blocked solar radiation and lowered temperatures. local effects. Deposits of more than 30 cm usually kill Widespread famine became the new normal and direct plants (Tsuyuzaki & del Moral 1994, Tsuyuzaki & Goto poisoning from water polluted by sulfur deposits contin- 2001), thus initiating primary succession. In March 1907, ued for years. Extreme winter storms killed many more Mt. Ksudach near the southern tip of Kamchatka (Russia) people and livestock than normal; spring floods devas- erupted in a classic Plinian way. Because strong southerly tated swaths of fertile land. Europe reeled from drought, winds prevailed, the plume spread for over 200 km and frigid winters, violent hailstorms and floods. Laki almost coated the only port city, Petropavlovsk. The birch forest surely triggered the French Revolution, which was, after was eliminated across a narrow swath that extended for all, more about poverty and famine than about the rights over 30 km, but the tephra deposit thinned rapidly east of man. Undeniably, the effects transcended Europe. and west of the main axis of deposition. Studies of this Droughts developed in India and Africa causing intense, tephra deposit allowed detailed assessments of selective extensive famines. Even deep in the balmy American effects along a depth gradient (Grishin et al. 1996). Rhi- South, plantation owners could ice-skate in Charleston zomatous species were able to penetrate deposits less than Harbor, while food was scarce for most others during that 40 cm deep but otherwise perished. Ground layer species protracted, cold winter of 1784. These catastrophes re- were killed by thinner deposits.
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