2008-超级火山Frequently Asked Questions About Supervolcanoes and Supereruptions

1How many supervolcanoes/

supereruptions are known?

Are there many more than are mentioned

in the papers in this issue?

There have been far more supereruptions than

are mentioned in this issue. Those discussed

here are the most famous, best documented,

and best known to the authors. A minimum

total number cited by Mason et al. (2004) is

47. Many other candidates with less well-

documented deposit volumes are known,

many others are as yet unrecognized, and

many more, we can assume, have had their

record obliterated. Natural processes cover

the tracks of supereruptions. Their erupted

products are spread over enormous areas, in

some cases including the seafloor and distant

continents, and they are easily eroded or

turned into soil. Much of their deposits may

be retained within the caldera, where their

great thickness is easily underestimated.

Thus,the enormous erupted volumes—and

therefore their “super” character—may not

be recognized.

2What are their ages? Are they all

“young” geologically speaking?Which is

the oldest known? Will we start recogniz-

ing them in older and older rocks?

Among the 47 eruptions listed in the Mason

et al. compilation, only four are older than

50Ma and three older than 100 Ma (three

gigantic, altered ash-fall tuff deposits in the

eastern United States and Europe, all about

454 Ma). Ages cited by Mason et al. (2004)

range from 26 ka (Oruanui) to these

Ordovician tuffs. Candidates dating back to

almost 2 billion years are known, but their

volumes or the nature of the eruptions remain

open to question (e.g. Hildebrand 1984; Allen

et al. 2003). The relative paucity of ancient

supereruptions is certainly due to a lack of

preservation. The older the deposit, the likelier

it is to be buried or eroded. Furthermore,

volcanologists typically focus on younger

deposits because of their closer link with

active volcanism and because of their better

preservation. It is likely that the products of

supereruptions will be recognized in older and

older rocks, but because of their vulnerability

they will always be underrepresented in the

ancient rock record.

Given the current estimates of frequency of

supereruptions (on the order of 10 per million

years), it is nonetheless reasonable to infer that

there have been many thousands—probably

tens of thousands—over the course of Earth

history.

3You mention several post 100 ka candidates. What are they?In addition to Oruanui and Toba, possible candidates include the Los Chocoyos eruption from Lake Atitlan caldera, Guatemala (84 ka),and several in Japan [from Aso (90 ka), Kikai-Akahoya (7 ka), and Aira (~25 ka) calderas—Blake and Self 2007]. The eruption of the Campanian Tuff from Campi Flegrei (39 ka) is of particular interest because it demonstrates the threat of a great eruption to a nearby city (Naples, Italy), but it falls short of the criteria for a true supereruption (Marianelli et al. 2006).4What is the tectonic setting of supervolcanoes? One common characteristic is that all known supereruptions occurred within thick,continental-type crust. As suggested in the papers in this issue, input of hot, mantle-derived basaltic magma into this thick, more-silicic crust appears to be required.Supereruptions occur in subduction zone settings, like Japan, Indonesia, New Zealand,and the Andes, and in plate interiors, both at hot spots like Yellowstone and in zones of extension like Long Valley, California. In fact,even where they occur in broadly convergent regions—subduction zones—supereruptions appear to be commonly and perhaps invari-ably associated with local extension.5Do calderas always indicate supereruptions?No. While all well-known supervolcanoes have calderas, this does not mean that calderas necessarily indicate a supereruption.In fact, calderas range widely in size, and a great majority formed during more modest eruptions (well-known examples: Crater Lake,Oregon, United States; Laacher See, Germany;Santorini, Greece).REFERENCES Allen SR, Simpson CJ, McPhie J, Daly SJ (2003)Stratigraphy, distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics, South Australia. Australian Journal of Earth Sciences 50:97-112Blake S, Self S (2007) Super-eruptions and super-volcanoes. Kagaku 77: 121293-121297 [in Japanese]Hildebrand RS (1984) Folded cauldrons of the early Proterozoic Labine Group, northwestern Canadian Shield. Journal of Geophysical Research 89: 8429-8440Marianelli P, Sbrana A, Proto M (2006) Magma chamber of the Campi Flegrei supervolcano at the time of eruption of the Campanian Ignimbrite.Geology 34: 937-940Mason BG, Pyle DM, Oppenheimer C (2004) The size and frequency of the largest explosive eruptions on Earth. Bulletin of Volcanology 66: 735-74816E L E M E N T S F EBRUARY 2008This cartoon illustrates the main components of a resurgent caldera like those that form during explo-sive supereruptions from large-volume, crustal magma systems (see F ig. 5 on page 14). Precaldera rocks are shown as three layers (shaded lavender) that are offset along faults (ring fractures ) defining the caldera margins. Overlying the precaldera rocks are pyroclastic deposits (green) laid down during the supereruption that accompanied caldera collapse.These include (a) thick intracaldera deposits (reaching thicknesses of 1000s of meters in some instances) that accumulated as the caldera was forming, and (b) thin-ner outflow deposits, which blanket the surrounding countryside. Thinning with increasing distance from the source, the outflow deposits can be tens to hun-dreds of meters thick near the caldera margin. In this example—as in most large calderas—a resur-gent dome formed sometime after collapse as the caldera floor was pushed upward by rising residual,unerupted magma in the underlying chamber. Sur-rounding the resurgent dome, and separating it from the caldera margins, or walls , is a lowland known as the moat . The moat often fills at least partially with water (leaving the resurgent dome exposed as an island), and traps sediments eroding from adjoining highlands as well as ash erupted from nearby volca-noes. The lowermost (oldest) moat sediments (yellow)often tilt radially outward (away from the resurgent dome), indicating deposition before dome uplift had ceased. The ring fractures bounding the collapsed region of the caldera are zones of weakness through which lavas are often extruded. Ring-fracture lavas (red in fig-ure) are typically composed of viscous, highly differ-entiated melt (rhyolite) from the same magma cham-ber that fed the earlier supereruption. F lowing only short distances, these lavas pile up, forming steep-sided, dome-shaped hills. Also shown are the less-dif-ferentiated basalt lavas (in black) often associated with (and at depth, critical to the survival of) supervolcano systems, but which only erupt some distance from the caldera margin, where they form small domes, flows,and cinder cones.(Potentially) Frequently Asked Questions About Supervolcanoes and Supereruptions

(RESPONSES BY CALVIN MILLER, DAVE WARK, STEVE SELF, STEVE BLAKE, AND DAVE JOHN)