Tick Tock ROROBEBERTRT NAEAEYEYE term technology. A giant asteroid impact 65 million years ago extinguished all dinosaur lineages except birds. But Asteroid impacts, gamma-ray bursts, the probability of such a large-scale calamity occurring in the next century is about one in a million. Moreover, supervolcanoes, and mega-tsunamis astronomers are well on their way toward cataloging represent threats to humanity’s future, almost all of the near-Earth asteroids that are big enough to threaten our collective survival, which will leave even but don’t lose sleep worrying about them. rarer comet impacts as the remaining potential threat. A much more likely disaster is a smaller impact or airburst, such as the explosion over Siberia on June 30, AS E. C. KRUPP eloquently explains in his article (page not predicting the world to 1908. These events occur perhaps once every 100 to 1,000 22), Mayan astronomers were years, on average. Impactors on the order of 100 meters end in 2012. But you might be asked if there are any loom- are so diffi cult to detect that we might atten have a city no oradvance region, or ing disasters that portend mass death and destruction. warning. Yet these objects could fl The short answer is “probably not.” Scientists are inundate coastlines if a big chunk splashes into an ocean studying several potential cataclysms that could threaten and spawns a tsunami. The sudden death of ect millions on civiliza- of humanity’s collective survival, or at least disrupt our civi- people would certainly have a profound eff lization on a massive scale. Fortunately, none are likely to tion. But such a tragedy would not threaten humanity’s occur within the lifetime of anyone reading this article. survival unless it triggered an accidental nuclear war. Jupiter recently experienced the most obvious threat: Television programs have touted the danger posed by the impact of an asteroid or comet (page 34). As scientists a more distant astronomical phenomenon: gamma-ray have pointed out, the impact of a kilometer-scale object is bursts. A massive star exploding in our galaxy as a GRB, the sole natural disaster that could annihilate our species. But it’s also the only one that we could prevent with near- sky & telescope 28 November 2009 © 2009 New Track Media, LLC. All rights reserved. S&T Photo Illustration Casey Reed SOURCE: © ISTOCKPHOTO.COM / PJARD (STATUE OF LIBERTY) & MIKEUK (OCEAN) and with a jet aimed precisely at Earth, could seriously As dramatized reasonably accurately in the TV movie damage Earth’s atmosphere. Researchers have speculated Supervolcano, Yellowstone National Park contains a that a GRB caused the Ordovician mass extinction 450 volcanic caldera roughly 45 miles across, and lurking million years ago, because the pattern of species extinc- beneath it is a giant, seething mass of molten rock. The tion is consistent with what one might expect from a Yellowstone supervolcano last went off 640,000 years ago, GRB. But there is no direct evidence that a GRB (or a pumping 1,000 cubic kilometers of volcanic ash and gases supernova) has ever aff ected life on Earth. into the atmosphere, covering much of North America. A Hubble Space Telescope survey of GRB host galaxies indicates that bursts are exceedingly rare in mature spiral galaxies such as the Milky Way. The chances of our planet being in a nearby GRB’s line of fi re are much lower than the odds that I’ll win the decathlon gold medal at the 2012 Summer Olympics. And there are no supernova progeni- tors close enough to Earth to substantially harm our biosphere should any of these stars explode in the next INCOMING! several centuries (S&T: March 2007, page 26). Asteroid The threat posed by wandering stars, pulsars, black fragments holes, and other interstellar exotica is even more minus- heading toward Earth could cule, given their low density in space. But extraterrestrial wreak regional objects are not the only causes for concern. Geologists havoc. have discovered several ticking time bombs lurking within ISTOCKPHOTO.COM / DUUUNA our planet. © SkyandTelescope.com November 2009 29 © 2009 New Track Media, LLC. All rights reserved. Tick Tock Mount St. Helens, 1980 / 0.24 cubic mile This material blotted out the Sun, cooling the planet for at Mount Pinatubo, 1991 / 2.4 cubic miles least several years. If Yellowstone blows its stack next year, at least half of the U.S. will be buried under volcanic ash, and the vast amounts of atmospheric material will snuff out agriculture around much of the world, perhaps cull- Krakatoa, 1883 / 4.3 cubic miles ing the human population by tens of percent. The last three Yellowstone eruptions were spaced roughly 700,000 years apart, which sounds ominous. Fortunately, there is no large-scale ground deformation or greatly increased seismic or geyser activity that signals an Mount Mazama (Crater Lake, Oregon) impending supereruption. Given the rarity of these cata- ~7,700 years ago / 18 cubic miles clysms, we’re probably safe for tens of thousands of years. And that’s a good thing. About 74,000 years ago, Lake Toba, a 20-by-60-mile-wide supervolcano on the Indone- sian island of Sumatra, exploded and perhaps gave Homo sapiens its closest shave with extinction. Genetic studies • Tambora 1815 36 cubic miles MONTANA IDAHO Caldera rim Canyon Yellowstone s Old Faithful National Park • Yellowstone caldera Plume mile 0 1.3 million years ago 7 WYOMING 2 67 cubic miles 10 miles 30 miles Magma chamber YELLOWSTONE TIME BOMB The Yellowstone caldera is about 45 miles across. A magma chamber containing hundreds of cubic miles of molten rock underlies the • Yellowstone caldera National Park. If the supervolcano vents this material into the atmosphere, it will pre- 640,000 years ago cipitate a global ecological crisis that will radically alter the course of civilization. 240 cubic miles Mount St. Helens MajorMajMajorjoor ashfallasshfallhhffa fromf frororomom Yellowstone MountMoMountou t St.StSt. HelensHHe Helene eensns 198019898800 s ile m 500 s Major ashfall e il from MajorMajMaajjoor ashfalla hffllffa fromromo m 0 Yellowstone YellowstoneYe owwssto eruptionerueruptpptiontionon 00 1, eruption • Yellowstone caldera 2.12. 1 million millio yearsyyg agoa gogo 640,000 2.1 million years ago 640,000 years ago 600 cubic miles CASEY REED (3) S&T: ASHFALL COMPARED The 1980 Mount St. Helens eruption devastated 600 square kilometers of forest, but its volume of ejected material was puny com- Volcanic Eruption Volumes pared to a Yellowstone supereruption. © 2009 New Track Media, LLC. All rights reserved. LAKE TOBA The most SOUTH CHINA SEA recent supervolcanic erup- tion occurred in Lake Toba M A (far left), Sumatra, about L A Y S I A 74,000 years ago. The cata- Lake strophic 1-gigaton explosion Toba EO (3,000 times the energy of Equator RN BO S U the 1980 Mount St. Helens M A eruption) might have cooled T R INDIAN A global temperatures 3 to 5°C OCEAN Jakarta JAVA SEA (5 to 9°F) for several years, Krakatoa a volcanic winter that may JAVA Tambora I N D have brought early modern O N E S I A humans to the brink of SUMBAWA NASA / GSFC / MITI / ERSDAC / JARUS / JAPAN ASTER SCIENCE TEAM extinction. indicate that the human population passed through a bottleneck sometime between 70,000 and 80,000 years ago, when there were perhaps only a few thousand living individuals. Although there is no proven link between the Lake Toba supereruption and the genetic bottleneck, the close association in time may not be coincidental. A recent computer simulation indicates that the eruption produced a decade-long “volcanic winter” that severely s r s r s u r aff ected global ecosystems, posing a formidable challenge u o u o o h h h 7 to Paleolithic human societies. 6 The media has recently focused a great deal of attention 3 on a more typical volcano, Cumbre Vieja, on La Palma in the Canary Islands. The 6,400-foot-high mountain appears min RY O to be unstable, leading to fears of a massive landslide that 30 CANA C DS C ISLAN O could trigger a mega-tsunami that would cross the Atlantic R O in 6 to 7 hours. Large cities such as New York could not be M ARA AH S evacuated quickly enough, so giant waves sweeping over A Landslide N I R N E A Atlantic coastlines would drown millions of people and T T S I E R U CASEY REED (3) W A leave behind incalculable economic destruction. But recent M S&T: work by geologists at the Delft University of Technology in The Netherlands strongly suggests that Cumbre Vieja will TSUNAMI IN remain stable for the next 10,000 years. WAITING? Another inevitable geological threat comes from a mag- The satellite image netic-fi eld reversal. Earth’s north and south poles occasion- on the left shows ally fl ip polarity, but without any regular periodicity. The La Palma, in the most recent reversal occurred 780,000 years ago, which is Canary Islands. At longer than the normal timescale between events. Are we the southern end overdue? Earth’s magnetic-fi eld strength has weakened lies the volcano by about a third in the past 2,000 years, raising legitimate Cumbre Vieja, concerns that we might be heading toward a big switcheroo whose west- in the next 1,000 to 2,000 years. ern fl anks may someday collapse Although we don’t really know what happens during into the Atlantic, these reversals, most geologists think the global dipole triggering a mega- fi eld shuts down temporarily, and then it turns back on tsunami (above) with opposite polarity.