Dynamical studies of the asteroid belt reveal it to be an inadequate source of terrestrial impactors of more than a few kilometers in diameter. A more promising source for large impactors is an unstable reservoir of comets orbiting between Jupiter and Neptune. Comets 100–300 km across leak from this reservoir into potentially hazardous orbits on relatively short time scales. With a mass typically 10 3 –10 4 times that of a Chicxulub-sized impactor, the fragmentation of a giant comet yields a highly enhanced impact hazard at all scales, with a prodigious dust influx into the stratosphere over the duration of its breakup, which could be anywhere from a few thousand to a few hundred thousand years. Repeated fireball storms of a few hours' duration, occurring while the comet is fragmenting, may destroy stratospheric ozone and enhance incident ultraviolet light. These storms, as much as large impacts, may be major contributors to biological trauma. Thus, the debris from such comets has the potential to create mass extinctions by way of prolonged stress. Large impact craters are expected to occur in episodes rather than at random, and this is seen in the record of well-dated impact craters of the past 500 m.y. There is a strong correlation between these bombardment episodes and mass extinctions of marine genera.

We use the occurrence of unusual or out-of-season dust storms and dissolved ion data as proxies for dust to propose a calendar-year chronology for a portion of the Greenland Ice Sheet Project 2 (GISP2) ice core during the early sixth century A.D. Our new time scale moves a small sulfate peak to early 537 A.D., which is more consistent with recent findings of a 6 mo to 18 mo time lag between volcanic eruptions and atmospheric fallout of their sulfate aerosols. Our new time scale is consistent with a small volcanic input to the A.D. 536–537 climate downturn. We use the time range of Ni-rich fragments and cosmic spherules to provide an independent test of the chronology. The time range of Ni-rich fragments and cosmic spherules matches historical observations of “dancing stars” starting in the summer of A.D. 533 and lasting until A.D. 539 or 540. These dancing stars have been previously attributed to cosmogenic dust loading of Earth's atmosphere. The time scale cannot be shifted to be either younger or older by 1 yr without destroying the match to historical accounts of dancing stars.

Catastrophic and unusual events on Earth such as bolide impacts, megafloods, supereruptions, flood volcanism, and subice volcanism may have devastating effects when they occur. Although these processes have unique characteristics and form distinctive features and deposits, we have difficulties identifying them and measuring the magnitude of their effects. Our difficulties with interpreting these processes and identifying their consequences are understandable considering their infrequency on Earth, combined with the low preservation potential of their deposits in the terrestrial rock record. Although we know these events do happen, they are infrequent enough that the deposits are poorly preserved on the geologically active face of the Earth, where erosion, volcanism, and tectonism constantly change the surface. Unlike the Earth, on Mars catastrophic and unusual features are well preserved because of the slow modification of the surface. Significant precipitation has not occurred on Mars for billions of years and there appears to be no discrete crustal plates to have undergone subduction and destruction. Therefore the ancient surface of Mars preserves geologic features and deposits that result from these extraordinary events. Also, unlike the other planets, Mars is the most similar to our own, having an atmosphere, surface ice, volcanism, and evidence of onceflowing water. So although our understanding of precursors, processes, and possible biological effects of catastrophic and unusual processes is limited on Earth, some of these mysteries may be better understood through investigating the surface of Mars.

Volcanic features are common on geologically active earthlike planets. Megascale or “super” eruptions involving >1000 Gt of magma have occurred on both Earth and Mars in the geologically recent past, introducing prodigious volumes of ash and volcanic gases into the atmosphere. Here we discuss felsic (explosive) and mafic (flood lava) supereruptions and their potential atmospheric and environmental effects on both planets. On Earth, felsic supereruptions recur on average about every 100–200,000 years and our present knowledge of the 73.5 ka Toba eruption implies that such events can have the potential to be catastrophic to human civilization. A future eruption of this type may require an unprecedented response from humankind to assure the continuation of civilization as we know it. Mafic supereruptions have resulted in atmospheric injection of volcanic gases (especially SO 2 ) and may have played a part in punctuating the history of life on Earth. The contrast between the more sustained effects of flood basalt eruptions (decades to centuries) and the near-instantaneous effects of large impacts (months to years) is worthy of more detailed study than has been completed to date. Products of mafic supereruptions, significantly larger than known from the geologic record on Earth, are well preserved on Mars. The volatile emissions from these eruptions most likely had global dispersal, but the effects may not have been outside what Mars endures even in the absence of volcanic eruptions. This is testament to the extreme variability of the current Martian atmosphere: situations that would be considered catastrophic on Earth are the norm on Mars.

Abstract The 1980 eruption of Mount St. Helens caused instantaneous landscape disturbance on a grand scale. On 18 May 1980, an ensemble of volcanic processes, including a debris avalanche, a directed pyroclastic density current, voluminous lahars, and widespread tephra fall, abruptly altered landscape hydrology and geomorphology, and created distinctive disturbance zones having varying impacts on regional biota. Response to the geological and ecological disturbances has been varied and complex. In general, eruption-induced alterations in landscape hydrology and geomorphology led to enhanced stormflow discharge and sediment transport. Although the hydrolog-ical response to landscape perturbation has diminished, enhanced sediment transport persists in some basins. In the nearly 30 years since the eruption, 350 million (metric) tons of suspended sediment has been delivered from the Toutle River watershed to the Cowlitz River (roughly 40 times the average annual preeruption suspended-sediment discharge of the Columbia River). Such prodigious sediment loading has wreaked considerable socioeconomic havoc, causing significant channel aggradation and loss of flood conveyance capacity. significant and ongoing engineering efforts have been required to mitigate these problems. The overall biological evolution of the eruption-impacted landscape can be viewed in terms of a framework of survivor legacies. Despite appearances to the contrary, a surprising number of species survived the eruption, even in the most heavily devastated areas. With time, survivor “hotspots” have coalesced into larger patches, and have served as stepping stones for immigrant colonization. The importance of biological legacies will diminish with time, but the intertwined trajectories of geophysical and biological successions will influence the geological and biological responses to the 1980 eruption for decades to come.

Abstract Salt (halite, NaCl) is the most soluble of common rocks; it is dissolved readily and forms a range of subsidence or collapse features as a result of human activities. Bedded or domal salt deposits are present in 25 of the 48 contiguous United States and underlie nearly 20% of the land area. These salts occur in 17 separate structural basins or geographic districts in the United States, and either local or extensive examples of natural or man-made salt karst are known in almost all of these basins or districts. Human activities have contributed to the development of salt karst. Boreholes or underground mines may enable (either intentionally or inadvertently) unsaturated water to flow through or against the salt deposits, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause land subsidence or catastrophic collapse. Because salt dissolution proceeds rapidly, human-induced karst features often develop quickly and with dramatically adverse impacts. Industries associated with local salt-dissolution and collapse features include solution mining (e.g., Cargill sink, Kansas; and Grand Saline sink, Texas), petroleum activities (e.g., the Wink sinks, Texas; Panning sink, Kansas; and Gorham oil field, Kansas), and underground, dry mining of salt (e.g., Jefferson Island mine, Louisiana; and Retsof mine, New York).

Abstract This chapter summarizes the processes and effects of the most notable catastrophic mass movement events in South America in the twentieth century. We present 23 case histories of individual and regional landslide events, beginning at the northeast terminus of the Andes Mountains in Venezuela, proceeding counterclockwise down the Pacific Coast to the southern Andes of Chile and Argentina, and ending with discussion of catastrophic regional mass movements in the Brazilian Highlands. The types of landslides involved in these disasters ranged from high-velocity rockslides and rock or debris avalanches to high- to medium-velocity debris flows and mudflows. Most casualties were caused by high-velocity debris avalanches and high-to medium-velocity, highly mobile, long-runout debris flows. A common, and particularly devastating, regional occurrence consisted of earthquake-triggered slides on steep slopes covered with saturated residual soils; these slides were rapidly transformed into very fluid, high-velocity debris avalanches, which in turn changed into devastating debris flows that ran out into populated areas on valley bottoms.

Abstract On the mountainous island of Taiwan, where earthquakes and typhoons are common, many landslide disasters occur. The Tsao-Ling rockslides are perhaps the best-known landslides in Taiwan. Between 1862 and 1979, four large catastrophic rockslides occurred on the southwest slope of Mount Tsao-Ling. These multimillion cubic meter mass movements created substantial landslide dams on the Ching-Shui River, which flows at the toe of the Tsao-Ling slope. The failures of these landslide dams resulted in additional catastrophes, which, in three cases, occurred years after the rockslide event. On December 17, 1941, a rockslide involving a mass movement of more than 80 × 10 6 m 3 occurred on the dip slope forming the southwest flank of Mount Tsao-Ling, triggered by a strong earthquake. On August 10, 1942, heavy rain caused another rockslide on the same slope, and more than 100 × 10 6 m 3 of rock slid down the Tsao-Ling dip slope. The Ching-Shui River was dammed with rock debris. The landslide dam (140–200 m high, 4800 m wide at base) was overtopped on May 18, 1951 and 120 × 10 6 m 3 of impounded water was suddenly released. More than 100 human lives were lost. On August 15, 1979, heavy rain caused a breakaway failure from the lower part of the remaining slope. The slide mass collided with the original debris dam and the Ching-Shui River was once again dammed. The landslide dam was overtopped on August 24, 1979. From 1980 to 1993, intensive investigations in the greater Tsao-Ling area were undertaken. We conclude that a further rockslide involving a mass movement of 50 × 10 6 m 3 or so is possible.

Abstract Unpublished reports of investigations carried out after the 1983 Sale Mountain landslide and some of the published papers related to the occurrence, mechanism, and mobility of the landslide are reviewed herein. The landslide occurred on the high, steep south slope of Sale Mountain, which comprises nearly horizontal Pliocene siltstones and mudstones covered by 120 m of Pleistocene eolian loess. The volume of loess involved in the slide was less than one-third of the total volume of the sliding mass, so it is was not a loess landslide, but a loess-covered mudstone landslide. Although the landslide occurred suddenly, before its occurrence there was a long-term preparatory stage, in which gravitational creep and tension fracturing were important processes leading to the final abrupt failure of the slope. Most researchers have suggested that the mechanism of the landslide was progressive failure that began with extremely slow sliding and tension fracturing, and ended with shearing through resisting elements across the bedding of the Pliocene sediments. The sliding velocity of the landslide was extremely rapid. The average velocity was ~20 m/s. The Fahrböschung is 11°, which represents an excessive travel distance of 1120 m.

Abstract Despite having a low recorded historical incidence of landsliding, the Sierra Nevada has undergone large prehistoric and historical rockfalls and rockslides that could be potentially catastrophic if they occurred today in the more densely populated parts of the region. Several large documented rockfall and rockslides have been triggered either by strong seismic shaking or long periods of unusually wet weather; however, in several instances no obvious triggering event can be identified. The glaciated topography of the higher elevations of the Sierra Nevada has produced many relatively small falls and slides within relatively hard, massively jointed, granitic rocks; however, where exposed to weathering for long periods after glaciation, the oversteepened rock slopes are prone to uncommonly large falls and slides. At lower elevations on the nonglaciated slopes of the Sierra Nevada, rockslides commonly occur within more weathered granitic rocks, where the strength of the rock mass is typically affected by joint weathering and alteration of the intact rock to saprolite. Historical large rockfalls and rockslides in the Sierra Nevada have created additional secondary natural hazards, including debris flows and floods from the breaching of landslide dams that can be as hazardous as the initial rockfalls and rockslides.

Abstract Rock avalanches have been widespread in the Karakoram Himalaya. More than 100 events have been identified in recent surveys and they include the largest catastrophic landslides known in the region. Some occurred in the present-day glacierized zone and during the past decade. Most have been identified and reconstructed from more or less ancient deposits at lower elevations. The incidences of these deposits and features that place them in the general class of rock-avalanche fragmentites are described. However, the main focus is on certain morphological and sedimentary phenomena that reflect interactions of rock avalanches with rugged terrain. In most cases, relations between runout path and geometry of opposing valley walls and interfluves have had major effects on the overall shape, surface morphology, and internal structures of the deposits. In some cases there are complex interactions with erodible substrates. Further complications arise from later erosion and burial of rockavalanche deposits, and the large changes they bring about in other geomorphic conditions. Styles of rock avalanche are proposed based upon the configuration of depositional complexes and their relations to surrounding topography, especially to local valley systems. The analysis and terminology for such topography-constrained forms are developed, in particular, from the work of Albert Heim. An understanding of these forms provides a useful guide to field identification and reconstruction of past rock avalanches in rugged mountain valleys, and necessary background for assessing future risks from catastrophic landslides.