BLOCKS AND LAPILLI AND ASH, OH MY! INTERPRETATION OF VOLCANICLASTIC DEPOSITS

Volcanoes can unleash an ensemble of geological hazards involving volcaniclastic sediment—tephra falls, pyroclastic density currents (PDCs), debris avalanches, lahars, and sediment-charged water floods—all of which can emplace deposits having many similarities. Though decades of research have identified key characteristics of deposits emplaced by each, it can sometimes be difficult to distinguish one type of deposit from another, or even primary from secondary. Recognizing subtle differences among particle composition, angularity, and sorting; stratification and types of bedding; nuances such as matrix vesicles, diffuse stratification, or thermoremanent magnetization; and deposit distribution is required for proper identification. Assessing risks to communities downstream and downwind of volcanoes relies on proper process identification, and the socioeconomic implications can be profound. For example, a thin, faintly laminated fine ash (silty sand) on a terrace could be evidence of a past flood just large enough to overtop the terrace, a nuisance-level tephra fall, or a lethal low-concentration PDC. A coarse breccia could be lahar, glacial till, or high-concentration PDC. Proper identification hinges on a number of textural and spatial clues, and correct identification has direct implications for vulnerable communities. Aberrant events can leave behind deposits most perplexing. For example, massive lahars composed mainly of ice fragments leave behind thin (1–3 cm), inconspicuous fine ash and lapilli layers difficult to interpret and highly susceptible to subsequent erosion (e.g., Redoubt volcano). Alluvial channel fills may represent meters of aggradation by floods over decades (e.g., Mount Hood) or only hours (e.g, Chaitén volcano). A many-cm-thick tephra layer, if undisturbed, can represent a moderately hazardous eruption, or a catastrophic eruption if only an erosional remnant of a much thicker layer. Recognizing subtleties of textural, physical, and spatial characteristics of volcaniclastic deposits and properly identifying process is crucial to fully grasping histories and hazards of volcanic eruptions.