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Primary terms
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carbon
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Tertiary
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ash flows
Distal ash hurricane (pyroclastic density current) deposits from a ca. 2000 yr B.P. Plinian-style eruption of Mount Pelée, Martinique: Distribution, grain-size characteristics, and implications for future hazard
Quaternary subglacial and explosive volcanism in the Canadian Cascade arc (Sea-to-Sky Corridor), British Columbia
Abstract This field guide examines Quaternary volcanism in the Canadian Cascade arc informally known as the Garibaldi Volcanic Belt. During the first day, the trip proceeds along the Sea-to-Sky Highway corridor from Vancouver to Pemberton and focuses on Quaternary glaciovolcanic deposits and lavas. Interactions between volcanoes and ice in the Garibaldi Volcanic Belt have been common during the past two million years, which has resulted in a diverse array of landforms, including subglacial domes, tuyas, impounded lava masses, and sinuous lavas that exploited within-ice drainage systems. On Days 2 and 3, the trip heads northwest of Pemberton, British Columbia, along logging roads to see deposits from the 2360 yr B.P. eruption of the Mount Meager volcanic complex. The eruption began Plinian-style, generating pyroclastic fall and density current deposits (Day 2) and ended with the production of welded and nonwelded block and ash flows by explosive (Vulcanian) collapse of a lava (Day 3). The guide will examine the deposits of the temporary lake impounded by a block and ash flow deposit dam, and the evidence of the catastrophic failure of the dam and the generation of a huge outburst flood and lahar. Many of the traits of the deposits seen on this three-day trip are a reflection of both the style of eruption and the nature of the surrounding landscape. In this regard, the trip provides a spectacular window into the nature and hazards of effusive and explosive volcanism occurring in mountainous terrains and the role of water and ice.
Explosive volcanic history and hazard zonation maps of Boquerón Volcano (San Salvador volcanic complex, El Salvador)
Boquerón Volcano, formed on the old San Salvador Volcano, is the youngest and active central edifice of the San Salvador volcanic complex, which also includes 25 secondary vents. The San Salvador volcanic complex is located in the vicinity of the San Salvador metropolitan area and is considered one of the most hazardous volcanic centers in El Salvador and Central America. Boquerón Volcano has a long record of effusive and explosive eruptions spanning 36,000 yr; the most recent eruption was in 1917. We reviewed and updated its eruptive history through detailed fieldwork, allowing the recognition of up to 25 different eruptions. Lava flows, ash-fall, and ballistic projectile deposits produced by Strombolian or violent-Strombolian eruptions are the most recurrent events preserved in the stratigraphic record of Boquerón Volcano. Pyroclastic-flow, and especially pyroclastic-surge, deposits are also present, indicative of explosive subplinian and Plinian eruptions, some of which had significant phreatomagmatic components. We define three hazard scenarios regarding ash fall, ballistic projectiles, and pyroclastic density currents for Boquerón Volcano and constrain them using its documented explosive eruptive history, fieldwork, and computer simulations. Each scenario is characterized by a likelihood of occurrence (high, medium, low), assigned to eruptive events of small, intermediate, or large magnitude, which are mainly characterized for the areal distribution of the related volcanic products. Resulting hazard maps show areas likely to be affected by future eruptions, enabling decision makers and the general public to consider volcanic hazards in land development and risk mitigation planning.
Geology and Age of Solander Volcano, Fiordland, New Zealand
Timing and Rates of Flysch Sedimentation In the Stanley Group, Ouachita Mountains, Oklahoma and Arkansas, U.S.A.: Constraints from U-Pb Zircon Ages of Subaqueous Ash-Flow Tuffs
Warford Ranch is a small “drive-in” shield volcano covering an area of ~2 by 3 km west of Phoenix, and it is accessible from Interstate Highway 8 near Gila Bend, Arizona. The basaltic shield is superposed on silicic lavas, granodiorites, and alluvial deposits and is part of the Sentinel-Arlington volcanic field. Dated at 3.19 Ma, the shield volcano is sufficiently young to preserve the original morphology, but it also shows the effects of moderate weathering, development of desert varnish, and the formation of caliche deposits. Imaged in both color near-infrared (IR) and in thermal infrared multispectral scanner (TIMS) data, these various units afford the opportunity to conduct simple remote-sensing mapping, which can then be field tested. In addition to the lava flows comprising the shield, pyroclastic deposits and dikes are also present. The compact size of the volcano enables the entire feature to be examined in the field in one day. With short introductory discussion, participants of nearly any background can be introduced to the fundamentals of remote sensing, igneous rocks, field methods, and evaluation of the volcanic history of a small volcano.
Kinematic characteristics of pyroclastic density currents at Merapi and controls on their avulsion from natural and engineered channels
Probabilistic digital hazard maps for avalanches and massive pyroclastic flows using TITAN2D
Geologists create volcanic hazard maps using scientific data to portray potential future geological events; the end users are principally public safety officials. Typical maps use a few simple polygons to outline areas of potential inundation or cover by a few categories of flows based on past frequency and size. Uncertainties in data regarding flow characteristics complicate the construction of accurate hazard maps. Generally, there are inadequate exposures of good sections, poorly known extents of units, and imprecise volumes for deposits. Crisis conditions limit the time available for field and laboratory work. Computer models can simulate possible scenarios, but the volumes, styles of emplacement, and source starting locations are poorly known in many cases. The large uncertainty in initial conditions is seldom taken into account in the construction of hazard maps, and these uncertainties are rarely passed on to the end users of the maps. TITAN2D is a computational model for volcanic block-and-ash flows and rock avalanches of various types and scales, and it forms the core of the TITAN toolkit for volcanic hazard analysis, which can integrate high-performance computing, database management, and visualization to a very sophisticated level. TITAN provides a solution to mapping problems by providing a probabilistic calculation of inundation depth that takes into account many of the critical uncertainties.
Organic block coatings in block-and-ash flow deposits at Merapi Volcano, central Java
Deciphering Magma Chamber Dynamics from Styles of Compositional Zoning in Large Silicic Ash Flow Sheets
Engineering geology of landslides on the volcanic island of Montserrat, West Indies
Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA
Paleomagnetic and geochronologic data from mafic intrusive rocks, inferred to contain magnetizations of early Late Cretaceous age, and upper Tertiary volcanic rocks, all part of the upper plate of the Silver Peak extensional complex in the southern Silver Peak Range, add to the growing body of results suggesting that Neogene displacement transfer within the central Walker Lane involved components of modest magnitude crustal tilting and, at least locally, rotation of structural blocks. Mesozoic intrusions and upper Tertiary volcanic rocks yield paleomagnetic data that are discordant to expected field directions. The data from 49 accepted sites in mafic dikes that cut granitic rocks, 4 sites in a single Oligocene(?) ash flow tuff, 20 sites in mid-Miocene andesite flows, and 28 sites in upper Miocene to lower Pliocene pyroclastic rocks may imply a systematic progression in the magnitude of vertical axis rotation and tilting with age. At a minimum, the data are consistent with at least some 20° of clockwise rotation of upper-plate rocks in this part of the Silver Peak Range and demonstrate a greater regional extent to the area affected by clockwise rotation during Neogene displacement transfer. Eight new 40 Ar/ 39 Ar age determinations from the mafic dikes and adjacent host rocks, all somewhat disturbed age spectra, imply that these rocks cooled below ∼300 °C during the Late Cretaceous between about 90 and 80 Ma. Four mafic dike groundmass concentrates yield integrated apparent ages between 86.31 Ma ± 0.12 Ma and 80.80 Ma ± 0.11 Ma, and four age spectra from biotite from the host granite yield integrated values between 93.6 ± 0.9 Ma and 78.6 Ma ± 0.2 Ma. The mafic dikes yield in situ exclusively normal polarity results consistent with an early Late Cretaceous age of magnetization acquisition, with an overall group mean (D = 25.1°, I = 55.4°, α 95 = 3.4°) that is discordant to an early Late Cretaceous expected field (D = 337°, I = 66°). Ten of 20 sites from steeply dipping mid-Miocene andesite flows and 21 of 28 sites in gently tilted upper Miocene ash flow tuffs yield overall stratigraphically corrected group means (D = 24.4°, I = 36.7°, α 95 = 7.1°) and (D = 16.5°, I = 53.5°, α 95 = 7.6°, respectively) that are discordant in a clockwise sense to the Miocene expected direction (D = 358°, I = 55°). The paleomagnetic data support a history of tilting and vertical axis rotation of the southern Silver Peak Range, most of which occurred coincidently with latest Miocene and Pliocene exhumation of the lower-plate rocks in the extensional complex. In addition, it is possible that the paleomagnetic data from Mesozoic intrusions record an additional, modest phase of deformation that predated development of the extensional complex. The observations are consistent with a tectonic model where deformation of upper-plate rocks in this area involved a small component of west- to southwest-side-down tilting, likely related to range-scale folding during the late Miocene and Pliocene, accompanied by modest clockwise vertical axis rotation.
Prehistoric Stratigraphy of the Soufrière Hills–South Soufrière Hills Volcanic Complex, Montserrat, West Indies
A new numerical code for simulating flows of granular material, TITAN2D, is used to model the Merapi-type block and ash flows resulting from the 1991 eruption of Colima Volcano, México. The 1991 block and ash flows reached distances of up to 4 km from the vent with a total estimated volume of 8 × 10 5 m 3 . The block and ash flows were modeled using a digital elevation model (DEM) of the region and compared to field data using a quantitative center-line comparison method. Input parameter values, which dictate flow dynamics, were varied to demonstrate the effect these parameters have on the program output. Analysis showed that the TITAN2D model performs best in replicating the center lines of the 1991 Colima block and ash flows when using an initial volume value representative of a single flow event rather than a total deposit volume.
Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.