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New publication: Postglacial eruptive history and geochemistry of Semisopochnoi volcano, western Aleutian Islands, Alaska New publication: Geochemistry of some Quaternary lavas from the Aleutian Arc and Mt. Wrangell New publication: Geologic map of Chiginagak volcano New publication: Major-element glass compositions of tephra from the circa 3.6 ka eruption of Aniakchak volcano, Alaska Peninsula, Alaska New publication: The 2014 eruptions of Pavlof Volcano, Alaska New publication: Historically active volcanoes of Alaska reference deck, v.2

Bogoslof Volcano, Alaska: ongoing eruption through the Bering Sea Thank you Unalaska! Bogoslof Summary of Current Activity Bogoslof Summary of Current Activity, through 19 January 2017

AVO studies resuspended volcanic ash from the Katmai region to Kodiak Island, Alaska Citizen Science - Volcanic Ash Collection Workshop and Public Talk, Kodiak January 30, 2016 Fieldwork at Iliamna and Spurr New publication highlights the importance of ash scrubbing in the evaluation of hazards from explosive eruptions

Critical Volcano Monitoring Systems Returned to Operation in Alaska Resuspended Volcanic Ash from the Katmai Region to Kodiak Island Remobilized Katmai 1912 ash: community events and health hazard analysis Makushin 2015 Geology Blog Sixth Anniversary of the Redoubt 2009 Eruption Happy Anniversary, Shishaldin 1967 and 2014!

New Publication on Aniakchak Volcano Available Online 25th Anniversary of the 1989-90 Eruption of Redoubt Volcano AVO geochemical database now available AVO Scientists Discuss Cook Inlet Volcanoes on Frontier Scientists TV Series Announcing new monitoring equipment for Cleveland volcano 22nd anniversary of Crater Peak (Mt Spurr) June 27 eruption Revised Alaska Interagency Operating Plan for Volcanic Ash Episodes Anniversary of Aniakchak 1931 eruption! April 19th - anniversary of Shishaldin 1999 and Pavlof 1986! Ground-coupled airwaves and explosion signals at Shishaldin 5th anniversary of the Redoubt 2009 eruption Loss of Critical Volcano Monitoring Information in Alaska NEW VOLCANO NUMBERING SYSTEM IMPLEMENTED Loss of Critical Volcano Monitoring Information in Alaska Report released: Geochemical investigations of the hydrothermal system on Akutan Island, July 2012

24th Anniversary of the 1989-90 eruption of Redoubt Volcano Veterans Day slideshow Call for images from active and retired service members! AVO operations during lapse of federal government appropriations New Tool for Reporting Alaska Volcanic Ash Fall Allows Residents to Assist Scientific Monitoring 25 years monitoring Alaska volcanoes - press release

AVO slideshow for Veterans Day Large ash eruptions: when volcanoes reshape valleys -- free public lecture Father Hubbard and the history of exploration in the Valley of Ten Thousand Smokes - free lecture Remote sensing and volcanoes - free public lecture The Great Eruption of 1912 - free public lecture Infrasound Detection of Volcanic Explosions Archaeology of Katmai area and the impact of past eruptions - free public lecture Historical Photography of the Great 1912 Eruption - free public lecture Catastrophic Eruptions and People -- free public lecture Eruption of an Island Volcano: Kasatochi, 2008 -- free public lecture Exploring the Plumbing System of Katmai Volcanoes Exploration of Katmai and the Valley of Ten Thousand Smokes - free public lecture Commemorative presentation in Kodiak: Be Prepared! Earthquakes Below Alaskan Volcanoes - free public lecture DisaStory - A Day of Oral History 1912 Katmai Eruption Children's Program Monitoring Alaska's Volcanoes - free public lecture Landmark volcano study: Katmai Centennial Perspectives free download Special activities on AVO's website for 1912 centennial Alaska Park Science - Volcanoes of Katmai and the Alaska Peninsula AVO at the Alaska Aviation Trade Show and Conference May 5-6 The Great Katmai Eruption of 1912 - a free lecture in Anchorage: April 24, 2012 The Great Katmai Eruption of 1912: A Century of Research Tracks Progress in Volcano Science April 25 -- The Novarupta - Katmai 1912 eruption: a free lecture in Fairbanks by Judy Fierstein Summer lecture series on Alaskan volcanism Poster contest celebrates anniversary of Katmai eruption! Mark your calendar: April 24 public lecture on the great Novarupta-Katmai eruption of 1912 An important volcanic anniversary in Alaska! PUBLISHED: The 2009 Eruption of Redoubt Volcano, Alaska

2011 Alaska Interagency Operating Plan for Volcanic Ash Episodes now available How does Cleveland's lava dome compare to Redoubt's 2009 lava dome? Alaska Volcanoes Guidebook for Teachers

New Fact Sheet on Kasatochi How big is the 2009 Redoubt lava dome?

New map: Historically active volcanoes of Alaska Steaming at Augustine Sarychev Volcano: Active Volcanoes of the Kurile Islands Footage of Alaska's Redoubt Volcano taken on Monday, March 23, 2009. Pre-eruption footage of Redoubt Volcano, Saturday, March 20, 2009 Redoubt Volcano B-Roll Footage

Kasatochi 2008 eruption summary 6th Biennial Workshop on Subduction Processes emphasizing the Kurile-Kamchatka-Aleutian Arcs Geophysical Institute, University of Alaska, Fairbanks, Alaska Chiginagak volcano's acid crater-lake continues to supply acidic, metal-laden water to salmon spawning habitat on the Alaska Peninsula ALASKA VOLCANOES - TEACHER ACTIVITY GUIDEBOOK & KIT 20 years of AVO Viewing earthquake information for Alaska volcanoes

Pavlof webcam added Activity at Pavlof volcano Pavlof thermal anomaly AVO Scientists present at U.S. Department of Education Teacher-to-Teacher Workshop Cleveland webcam available Activity at Cleveland volcano Cleveland satellite images Sheveluch Eruption U.S. Geological Survey's alert notification system for volcanic activity KVERT Volcanic Warnings Ceased

New alert system for volcanic activity Three new webcams added AGU presentations requested New webcam available
New publication: Postglacial eruptive history and geochemistry of Semisopochnoi volcano, western Aleutian Islands, Alaska
Posted: February 26, 2018

Sugarloaf Peak, as viewed from Semisopochnoi caldera
Postglacial eruptive history and geochemistry of Semisopochnoi volcano, western Aleutian Islands, Alaska, is available for download here.

Semisopochnoi Island, located in the Rat Islands group of the western Aleutian Islands and Aleutian volcanic arc, is a roughly circular island composed of scattered volcanic vents, the prominent caldera of Semisopochnoi volcano, and older, ancestral volcanic rocks. The oldest rocks on the island are gently radially dipping lavas that are the remnants of a shield volcano and of Ragged Top, which is an eroded stratocone southeast of the current caldera. None of these oldest rocks have been dated, but they all are likely Pleistocene in age. Anvil Peak, to the caldera’s north, has the morphology of a young stratocone and is latest Pleistocene to early Holocene in age. The oldest recognized Holocene deposits are those of the caldera-forming eruption, which produced the 7- by 6-km caldera in the center of the island, left nonwelded ignimbrite in valleys below the edifice, and left welded ignimbrite high on its flanks. The caldera-forming eruption produced rocks showing a range of intermediate whole-rock compositions throughout the eruption sequence, although a majority of clasts analyzed form a fairly tight cluster on SiO2-variation diagrams at 62.9 to 63.4 weight percent SiO2. This clustering of compositions at about 63 weight percent SiO2 includes black, dense, obsidian-like clasts, as well as tan, variably oxidized, highly inflated pumice clasts. The best estimate for the timing of the eruption is from a soil dated at 6,920±60 14C years before present underlying a thin facies of the ignimbrite deposit on the island’s north coast. Shortly after the caldera-forming eruption, two scoria cones on the northwest flank of the volcano outside the caldera, Ringworm crater and Threequarter Cone, simultaneously erupted small volumes of andesite.

The oldest intracaldera lavas, on the floor of the caldera, are andesitic to dacitic, but are mostly covered by younger lavas and tephras. These intracaldera lavas include the basaltic andesites of small Windy cone, as well as the more voluminous basaltic andesites of three-peaked Mount Cerberus, which takes up most of the west half of the caldera and has erupted lavas that flowed to the sea on the southwestern coast of the island. Apparently active at the same time as Mount Cerberus, extracaldera Sugarloaf Peak at the southern point of the island has exclusively erupted basalts. Its young satellite peak, Sugarloaf Head, has erupted morphologically young lavas and cinder cones and may be the source of the last historical eruption in 1987. Several tephra sections on the east half of the island record as many as 50 tephras, mostly from Mount Cerberus, Sugarloaf Peak, and Sugarloaf Head, over the past several thousand years.

Eruptive products of Semisopochnoi Island show an overall compositional range of basalt to dacite, though basaltic andesite and andesite constitute the largest proportions of rock types. They are tholeiitic, low to medium K, and have geochemical characteristics typical of magmatic arcs. The earliest Pleistocene lavas are mostly basalts that show the greatest geochemical diversity, as illustrated by, for example, LaN/YbN ratios of 1.9 to 3.5, suggesting fluctuations in the magma source region over the hundreds of thousands of years recorded by these older lavas. The Holocene rocks, in contrast, follow arrays in compositional space that suggest crystallization differentiation from discrete, subtly different batches of magma under varying pressure and temperature conditions. Increasingly negative Eu anomalies and an only modestly increasing alumina saturation index value with differentiation suggest that plagioclase and mafic silicates (amphibole and pyroxene) were involved to varying degrees in fractional crystallization to produce Semisopochnoi’s magmatic diversity. The crystal-poor, andesitic magmas that erupted during caldera formation likely separated from a plagioclase-, amphibole-, and clinopyroxene-dominated crystal residue in the upper crust at less than 900 °C, possibly following a period of decreased magmatic flux. During the Holocene, basaltic Sugarloaf Peak appears to bypass any upper crustal magmatic storage region and erupt crystal-rich basalts. Recent seismic swarms and long-lived warm springs attest to ongoing magmatic activity.

The Holocene eruptive record at Semisopochnoi volcano is one of diverse eruptive styles as well as frequent eruptions from multiple vents located within and outside the caldera. The number and diversity of postcaldera vents means that the sites of future eruptions cannot be predicted with certainty. Future eruptions of ash similar in magnitude to the VEI 3 or less eruptions recorded in the documented tephra deposits would pose a hazard to aircraft in the region.
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