Eruption and deposition of the fisher tuff (Alaska) : evidence for the evolution of pyroclastic flows.

International audienceRecognition that the Fisher Tuff (Unimak Island, Alaska) was deposited on the leeside of ~500-700 m high mountain range (Tugamak Range) more than 10 km away from its source played a major role in defining pyroclastic flows as momentum-driven currents. We re-examined the Fisher Tuff to evaluate whether deposition from expanded turbulent clouds can better explain its depositional features. We studied the tuff at 89 sites, and sieved bulk samples from 27 of those sites. We find that the tuff consists of a complex sequence of deposits that record the evolution of the eruption from a buoyant plume (22 km) that deposited ~0.2 km3 of dacite magma as a pyroclastic fall layer to erupting ~10-100 km3 of andesitic magma as scoria-rich pyroclastic falls and flows that were mainly deposited to the north and northwest of the caldera, including those in valleys within the Tugamak Range. The distribution of the flow deposits and their welding, internal stratification, and occurrence of lithic breccia, all suggest that the pyroclastic flows were fed from a fountaining column that vented from an inclined conduit, the first time such a conduit has been recognized during a large volume caldera eruption. Pyroclastic flow deposits before and after the mountain range, and thin veneer deposits high in the Range, are best explained by a flow that was stratified into a dense undercurrent and an over-riding dilute turbulent cloud, from which deposition before the range was mainly from the undercurrent. When the flow ran into the mountain range, however, the undercurrent was blocked, but the turbulent cloud continued on. As the flow continued north, it re-stratified, forming another undercurrent. The Fisher Tuff thus records the passing of a flow that was significantly higher (800-1100 m thick) than the mountain range, and thus did not require excessive momentum

The InVEST Volcanic Concept Survey: Exploring Student Understanding About Volcanoes

Results from the Volcanic Concept Survey (VCS) indicated that many undergraduates do not fully understand volcanic systems and plate tectonics. During the 2006 academic year, a ten-item conceptual survey was distributed to undergraduate students enrolled in Earth science courses at five U.S. colleges and universities. A trained team of graders scored 672 completed surveys, coding responses to each item with a score, out of 3, based on accuracy and comprehensiveness. Questions requiring only basic content knowledge (e.g., terminology, volcano topology) received more high scoring responses than questions requiring higher thinking and deeper conceptual connections (association with plate tectonics, prediction of hazards and impacts on the environment). The mechanics of eruptions also appeared to be poorly understood. Special attention was paid to students’ alternate conceptions about where volcanoes are likely to form. Male students, students highly interested in science, and students who lived in a volcanically active area received significantly higher total scores than other student groups. Science, technology, engineering, and mathematics (STEM) majors also performed significantly better than non-STEM majors. Understanding the nature of student comprehension and misconception may be useful for geoscience educators seeking to address student preconceptions and promote conceptual change.This article is from Journal of Geoscience Education 58 (2010): 177, doi:10.5408/1.3544298. Posted with permission.</p