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GSA Field Trip: Tuff Cones, Tuff Rings, and Maars of the Fort Rock-Christmas Valley Basin, Oregon: Exploring the Vast Array of Pyroclastic Features that Record Violent Hydrovolcanism at Fort Rock and the Table Rock Complex
From: Brittany Brand bbrand@xxxxxxxxxxxxxxxx
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Feel free to email the field trip leaders for more information, or for a pdf
flyer with photos (bbrand@xxxxxxxxxxxxxxxx).
Field Trip Announcement
Geological Society of America
Portland, Oregon – USA
October 22 – 24, 2009
http://www.geosociety.org/meetings/2009/ft-post.htm
Tuff Cones, Tuff Rings, and Maars of the Fort Rock-Christmas Valley
Basin, Oregon: Exploring the Vast Array of Pyroclastic Features
that Record Violent Hydrovolcanism at Fort Rock and the Table Rock Complex
*Brittany D Brand (Earth and Space Sciences, University of Washington,
Seattle, WA 98195, bbrand@xxxxxxxxxxxxxxxx);
*Grant Heiken (Freeland, Washington; heiken@xxxxxxxxxxx)
Abstract
Hydrovolcanic eruptions occur when rising magma violently fragments while
mixing with shallow surface water or groundwater. These eruptions, among the
most violent on Earth, generate hundreds-to-thousands of explosions
throughout the course of an eruptive event. Each of these explosions ejects
a mixture of juvenile and accidental clasts, gas, and water droplets. The
solid materials either fall to the ground from tephra jets, or collapse to
form pyroclastic density currents (PDCs). The deposits of these eruptions
build up rings of bedded tuff around the vent, recording both a wide variety
of pyroclastic depositional mechanisms, and important changes in the
eruptive style with time. This field trip will explore the deposits of
basaltic, hydrovolcanic eruptions in the Fort Rock-Christmas Valley basin,
the location of a widespread Pleistocene lake. Basaltic volcanoes within the
ancient lake are characterized by Surtseyan eruptions (standing water),
along the lake margins by maar eruptions (groundwater), and beyond the lake
margin by scoria cones. The focus of the trip will be Fort Rock (Surtseyan)
and the Table Rock Complex (large Surtseyan-tuff cone, large maar, and seven
minor eruptions). This trip offers the opportunity to examine (1) settings
under which explosive hydrovolcanic eruptions occur, (2) depositional
characteristics that infer eruptive conditions, (3) a wide variety of
pyroclastic deposits (i.e., fallout through air or a body of water,
eruption-fed subaqueous sediment gravity current deposits, PDC deposits), and
(4) mega-dunes; 200 m-wavelength dunes associated with large scale, dilute,
PDCs from what may be one of the largest mafic hydrovolcanic eruptions ever
documented.
--
Brittany D Brand
University of Washington
Earth and Space Sciences
4000 15th Avenue NE
Box 351310
Seattle, WA 98195
Office: JHN 434
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