VOLCANIC HAZARDS RELATED TO THE GROWTH OF LACCOLITHS: EXAMPLES FROM MIOCENE IRON AXIS LACCOLITHS, SOUTHWEST UTAH

Volcanic eruptions are one of Earth's most dramatic geologic agents of change, with some violent explosive eruptions altering the landscape for tens of kilometers around a volcano. There are many historically documented volcanic activities that pose hazards to the lives of people and property, including those associated with lava flows, pyroclastic flows, tephra, landslides, lahars, gases, and tsunamis. Much is also known from the geologic record concerning caldera eruptions (e.g., Yellowstone Caldera) and the potential risks of these lethal destructive forces despite the lack of historical eruption records. Less is known from the geologic and historic record about hazards associated with the emplacement and growth of shallow-level intrusions such as laccoliths, despite modern examples of their growth (e.g., Syowa Sinzan at Usu-san, Hakkaido Island, Japan). Our work on Miocene laccoliths of southwest Utah shows that laccoliths can pose potential hazards similar to explosive volcanoes and should be included in volcanic assessments of likely future hazards in areas of modern dome growth.

Geologic mapping and field studies of five quartz monzonite laccoliths (Pinto Peak, Iron Mountain, Bull Valley-Big Mountain, Stoddard Mountain, and Pine Valley) that make up a portion of the 22 to 20 Ma Iron Axis magmatic province of southwest Utah has delineated a series of gravity slides (i.e., landslides), ash-flow tuffs, and lava flows that resulted from the catastrophic slope failure of the laccolith domes produced by the rapidly inflating intrusions (Hacker et. al., 2002). The largest slide mass covers >150 km2, is more than 550 m thick, and extends >20 km from its parent laccolith. The largest ash-flow tuff covers an area of >700 km2, is more than 100 m thick, and flowed >25 km from its parent laccolith. In all cases, the gravity slides are blanketed by volcanic products indicating a rapid sequence of events (1-doming, 2-gravity sliding, and 3-volcanism) in which the release of confining pressure by gravity sliding on laccolith magma chambers initiated violent eruptions. This sequence of events is well documented in historic volcano eruptions like Mount St. Helens (Lipman and Mullineaux, 1981), but this is the first study to show these hazards associated with laccolith doming.