Effects of topography on facies and compositional zonation in caldera-related ignimbrites

Research Article| February 01, 1992 Effects of topography on facies and compositional zonation in caldera-related ignimbrites GREG A. VALENTINE; GREG A. VALENTINE 1Geoanalysis Group, M.S. F665, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Search for other works by this author on: GSW Google Scholar KENNETH H. WOHLETZ; KENNETH H. WOHLETZ 2Geology-Geochemistry Group, M.S. D462, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Search for other works by this author on: GSW Google Scholar SUSAN W. KIEFFER SUSAN W. KIEFFER 3Department of Geology, Arizona State University, Tempe, Arizona 85287 Search for other works by this author on: GSW Google Scholar Author and Article Information GREG A. VALENTINE 1Geoanalysis Group, M.S. F665, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 KENNETH H. WOHLETZ 2Geology-Geochemistry Group, M.S. D462, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 SUSAN W. KIEFFER 3Department of Geology, Arizona State University, Tempe, Arizona 85287 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (2): 154–165. https://doi.org/10.1130/0016-7606(1992)104<0154:EOTOFA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation GREG A. VALENTINE, KENNETH H. WOHLETZ, SUSAN W. KIEFFER; Effects of topography on facies and compositional zonation in caldera-related ignimbrites. GSA Bulletin 1992;; 104 (2): 154–165. doi: https://doi.org/10.1130/0016-7606(1992)104<0154:EOTOFA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Large-scale fluid dynamical processes during explosive eruptions within calderas are examined numerically by solving the full set of two-phase hydrodynamic equations with a topographic barrier, representing the rim of a caldera. The effect of the caldera rim on eruption dynamics depends on the relative locations of the rim and the impact zone where tephra collapsing from the eruption column strikes the ground. The distance of the impact zone from the vent is proportional to the collapse (fountain) height of the eruption column. Three significantly different eruption patterns have been observed in the simulations: (1) If the impact zone is outside the caldera rim, relatively continuous pyroclastic flow occurs outside the caldera. (2) If the impact zone is on or near the caldera rim, an initial pyroclastic current flows out of the caldera and is followed by a lapse in outflow during which the caldera fills up with ash. (3) If the impact zone is inside the rim, all initial pyroclastic flows are contained within the caldera unless the flows have sufficiently high initial densities and velocities to carry them over the rim. In most cases, recirculation of pyroclasts into the base of the column causes fountain height to decrease dramatically with time due to the "choking" effect of the ash. This recycling of ash in turn reduces the ability of pyroclastic flows to surmount the rim. The numerical models suggest several processes that cause the formation of multiple cooling and flow units in deposits outside a caldera from a single eruption of steady discharge. Compositional gaps may occur in outflow ignimbrite due entirely to interaction of eruption and emplacement dynamics with topography; sharp compositional gradients within a magma chamber are not necessarily implied by compositional gaps in outflow units. Outflow and intracaldera facies tuffs from the same eruptions can show different cooling histories and compositional variations because of these large-scale fluid dynamical processes. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.