Boulder, CO, USA - "Setting the scene for self destruction" summarizes well the theme for GEOSPHERE this month. Edited by Sergio Rocchi, Andrea Dini, Francesco Mazzarini, and David S. Westerman, "LASI III - Magma pulses and sheets in tabular intrusions" draws on results from the 2008 LASI III conference, detailing stratovolcanoes in Italy and Chile; volcanic intrusions in South Africa's Karoo Basin; magma reservoirs at Elba Island, Italy; and the subsurface geology of the Santa Rosa Plain, California, USA.
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Introduction: Magma pulses and sheets in tabular intrusions
Sergio Rocchi et al., Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
Excerpt from the introduction: The origins of granites and intrusive rocks have been widely discussed for a couple of centuries, and the way volcanoes work and their magma forms have attracted scientists, naturalists, and laymen since the dawn of humankind. However, shallow igneous intrusions, representing the obvious link between the hidden kingdom of Pluto and the fiery realm of Vulcanus, have been partly overlooked, leading to some lack of communication between "plutonic" and "volcanic" researchers. An effort devoted to heal this breach has been contributed to by the establishment of the LASI conferences (named after laccolith and sill, the main types of shallow igneous intrusions).
Setting the scene for self destruction: From sheet intrusions to the structural evolution of rifted stratovolcanoes
Alessandro Tibaldi et al., Dipartimento di Scienze Geologiche e Geotecnologie, Università di Milano-Bicocca, Piazza della Scienza 4, Milan 20126, Italy
The evolution of volcanoes can be very complicated because they alternate phases of growth, both by lava piling at the surface and magma intrusions within the volcano, with phases of deep and surface land sliding. This team from Italy and the UK investigates the geological processes that produce these phenomena and demonstrate that they are intimately linked. Alessandro Tibaldi of the University of Milano-Bicocca and colleagues collected field data from four volcanoes located in Italy and Chile that are affected by rectilinear zones of fracturing, faulting, and magma upwelling, known as volcanic-rift zones. They also integrated these data with scaled physical models and numerical 3-D modeling. The results show that magma propagates from a magma chamber upward with a geometry given by planar magma intrusions (known as intrusive sheets). These intrusive sheets mostly occur along the volcanic-rift zones and produce a gradual expansion of the volcano perpendicularly to the rift that represents the internal growth of volcanoes. Along the rift, where intrusive sheets intersect the topographic surface, eruptions occur and lavas pile up. These constructive processes are partially or fully balanced by destructive processes that occur both perpendicular to the rift and along the rift zone, and that are initiated by the sheet intrusions. Field data and numerical and scaled physical models indicate that the intrusion of these sheets produces a magma push perpendicular to the rift, causing lateral deformation of the volcano flank and possibly triggering the failure of the entire cone flank. This creates a valley depression on the volcano flank. If this failure depression is refilled by new volcanic products, the cone attains a new critical height (and mass), setting the scene for renewed failure. This sequence inhibits the cone from widening normal to the rift zone. Our study also indicates that intrusive sheeting within the rift zone destabilizes the volcano flanks along the rift, possibly producing smaller and more surface landslides. This, in turn, produces debuttressing above the intruding sheet, favoring effusive eruptions and lavas flowing within the landslide depression. When volcanic activity ends, cone consumption is concentrated along the rift and is favored by fault slip or channeled erosion.
Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and sediment mobilization during solidification of magmatic sheet intrusions in sedimentary basins
Henrik Svensen et al., Physics of Geological Processes, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
What happens when magma intrudes the Earth's sedimentary cover? The Karoo Basin in South Africa was affected by widespread volcanic activity 183 million years ago, and this had a profound influence on the sedimentary rock that were intruded. Gas production and boiling of water occurred near the intrusions. This new study of the Karoo Basin by Norwegian scientists investigates layered volcanic intrusions and how the temperature and pressure evolved in the combined volcanic-sedimentary system. Modeling shows that huge pressure gradients developed soon after the sills were emplaced, with the potential to "suck" the surrounding sediments into the sills. Case studies from the Karoo Basin confirm this model, as spectacular sand-filled dikes are cross-cutting the volcanic sills. Thus, this work presents a new explanation for how sand-dikes in volcanic rocks are formed, and the model can be applied to similar geological systems.
Intrusive sheets and sheeted intrusions at Elba Island, Italy
Sergio Rocchi et al., Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
Magma ascending through the crust can rest forever in subterranean reservoirs or give way to catastrophic volcanic eruptions, such as that of Mount St. Helens thirty years ago. The understanding of the mechanisms driving the process to such different outcomes is based on both the investigation of recent volcanic products and old, exhumed reservoirs of magma in the crust. Headed by Sergio Rocchi of the University of Pisa in Italy, this study examines several magma reservoirs as old as 7 million years at Elba Island, Italy, show them having a simple tabular shape as well as a complex build-up history, made of multiple episodes of magma supply. The different magma batches formed separate sheet intrusions or merged in a single stack of magma sheets, depending on physical parameters such as the pressure driving magma upwards, the weight of the rock overburden, and the availability of physical discontinuities in the crust.
Also in this issue:
Three-dimensional geologic modeling of the Santa Rosa Plain, California
Donald S. Sweetkind et al., U.S. Geological Survey, Denver Federal Center, Mail Stop 973, Denver, Colorado 80225, USA
This study by geoscientists from the USGS and ESRI summarizes new interpretations of the subsurface geology of the Santa Rosa Plain in the northern part of the San Francisco Bay area, California, USA. Geologic data from 2,683 wells were combined with surface geologic information and geophysical data to develop an understanding of subsurface lithologic (rock type) and stratigraphic (geologic formation) variations. Numerical methods were used to interpolate geologic information between the well data to create a three-dimensional portrayal of the subsurface geology. The subsurface geologic results highlight a transition from continental to marine depositional environments in the younger sedimentary rocks within the basin; suggest the presence of abundant fine-grained sedimentary rocks at depth throughout much of the basin; and highlight the importance of the Trenton Ridge, a structural uplift that bisects the basin, in separating specific geologic units within the basin. The three-dimensional subsurface interpretations provide a suitable subsurface geologic framework to support ongoing groundwater resource assessment of the Santa Rosa Plain.
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Geosphere