V33A-1457 1340h
Emplacement of Large-volume rhyolite lavas in the Eastern Snake River Plains: The Reynolds Creek flow
Although less well known than their smaller counterparts, large-volume (>4 km3) evolved lavas (SiO2 > 60%) are common and contribute significantly to the volume of continental crust. However, little is known about the eruption and emplacement of evolved flows as they have not yet been observed while active. In the western Snake River Plains (SRP), Idaho, there are a number (~10-15) of rhyolite flows each with volumes of ~10-200 km3; we are investigating these to understand the eruption and emplacement of large-volume evolved flows. Here, we present the preliminary results of our investigation of the Reynolds Creek Rhyolite in the western SRP. This flow is useful for investigation because it is well preserved and accessible. Previous work revealed that the Reynolds Creek flow is smaller than most of the rhyolites of the SRP at 2.5-3.5 km3 and was probably not more than twice this volume at its greatest. It is a high-silica rhyolite (SiO2 = 75%) which erupted from a fissure ~0.4 km long and flowed mostly to the NE. In total, it is about 9 km long, smaller in the proximal region (0.2 km), widening distally (4 km in the NE) and ranges from 50-150 m thick. Results from Jeffrey's equation suggest that the Reynolds Creek flow may have had a flow velocity of up to 9 m/day, indicating emplacement in about 2-3 years. Graetz number calculations yield an effusion rate of ~40 m3s-1, suggesting an eruption duration of 1.5-3 years. These calculations all suggest that the volume of this flow was emplaced in months to years, rather than decades. These eruption duration calculations are comparable to that estimated for the 15 km3 Badlands flow (also in the Eastern Snake River Plains), at 6 - 16 years; and the Ben Lomond Flow, New Zealand, calculated at 1.5 - 6 years. In contrast, the Mount St Helens dacite has an estimated effusion rate of 1.4-40 m3s-1 for single lobes and the Santiaguito dacite, Guatemala, is estimated at 0.6-1.9 m3s-1; this may be due to periods of repose still included in the estimates. Yield strength estimates for the Reynolds Creek flow, based on flow thickness and underlying slope, range from 104-105 Pa which are comparable to the 8 x 105 Pa estimated for the large-volume (26 km3) Chao Dacite, Chile, suggesting similar rheologies and emplacement behaviors. The Mount St Helens dacite also falls within the same yield strength rate on the order of 1-2 x 105 Pa. Continued investigations will reveal whether the large-volume evolved flows have fundamentally different emplacement styles than the more familiar, small-volume domes.
V33A-1458 1340h
Melt Inclusions From a Basaltic Tuya in British Columbia: Contrasts in Early- and Late-Stage Degassing Processes
Volcanic tuyas are of interest because their formation beneath glaciers preserves information on ice-magma interaction and magma degassing processes. In northern British Columbia (58-$60\deg$ N latitude) there exists a small volcanic field of Quaternary basaltic tuyas. Ash Mountain is one example that is 700m in height, 4-5 km in basal diameter and has a lower section of pillow lavas and an upper section of unconsolidated hyaloclastite. This distinctive morphological change, common to many of the tuyas, has been interpreted as corresponding to a shift from initial subglacial eruption to later subaqueous to subaerial eruption. Ash Mountain also displays the common shift in composition from early tholeiitic lavas to later alkalic lavas. Melt inclusions from the upper ash unit have been analyzed to investigate the later stages of volcano formation. Olivine crystals (Fo$_{76-80}$) separated from bulk ash from the upper flank of the volcano contain abundant melt inclusions and large glass-filled re-entrants. The melt inclusions are alkalic in character and closely match the groundmass glass composition indicating that the crystals and melt inclusions are representative of late-stage crystallization. FTIR analysis shows that the average water content of melt inclusions is 0.9 wt.% with variable CO$_{2}$ (0 to 1200 ppm). The variation in H$_{2}$O and CO$_{2}$ resembles an open-system degassing trend consistent with magma crystallization during transport within the shallow crust. Previously determined water abundance (0.5 to 0.7 wt.%) and hydrogen isotopic composition (\deltaD -87.2 to -- 102.0 \permil) for Ash Mountain pillow lavas are consistent with shallow level degassing during early volcano formation. The melt inclusion data indicates the later alkalic magma,which erupted with greater energy, was derived from greater depth.
V33A-1459 1340h
Numerical Modelling of Formation of Sheeted-Dyke Complex at Different Types of Mid-Ocean Ridges
Magma dykes are the most important structural elements of oceanic crust. In this work, attention is concentrated on the origin of the sheeted-dyke complex. Generally, it is formed by solidification of episodic intrusions from magma reservoirs. Up to now, most models are qualitative and tend to neglect stress fields arising from dyke openings. In this model, heterogeneities of the stress field are calculated quantitatively and are taken into account. A modified boundary element code capable of simulating quasi-static finite-volume fluid-filled fracture growth and movement in an elastic lithosphere under inhomogeneous stress loading is used. Numerical models for slow to fast spreading ridges are presented. Slow spreading ridges, possessing magma reservoirs at the crust-mantle boundary with large extension perpendicular to the ridge, demand broad dyke injection zones with continuous magma supply, whereas melt ascent from crustal magma chambers at fast spreading ridges generates only narrow dyke injection zones. Despite the different origins of magma at slow and fast spreading ridges, the sheeted-dyke complex looks similar. Finally, ridges spreading at intermediate rates provide discontinuous magma supply for crust formation either by unstable crustal magma chambers or by feeding of the sheeted-dyke complex by underlying sills. Results of modelling are that stress fields caused by dyke emplacement cannot be neglected in any models for the formation of oceanic crust, since self-induced stress fields can locally surmount the effect of regional stress fields. For example, crossing and focussing of dykes may either lead to the development of volcanic centres with different spacing or to pooling of sills building up a whole layer, the behaviour being dependent e.g. on dyke lenghts. Moreover, the focussing of dykes may generate a cumulative stress field similar to that of an elliptical magma chamber in a kind of self-organising way. If a regional, extensional stress field is introduced additionally, both narrow and broad dyke injection zones result in parallelism of ascending dykes and thus favour the construction of a sheeted-dyke complex. Acknowledgments: The project is supported by the German Research Foundation (DFG).
V33A-1460 1340h
Dendritic lava flows, landslides and terraces around the central Azores islands
Surveying around volcanic ocean islands with sonars has recovered important information on giant landslides, faults and primary volcanic features, but efforts so far have largely been unable to image shallow water coastal areas because of vessel safety. Here we report surveying with a Reson 8160 multibeam sonar aboard a shallow draft research vessel, R/V Arquipelago, which enabled us to survey to less than 10 m water depth around the coasts of Faial, Pico and Sao Jorge islands of the Azores. The data cover coasts that have been growing volcanically, some during historical times. Where the coast has a finite abrasion shelf, the new data show that lava reaching the shore can breach the surf zone and develop a variety of submarine lava structures on the shelf. Many are dendritic in plan-view and some with transverse ribbing similar to pahoehoe flows on land but much larger scale. A variety of divergent flow paths are clearly indicated in the data. Some flows cross the shelf and descend the upper slope beyond the shelf break, providing evidence that a component of growth of the submarine island can include subaerially-originating lava as inferred from sulphur contents in submarine lava dredged from around Hawai'i. Where the abrasion shelf is very narrow or absent, the upper slope of the island contains abundant shallow landslides in the new unstable and steep volcanic material. The data show a variety of other interesting features, such as terraces, volcanic cones, collapse structures, tumuli, faults associated with the Azores plate boundary and sedimentary bedforms produced by interaction of oceanic currents with the island topography and from turbidity currents descending island slopes.
V33A-1461 1340h
Implications of Viscosity-Contrast for Co-Extruding Two-Component Magmas, Triggering Eruptions and Forming Layered Domes
Polymer co-extrusion experiments represent excellent dynamical analogies with two-magma transport and the effusion of composite lava domes. They demonstrate that the co-extrusion of magmas having different viscosity can explain not only the observed normal zoning in magma dikes and conduits but also the compositional layering observed in effused lava domes. New results indicate that dike and conduit zoning along with dome layering are strongly dependent on the viscosity contrast between the flowing magmas. Realistic models of magma storage and dike formation show that co-extrusion of magmas is both more readily explained and energetically preferred over serial intrusion processes. Co-extrusion during the formation of dikes may play an important role in triggering larger volcanic eruptions. Lubrication of the flow by a typically, more mafic, lower-viscosity component allows a more viscous but also more highly volatile-charged magma to be transported greater distances upward in the dike resulting in exsolution of a gas phase and the formation of a magma foam. Transition to a foam lowers the bulk density of the magma enabling dikes to propagate greater vertical distances for a given back pressure. Our new results suggest that a dike propagating across a sloping magma-chamber roof intersecting both "wet" silicic and relatively "dry" mafic layers has the greatest probability of reaching the surface in the dike segment where the magmas flow co-extrusively. Thus, bimodal eruptive compositions are dynamically preferred in such a petrologically common magmatic regime. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
V33A-1462 1340h
Magnetic petrology through Fe-Ti oxide minerals in various modes of eruption: case studies of Unzen and other volcanoes
Volcanic eruptions generate a variety of materials depending on varying magma, degrees of degassing and different cooling process. Therefore, it is important to understand what condition the material is generated from. Iron-titanium oxide minerals are very useful for studying the generation condition. Their initial chemical compositions record the equilibrium state. Successively they are oxidized in deuteric cooling process and transformed into composite multiphase grains whose phases have distinct chemical compositions. In addition, two solid solution series, titanomagnetite and titanohematite, show magnetic properties and acquire thermoremanent magnetization during cooling from above the Curie temperature. Their magnetic properties depend on compositions, grain sizes and amount of oxide minerals. Therefore, if we identify iron-titanium oxides and determine their properties, we can estimate the equilibrium state and the oxidation process of iron-titanium oxides during eruption. Such a method of study has been recently graced with a formal name "magnetic petrology". We have carried out magnetic petrological study in some volcanoes in order to understand a variety of volcanic materials. In order to understand the oxidation state in lava dome, magnetic petrological analyses were carried out on lava samples from the lava domes and block-and-ash-flow deposits of the 1990-1995 eruption of Unzen volcano, Japan. As a result, we found that lava samples were classified into three types with different iron-titanium oxide mineral assemblages and deuteric oxidation in the lava dome separated samples of each type. By field survey of Unzen lava dome, we found that the exogenous dome consists of fresh lava, while the endogenous dome oxidizes and produces oxidized blocks in the surface. This is because the exogenous dome generated much fresh lava due to successive highly supply of new lava, while the endogenous dome was exposed to the air for a long time at high temperature and oxidized because the hot lava slowly intruded within the dome. We will show some evidences in terms of application of our methods in magnetic petrology to other volcanoes including Suwanose volcano in SW Japan and Kilauea volcano in Hawaii.
V33A-1463 1340h
Length Scales in Columnar Jointing of the Columbia Basalt Group
We report a set of observations and interpretations of the pattern of columnar jointing as displayed in the Columbia Basalt Group of Washington and Oregon. This pattern of nearly hexagonal prismatic columns of rock originates when lava pools, and slowly cools from its surfaces. Fractures initiate at the cooling surfaces, and advance periodically into the flow. The length scale of each fracture advance is believed to be related to the height of the elastic layer between the already fractured rock, and the still compliant interior of the flow. This sequential advance leaves behind alternating bands of rough and smooth rock, called striae, on the sides of the columns. Field observations were made along the Grand Coulee, and in the Snake and Columbia River valleys, investigating the relationship between striae height and the face widths of individual columns. We confirm that, on average, there is a significant correlation between the striae height and the column width. However, this correlation can be misleading, as striae heights can vary by over a factor of three on some columns -- without corresponding changes in column widths. Observations made near the edges of flow units show a tendency of the striae width to increase away from the flow margins. We suggest that, although both striae and face length scales may be linked to the cooling rate of the flow, the striae length scale can more readily adapt to changing cooling conditions than can the column width. This hypothesis is supported by recent experiments in laboratory analog systems. We also report tentative observations of a zig-zag modulation visible on some column faces. The height of each zig (or zag) presents a length scale 3 to 5 times larger than that of corresponding striae heights, and is about half the size of the column widths.