Geologic map of Medicine Lake volcano, northern California (original) (raw)
Related papers
Journal of Volcanology and Geothermal Research, 2008
Medicine Lake Volcano (MLV), located in the southern Cascades ∼ 55 km east-northeast of contemporaneous Mount Shasta, has been found by exploratory geothermal drilling to have a surprisingly silicic core mantled by mafic lavas. This unexpected result is very different from the long-held view derived from previous mapping of exposed geology that MLV is a dominantly basaltic shield volcano. Detailed mapping shows that b 6% of the ∼ 2000 km 2 of mapped MLV lavas on this southern Cascade Range shield-shaped edifice are rhyolitic and dacitic, but drill holes on the edifice penetrated more than 30% silicic lava. Argon dating yields ages in the range ∼ 475 to 300 ka for early rhyolites. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame as well, indicating that volcanism at MLV began about half a million years ago. Mafic compositions apparently did not dominate until ∼ 300 ka. Rhyolite eruptions were scarce post-300 ka until late Holocene time. However, a dacite episode at ∼ 200 to ∼ 180 ka included the volcano's only ash-flow tuff, which was erupted from within the summit caldera. At ∼ 100 ka, compositionally distinctive high-Na andesite and minor dacite built most of the present caldera rim. Eruption of these lavas was followed soon after by several large basalt flows, such that the combined area covered by eruptions between 100 ka and postglacial time amounts to nearly two-thirds of the volcano's area. Postglacial eruptive activity was strongly episodic and also covered a disproportionate amount of area. The volcano has erupted 9 times in the past 5200 years, one of the highest rates of late Holocene eruptive activity in the Cascades. Estimated volume of MLV is ∼ 600 km 3 , giving an overall effusion rate of ∼ 1.2 km 3 per thousand years, although the rate for the past 100 kyr may be only half that. During much of the volcano's history, both dry HAOT (high-alumina olivine tholeiite) and hydrous calcalkaline basalts erupted together in close temporal and spatial proximity. Petrologic studies indicate that the HAOT magmas were derived by dry melting of spinel peridotite mantle near the crust mantle boundary. Subduction-derived H 2 O-rich fluids played an important role in the generation of calcalkaline magmas. Petrology, geochemistry and proximity indicate that MLV is part of the Cascades magmatic arc and not a Basin and Range volcano, although Basin and Range extension impinges on the volcano and strongly influences its eruptive style. MLV may be analogous to Mount Adams in southern Washington, but not, as sometimes proposed, to the older distributed back-arc Simcoe Mountains volcanic field.
Crustal subsidence, seismicity, and structure near Medicine Lake Volcano, California
Journal of Geophysical Research, 1991
The pattern of historical ground deformation, seismicity, and crustal structure near Medicine Lake volcano illustrates a close relation between magmatism and tectonism near the margin of the Cascade volcanic chain and the Basin and Range tectonic province. Between leveling surveys in 1954 and 1989 the summit of Medicine Lake volcano subsided 389 +--43 mm with respect to a reference bench mark 40 km to the southwest (average rate = 11.1 +--1.2 mm/yr). A smaller survey across the summit caldera in 1988 suggests that the subsidence rate was 15-28 mm/yr during 1988-1989. Swarms of shallow earthquakes (M < 4.6) occurred in the region during August 1978, January-February 1981, and September 1988. Except for the 1988 swarm, which occurred beneath Medicine Lake caldera, most historical earthquakes were located at least 25 km from the summit. The spatial relation between subsidence and seismicity indicates (1) radially symmetric downwarping of the volcano's summit and flanks centered near the caldera and (2) downfaulting of the entire edifice along regional faults located 25-30 km from the summit. We propose that contemporary subsidence, seismicity, and faulting are caused by (1) loading of the crust by more than 600 km3 of erupted products plus a large volume of mafic intrusives' (2) east-west extension in the western Basin and Range province; hnd, to a lesser extent, (3) crystallization or withdrawal of magma beneath the volcano. Thermal weakening of t•he subvolcanic crust by mafic intrusions facilitates subsidence and influences the distribution of earthquakes. Subsidence occurs mainly by aseismic creep within 25 km of the summit, where thi• crust has been heated and weakened by intrusions, and by normal faulting during episodic earthquake swarms in surrounding, cooler terrain. '
Volcano Hazards Assessment for Medicine Lake Volcano, Northern California
Scientific Investigations Report, 2007
electronic or motorized equipment. Once dry, volcanic ash deposits can be remobilized by wind and remain troublesome long after an eruption ceases. See plate 1 for a concise presentation of the material in this report, and see the glossary at the back of the report for explanation of geologic terms.
Journal of Geophysical Research, 1991
The Giant Crater lava field consists of >4 km 3 of basaltic lava, compositionally zoned from first-erupted calc-alkaline basaltic andesite to last-erupted primitive high-alumina basalt. On the FeO*/MgO (where FeO* is total Fe calculated as FeO) versus SiO2 discrimination diagram commonly used to distinguish tholeiitic from calc-alkaline series lavas the compositionally zoned eruption crosses from the tholeiitic field to the calc-alkaline field. The lavas erupted in a brief span of time about 10,500 years ago from several closely spaced vents on the south flank of Medicine Lake volcano in the southern Cascade Range. Six chemical-stratigraphic groups were mapped. Lower K20, higher MgO groups always overlie higher K20, lower MgO groups. Group 6 lavas erupted last and are aphyric, have high contents of MgO and Ni, and contain as little as 0.07% K20. Group 1 lavas are porphyritic and have as much as 1.10% K20. Major element contents of primitive group 6 Giant Crater basalt are very similar to a subset of primitive mid-ocean ridge basalts (MORB). Group 6 lava is more depleted in middle and heavy rare earth elements (REE) and Y than is primitive MORB, but it is enriched in large ion lithophile elements (LILE). These LILE enrichments may be a result of fluid from the subducting slab interacting with the mantle beneath Medicine Lake volcano. The group 6 REE pattern is parallel to the pattern of normal-type MORB, indicating a similar although perhaps more depleted mantle source. The location of Medicine Lake volcano in an extensional environment behind the volcanic front facilitates the rise of mantle-derived melts. Modification of the primitive group 6 basalt to more evolved compositions takes place in the upper crust by processes involving fractional crystallization and assimilation. The group 1 calc-alkaline Giant Crater basaltic andesite produced by these processes is similar to other Cascade basaltic andesites, implying that a similar high-alumina basalt may be parental.
Frontiers in Earth Science, 2016
The eastern boundary of the Southern Cascades (Hat Creek Graben region), California, USA, is an extensively faulted volcanic corridor between the Cascade Range and Modoc Plateau. The morphology of the region is a result of plate motions associated with different tectonic provinces, faulting, and recurring volcanic activity, making it an ideal place to study the interrelationship between tectonics, volcanoes, and geomorphology. We use the morphometry and spatial distribution of volcanoes and their interaction with regional structures to understand how long term regional deformation can affect volcano evolution. A database of volcanic centers and structures was created from interpretations of digital elevation models. Volcanic centers were classified by morphological type into cones, sub-cones, shields and massifs. A second classification by height separated the larger and smaller edifices, and revealed an evolutionary trend. Poisson Nearest Neighbor analysis showed that bigger volcanoes are spatially dispersed while smaller ones are clustered. Using volcano centroid locations, about 90 lineaments consisting of at least three centers within 6 km of one another were found, revealing that preferential north-northwest directed pathways control the transport of magma from the source to the surface, consistent with the strikes of the major fault systems. Most of the volcano crater and collapse scar openings are perpendicular to the north northwest-directed maximum horizontal stress, expected for extensional environments with dominant normal faulting. Early in the history of a volcano or volcano cluster, melt propagates to the surface using the easiest and most efficient pathway, mostly controlled by the pre-existing normal faults and near-surface stress fields, as indicated by the pervasive vent alignments. Volcano growth continues to be dependent on the regional structures as indicated by the opening directions, suggesting structural control on the growth of the volcanic edifices. The results present a particularly well-defined case in which extension of a volcanic region is accommodated mostly by faulting, and only partly by intrusion to form volcanoes. This is attributed to a low magma supply rate.
Pre-eruptive conditions beneath Medicine Lake Volcano, California, during the Pleistocene epoch
2000
This investigation uses the variations in composition of lavas and mineralogical evidence to characterize the preeruptive conditions beneath Medicine Lake Volcano (MLV) during the Pleistocene epoch. Prior to this investigation, the Pleistocene lavas of MLV had not been extensively studied because detailed time relations among them were not known. Recent work by Donnelly-Nolan has provided a time framework for relating these lavas, and has afforded us the opportunity to perform this study. Whole rock analyses for major elements, trace elements, and rare earth elements were performed, as well as electron microprobe analyses of individual minerals in the lavas. These analyses indicate that the lavas were formed by the mixing of three components. The first component was formed by fractional crystallization of a high-H20 magma (water content of 3-4 wt%) which was emplaced in the shallow crust, at approximately 1 kbar of pressure and temperatures between 980-1300K. The emplacement and cry...
Frontiers in Earth Science, 2021
The Cascades back-arc in northern California is dominated by monogenetic tholeiitic basalts that erupted throughout the Pleistocene. Elucidating their eruptive history and processes is important for understanding potential future eruptions here. We focus on the well-exposed monogenetic volcano that emplaced the Brushy Butte flow field, which constructed a ∼150 m tall edifice, has flow lobes up to >10 km long, and in total covers ∼150 km2 with an eruptive volume of 3.5 km3. We use a multidisciplinary approach of field mapping, petrography, geochemistry, paleomagnetism, geochronology, and lidar imagery to unravel the eruptive history and processes that emplaced this flow field. Tholeiitic basalts in northern California have diverse surface morphology and vegetation cover but similar petrographic appearances, which makes them hard to distinguish in the field. Geochemistry and paleomagnetism offer an independent means of distinguishing tholeiitic basalts. Brushy Butte flow field lava...
Young bimodal volcanism at Medicine Lake volcanic center, northern California
Geochimica et Cosmochimica Acta, 1975
The last 10,000 years of activity at the Medicine Lake volcanic center in northern California is characterized by bimodal mafic and siliceous volcanism. Interflow element variations are complex and exhibit a discontinuity for most elements between 57 and 62 per cent SiO,. No simple linear or curvilinear element trends exist between the mafic (Modoc) and siliceous (glass) volcanics. The geochemical variation patterns exhibited by volcanic rocks from the Medicine Lake volcanic center preclude any simple model for magma origin involving either varying degrees of melting or of fractional crystallization.