Search USGS⌕ Search

Geology topics

Michael P. Poland

Publications and source records attributed to Michael P. Poland.

143 records · Page 8Linked to original sources
USGS2008

Radar interferometry observations of surface displacements during pre- and coeruptive periods at Mount St. Helens, Washington, 1992-2005

We analyzed hundreds of interferograms of Mount St. Helens produced from radar images acquired by the ERS-1/2, ENVISAT, and RADARSAT satellites during the 1992-2004 preeruptive and 2004-2005 coeruptive periods for signs of deformation associated with magmatic activity at depth. Individual interferograms were often contaminated by atmospheric delay anomalies; therefore, we employed stacking to amplify any deformation patterns that might exist while minimizing random noise. Preeruptive interferograms show no signs of volcanowide deformation between 1992 and the onset of eruptive activity in 2004. Several patches of subsidence in the 1980 debris-avalanche deposit were identified, however, and are thought to be caused by viscoelastic relaxation of loosely consolidated substrate, consolidation of water-saturated sediment, or melting of buried ice. Coeruptive interferometric stacks are dominated by atmospheric noise, probably because individual interferograms span only short time intervals in 2004 and 2005. Nevertheless, we are confident that at least one of the seven coeruptive stacks we constructed is reliable at about the 1-cm level. This stack suggests deflation of Mount St. Helens driven by contraction of a source beneath the volcano.

Washington↗
USGS2008

Evolution of deformation studies on active Hawaiian volcanoes

Everything responds to pressure, even rocks. Deformation studies involve measuring and interpreting the changes in elevations and horizontal positions of the land surface or sea floor. These studies are variously referred to as geodetic changes or ground-surface deformations and are sometimes indexed under the general heading of geodesy. Deformation studies have been particularly useful on active volcanoes and in active tectonic areas. A great amount of time and energy has been spent on measuring geodetic changes on Kilauea and Mauna Loa Volcanoes in Hawai`i. These changes include the build-up of the surface by the piling up and ponding of lava flows, the changes in the surface caused by erosion, and the uplift, subsidence, and horizontal displacements of the surface caused by internal processes acting beneath the surface. It is these latter changes that are the principal concern of this review. A complete and objective review of deformation studies on active Hawaiian volcanoes would take many volumes. Instead, we attempt to follow the evolution of the most significant observations and interpretations in a roughly chronological way. It is correct to say that this is a subjective review. We have spent years measuring and recording deformation changes on these great volcanoes and more years trying to understand what makes these changes occur. We attempt to make this a balanced as well as a subjective review; the references are also selective rather than exhaustive. Geodetic changes caused by internal geologic processes vary in magnitude from the nearly infinitesimal - one micron or less, to the very large - hundreds of meters. Their apparent causes also are varied and include changes in material properties and composition, atmospheric pressure, tidal stress, thermal stress, subsurface-fluid pressure (including magma pressure, magma intrusion, or magma removal), gravity, and tectonic stress. Deformation is measured in units of strain or displacement. For example, tilt of the ground surface on the rim of Kilauea Caldera is measured in microradians, a strain unit that gives the change in angle from some reference. The direction in which the tilt is measured must be defined - north or south, or some direction normal to the maximum changes. For displacements related to surface faulting, the changes are normally given in linear measures of offset. Changes in the diameter of a caldera can be given in either displacements or strain units. In the later case, the displacement divided by the 'original' diameter gives the strain ratio. Strains are dimensionless numbers; displacements have the dimensions of length. Vectors commonly are used to show the direction and amount of displacements in plan view. Strain results from stress. It can be elastic strain, when the strain is linearly related to stress and is recoverable; it can be viscous strain, where the rate of strain is proportional to the stress and is not recoverable; or it can be plastic strain that is often some complex stress-strain relationship, for example, elastic up to some yield strength and viscous beyond. Volcanic rocks are brittle when cold and under near-surface pressures but plastic to viscous under higher temperature and pressure regimes. It is important in deformation studies to try to define the nature of the strain and the rheology of the rocks being deformed. A good text on rheology is 'The Structure and Rheology of Complex Fluids' by R.G. Larson, 1999. Under changing tensional or compressional stresses, tiny cracks in brittle rocks may open or close, causing a quasielastic strain response. If the stresses exceed the breaking strength of the rock, brittle failure occurs, and the stress-strain relationship breaks down.

Scientific Investigations Report↗
USGS2008

Instrumentation recommendations for volcano monitoring at U.S. volcanoes under the national volcano early warning system

As magma moves toward the surface, it interacts with anything in its path: hydrothermal systems, cooling magma bodies from previous eruptions, and (or) the surrounding “country rock.” Magma also undergoes significant changes in its physical properties as pressure and temperature conditions change along its path. These interactions and changes lead to a range of geophysical and geochemical phenomena. The goal of volcano monitoring is to detect and correctly interpret such phenomena in order to provide early and accurate warnings of impending eruptions. Given the well-documented hazards posed by volcanoes to both ground-based populations (for example, Blong, 1984; Scott, 1989) and aviation (for example, Neal and others, 1997; Miller and Casadevall, 2000), volcano monitoring is critical for public safety and hazard mitigation. Only with adequate monitoring systems in place can volcano observatories provide accurate and timely forecasts and alerts of possible eruptive activity. At most U.S. volcanoes, observatories traditionally have employed a two-component approach to volcano monitoring: (1) install instrumentation sufficient to detect unrest at volcanic systems likely to erupt in the not-too-distant future; and (2) once unrest is detected, install any instrumentation needed for eruption prediction and monitoring. This reactive approach is problematic, however, for two reasons. 1. At many volcanoes, rapid installation of new ground-1. based instruments is difficult or impossible. Factors that complicate rapid response include (a) eruptions that are preceded by short (hours to days) precursory sequences of geophysical and (or) geochemical activity, as occurred at Mount Redoubt (Alaska) in 1989 (24 hours), Anatahan (Mariana Islands) in 2003 (6 hours), and Mount St. Helens (Washington) in 1980 and 2004 (7 and 8 days, respectively); (b) inclement weather conditions, which may prohibit installation of new equipment for days, weeks, or even months, particularly at midlatitude or high-latitude volcanoes; (c) safety factors during unrest, which can limit where new instrumentation can safely be installed (particularly at near-vent sites that can be critical for precursor detection and eruption forecasting); and (d) the remoteness of many U.S. volcanoes (particularly those in the Aleutians and the Marianas Islands), where access is difficult or impossible most of the year. Given these difficulties, it is reasonable to anticipate that ground-based monitoring of eruptions at U.S. volcanoes will likely be performed primarily with instruments installed before unrest begins. 2. Given a growing awareness of previously undetected 2. phenomena that may occur before an eruption begins, at present the types and (or) density of instruments in use at most U.S. volcanoes is insufficient to provide reliable early warning of volcanic eruptions. As shown by the gap analysis of Ewert and others (2005), a number of U.S. volcanoes lack even rudimentary monitoring. At those volcanic systems with monitoring instrumentation in place, only a few types of phenomena can be tracked in near-real time, principally changes in seismicity, deformation, and large-scale changes in thermal flux (through satellite-based remote sensing). Furthermore, researchers employing technologically advanced instrumentation at volcanoes around the world starting in the 1990s have shown that subtle and previously undetectable phenomena can precede or accompany eruptions. Detection of such phenomena would greatly improve the ability of U.S. volcano observatories to provide accurate early warnings of impending eruptions, and is a critical capability particularly at the very high-threat volcanoes identified by Ewert and others (2005). For these two reasons, change from a reactive to a proactive volcano-monitoring strategy is clearly needed at U.S. volcanoes. Monitoring capabilities need to be expanded at virtually every volcanic center, regardless of its current state of unrest, with particular emphases on real-time data transmission and increasing the diversity, quality, and quantity of instrumentation at U.S. volcano observatories. In this report, we present recommendations for the types and numbers of instruments that should be deployed to monitor U.S. volcanoes. These recommendations are the result of discussions among members of a panel of government- and university-based scientists about the status and future directions of volcano monitoring, with discussions framed by the U.S. Geological Survey (USGS)’s National Volcano Early Warning System (NVEWS) initiative (Ewert and others, 2005).

Scientific Investigations Report↗
USGS2008

Deformation and stress-change modeling at Sierra Negra volcano, Galapagos, from ENVISAT INSAR and GPS observations

We use radar interferograms and GPS observations to constrain models of magma accumulation and faulting at Sierra Negra volcano, Galápagos, during the years before its 2005 eruption. The data have shown ~5 m of pre-eruption uplift and multiple trapdoor faulting events on an intra-caldera fault system. We find the pattern of uplift to be consistent with an inflating sill at 2.2 km depth under the caldera. Our deformation modeling and stress-change calculations suggest that the inflating sill triggered faulting on an inward- dipping thrust fault and that the faulting in turn relieved the pressure within the sill. This sill-fault interaction tends to thicken the sill and limit its lateral extent within the area bounded by the fault.

Conference Paper↗
USGS2008

InSAR observations of deformation associated with new episodes of volcanism at Kilauea Volcano, Hawai'i, 2007

In June 2007, the Pu'u 'Ō'ō-Kūpaianaha eruption of Kīlauea Volcano was interrupted when magma intruded the east rift zone (ERZ), resulting in a small extrusion of lava near Makaopuhi Crater. Deformation associated with the activity was exceptionally well-documented by ASAR interferometry, which indicates deflation of the summit and uplift and extension of the ERZ. Models of co-intrusion interferograms suggest that the dike was emplaced in two distinct segments. The modeled volume of the dike greatly exceeds that of the deflation source, raising the possibility that magma from the downrift Pu'u 'Ō'ō vent (dominant extrusion site at Kīlauea since 1983) contributed to the eruption near Makaopuhi, or that the magma that fed the eruption from the summit was compressible. A month following the Makaopuhi eruption, an eruptive fissure opened on the east flank of Pu'u 'Ō'ō . Interferograms, processed within 48 hours of the event, were critical in demonstrating that the magma source feeding the eruption was shallow. The eruption probably resulted from overpressure in Pu'u 'Ō'ō 's magmatic system.

Hawaii↗
USGS2007

A model for radial dike emplacement in composite cones based on observations from Summer Coon volcano, Colorado, USA

We mapped the geometry of 13 silicic dikes at Summer Coon, an eroded Oligocene stratovolcano in southern Colorado, to investigate various characteristics of radial dike emplacement in composite volcanoes. Exposed dikes are up to about 7 km in length and have numerous offset segments along their upper peripheries. Surprisingly, most dikes at Summer Coon increase in thickness with distance from the center of the volcano. Magma pressure in a dike is expected to lessen away from the pressurized source region, which would encourage a blade-like dike to decrease in thickness with distance from the center of the volcano. We attribute the observed thickness pattern as evidence of a driving pressure gradient, which is caused by decreasing host rock shear modulus and horizontal stress, both due to decreasing emplacement depths beneath the sloping flanks of the volcano. Based on data from Summer Coon, we propose that radial dikes originate at depth below the summit of a host volcano and follow steeply inclined paths towards the surface. Near the interface between volcanic cone and basement, which may represent a neutral buoyancy surface or stress barrier, magma is transported subhorizontally and radially away from the center of the volcano in blade-like dikes. The dikes thicken with increasing radial distance, and offset segments and fingers form along the upper peripheries of the intrusions. Eruptions may occur anywhere along the length of the dikes, but the erupted volume will generally be greater for dike-fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance from the source. Observed eruptive volumes, vent locations, and vent-area intrusions from inferred post-glacial dike-fed eruptions at Mount Adams, Washington, USA, support the proposed model. Hazards associated with radial dike emplacement are therefore greater for longer dikes that propagate to the outer flanks of a volcano.

Colorado↗
USGS2007

The 2005 eruption of Sierra Negra volcano, Galápagos, Ecuador

Sierra Negra volcano began erupting on 22 October 2005, after a repose of 26 years. A plume of ash and steam more than 13 km high accompanied the initial phase of the eruption and was quickly followed by a ~2-km-long curtain of lava fountains. The eruptive fissure opened inside the north rim of the caldera, on the opposite side of the caldera from an active fault system that experienced an m b 4.6 earthquake and ~84 cm of uplift on 16 April 2005. The main products of the eruption were an `a`a flow that ponded in the caldera and clastigenic lavas that flowed down the north flank. The `a`a flow grew in an unusual way. Once it had established most of its aerial extent, the interior of the flow was fed via a perched lava pond, causing inflation of the `a`a. This pressurized fluid interior then fed pahoehoe breakouts along the margins of the flow, many of which were subsequently overridden by `a`a, as the crust slowly spread from the center of the pond and tumbled over the pahoehoe. The curtain of lava fountains coalesced with time, and by day 4, only one vent was erupting. The effusion rate slowed from day 7 until the eruption’s end two days later on 30 October. Although the caldera floor had inflated by ~5 m since 1992, and the rate of inflation had accelerated since 2003, there was no transient deformation in the hours or days before the eruption. During the 8 days of the eruption, GPS and InSAR data show that the caldera floor deflated ~5 m, and the volcano contracted horizontally ~6 m. The total eruptive volume is estimated as being ~150×10 6 m 3 . The opening-phase tephra is more evolved than the eruptive products that followed. The compositional variation of tephra and lava sampled over the course of the eruption is attributed to eruption from a zoned sill that lies 2.1 km beneath the caldera floor.

Galapagos↗
USGS2007

Interferogram formation in the presence of complex and large deformation

Sierra Negra volcano in Isabela island, Galápagos, erupted from October 22 to October 30 in 2005. During the 8 days of eruption, the center of Sierra Negra's caldera subsided about 5.4 meters. Three hours prior to the onset of the eruption, an earthquake (Mw 5.4) occurred, near the caldera. Because of the large and complex phase gradient due to the huge subsidence and the earthquake, it is difficult to form an interferogram inside the caldera that spans the eruption. The deformation is so large and spatially variable that the approximations used in existing InSAR software (ROI, ROI_PAC, DORIS, GAMMA) cannot properly coregister SAR image pairs spanning the eruption. We have developed here a two-step algorithm that can form intra-caldera interferograms from these data. The first step involves a "rubber-sheeting" SAR image coregistration. In the second step we use range offset estimates to mitigate the steep phase gradient. Using this new algorithm, we retrieve an interferogram with the best coverage to date inside the caldera of Sierra Negra.

Isabela island, Sierra Negra volcano↗
USGS2007

ASAR images a diverse set of deformation patterns at Kilauea volcano, Hawai'i

Since 2003, 27 independent look angles have been acquired by ENVISAT’s Advanced Synthetic Aperture Radar (ASAR) instrument over the island of Hawai`i, allowing for the formation of thousands of interferograms showing deformation of the ground surface. On Kīlauea volcano, a transition from minor to broad-scale summit inflation was observed by interferograms that span 2003 to 2006. In addition, radar interferometry (InSAR) observations of Kīlauea led to the discovery of several previously unknown areas of localized subsidence in the caldera and along the volcano’s east rift zone. These features are probably caused by the cooling and contraction of accumulated lavas. After November 2005, a surface instability near the point that lava entered the ocean on the south flank of Kīlauea was observed in interferograms. The motion is most likely a result of unbuttressing of a portion of the coast following the collapse of a large lava delta in November 2005. InSAR data can also be used to map lava flow development over time, providing ~30 m spatial resolution maps at approximately monthly intervals. Future applications of InSAR to Kīlauea will probably result in more discoveries and insights, both as the style of volcano deformation changes and as data from new instruments are acquired.

Hawaii↗
USGS2006

Constraints on the mechanism of long-term, steady subsidence at Medicine Lake volcano, northern California, from GPS, leveling, and InSAR

Leveling surveys across Medicine Lake volcano (MLV) have documented subsidence that is centered on the summit caldera and decays symmetrically on the flanks of the edifice. Possible mechanisms for this deformation include fluid withdrawal from a subsurface reservoir, cooling/crystallization of subsurface magma, loading by the volcano and dense intrusions, and crustal thinning due to tectonic extension (Dzurisin et al., 1991 [Dzurisin, D., Donnelly-Nolan, J.M., Evans, J.R., Walter, S.R., 1991. Crustal subsidence, seismicity, and structure near Medicine Lake Volcano, California. Journal of Geophysical Research 96, 16, 319-16, 333.]; Dzurisin et al., 2002 [Dzurisin, D., Poland, M.P., Bürgmann, R., 2002. Steady subsidence of Medicine Lake Volcano, Northern California, revealed by repeated leveling surveys. Journal of Geophysical Research 107, 2372, doi:10.1029/2001JB000893.]). InSAR data that approximate vertical displacements are similar to the leveling results; however, vertical deformation data alone are not sufficient to distinguish between source mechanisms. Horizontal displacements from GPS were collected in the Mt. Shasta/MLV region in 1996, 1999, 2000, 2003, and 2004. These results suggest that the region is part of the western Oregon block that is rotating about an Euler pole in eastern Oregon. With this rotation removed, most sites in the network have negligible velocities except for those near MLV caldera. There, measured horizontal velocities are less than predicted from ∼10 km deep point and dislocation sources of volume loss based on the leveling data; therefore volumetric losses simulated by these sources are probably not causing the observed subsidence at MLV. This result demonstrates that elastic models of subsurface volume change can provide misleading results where additional geophysical and geological constraints are unavailable, or if only vertical deformation is known. The deformation source must be capable of causing broad vertical deformation with comparatively smaller horizontal displacements. Thermoelastic contraction of a column of hot rock beneath the volcano cannot reproduce the observed ratio of vertical to horizontal surface displacements. Models that determine deformation due to loading by the volcano and dense intrusions can be made to fit the pattern of vertical displacements by assuming a weak upper crust beneath MLV, though the subsidence rates due to surface loading must be lower than the observed displacements. Tectonic extension is almost certainly occurring based on fault orientations and focal mechanisms, but does not appear to be a major contributor to the observed deformation. We favor a model that includes a combination of sources, including extension and loading of a hot weak crust with thermal contraction of a cooling mass of rock beneath MLV, which are processes that are probably occurring at MLV. Future microgravity surveys and the planned deployment of an array of continuous GPS stations as part of a Plate Boundary Observatory volcano cluster will help to refine this model.

California↗
USGS2006

Geodetic observations and modeling of magmatic inflation at the Three Sisters volcanic center, central Oregon Cascade Range, USA

Tumescence at the Three Sisters volcanic center began sometime between summer 1996 and summer 1998 and was discovered in April 2001 using interferometric synthetic aperture radar (InSAR). Swelling is centered about 5 km west of the summit of South Sister, a composite basaltic-andesite to rhyolite volcano that last erupted between 2200 and 2000 yr ago, and it affects an area ∼20 km in diameter within the Three Sisters Wilderness. Yearly InSAR observations show that the average maximum displacement rate was 3–5 cm/yr through summer 2001, and the velocity of a continuous GPS station within the deforming area was essentially constant from June 2001 to June 2004. The background level of seismic activity has been low, suggesting that temperatures in the source region are high enough or the strain rate has been low enough to favor plastic deformation over brittle failure. A swarm of about 300 small earthquakes ( M max = 1.9) in the northeast quadrant of the deforming area on March 23–26, 2004, was the first notable seismicity in the area for at least two decades. The U.S. Geological Survey (USGS) established tilt-leveling and EDM networks at South Sister in 1985–1986, resurveyed them in 2001, the latter with GPS, and extended them to cover more of the deforming area. The 2001 tilt-leveling results are consistent with the inference drawn from InSAR that the current deformation episode did not start before 1996, i.e., the amount of deformation during 1995–2001 from InSAR fully accounts for the net tilt at South Sister during 1985–2001 from tilt-leveling. Subsequent InSAR, GPS, and leveling observations constrain the source location, geometry, and inflation rate as a function of time. A best-fit source model derived from simultaneous inversion of all three datasets is a dipping sill located 6.5 ± 2.5 km below the surface with a volume increase of 5.0 × 10 6 ± 1.5 × 10 6 m 3 /yr (95% confidence limits). The most likely cause of tumescence is a pulse of basaltic magma intruding the upper crust along the brittle–ductile interface — a process that must occur episodically beneath the Cascade Range but in the past would have escaped detection in the absence of unusual seismicity. We speculate that such intrusive episodes last from days to years and are separated by quiescent periods of decades to centuries. The likelihood that the current episode at Three Sisters will culminate in an eruption is judged to be low, but the impact of an eruption could be great. The USGS has updated its volcano hazards assessment for the Three Sisters region, notified appropriate agencies and the public, and is helping to prepare an emergency coordination and communication plan.

Central Oregon Cascade Range, Three Sisters volcan↗
USGS2006

The changing shapes of active volcanoes: History, evolution, and future challenges for volcano geodesy

At the very heart of volcanology lies the search for the 'plumbing systems' that form the inner workings of Earth’s active volcanoes. By their very nature, however, the magmatic reservoirs and conduits that underlie these active volcanic systems are elusive; mostly they are observable only through circumstantial evidence, using indirect, and often ambiguous, surficial measurements. Of course, we can infer much about these systems from geologic investigation of materials brought to the surface by eruptions and of the exposed roots of ancient volcanoes. But how can we study the magmatic processes that are occurring beneath Earth’s active volcanoes? What are the geometry, scale, physical, and chemical characteristics of magma reservoirs? Can we infer the dynamics of magma transport? Can we use this information to better forecast the future behavior of volcanoes? These questions comprise some of the most fundamental, recurring themes of modern research in volcanology. The field of volcano geodesy is uniquely situated to provide critical observational constraints on these problems. For the past decade, armed with a new array of technological innovations, equipped with powerful computers, and prepared with new analytical tools, volcano geodesists have been poised to make significant advances in our fundamental understanding of the behavior of active volcanic systems. The purpose of this volume is to highlight some of these recent advances, particularly in the collection and interpretation of geodetic data from actively deforming volcanoes. The 18 papers that follow report on new geodetic data that offer valuable insights into eruptive activity and magma transport; they present new models and modeling strategies that have the potential to greatly increase understanding of magmatic, hydrothermal, and volcano-tectonic processes; and they describe innovative techniques for collecting geodetic measurements from remote, poorly accessible, or hazardous volcanoes. To provide a proper context for these studies, we offer a short review of the evolution of volcano geodesy, as well as a case study that highlights recent advances in the field by comparing the geodetic response to recent eruptive episodes at Mount St. Helens. Finally, we point out a few areas that continue to challenge the volcano geodesy community, some of which are addressed by the papers that follow and which undoubtedly will be the focus of future research for years to come.

Journal of Volcanology and Geothermal Research↗
USGS2006

A volcano bursting at the seams: Inflation, faulting, and eruption at Sierra Negra volcano, Galápagos

The results of geodetic monitoring since 2002 at Sierra Negra volcano in the Galápagos Islands show that the filling and pressurization of an ∼2-km-deep sill eventually led to an eruption that began on 22 October 2005. Continuous global positioning system (CGPS) monitoring measured >2 m of accelerating inflation leading up to the eruption and contributed to nearly 5 m of total uplift since 1992, the largest precursory inflation ever recorded at a basaltic caldera. This extraordinary uplift was accommodated in part by repeated trapdoor faulting, and coseismic CGPS data provide strong constraints for improved deformation models. These results highlight the feedbacks between inflation, faulting, and eruption at a basaltic volcano, and demonstrate that faulting above an intruding magma body can relieve accumulated strain and effectively postpone eruption.

Galapagos↗
USGS2006

InSAR captures rifting and volcanism in East Africa

In the past decade, synthetic aperture radar interferometric (InSAR) has enjoyed increasing use as a tool for detecting and characterizing surface deformation associated with volcanoes, earthquakes, glaciers, and other geological processes. Though InSAR can only image deformation that occurs along the radar line-of-sight and is subject to atmospheric, orbital, and other errors that can be difficult to quantify, the method has the advantage of high spatial resolution (especially in arid, unvegetated environments) without requiring equipment on the ground. As a result, InSAR is extremely useful for mapping deformation in poorly accessible or unmonitored parts of the world.

Report↗
USGS2004

Patterns of magma flow in segmented silicic dikes at Summer Coon volcano, Colorado: AMS and thin section analysis

A complex pattern of magma flow is found in two silicic dikes of a radial swarm at Summer Coon, an eroded stratovolcano in southern Colorado. The two intrusions are broken into multiple segments that suggest vertical dike propagation. However, anisotropy of magnetic susceptibility (AMS) measurements and thin section observations suggest that magma flow was often subhorizontal and away from the center of the volcano. Segments that are proximal to the central intrusion are characterized by magma that flowed steeply upward at the proximal segment extremity, then laterally along the segment, and finally downward at the distal end of the segment. Magma flow in offset segment tips located far from the volcano center was subhorizontal towards the adjacent segment, implying lateral propagation of segment tips towards one another. This observation suggests relatively high driving pressure in distal dike segments, as supported by dike thickening with radial distance from the center of the volcano. The present study indicates that radial dike evolution at stratovolcanoes is dominated by lateral flow of magma and dike segmentation is a poor magma flow indicator. A horizontally propagating radial dike has the potential to cause an eruption low on the flank of a composite cone, which poses a significant yet largely unrecognized hazard to population centers and infrastructure that may surround the volcano.

Colorado↗
USGS2002

Steady subsidence of Medicine Lake volcano, northern California, revealed by repeated leveling surveys

Leveling surveys of a 193‐km circuit across Medicine Lake volcano (MLV) in 1954 and 1989 show that the summit area subsided by as much as 302 ± 30 mm (−8.6 ± 0.9 mm/yr) with respect to a datum point near Bartle, California, 40 km to the southwest. This result corrects an error in the earlier analysis of the same data by Dzurisin et al. [1991] , who reported the subsidence rate as −11.1 ± 1.2 mm/yr. The subsidence pattern extends across the entire volcano, with a surface area of nearly 2000 km 2 . Two areas of localized subsidence by as much as 20 cm can be attributed to shallow normal faulting near the volcano's periphery. Surveys of an east–west traverse across Lava Beds National Monument on the north flank of the volcano in 1990 and of a 23‐km traverse across the summit area in 1999 show that subsidence continued at essentially the same rate during 1989–1999 as 1954–1989. Volcano‐wide subsidence can be explained by either a point source of volume loss (Mogi) or a contracting horizontal rectangular dislocation (sill) at a depth of 10–11 km. Volume loss rate estimates range from 0.0013 to 0.0032 km 3 /yr, depending mostly on the source depth estimate and source type. Based on first‐order quantitative considerations, we can rule out that the observed subsidence is due to volume loss from magma withdrawal, thermal contraction, or crystallizing magma at depth. Instead, we attribute the subsidence and faulting to: (1) gravitational loading of thermally weakened crust by the mass of the volcano and associated intrusive rocks, and (2) thinning of locally weakened crust by Basin and Range deformation. The measured subsidence rate exceeds long‐term estimates from drill hole data, suggesting that over long timescales, steady subsidence and episodic uplift caused by magmatic intrusions counteract each other to produce the lower net subsidence rate.

California↗