Decadal‐scale change of infiltration characteristics of a tephra‐mantled hillslope at Mount St Helens,Washington

The cataclysmic 1980 eruption of Mount St Helens radically reduced the infiltration characteristics of ∼60 000 ha of rugged terrain and dramatically altered landscape hydrology. Two decades of erosional, biogenic, cryogenic, and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption. We assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra, but now revegetated primarily with grasses and other plants, to evaluate hydrological modifications due to erosion and natural turbation. Eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50‐fold. Between 1980 and 2000, apparent infiltration capacities of plots on the hillslope increased as much as ten fold, but remain approximately three to five times less than the probable pre‐eruption capacities. Common regional rainfall intensities and snowmelt rates presently produce little surface runoff; however, high‐magnitude, low‐frequency storms and unusually rapid snowmelt can still induce broad infiltration‐excess overland flow. After 20 years, erosion and natural mechanical turbation have modulated, but not effaced, the hydrological perturbation caused by the cataclysmic eruption. Copyright © 2005 John Wiley & Sons, Ltd.     

Results of repeated leveling surveys at Newberry Volcano, Oregon, and near Lassen Peak Volcano, California

 Personnel from the U.S. Geological Survey's Cascades Volcano Observatory conducted first-order, class-II leveling surveys near Lassen Peak, California, in 1991 and at Newberry Volcano, Oregon, in 1985, 1986, and 1994. Near Lassen Peak no significant vertical displacements had occurred along either of two traverses, 33 and 44 km long, since second-order surveys in 1932 and 1934. At Newberry, however, the 1994 survey suggests that the volcano's summit area had risen as much as 97±22 mm with respect to a third-order survey in 1931. The 1931 and 1994 surveys measured a 37-km-long, east–west traverse across the entire volcano. The 1985 and 1986 surveys, on the other hand, measured only a 9-km-long traverse across the summit caldera with only one benchmark in common with the 1931 survey. Comparison of the 1985, 1986, and 1994 surveys revealed no significant differential displacements inside the caldera. A possible mechanism for uplift during 1931–1994 is injection of approximately 0.06 km³ of magma at a depth of approximately 10 km beneath the volcano's summit. The average magma supply rate of approximately 1×10^–3 km³/year would be generally consistent with the volcano's growth rate averaged over its 600,000-year history (0.7–1.7×10^–3 km³/year). Received: 10 September 1998 / Accepted: 4 December 1998     

地壳中岩浆沿已有断层运移的数值模拟——在长白山天池火山的应用

本文利用有限元及有限差分的方法,对壳内岩浆房或岩浆囊中的岩浆在构造应力及由于围岩与岩浆的密度差产生的浮力作用下,沿已有断层向上运移的动力学过程进行了数值模拟.在岩浆囊顶部与上覆岩层接触处,沿着已有微小破裂,岩浆在一定超压力条件下使已有断层张开并继续向上延伸,从而形成岩浆向上运移的通道.研究了岩浆黏度、密度差、模型深度对最小超压力（岩浆运移到地表所需的最小岩浆房超压力）的影响.在10 km深度的地壳中,若岩浆黏度为0.1~10³ Pa·s,当超压力达到17~20 MPa时,岩浆压力可以驱动岩浆运移到地表层;同时,岩浆动力黏度越大,使岩脉运移到地表需要的超压力就越大.当密度差为300~700 kg·m^-3,其变化对超压力的影响比较小.本文亦对比了三维应力条件和二维平面应变条件下不同结果,比较了不同条件下岩浆运移造成的地表垂直位移变化.结合长白山天池火山地区的区域地质环境,对长白山天池火山岩浆运移条件进行了参数试验性计算分析,估算了在给定长白山天池火山模型条件下地下可能存在的岩浆囊的大小,其结果对认识长白山天池火山地区岩浆活动及相关的预测和监控有参考意义.     

Potential Hazards of Eruptions around the Tianchi Caldera Lake, China

Since the eruption of the Tianchi volcano about 1000 years ago, there have been at least 3 to 5 eruptions of small to moderate size. In addition, hazardous avalanches, rock falls and debris flows have occurred during periods between eruptions. A future eruption of the Tianchi volcano is likely to involve explosive interaction between magma and the caldera lake. The volume of erupted magma is almost in a range of 0.1-0.5 km3. Tephra fallout may damage agriculture in a large area near the volcano. If only 1% of the lake water were ejected during an eruption and then precipitated over an area of 200 km2, the average rainfall would be 100 mm. Moreover, lahars are likely to occur as both tephra and water ejected from the caldera lake fall onto flanks of the volcano. Rocks avalanching into the caldera lake also would bring about grave hazards because seiches would be triggered and lake water with the volume equal to that of the landslide would spill out of the existing breach in the caldera and cause flooding     

基于差分干涉与GPS测量的夏威夷基拉韦厄火山形变监测

利用欧空局ENVISAT-ASAR影像数据,提取夏威夷基拉韦厄火山区域由2007-06-17～19小规模火山喷发引起的地表形变场,并结合GPS时间序列分析喷发前后形变特征。结果表明,此次小规模喷发造成Makaopuhi火山口附近发生明显地表形变,LOS向形变值最大超过30 cm。将研究区内同期GPS观测值投影至LOS方向,其结果与差分干涉所得形变量具有较高的一致性,均方根误差为1.8 cm。     

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.     

Transport and sedimentation dynamics of transitional explosive eruption columns: The example of the 800 BP Quilotoa plinian eruption (Ecuador)

Impact of large-scale explosive eruptions largely depends on the dynamics of transport, dispersal and deposition of ash by the convective system. In fully convective eruptive columns, ejected gases and particles emitted at the vent are vertically injected into the atmosphere by a narrow, buoyant column and then dispersed by atmosphere dynamics on a regional scale. In fully collapsing explosive eruptions, ash partly generated by secondary fragmentation is carried and dispersed by broad co-ignimbrite columns ascending above pyroclastic currents. In this paper, we investigate the transport and dispersion dynamics of ash and lapillis during a transitional plinian eruption in which both plinian and co-ignimbrite columns coexisted and interacted. The 800 BP eruptive cycle of Quilotoa volcano (Ecuador) produced a well-exposed tephra sequence. Our study shows that the sequence was accumulated by a variety of eruptive dynamics, ranging from early small phreatic explosions, to sustained magmatic plinian eruptions, to late phreatomagmatic explosive pulses. The eruptive style of the main 800 BP plinian eruption (U1) progressively evolved from an early fully convective column (plinian fall bed), to a late fully collapsing fountain (dense density currents) passing through an intermediate transitional eruptive phase (fall + syn-plinian dilute density currents). In the transitional U1 regime, height of the convective plinian column and volume and runout of the contemporaneous pyroclastic density currents generated by partial collapses were inversely correlated. The convective system originated from merging of co-plinian and co-surge contributions. This hybrid column dispersed a bimodal lapilli and ash-fall bed whose grain size markedly differs from that of classic fall deposits accumulated by fully convective plinian columns. Sedimentological analysis suggests that ash dispersion during transitional eruptions is affected by early aggregation of dry particle clusters.     

Caldera-scale inflation of the Lazufre volcanic area, South America: Evidence from InSAR

Collapsed calderas are the structural surface expression of the largest volcanic eruptions on Earth and may reach diameters of tens of kilometres while erupting volumes larger than 1000 km³. Remnants of collapse calderas can be found along the South American volcanic arc and are thought to be inactive. However, this study shows that systems of such dimension may become active in a relatively short period of time without attracting much attention. Using satellite-based InSAR data, a 45 km wide elongated area of ground deformation was observed in the Lazufre volcanic region (Chile), where no deformation was detected 10 years ago. The deformation signal shows an uplift of up to ~ 3 cm yr^− 1 during 2003–2006, affecting an area of about 1100 km², comparable in size to super-volcanoes such as Yellowstone or Long Valley. This deformation signal can be explained by an inflating magma body at about 10 km depth, expanding and propagating laterally at a velocity of up to 4 km per year. Although it is not clear whether this intrusion will lead to an eruption, its dimensions and the rapid deformation rate insinuate that a potentially large volcanic system is forming.     

Progressive cooling of the hyaloclastite ridge at Gjálp, Iceland, 1996–2005

In the subglacial eruption at Gjálp in October 1996 a 6 km long and 500 m high subglacial hyaloclastite ridge was formed while large volumes of ice were melted by extremely fast heat transfer from magma to ice. Repeated surveying of ice surface geometry, measurement of inflow of ice, and a full Stokes 2-D ice flow model have been combined to estimate the heat output from Gjálp for the period 1996–2005. The very high heat output of order 10⁶ MW during the eruption was followed by rapid decline, dropping to  2500 MW by mid 1997. It remained similar until mid 1999 but declined to 700 MW in 1999–2001. Since 2001 heat output has been insignificant, probably of order 10 MW. The total heat carried with the 1.2 × 10¹² kg of basaltic andesite erupted (0.45 km³ DRE) is estimated to have been 1.5 × 10¹⁸ J. About two thirds of the thermal energy released from the 0.7 km³ edifice in Gjálp occurred during the 13-day long eruption, 20% was released from end of eruption until mid 1997, a further 10% in 1997–2001, and from mid 2001 to present, only a small fraction remained. The post-eruption heat output history can be reconciled with the gradual release of 5 × 10¹⁷ J thermal energy remaining in the Gjálp ridge after the eruption, assuming single phase liquid convection in the cooling edifice. The average temperature of the edifice is found to have been approximately 240 °C at the end of the eruption, dropping to  110 °C after 9 months and reaching  40 °C in 2001. Although an initial period of several months of very high permeability is possible, the most probable value of the permeability from 1997 onwards is of order 10^− 12 m². This is consistent with consolidated/palagonitized hyaloclastite but incompatible with unconsolidated tephra. This may indicate that palagonitization had advanced sufficiently in the first 1–2 years to form a consolidated hyaloclastite ridge, resistant to erosion. No ice flow traversing the Gjálp ridge has been observed, suggesting that it has effectively been shielded from glacial erosion in its first 10 years of existence.     

Mechanism of explosive eruption revealed by geophysical observations at the Sakurajima,Suwanosejima and Semeru volcanoes

A common sequence of phenomena associated with volcanic explosions is extracted based on seismic and ground deformation observations at 3 active volcanoes in Japan and Indonesia. Macroscopic inflation-related ground deformations are detected prior to individual explosions, while deflations are observed during eruptions. Precursory inflation occurs 5 min to several hours before eruption at the Sakurajima volcano, but just 1–2 min at Suwanosejima and 3–30 min at the Semeru volcano. The sequence commences with minor contraction, which is detected by extensometers 1.5 min before eruption at Sakurajima, as a dilatant first motion of the explosion earthquakes 0.2–0.3 s before surface explosions at Suwanosejima, and as downward tilt 4–5 s prior to eruption at the Semeru volcano. The sequence is detected for explosive eruptions with > 0.1 μrad tilt change at Sakurajima, 90% at Suwanosejima and 75% at Semeru volcanoes. It is inferred that the minor contraction is caused by a volume and pressure decrease due to the release of gas from a pocket at the top of the conduit as the gas pressure exceeds the strength of the confining plug. The subsequent violent expansion may be triggered by sudden outgassing of the water-saturated magma induced by the decrease in confining pressure.