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1.
N.N.Myagkov 《Geophysical Journal International》1998,133(1):209-211
A hydrodynamic model of a strong volcanic blast associated with large (caldera-forming) ash–gas-flow eruptions is presented. We show that the mass erupted from a vent influences considerably the damping of the shock wave. If the ejected mass flux is constant, the shock amplitude as a function of distance will decrease slower (in accordance with the inverse 4/3 power law) than that of an atomic explosion (the inverse cube power law). From the modelling, we extract a relation connecting the effective energy of a blast with the mass ejected during the blast phase. The mass released during the largest explosion of the 1883 eruption of Krakatau, and the lateral blast energy at Mount St. Helens on 1980 May 18 are estimated on the basis of the equation obtained. 相似文献
2.
Comparison of short-term seacliff retreat measurement methods in Del Mar, California 总被引：1，自引：0，他引：1
Seacliff retreat has been variously characterized as the recession rate of the cliff top, of the cliff base, and as the bulk recession rate based on volumetric changes of the entire cliff face. Here, these measures of retreat are compared using nine semi-annual airborne LiDAR (Light Detection And Ranging) surveys of southern California seacliffs. Changes in the cliff base location (where the steeply sloping cliff face intersects the beach) include cliff retreat owing to basal erosion, but also reflect changes in beach sand level and basal talus deposits. Averaged over the 2.5 km alongshore study span, the cliff base actually prograded seaward about 12 cm during the 4-year study. Cliff top change was dominated by few, relatively large (several meters) localized retreats. Cliff face changes, that include failures and deposits anywhere on the cliff profile, had a relatively small mean magnitude compared to cliff top changes and were more widely distributed alongshore. However, the similar alongshore averaged, cumulative cliff top and net bulk cliff face end-point retreat (14 and 19 cm, respectively) suggest that mean cumulative cliff top retreat can potentially be a viable surrogate for mean net cumulative cliff-wide erosion (and vice versa) over relatively short time periods. Cliff face erosion occurred repeatedly at some locations, confirming the presence of seacliff erosion hot-spots during the study period. 相似文献
3.
HIROYUKI OBANAWA YUICHI S. HAYAKAWA YUKINORI MATSUKURA 《Geografiska Annaler: Series A, Physical Geography》2009,91(4):269-278
On the basis of a space–time substitution, rates of cliff retreat and talus development overthe pasttens ofthousands of years were quantified for valley‐side slopes along the Shomyo River with a known rate of waterfall recession. Detailed profiles of the valley‐side slopes were obtained at 19 sites along the river by map reading and field survey, and the morphologic characteristics of the slopes were then measured. By combining the locations of the slopes with the recession rate of Shomyo Falls, it is estimated that the growth rate of the talus slope is 3.9–7.2 mm/yr, the rate of decline of the cliff is 2.0–4.0 × 10?4 deg/yr, and the retreat rate of the cliff top is 6.2–11.6 mm/yr. 相似文献
4.
M. E. Dickson 《The Australian geographer》2004,35(2):223-238
Lord Howe Island is a small eroded remnant of a Late Miocene shield volcano. A fringing coral reef dissipates wave energy along a portion of the shoreline, but the remainder of the coast is rugged with spectacular high basaltic sea cliffs. This paper investigates the evolution of talus slopes that occur beneath the loftiest cliffs, and places this analysis within the context of a longer history of island planation that has resulted in a wide truncated shelf around the island. During the Last Glacial, when the sea level was lower than at present, talus slopes accumulated around the extent of the island's cliffed coast because material eroded from cliffs by subaerial processes could not be removed by marine processes. The survival of these slopes during the Holocene has depended on a balance achieved between rates of subaerial and marine erosion. This balance is fundamentally influenced by cliff height, as cliffs higher than 200 m are plunging or veneered by talus slopes, whereas lower cliffs have erosional shore platforms. On comparison with published erosion rates from inland basalt scarps it appears that marine processes may account for over 90 per cent of the total cliff retreat that has occurred at Lord Howe Island, yet contemporary coastal morphology attests to the significance of subaerial processes in recent times. It is likely that marine cliffing was very rapid soon after volcanism ceased, but rates of erosion decreased through time as wave energy became increasingly attenuated across a widening planation surface, and as increasing cliff heights yielded greater quantities of talus that provided protection from rapid marine erosion. 相似文献
5.
6.
Summary. Atmospheric pressure waves from the Mount St Helens eruption 1980 May 18 have been clearly recorded by a sensitive microbarograph at Berkeley, California. The record shows three types of waves with different group velocities. The pressure waves can be interpreted in terms of direct waves A1, antipodean travelling waves A2 and circumnavigating waves A3, all of which are composed of several acoustic-gravity modes propagated in the lower atmosphere. Synthetic barograms appropriate to the Berkeley station have been calculated on the basis of the dynamic response of the lower atmospheric structure, together with various assumptions of source properties. Comparisons between synthetic and observed barograms provide estimates for ranges of the time history of upward particle velocity at the source, source dimensions and the velocity of the source spreading over the blast zone, as well as for the average dissipation effects over the circumferential path. The results suggest that two major compression pulses on the A1 record correlate with the arrival of pressure waves from the first (lateral) blast and second (vertical) blast, although the inferred interblast time interval is not consistent with that estimated from seismic observations. 相似文献
7.
Lateglacial and Holocene talus slope development and rockwall retreat on Mynydd Du, UK 总被引：1，自引：0，他引：1
Modification of Lateglacial and Holocene talus sheets by debris flows and gully incision on Mynydd Du, Wales, has resulted in a convergence of upper slope form characterised by an upper rectilinear slope gradient of 36°±3° and a range of concavities of c. 0.1–0.2. In most cases, gully incision and accumulation of debris cones have led to an increase in slope concavity. Evidence for talus erosion, reworking and redeposition on the upper slope emphasises secondary reworking processes, as well as primary talus accumulation on the upper slope, and permits construction of a model of talus development at Mynydd Du. On the basis of talus volume, calculation of the first rockwall retreat data set for southern Britain suggests that c. 7.1 m (84%) of overall rockwall retreat (8.5 m) took place during the Lateglacial, and only c. 1.4 m (16%) occurred during the Holocene. These figures imply that Lateglacial retreat rates ranged from 1.01 to 2.44 m ka−1, with an overall mean rate of 1.23 m ka−1. In contrast, Holocene rockwall retreat rates range from 0.10 to 0.17 m ka−1, with a mean rate of 0.12 m ka−1. Approximately 27% of cliff retreat is attributed to microgelivation. While similar to Holocene and present-day alpine environments, these Lateglacial retreat rates are one order of magnitude higher than most equivalent values for arctic sites. This reflects both ‘alpine-style’ diurnal freeze–thaw activity on Mynydd Du during the Younger Dryas and paraglacial rock-mass instability following deglaciation. Assuming an exponential decline in rockwall sediment release, it is estimated that approximately half the talus had accumulated within c. 1 ka of deglaciation. At one site, paraglacial talus accumulation appears to have contributed significantly to the glacial sediment transport system of a subsequent ice advance. Present-day rates of rockwall retreat and talus accumulation by rockfall are estimated to be 0.014 and 0.022 mm yr−1 (m ka−1), respectively, similar to values for other British sites and markedly lower than Holocene rates of cliff recession due to microgelivation. By implication, the geomorphic significance of microgelivation may have been greatly underestimated in studies of inland rock-slope evolution in temperate, mid-latitude environments. 相似文献
8.
Effect of the development of notches and tension cracks on instability of limestone coastal cliffs in the Ryukyus, Japan 总被引：1，自引：0，他引：1
Tetsuya Kogure Hisashi Aoki Akira Maekado Takashi Hirose Yukinori Matsukura 《Geomorphology》2006,80(3-4):236-244
Notches cut by waves are currently developing at the base of vertical coastal limestone cliffs in Okinawa, Japan. The cliff height varies from 3.0–22.1 m, and the maximum notch depth is 8.8 m. Many rectangular or cubic blocks, which appear to have originated from cliff failures, are found on platforms in front of the cliffs. On the flat top surface of the cliff, tension cracks often run parallel to the cliff face. The vertical face of the cliffs displays small undulations but no sliding striation, suggesting that cliff failures have been caused by toppling rather than by shearing or sliding. We use slope stability analysis to determine the critical condition for toppling failure. Physical and mechanical properties of the cliff material were first obtained from laboratory tests. The results indicate that the strength of limestone shows a scale effect, such that the strength decreases with increasing size of the test specimens. Based on this result, we estimated the strength of a rock mass corresponding to the size of the coastal cliff. Cliff stability was then analyzed using a cantilever beam model. Comparison of the stability analysis and the dimension of fallen blocks indicates that toppling failure is strongly associated with the development of notches and tension cracks. 相似文献
9.
The generation and degradation of marine terraces 总被引：2，自引：0，他引：2
Marine terraces are ephemeral planar landforms. While tectonic and climatic forcings responsible for the generation of existing marine terraces have operated for at least 1 Myr, terraces have been completely removed by erosion above a given altitude (and hence above a given age). Above this altitude, the landscape has forgotten that it was once terraced. We ask what controls this characteristic time-scale, which we term the ‘forget time’, in a landscape. We approach the problem with simple scaling arguments, and 1-D numerical models of landscape evolution. Using a simple cliff erosion model with a realistic sea-level history, rock uplift and a cliff retreat rule, we find that the most important means of terrace removal is through the deeper transgression of a subsequent sea cliff into the landmass. The sequence of preserved terraces depends upon the history of sea cliff incursion into the landmass. The extent of sea cliff incursion depends on the duration of the sea-level highstand, the far-field wave energy input and the degree to which bathymetric drag dissipates wave energy. This portion of the marine terrace survival problem is an example of a common problem in geomorphology, in which the record of past tectonic or climatic events is rendered incomplete by the potential for younger events to wipe the topographic slate clean. While sea cliffs decay through time, their form can still be recognized many hundreds of thousands of years after formation. This reflects the diffusive nature of their decay: early rapid evolution and lowering of maximum slopes yields to slower rates through time. Incision by streams, on the other hand, is rapid, as the streams respond to base-level history driven by sea-level changes. The rate of incision reflects the local climate conditions, and is limited by the rate of base-level fall. The principal means of vanquishing a marine terrace is by backwearing of slopes adjacent to these incising streams. The forget time should be proportional to the spacing between major incising streams and to the angle of hillslope stability, and should be inversely proportional to the rate of channel incision. This yields an overestimate of the forget time, as the terraced interfluves are reduced as well by the headward incision of tributary streams. The resulting landscape may be viewed as a terraced fringe separating the sea from the fully channellized landscape. Over time-scales corresponding to many glacial–interglacial sea-level oscillations, this fringe can achieve a nearly steady width. The rate of generation of new terraced landscape, reflecting the uplift rate pattern, is then balanced by the rate at which the terraces are erased beyond recognition by channel and hillslope processes. The width of this fringe should depend upon the precipitation, and upon the distance to the nearest drainage divide, both of which limit the maximum power available to drive channel incision. 相似文献
10.
Magnetic history of a dyke on Mount Etna (Sicily) 总被引：1，自引：0，他引：1
During the 1989 eruption of Mount Etna, two fracture systems, trending c. N45°E and N150°E, opened at the foot of its 3000 m high SE Crater and propagated quickly downslope to distances of ≈3 and 7 km, respectively. The northeastern fracture fed a flank eruption, whereas the southeastern fracture remained dry and offered contrasting volcanological and geophysical evidence of the presence of magma at a shallow depth. During the opening of this non-eruptive fracture system, a differential magnetic network was set up on a short profile across its distant extremity. Initially, the magnetic field did not display any change along the profile between frequent surveys. However, repeated measurements at intervals of about 3 months for two years revealed the slow build-up of a 130 nT anomaly. The anomaly vanishes laterally within 0.2 km of the surface expression of the fracture system. This exceptional set of observations constrains the location and time of cooling of a shallow dyke. The increase in magnetization of the dyke inferred by the rate of growth of the anomaly leads to the interpretation that the dyke was emplaced near the end of the eruption. 相似文献
11.
莫高窟崖顶防沙工程的效益分析 总被引：6，自引：7，他引：6
４年防沙实践证明：莫高窟崖顶防沙试验的工程设计构思是正确的，设计依据科学可靠，防护效益显著。对保护石窟和壁画已经起到重要的作用．其具体防护作用是：１）直接控制偏西风向洞窟搬运沙量的９５％左右；２）洞前夜间积沙减少了８０％以上；３）外围栅栏对来自主风向的外侧积沙的侧导率平均为３５％；对内侧积沙的侧导率，在偏东风作用下可达５７．５１％，在偏西风作用下平均为１５．８９％．上述防沙作用均具有明显的季节变化。 相似文献
12.
Debris flows are one of the many active slope-forming processes within Glacier National Park, Montana. Most debris flow landforms exhibit classic morphology with a distinct failure scarp, incised channel, channel levees, and toe deposits that often develop a lobate form. The Precambrian metasediments that dominate Glacier National Park's geology weather into angular clasts that range in size from platy gravels to boulders. Classic debris flows occur in areas where the topographic expression provides a debris source from cliff faces and an accumulation of regolith, often in the form of talus slopes. Many of these debris flows have long runout zones and can travel many hundreds of meters. Often they cross hiking trails or roads, including the main east–west highway, Going-to-the-Sun Road. Debris flows impacting the road have resulted in several near fatalities, and hikers have been forced to cross active debris flows to reach safe ground. The magnitude of debris flows varies between high magnitude channel incising events and low magnitude channel filling and/or reworking events. The frequency of debris flow events is irregular and appears to be controlled by the hydrology of triggering storms and antecedent moisture conditions, not by the debris supply. As a result, debris flow magnitude is not a function of frequency, but is more closely related to the characteristics of antecedent conditions and individual storms. 相似文献
13.
The main purpose of this research was to measure the near surface ground temperatures and describe the main characteristics of the thermal regime of a sheltered arctic coastal cliff. Measurements were made near Ny-Alesund, western Spitsbergen, Svalbard (79°N, 12°E), in a transect across sediments on top of a coastal cliff, in a 8 m high coastal cliff and across the beach below the cliff. Temperatures were logged hourly from August 1987 to August 1988. A local snow cover of 1–4 m on the beach had a strong influence on surface heat exchange, and hence the dynamics of the ground thermal regime. Late winter temperatures in the cliff and backshore sediments were stable and ranged from -5°C to -9°C. At the first snow melt events in spring there was a large heat flux into the ground due to the release of latent heat from refreezing of meltwater. The interpretation of the temperature records leads to a discussion on some aspects of cryogenic weathering. The snow-melt period in spring and summer may be a period of rock fracturing, based on the model of segregation ice growth at subzero temperatures. Steep temperature gradients and available meltwater at the surface favor water migration in the heavily fractured dolomitic limestone even at the low hydraulic conductivities expected at below zero temperatures. 相似文献
14.
流域输沙过程是地貌学和地表动力学的重要研究内容,但传统的输沙过程监测方法仅能得到某个区域的总输沙率,无法推算其空间分布。论文以黄土高原绥德县窑家湾小流域为例,利用无人机摄影测量技术得到其2006年和2019年2期数字高程模型(DEM)并计算地形变化量;然后,根据质量守恒原理和多流向算法建立泥沙在空间上的输送模型,进而计算小流域输沙率的空间分布。实验结果表明,该模型能有效模拟泥沙在空间上的输送情况,输沙率出现质量不守恒的区域面积占比小于4%,且不守恒区域多为人类活动影响区。同时,论文讨论了DEM的选择和不同地形变化检测水平对模型结果的影响。当使用第一期DEM进行泥沙搬运路径推算时,质量不守恒区域的面积显著降低。使用误差空间分布图进行地形变化检测得到的输沙率结果鲁棒性更强。使用中误差进行地形检测得到的结果在不同置信度下变化较大。基于无人机地形变化检测的空间输沙模型能方便、快捷地提供详尽的输沙率空间分布,为地表过程研究带来了新的机遇。 相似文献
15.
While the eruptive record of Mount Etna is reasonably complete for the past 400 years, the activity of the early and late 1960s, which took place at the summit, is poorly documented in the scientific literature. From 1955 to 1971, the Central and Northeast Craters were the sites of long-lived mild Strombolian and effusive activity, and numerous brief episodes of vigorous eruptive activity, which led to repeated overflows of lava onto the external flanks of the volcano. A reconstruction of the sequence of the more important of these events based on research in largely obscure and nearly inaccessible sources permits a better understanding of the eruption dynamics and rough estimates of erupted volumes and of the changes to the morphology of the summit area. During the first half of 1964, the activity culminated in a series of highly dynamic events at the Central Crater including the opening of a fissure on the E flank of the central summit cone, lava fountains, voluminous tephra emission, prolonged strong activity with continuous lava overflows, and growth of large pyroclastic intracrater cones. Among the most notable processes during this eruption was the breaching of a section of the crater wall, which was caused by lateral pressure of lava ponding within the crater. Comparison with the apparently similar summit activity of 1999 allows us to state that (a) lava overflows from large pit craters at the summit are often accompanied by breaching of the crater walls, which represents a significant hazard to nearby observers, and that (b) eruptive activity in 1999 was much more complex and voluminous than in 1964. For 1960s standards however, the 1964 activity was the most important summit eruption in terms of intensity and output rates for about 100 years, causing profound changes to the summit morphology and obliterating definitively the former Central Crater. 相似文献
16.
地形分析与地理信息系统在水文地貌学中的应用 总被引：3，自引：0，他引：3
本文阐述了利用地理信息系统进行大面积及高分辨率的地形分布方面的最新进展，并且讨论了一些由地理信息系统获得地形参数的规律，并提供了作者本人的一些设想。 相似文献
17.
通过多波束水深资料,研究了南海西沙海域甘泉海台地形特征。甘泉海台顶面平坦,坡度通常＜1°,但存在3级阶梯状地形,水深范围介于550~700 m,这3级阶梯状地形总体向南西倾斜。在600~850 m水深范围内,甘泉海台普遍发育水下阶地,识别出的水下阶地多达5级,其中西侧阶地发育程度优于东侧。初步认为：甘泉海台顶部和周缘阶梯状地形是冰期―间冰期旋回尺度上全球海平面变化和南海地壳沉降共同作用的结果,这２个因素可能也控制着南海其他珊瑚礁的发育演化。在850~1 300 m水深范围内,甘泉海台周缘存在倒石堆状地形,推测主要是珊瑚礁碎屑从礁顶搬运沉积的产物,甘泉海台西北侧比东南侧倒石堆地形更加发育的现象估计与甘泉海台顶面向南西倾斜直接相关。 相似文献
18.
Effect of Rock Hardness and Moisture Content on Tafoni Weathering in the Granite of Mount Doeg-sung, Korea 总被引：2，自引：0，他引：2
Yukinori Matsukura & Yukiya Tanaka 《Geografiska Annaler: Series A, Physical Geography》2000,82(1):59-67
Many tafoni occur on the granite tors of Mount Doeg-sung, Korea. Relationships are investigated between growth processes of granite tafoni and rock properties such as rock hardness and moisture content. Field evidence shows that the rock surface of the inner backwalls and ceiling of tafoni have reduced hardness and higher moisture content compared to the outer visor of tafoni and to vertical cliff walls with no tafoni. This suggests that tafoni arise in weathered rock where conditions of low strength and high moisture content are both satisfied. 相似文献
19.
Historical geographic information systems (GIS) scholarship has provided important insights into a broad spectrum of historical questions in recent years. This article contributes to the emerging field of historical GIS by employing map algebra and transect analysis to quantitatively assess topographical change resulting from rapid urban growth in New York City since the first quarter of the nineteenth century. These methodological techniques enable a systematic comparison of Manhattan's historical and contemporary topography through a spatial analysis of various topographic metrics, including the extent of positive and negative elevation change, shifts in elevation range, the percentage of geographic area that experienced elevation changes, and the estimated volume of Earth added to and removed from particular areas within the study site. The results indicate that there were more valleys filled than hills leveled on Manhattan's East Side, whereas the reverse was the case on the West Side. The West Side, however, experienced considerably more topographic changes than its eastern counterpart, in large part due to the former's more rugged terrain. Overall, the evidence suggests that although Manhattan's topography has been radically transformed in certain respects, the broader spatial pattern of topographic variation has remained largely constant. These findings, therefore, can be used to support competing narrative claims concerning the significance of the topographical transformations that resulted from the urbanization process in New York City. 相似文献
20.
Curt Fredén 《Geografiska Annaler: Series A, Physical Geography》1999,81(3):383-385
During the investigation of the erosional effects of the rainstorm on 30-31 August 1997 on Mount Fulufjället, two samples have been radiocarbon age determined. The results give a maximum age of events occurring earlier. One sample of an exposed Picea trunk in the sandur of the stream St Göljån was radiocarbon dated to 1320±120 14 C years BP. The lowermost part of a peat sequence in a soligenous mire in the valley of the stream Tangån was radiocarbon dated to 1455±70 14 C years. 相似文献