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1.
A short length of channel on Pico Partido volcano, Lanzarote, provides us the opportunity to examine the dynamics of lava flowing in a channel that extends over a sudden break in slope. The 1–2-m-wide, 0.5–2-m-deep channel was built during the 1730–1736 eruptions on Lanzarote and exhibits a sinuous, well-formed channel over a steep (11° slope) 100-m-long proximal section. Over-flow units comprising smooth pahoehoe sheet flow, as well as evidence on the inner channel walls for multiple (at least 11) flow levels, attest to unsteady flow in the channel. In addition, superelevation is apparent at each of the six bends along the proximal channel section. Superelevation results from banking of the lava as it moves around the bend thus causing preferential construction of the outer bank. As a result, the channel profile at each bend is asymmetric with an outer bank that is higher than the inner bank. Analysis of superelevation indicates flow velocities of ~8 m s–1. Our analysis of the superelevation features is based on an inertia-gravity balance, which we show is appropriate, even though the down-channel flow is in laminar flow. We use a viscosity-gravity balance model, together with the velocities calculated from superelevation, to obtain viscosities in the range 25–60 Pa s (assuming that the lava behaved as a Newtonian liquid). Estimated volume fluxes are in the range 7–12 m3 s–1. An apparent down-flow increase in derived volume flux may have resulted from variable supply or bulking up of the flow due to vesiculation. Where the channel moves over a sharp break in slope and onto slopes of ~6°, the channel becomes less well defined and widens considerably. At the break of slope, an elongate ridge extends across the channel. We speculate that this ridge was formed as a result of a reduction in velocity immediately below the break of slope to allow deposition of entrained material or accretion of lava to the channel bed as a result of a change in flow regime or depth. 相似文献
2.
L. Lodato L. Spampinato A. Harris S. Calvari J. Dehn M. Patrick 《Bulletin of Volcanology》2007,69(6):661-679
The use of a hand-held thermal camera during the 2002–2003 Stromboli effusive eruption proved essential in tracking the development
of flow field structures and in measuring related eruption parameters, such as the number of active vents and flow lengths.
The steep underlying slope on which the flow field was emplaced resulted in a characteristic flow field morphology. This comprised
a proximal shield, where flow stacking and inflation caused piling up of lava on the relatively flat ground of the vent zone,
that fed a medial–distal lava flow field. This zone was characterized by the formation of lava tubes and tumuli forming a
complex network of tumuli and flows linked by tubes. Most of the flow field was emplaced on extremely steep slopes and this
had two effects. It caused flows to slide, as well as flow, and flow fronts to fail frequently, persistent flow front crumbling
resulted in the production of an extensive debris field. Channel-fed flows were also characterized by development of excavated
debris levees in this zone (Calvari et al. 2005). Collapse of lava flow fronts and inflation of the upper proximal lava shield made volume calculation very difficult. Comparison
of the final field volume with that expecta by integrating the lava effusion rates through time suggests a loss of ~70% erupted
lava by flow front crumbling and accumulation as debris flows below sea level. Derived relationships between effusion rate,
flow length, and number of active vents showed systematic and correlated variations with time where spreading of volume between
numerous flows caused an otherwise good correlation between effusion rate, flow length to break down. Observations collected
during this eruption are useful in helping to understand lava flow processes on steep slopes, as well as in interpreting old
lava–debris sequences found in other steep-sided volcanoes subject to effusive activity. 相似文献
3.
Crust formation on basaltic lava flows dictates conditions of both flow cooling and emplacement. For this reason, flow histories are dramatically different depending on whether lava is transported through enclosed lava tubes or through open channels. Recent analog experiments in straight uniform channels (Griffiths et al. J Fluid Mech 496:33–62, 2003) have demonstrated that tube flow, dictated by a stationary surface crust, can be distinguished from a mobile crust regime, where a central solid crust is separated from channel walls by crust-free shear zones, by a simple dimensionless parameter ϑ, such that ϑ<25 produces tube flow and ϑ>25 describes the mobile crust regime. ϑ combines a previously determined parameter ψ, which describes the balance between the formation rate of surface solid and the shear strain that disrupts the solid crust, with the effects of thermal convection (described by the Rayleigh number Ra).Here we explore ways in which ϑ can be used to describe the behavior of basaltic lava channels. To do this we have extended the experimental approach to examine the effects of channel irregularities (expansions, contractions, sinuosity, and bottom roughness) on crust formation and disruption. We find that such changes affect local flow behavior and can thus change channel values of ϑ. For example, gradual widening of a channel results in a decrease in flow velocity that causes a decrease in ϑ and may allow a down-flow transition from the mobile crust to the tube regime. In contrast, narrowing of the channel causes an increase in flow velocity (increasing ϑ), thus inhibiting tube formation.We also quantify the fraction of surface covered by crust in the mobile crust regime. In shallow channels, variations in crust width (d
c) with channel width (W) are predicted to follow d
c∼W
5/3. Analysis of channelized lava flows in Hawaii shows crustal coverage consistent with this theoretical result along gradually widening or narrowing channel reaches. An additional control on crustal coverage in both laboratory and basaltic flows is disruption of surface crust because of flow acceleration through constrictions, around bends, and over breaks in slope. Crustal breakage increases local rates of cooling and may cause local blockage of the channel, if crusts rotate and jam in narrow channel reaches. Together these observations illustrate the importance of both flow conditions and channel geometry on surface crust development and thus, by extension, on rates and mechanisms of flow cooling. Moreover, we note that this type of analysis could be easily extended through combined use of FLIR and LiDAR imaging to measure crustal coverage and channel geometry directly.Editorial responsibility: A. Harris 相似文献
4.
Andrew J. L. Harris 《Bulletin of Volcanology》2009,71(5):541-558
Lava flowing into a pit crater will become entrapped to form an inactive lava lake. At Masaya volcano (Nicaragua) pit filling
lavas are exposed in the walls of Nindiri, Santiago and San Pedro pits. Mapping of these lavas shows that fill can involve
emplacement of both ’a’a and pahoehoe, with single fill units ranging in thickness from 2 to 22 m. Thick units with columnar
joints were emplaced as simple inactive lava lakes during high effusion rate episodes. Sequences of thinner units, which can
form pit floor shields or compound lakes, were emplaced at lower effusion rates. Lava withdrawal caused unsupported sections
of three 20-m-thick units to subside, resulting in unit flexure and faulting, and viscous peeling features reveal that subsidence
occurred while at least one unit was still partially molten. Where withdrawal has not occurred, fill sequences are flat lying
and symmetrically distributed around the feeder structures (cinder cones and dykes). The filled Nindiri pit holds 5 × 107 m3 of lava in a 215-m-thick sequence. Partial fill of Santiago pit with 1 × 107 m3 of lava has filled the pit with a 110-m-thick lava sequence, of which ∼50% has been consumed by formation of a secondary
pit. Altogether, 6.4 × 107 m3 of lava was erupted into Nindiri and Santiago during 1525–1965, with 94% of this volume remaining pit-contained; the remainder
forms a north flank lava flow field. Pit development and filling is a dynamic and ephemeral process, having short-lived effects
on volcano morphology, where pits develop and fill over hours-to-centuries. However, pits play an important role in shaping
an edifice, representing lava sinks and controlling whether lavas are trapped or able to spread onto the flanks. 相似文献
5.
Andrew J. L. Harris Massimiliano Favalli Francesco Mazzarini Christopher W. Hamilton 《Bulletin of Volcanology》2009,71(4):459-474
We use a kinematic GPS and laser range finder survey of a 200 m-long section of the Muliwai a Pele lava channel (Mauna Ulu,
Kilauea) to examine the construction processes and flow dynamics responsible for the channel–levee structure. The levees comprise
three packages. The basal package comprises an 80–150 m wide ′a′a flow in which a ∼2 m deep and ∼11 m wide channel became
centred. This is capped by a second package of thin (<45 cm thick) sheets of pahoehoe extending no more than 50 m from the
channel. The upper-most package comprises localised ′a′a overflows. The channel itself contains two blockages located 130 m
apart and composed of levee chunks veneered with overflow lava. The channel was emplaced over 50 h, spanning 30 May–2 June,
1974, with the flow front arriving at our section (4.4 km from the vent) 8 h after the eruption began. The basal ′a′a flow
thickness yields effusion rates of 35 m3 s−1 for the opening phase, with the initial flow advancing across the mapped section at ∼10 m/min. Short-lived overflows of fluid
pahoehoe then built the levee cap, increasing the apparent channel depth to 4.8 m. There were at least six pulses at 90–420 m3 s−1, causing overflow of limited extent lasting no more than 5 min. Brim-full flow conditions were thus extremely short-lived.
During a dominant period of below-bank flow, flow depth was ∼2 m with an effusion rate of ∼35 m3 s−1, consistent with the mean output rate (obtained from the total flow bulk volume) of 23–54 m3 s−1. During pulses, levee chunks were plucked and floated down channel to form blockages. In a final low effusion rate phase,
lava ponded behind the lower blockage to form a syn-channel pond that fed ′a′a overflow. After the end of the eruption the
roofed-over pond continued to drain through the lower blockage, causing the roof to founder. Drainage emplaced inflated flows
on the channel floor below the lower blockage for a further ∼10 h. The complex processes involved in levee–channel construction
of this short-lived case show that care must be taken when using channel dimensions to infer flow dynamics. In our case, the
full channel depth is not exposed. Instead the channel floor morphology reflects late stage pond filling and drainage rather
than true channel-contained flow. Components of the compound levee relate to different flow regimes operating at different
times during the eruption and associated with different effusion rates, flow dynamics and time scales. For example, although
high effusion rate, brim-full flow was maintained for a small fraction of the channel lifetime, it emplaced a pile of pahoehoe
overflow units that account for 60% of the total levee height. We show how time-varying volume flux is an important parameter
in controlling channel construction dynamics. Because the complex history of lava delivery to a channel system is recorded
by the final channel morphology, time-varying flow dynamics can be determined from the channel morphology. Developing methods
for quantifying detailed flux histories for effusive events from the evidence in outcrop is therefore highly valuable. We
here achieve this by using high-resolution spatial data for a channel system at Kilauea. This study not only indicates those
physical and dynamic characteristics that are typical for basaltic lava flows on Hawaiian volcanoes, but also a methodology
that can be widely applied to effusive basaltic eruptions. 相似文献
6.
Ciro Del Negro Luigi Fortuna Alexis Herault Annamaria Vicari 《Bulletin of Volcanology》2008,70(7):805-812
Since the mechanical properties of lava change over time, lava flows represent a challenge for physically based modeling.
This change is ruled by a temperature field which needs to be modeled. MAGFLOW Cellular Automata (CA) model was developed
for physically based simulations of lava flows in near real-time. We introduced an algorithm based on the Monte Carlo approach
to solve the anisotropic problem. As transition rule of CA, a steady-state solution of Navier-Stokes equations was adopted
in the case of isothermal laminar pressure-driven Bingham fluid. For the cooling mechanism, we consider only the radiative
heat loss from the surface of the flow and the change of the temperature due to mixture of lavas between cells with different
temperatures. The model was applied to reproduce a real lava flow that occurred during the 2004–2005 Etna eruption. The simulations
were computed using three different empirical relationships between viscosity and temperature. 相似文献
7.
Measurement of effusion rate is a primary objective for studies that model lava flow and magma system dynamics, as well as
for monitoring efforts during on-going eruptions. However, its exact definition remains a source of confusion, and problems
occur when comparing volume flux values that are averaged over different time periods or spatial scales, or measured using
different approaches. Thus our aims are to: (1) define effusion rate terminology; and (2) assess the various measurement methods
and their results. We first distinguish between instantaneous effusion rate, and time-averaged discharge rate. Eruption rate
is next defined as the total volume of lava emplaced since the beginning of the eruption divided by the time since the eruption
began. The ultimate extension of this is mean output rate, this being the final volume of erupted lava divided by total eruption
duration. Whether these values are total values, i.e. the flux feeding all flow units across the entire flow field, or local,
i.e. the flux feeding a single active unit within a flow field across which many units are active, also needs to be specified.
No approach is without its problems, and all can have large error (up to ∼50%). However, good agreement between diverse approaches
shows that reliable estimates can be made if each approach is applied carefully and takes into account the caveats we detail
here. There are three important factors to consider and state when measuring, giving or using an effusion rate. First, the
time-period over which the value was averaged; second, whether the measurement applies to the entire active flow field, or
a single lava flow within that field; and third, the measurement technique and its accompanying assumptions. 相似文献
8.
The 1990 Kalapana flow field is a complex patchwork of tube-fed pahoehoe flows erupted from the Kupaianaha vent at a low effusion rate (approximately 3.5 m3/s). These flows accumulated over an 11-month period on the coastal plain of Kilauea Volcano, where the pre-eruption slope angle was less than 2°. the composite field thickened by the addition of new flows to its surface, as well as by inflation of these flows and flows emplaced earlier. Two major flow types were identified during the development of the flow field: large primary flows and smaller breakouts that extruded from inflated primary flows. Primary flows advanced more quickly and covered new land at a much higher rate than breakouts. The cumulative area covered by breakouts exceeded that of primary flows, although breakouts frequently covered areas already buried by recent flows. Lava tubes established within primary flows were longer-lived than those formed within breakouts and were often reoccupied by lava after a brief hiatus in supply; tubes within breakouts were never reoccupied once the supply was interrupted. During intervals of steady supply from the vent, the daily areal coverage by lava in Kalapana was constant, whereas the forward advance of the flows was sporadic. This implies that planimetric area, rather than flow length, provides the best indicator of effusion rate for pahoehoe flow fields that form on lowangle slopes. 相似文献
9.
10.
Factors which control lava flow length are still not fully understood. The assumption that flow length as mainly influenced by viscosity was contested by Walker (1973) who proposed that the length of a lava flow was dependent on the mean effusion rate, and by Malin (1980) who concluded that flow length was dependent on erupted volume. Our reanalysis of Malin's data shows that, if short duration and tube-fed flows are eliminated, Malin's Hawaiian flow data are consistent with Walker's assertion. However, the length of a flow can vary, for a given effusion rate, by a factor of 7, and by up to 10 for a given volume. Factors other than effusion rate and volume are therefore clearly important in controlling the lengths of lava flows. We establish the relative importance of the other factors by performing a multivariate analysis of data for recent Hawaiian lava flows. In addition to generating empirical equations relating flow length to other variables, we have developed a non-isothermal Bingham flow model. This computes the channel and levee width of a flow and hence permits the advance rates of flows and their maximum cooling-limited lengths for different gradients and effusion rates to be calculated. Changing rheological properties are taken into account using the ratio of yield strength to viscosity; available field measurements show that this varies systematically from the vent to the front of a lava flow. The model gives reasonable agreement with data from the 1983–1986 Pu'u Oo eruptions and the 1984 eruption of Mauna Loa. The method has also been applied to andesitic and rhyolitic lava flows. It predicts that, while the more silicic lava flows advance at generally slower rates than basaltic flows, their maximum flow lengths, for a given effusion rate, will be greater than for basaltic lava flows. 相似文献
11.
We evaluated the quantitative effects of artificial barriers, water-cooling and guiding channels on lava flow using the lava
simulation program LavaSIM. Lava flow is basically subject to the topography around the path, effusive rate and viscosity.
To prevent damage due to lava flow, we conducted experiments in controlling the flow direction, velocity and temperature.
The simulation demonstrated that artificial barriers can successfully change the direction of a lava flow and is more effective
when placed nearly parallel to the flow direction at a point where the topography is not very steep, while a barrier placed
perpendicular to the flow direction can only stop the flux temporarily, ultimately allowing the solidified crust to accumulate
and causing the following mass to go over the barrier. The water-cooling trial was also effective in controlling the direction
and temperature, although the amount of water was as much order as 105 m3. The guiding channels successfully control the direction and inundated area but only in local areas. 相似文献
12.
Andrew J. L. Harris Anna L. Butterworth Richard W. Carlton Ian Downey Peter Miller Pedro Navarro David A. Rothery 《Bulletin of Volcanology》1997,59(1):49-64
Satellite data offer a means of supplementing ground-based monitoring during volcanic eruptions, especially at times or locations
where ground-based monitoring is difficult. Being directly and freely available several times a day, data from the advanced
very high resolution radiometer (AVHRR) offers great potential for near real-time monitoring of all volcanoes across large
(3000×3000 km) areas. Herein we describe techniques to detect and locate activity; estimate lava area, thermal flux, effusion
rates and cumulative volume; and distinguish types of activity. Application is demonstrated using data for active lavas at
Krafla, Etna, Fogo, Cerro Negro and Erebus; a pyroclastic flow at Lascar; and open vent systems at Etna and Stromboli. Automated
near real-time analysis of AVHRR data could be achieved at existing, or cheap to install, receiving stations, offering a supplement
to conventional monitoring methods.
Received: 21 January 1997 / Accepted: 3 April 1997 相似文献
13.
During long-lived basaltic eruptions, overflows from lava channels and breaching of channel levées are important processes
in the development of extensive 'a'ā lava flow-fields. Short-lived breaches result in inundation of areas adjacent to the
main channel. However, if a breach remains open, lava supply to the original flow front is significantly reduced, and flow-field
widening is favoured over lengthening. The development of channel breaches and overflows can therefore exert strong control
over the overall flow-field development, but the processes that determine their location and frequency are currently poorly
understood. During the final month of the 2008–2009 eruption of Mt. Etna, Sicily, a remote time-lapse camera was deployed
to monitor events in a proximal region of a small ephemeral lava flow. For over a period of ~10 h, the flow underwent changes
in surface elevation and velocity, repeated overflows of varying vigour and the construction of a channel roof (a required
prelude to lava tube formation). Quantitative interpretation of the image sequence was facilitated by a 3D model of the scene
constructed using structure-from-motion computer vision techniques. As surface activity waned during the roofing process,
overflow sites retreated up the flow towards the vent, and eventually, a new flow was initiated. Our observations and measurements
indicate that flow surface stagnation and flow inflation propagated up-flow at an effective rate of ~6 m h−1, and that these processes, rather than effusion rate variations, were ultimately responsible for the most vigorous overflow
events. We discuss evidence for similar controls during levée breaching and channel switching events on much larger flows
on Etna, such as during the 2001 eruption. 相似文献
14.
John E. Bailey Andrew J. L. Harris Jonathan Dehn Sonia Calvari Scott K. Rowland 《Bulletin of Volcanology》2006,68(6):497-515
An open channel lava flow on Mt. Etna (Sicily) was observed during May 30–31, 2001. Data collected using a forward looking
infrared (FLIR) thermal camera and a Minolta-Land Cyclops 300 thermal infrared thermometer showed that the bulk volume flux
of lava flowing in the channel varied greatly over time. Cyclic changes in the channel's volumetric flow rate occurred over
several hours, with cycle durations of 113–190 min, and discharges peaking at 0.7 m3 s−1 and waning to 0.1 m3 s−1. Each cycle was characterized by a relatively short, high-volume flux phase during which a pulse of lava, with a well-defined
flow front, would propagate down-channel, followed by a period of waning flow during which volume flux lowered. Pulses involved
lava moving at relatively high velocities (up to 0.29 m s−1) and were related to some change in the flow conditions occurring up-channel, possibly at the vent. They implied either a
change in the dense rock effusion rate at the source vent and/or cyclic-variation in the vesicle content of the lava changing
its bulk volume flux. Pulses would generally overspill the channel to emplace pāhoehoe overflows. During periods of waning
flow, velocities fell to 0.05 m s–1. Blockages forming during such phases caused lava to back up. Occasionally backup resulted in overflows of slow moving ‘a‘ā
that would advance a few tens of meters down the levee flank. Compound levees were thus a symptom of unsteady flow, where
overflow levees were emplaced as relatively fast moving pāhoehoe sheets during pulses, and as slow-moving ‘a‘ā units during
backup. Small, localized fluctuations in channel volume flux also occurred on timescales of minutes. Volumes of lava backed
up behind blockages that formed at constrictions in the channel. Blockage collapse and/or enhanced flow under/around the blockage
would then feed short-lived, wave-like, down-channel surges. Real fluctuations in channel volume flux, due to pulses and surges,
can lead to significant errors in effusion rate calculations.
Editorial responsibility: A. Woods 相似文献
15.
In an attempt to model the effect of slope on the dynamics of lava flow emplacement, four distinct morphologies were repeatedly produced in a series of laboratory simulations where polyethylene glycol (PEG) was extruded at a constant rate beneath cold sucrose solution onto a uniform slope which could be varied from 1° through 60°. The lowest extrusion rates and slopes, and highest cooling rates, produced flows that rapidly crusted over and advanced through bulbous toes, or pillows (similar to subaerial “toey” pahoehoe flows and to submarine pillowed flows). As extrusion rate and slope increased, and cooling rate decreased, pillowed flows gave way to rifted flows (linear zones of liquid wax separated by plates of solid crust, similar to what is observed on the surface of convecting lava lakes), then to folded flows with surface crusts buckled transversely to the flow direction, and, at the highest extrusion rates and slopes, and lowest cooling rates, to leveed flows, which solidified only at their margins. A dimensionless parameter, Ψ, primarily controlled by effusion rate, cooling rate and flow viscosity, quantifies these flow types. Increasing the underlying slope up to 30° allows the liquid wax to advance further before solidifying, with an effect similar to that of increasing the effusion rate. For example, conditions that produce rifted flows on a 10° slope result in folded flows on a 30° slope. For underlying slopes of 40°, however, this trend reverses, slightly owing to increased gravitational forces relative to the strength of the solid wax. Because of its significant influence on heat advection and the disruption of a solid crust, slope must be incorporated into any quantitative attempt to correlate eruption parameters and lava flow morphologies. These experiments and subsequent scaling incorporate key physical parameters of both an extrusion and its environment, allowing their results to be used to interpret lava flow morphologies on land, on the sea floor, and on other planets. 相似文献
16.
Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā 总被引:1,自引:1,他引:0
Samples collected from a lava channel active at Kīlauea Volcano during May 1997 are used to constrain rates of lava cooling
and crystallization during early stages of flow. Lava erupted at near-liquidus temperatures (∼1150 °C) cooled and crystallized
rapidly in upper parts of the channel. Glass geothermometry indicates cooling by 12–14 °C over the first 2 km of transport.
At flow velocities of 1–2 m/s, this translates to cooling rates of 22–50 °C/h. Cooling rates this high can be explained by
radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel.
Crystallization of plagioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20–50%
per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of ∼104/cm3s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation
delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due
to stirring of the flow. The transition of the flow surface morphology from pāhoehoe to 'a'ā occurred at a distance of 1.9 km
from the vent. At this point, the flow was thermally stratified, with an interior temperature of ∼1137 °C and crystallinity
of ∼15%, and a flow surface temperature of ∼1100 °C and crystallinity of ∼45%. 'A'ā formation initiated along channel margins,
where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that
the transition involved crossing of a rheological threshold. We suggest this threshold to be the development of a lava yield
strength sufficient to prevent viscous flow of lava at the channel margin. We use this concept to propose that 'a'ā formation
in open channels requires both sufficiently high strain rates for continued disruption of surface crusts and sufficient groundmass
crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite
poor on eruption, these combined requirements help to explain two common observations on 'a'ā formation: (a) 'a'ā flow fields
are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava
issues from the vent as pāhoehoe and changes to 'a'ā only after flowing some distance, thus permitting sufficient crystallization.
Received: 3 September 1998 / Accepted: 12 April 1999 相似文献
17.
Estimating depths of buried lava tubes is important for determining the thermal budgets and effusion rates of basaltic volcanic
systems. This research used a laboratory experiment scaled to a lava tube system to measure the 3D temperature field surrounding
a hot viscous fluid flowing through a buried glass tube while varying conditions such as flow rate and temperature. The depth
of the glass tube was changed for different experimental runs. Numerical techniques were applied to model the laboratory experiment.
The surface thermal distributions from 166 thermal traverses, constrained to a depth to width ratio of 0.6 to 1.6, were analyzed
to empirically derive a depth estimation function using regression techniques. This “Linear Anomaly Surface Transect (LAST)”
depth function is a scaleable depth estimation technique which can be solved with thermal imaging data alone. The minimum
temperature, maximum temperature and width of a Lorentzian distribution fit to a surface thermal transect, are the only inputs
required for the LAST function to estimate the depths of the hot source. The input parameters were then applied to non-laboratory
situations including the Kuhio lava tube system in Hawai’i. The LAST function produced depth estimates of ∼ 0.3 m for the
Kuhio lava tube in Hawai’i, which did not agree with observations on the ground. This is the result of the complex composition
and geometry of an actual lava tube where heat transfer is controlled by more than a simple fluid filling a tube, but also
by convection of gasses and fluids in a partially filled passage. Though not effective for lava tubes at this time, the model
provides promising results for simple cases applied to engineering and underground coal fires. 相似文献
18.
Emplacement conditions of the c. 1,600-year bp Collier Cone lava flow, Oregon: a LiDAR investigation
A long-standing question in lava flow studies has been how to infer emplacement conditions from information preserved in solidified flows. From a hazards perspective, volumetric flux (effusion rate) is the parameter of most interest for open-channel lava flows, as the effusion rate is important for estimating the final flow length, the rate of flow advance, and the eruption duration. The relationship between effusion rate, flow length, and flow advance rate is fairly well constrained for basaltic lava flows, where there are abundant recent examples for calibration. Less is known about flows of intermediate compositions (basaltic andesite to andesite), which are less frequent and where field measurements are limited by the large block sizes and the topographic relief of the flows. Here, we demonstrate ways in which high-resolution digital topography obtained using Light Detection and Ranging (LiDAR) systems can provide access to terrains where field measurements are difficult or impossible to collect. We map blocky lava flow units using LiDAR-generated bare earth digital terrain models (DTMs) of the Collier Cone lava flow in the central Oregon Cascades. We also develop methods using geographic information systems to extract and quantify morphologic features such as channel width, flow width, flow thickness, and slope. Morphometric data are then analyzed to estimate both effusion rates and emplacement times for the lava flow field. Our data indicate that most of the flow outline (which comprises the earliest, and most voluminous, flow unit) can be well explained by an average volumetric flux ~14–18?m3/s; channel data suggest an average flux ~3?m3/s for a later, channel-filling, flow unit. When combined with estimates of flow volume, these data suggest that the Collier Cone lava flow was most likely emplaced over a time scale of several months. This example illustrates ways in which high-resolution DTMs can be used to extract and analyze morphologic measurements and how these measurements can be analyzed to estimate emplacement conditions for inaccessible, heavily vegetated, or extraterrestrial lava flows. 相似文献
19.
High-resolution bathymetric mapping has shown that submarine flat-topped volcanic cones, morphologically similar to ones
on the deep sea floor and near mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea),
Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constructed of tholeiitic basalt erupted
during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparently formed
of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana
Ridge, and offshore Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield stage flat-topped cones have steep
(∼25°) sides, remarkably flat horizontal tops, basal diameters of 1–3 km, and heights <300 m. The flat tops commonly have
either a low mound or a deep crater in the center. The rejuvenated-stage flat-topped cones have the same shape with steep
sides and flat horizontal tops, but are much larger with basal diameters up to 5.5 km and heights commonly greater than 200 m.
The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most
of the rejuvenated-stage flat-topped cones formed on slopes <10° and formed adjacent semicircular steps down the flank of
Ni'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive
eruptions lasting years to decades. These, and other submarine volcanic cones of similar size and shape, apparently form as
continuously overflowing submarine lava ponds. A lava pond surrounded by a levee forms above a sea-floor vent. As lava continues
to flow into the pond, the lava flow surface rises and overflows the lowest point on the levee, forming elongate pillow lava
flows that simultaneously build the rim outward and upward, but also dam and fill in the low point on the rim. The process
repeats at the new lowest point, forming a circular structure with a flat horizontal top and steep pillowed margins. There
is a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that
facilitate construction of such landforms include effusive eruption of lava with low volatile contents, moderate to high confining
pressure at moderate to great ocean depth, long-lived steady eruption (years to decades), moderate effusion rates (probably
ca. 0.1 km3/year), and low, but not necessarily flat, slopes. With higher effusion rates, sheet flows flood the slope. With lower effusion
rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure eruptions. If the eruption is too brief, it will
not consolidate activity at a point, and fissure-fed flows will form a pond with irregular levees. The pond will solidify
between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water
will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones, as occurs during the post-shield
stage. The steady effusion of lava on land constructs lava shields, which are probably the subaerial analogs to submarine
flat-topped cones but formed under different cooling conditions.
Received: 30 September 1999 / Accepted: 9 March 2000 相似文献
20.
Mary E. MacKay Scott K. Rowland Peter J. Mouginis-Mark Harold Garbeil 《Bulletin of Volcanology》1998,60(4):239-251
We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness
of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average
40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of
flow thickness, which we used to calculate the volume (up to 1 km3 for an individual flow, and 1.8 km3 for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity
based on the spacing of large folds (1.2×1012 to 5.5×1012 Pa s for surface viscosity, and 7.5×1010 to 5.2×1011 Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures
such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable
in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images.
Received: 6 September 1997 / Accepted: 15 May 1998 相似文献