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1.
The Pu'u 'O'o-Kupaianaha eruption (1983-present) is the longest lived rift eruption of either Kilauea or neighboring Mauna Loa in recorded history. The initial fissure opening in January 1983 was followed by three years of episodic fire fountaining at the Pu'u 'O'o vent on Kilauea's east rift zone 19km from the summit (episodes 4–47). These spectacular events gave way in July 1986 to five and a half years of nearcontinuous, low-level effusion from the Kupaianaha vent, 3km to the cast (episode 48). A 49th episode began in November 1991 with the opening of a new fissure between Pu'u 'O'o and Kupaianaha. this three week long outburst heralded an era of more erratic eruptive behavior characterized by the shut down of Kupaianaha in February 1992 and subsequent intermittent eruption from vents on the west flank of Pu'u 'O'o (episodes 50 and 51). The events occurring over this period are due to progressive shrinkage of the rift-zone reservoir beneath the eruption site, and had limited impact on eruption temperatures and lava composition.  相似文献   

2.
 The Pu'u 'Ō'ō-Kūpaianaha eruption on the east rift zone of Kīlauea began in January 1983. The first 9 years of the eruption were divided between the Pu'u 'Ō'ō (1983–1986) and Kūpaianaha (1986–1992) vents, each characterized by regular, predictable patterns of activity that endured for years. In 1990 a series of pauses in the activity disturbed the equilibrium of the eruption, and in 1991, the output from Kūpaianaha steadily declined and a short-lived fissure eruption broke out between Kūpaianaha and Pu'u 'Ō'ō. In February 1992 the Kūpaianaha vent died, and, 10 days later, eruptive episode 50 began as a fissure opened on the uprift flank of the Pu'u 'Ō'ō cone. For the next year, the eruption was marked by instability as more vents opened on the flank of the cone and the activity was repeatedly interrupted by brief pauses in magma supply to the vents. Episodes 50–53 constructed a lava shield 60 m high and 1.3 km in diameter against the steep slope of the Pu'u 'Ō'ō cone. By 1993 the shield was pockmarked by collapse pits as vents and lava tubes downcut as much as 29 m through the thick deposit of scoria and spatter that veneered the cone. As the vents progressively lowered, the level of the Pu'u 'Ō'ō pond also dropped, demonstrating the hydraulic connection between the two. The downcutting helped to undermine the prominent Pu'u 'Ō'ō cone, which has diminished in size both by collapse, as a large pit crater formed over the conduit, and by burial of its flanks. Intervals of eruptive instability, such as that of 1991–1993, accelerate lateral expansion of the subaerial flow field both by producing widely spaced vents and by promoting surface flow activity as lava tubes collapse and become blocked during pauses. Received: 1 July 1997 / Accepted: 23 October 1997  相似文献   

3.
 The 1991–1993 lava flow is the most voluminous flow erupted at Mount Etna, Sicily, in over 300 years. Estimates of the volume obtained by various methods range from 205×106 m3 (Tanguy 1996) to over 500×106 m3 (Barberi et al. 1993). This paper describes the results of an electronic distance measurement (EDM)-based field survey of the upper surface of the 1991–1993 flow field undertaken in 1995. The results were digitised, interpolated and converted into a digital elevation model and then compared with a pre-eruption digital elevation model, constructed from a 1 : 25 000 contour map of the area, based on 1989 aerial photographs. Our measurements are the most accurate to date and show that the 1991–1993 lava flow occupies a volume of 231±29×106 m3. Received: 20 July 1996 / Accepted: 5 November 1996  相似文献   

4.
 The Puu Oo eruption has been remarkable in the historical record of Kilauea Volcano for its duration (over 13 years), volume (>1 km3) and compositional variation (5.7–10 wt.% MgO). During the summer of 1986, the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser-like fountaining at Puu Oo to virtually continuous, relatively quiescent effusion at the Kupaianaha vent. This paper examines this next chapter in the Puu Oo eruption, episodes 48 and 49, and presents new ICP-MS trace element and Pb-, Sr-, and Nd-isotope data for the entire eruption (1983–1994). Nearly aphyric to weakly olivine-phyric lavas were erupted during episodes 48 and 49. The variation in MgO content of Kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of Kilauea, both of which probably were controlled by variations in Kilauea's magma supply rate. These lavas contain euhedral olivines which generally are in equilibrium with whole-rock compositions, although some of the more mafic lavas which erupted during 1990, a period of frequent pauses in the eruption, accumulated 2–4 vol.% olivine. The highest forsterite content of olivines (∼85%) in Kupaianaha lavas indicates that the parental magmas for these lavas had MgO contents of ∼10 wt.%, which equals the highest observed value for lavas during this eruption. The composition of the Puu Oo lavas has progressively changed during the eruption. Since early 1985 (episode 30), when mixing between an evolved rift zone magma and a more mafic summit reservoir-derived magma ended, the normalized (to 10 wt.% MgO) abundances of highly incompatible elements and CaO have systematically decreased with time, whereas ratios of these trace elements and Pb, Sr, and Nd isotopes, and the abundances of Y and Yb, have remained relatively unchanged. These results indicate that the Hawaiian plume source for Puu Oo magmas must be relatively homogeneous on a scale of 10–20 km3 (assuming 5–10% partial melting), and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued. The rate of variation of highly incompatible elements in Puu Oo lavas is much greater than that observed for Kilauea historical summit lavas (e.g., Ba/Y 0.09 a–1 vs ∼0.03 a–1). This rapid change indicates that Puu Oo magmas did not mix thoroughly with magma in the summit reservoir. Thus, except for variable amounts of olivine fractionation, the geochemical variation in these lavas is predominantly controlled by mantle processes. Received: 8 March 1996 / Accepted: 30 April 1996  相似文献   

5.
 Lava drainback has been observed during many eruptions at Kilauea Volcano: magma erupts, degasses in lava fountains, collects in surface ponds, and then drains back beneath the surface. Time series data for melt inclusions from the 1959 Kilauea Iki picrite provide important evidence concerning the effects of drainback on the H2O contents of basaltic magmas at Kilauea. Melt inclusions in olivine from the first eruptive episode, before any drainback occurred, have an average H2O content of 0.7±0.2 wt.%. In contrast, many inclusions from the later episodes, erupted after substantial amounts of surface degassed lava had drained back down the vent, have H2O contents that are much lower (≥0.24 wt.% H2O). Water contents in melt inclusions from magmas erupted at Pu'u 'O'o on the east rift zone vary from 0.39–0.51 wt.% H2O in tephra from high fountains to 0.10–0.28 wt.% H2O in spatter from low fountains. The low H2O contents of many melt inclusions from Pu'u 'O'o and post-drainback episodes of Kilauea Iki reveal that prior to crystallization of the enclosing olivine host, the melts must have exsolved H2O at pressures substantially less than those in Kilauea's summit magma reservoir. Such low-pressure H2O exsolution probably occurred as surface degassed magma was recycled by drainback and mixing with less degassed magma at depth. Recognition of the effects of low-pressure degassing and drainback leads to an estimate of 0.7 wt.% H2O for differentiated tholeiitic magma in Kilauea's summit magma storage reservoir. Data for MgO-rich submarine glasses (Clague et al. 1995) and melt inclusions from Kilauea Iki demonstrate that primary Kilauean tholeiitic magma has an H2O/K2O mass ratio of ∼1.3. At transition zone and upper mantle depths in the Hawaiian plume source, H2O probably resides partly in a small amount of hydrous silicate melt. Received: 31 March 1997 / Accepted: 17 November 1997  相似文献   

6.
 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   

7.
The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO2). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 105 m3. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.  相似文献   

8.
Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEVIRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ??0.006?km3. The mineralogy of the 2010 lava is plagioclase?+?clinopyroxene?+?olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H2O contents ??1,300?ppm and CO2 contents of ??200?ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7?C42?MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ??1,150?°C, consistent with previously published field measurements.  相似文献   

9.
The Puu Oo eruption of Kilauea Volcano in Hawaii is one of its largest and most compositionally varied historical eruptions. The mineral and whole-rock compositions of the Puu Oo lavas indicate that there were three compositionally distinct magmas involved in the eruption. Two of these magmas were differentiated (<6.8 wt% MgO) and were apparently stored in the rift zone prior to the eruption. A third, more mafic magma (9–10 wt% MgO) was probably intruded as a dike from Kilauea's summit reservoir just before the start of the eruption. Its intrusion forced the other two magmas to mix, forming a hybrid that erupted during the first three eruptive episodes from a fissure system of vents. A new hybrid was erupted during episode 3 from the vent where Puu Oo later formed. The composition of the lava erupted from this vent became progressively more mafic over the next 21 months, although significant compositional variation occurred within some eruptive episodes. The intra-episode compositional variation was probably due to crystal fractionation in the shallow (0.0–2.9 km), dike-shaped (i.e. high surface area/volume ratio) and open-topped Puu Oo magma reservoir. The long-term compositional variation was controlled largely by mixing the early hybrid with the later, more mafic magma. The percentage of mafic magma in the erupted lava increased progressively to 100% by episode 30 (about two years after the eruption started). Three separate magma reservoirs were involved in the Puu Oo eruption. The two deeper reservoirs (3–4 km) recharged the shallow (0.4–2.9 km) Puu Oo reservoir. Recharge of the shallow reservoir occurred rapidly during an eruption indicating that these reservoirs were well connected. The connection with the early hybrid magma body was cut off before episode 30. Subsequently, only mafic magma from the summit reservoir has recharged the Puu Oo reservoir.  相似文献   

10.
Volcanic gas samples were collected from July to November 1985 from a lava pond in the main eruptive conduit of Pu'u O'o from a 2-week-long fissure eruption and from a minor flank eruption of Pu'u O'o. The molecular composition of these gases is consistent with thermodynamic equilibrium at a temperature slightly less than measured lava temperatures. Comparison of these samples with previous gas samples shows that the composition of volatiles in the magma has remained constant over the 3-year course of this episodic east rift eruption of Kilauea volcano. The uniformly carbon depleted nature of these gases is consistent with previous suggestions that all east rift eruptive magmas degas during prior storage in the shallow summit reservoir of Kilauea. Minor compositional variations within these gas collections are attributed to the kinetics of the magma degassing process.  相似文献   

11.
 The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km3, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit. The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000 BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above the 'Ailā'au flows, cannot be older than this age. Received: 10 October 1998 / Accepted: 12 May 1999  相似文献   

12.
A Landsat Thematic Mapper (TM) image acquired on 23 July 1991 recorded widespread activity associated with the Episode 48 of the Pu'u 'O'o-Kupaianaha eruption of Kilauea Volcano, Hawaii. The scene contains a very large number (>3500) of thermally elevated near infrared (0.8–2.35 m) pixels (each 900 m2), which enable the spatial distribution of volcanic activity to be identified. This activity includes a lava lake within Pu'u 'O'o cone, an active lava tube system (7.9 km in length) with skylights between the Kupaianaha lava shield and several ocean entry points, and extensive active surface flows (total area of 1.3 km2) within a much larger area of cooling flows (total16 km2). The production of an average flux density map from the TM data of the flow field, wherein the average flux density is defined in units of Wm-2, allows for the chronology of emplacement of active and cooling flows to be determined. The flux density map reveals that there were at least three breakouts (>5000 Wm-2) feeding active flows, but on the day that the data were collected the TM recorded a waning phase of surface activity in this area, based on the relatively large amount of intermediate power-emitting (cooling) flows compared to high power-emitting (active) flows. The production of a comparable flux density map for future eruptions would aid in the assessment of volcanic hazards if the data were available in near-real time.  相似文献   

13.
Post-10 ka rhyolitic eruptions from the Haroharo linear vent zone, Okataina Volcanic Centre, have occurred from several simultaneously active vents spread over 12 km. Two of the three eruption episodes have tapped multiple compositionally distinct homogeneous magma batches. Three magmas totalling ~8 km3 were erupted during the 9.5 ka Rotoma episode. The most evolved Rotoma magma (SiO2=76.5–77.9 wt%, Sr=96–112 ppm) erupted from a southeastern vent, and is characterised by a cummingtonite-dominant mineralogy, a temperature of 739±14°C, and fO2 of NNO+0.52±0.11. The least evolved (SiO2=75.0–76.4 wt%, Sr=128–138 ppm, orthopyroxene+ hornblende-dominant) Rotoma magma erupted from several vents, and was hotter (764±18°C) and more reduced (NNO+0.40±0.13). The ~11 km3 Whakatane episode occurred at 5.6 ka and also erupted three magmas, each from a separate vent. The most evolved (SiO2=73.3–76.2 wt%, Sr=88–100 ppm) Whakatane magma erupted from the southwestern (Makatiti) vent and is cummingtonite-dominant, cool (745±11°C), and reduced (NNO+0.34±0.08). The least evolved (SiO2=72.8–74.1 wt%, Sr=132–134 ppm) magma was erupted from the northeastern (Pararoa) vent and is characterised by an orthopyroxene+ hornblende-dominant mineralogy, temperature of 764±18°C, and fO2 of NNO+0.40±0.13. Compositionally intermediate magmas were erupted during the Rotoma and Whakatane episodes are likely to be hybrids. A single ~13 km3 magma erupted during the intervening 8.1 ka Mamaku episode was relatively homogeneous in composition (SiO2=76.1–76.8 wt%, Sr=104–112 ppm), temperature (736±18°C), and oxygen fugacity (NNO+0.19±0.12). Some of the vents tapped a single magma while others tapped several. Deposit stratigraphy suggests that the eruptions alternated between magmas, which were often simultaneously erupted from separate vents. Both effusive and explosive activity alternated, but was predominantly effusive (>75% erupted as lava domes and flows). The plumbing systems which fed the vents are inferred to be complex, with magma experiencing different conditions in the conduits. As the eruption of several magmas was essentially concurrent, the episodes were likely triggered by a common event such as magmatic intrusion or seismic disturbance.  相似文献   

14.
The August 1991 eruptions of Hudson volcano produced ~2.7 km3 (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO2. A trend of decreasing SiO2 is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO2) to the end (unit D, 60–63 wt% SiO2). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log fO2 –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between 2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera. The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different vent 4 km to the south-southeast. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

15.
 The postglacial eruption rate for the Mount Adams volcanic field is ∼0.1 km3/k.y., four to seven times smaller than the average rate for the past 520 k.y. Ten vents have been active since the last main deglaciation ∼15 ka. Seven high flank vents (at 2100–2600 m) and the central summit vent of the 3742-m stratocone produced varied andesites, and two peripheral vents (at 2100 and 1200 m) produced mildly alkalic basalt. Eruptive ages of most of these units are bracketed with respect to regional tephra layers from Mount Mazama and Mount St. Helens. The basaltic lavas and scoria cones north and south of Mount Adams and a 13-km-long andesitic lava flow on its east flank are of early postglacial age. The three most extensive andesitic lava-flow complexes were emplaced in the mid-Holocene (7–4 ka). Ages of three smaller Holocene andesite units are less well constrained. A phreatomagmatic ejecta cone and associated andesite lavas that together cap the summit may be of latest Pleistocene age, but a thin layer of mid-Holocene tephra appears to have erupted there as well. An alpine-meadow section on the southeast flank contains 24 locally derived Holocene andesitic ash layers intercalated with several silicic tephras from Mazama and St. Helens. Microprobe analyses of phenocrysts from the ash layers and postglacial lavas suggest a few correlations and refine some age constraints. Approximately 6 ka, a 0.07-km3 debris avalanche from the southwest face of Mount Adams generated a clay-rich debris flow that devastated >30 km2 south of the volcano. A gravitationally metastable 2-to 3-km3 reservoir of hydrothermally altered fragmental andesite remains on the ice-capped summit and, towering 3 km above the surrounding lowlands, represents a greater hazard than an eruptive recurrence in the style of the last 15 k.y. Received: 24 June 1996 / Accepted: 6 December 1996  相似文献   

16.
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   

17.
 We analyze digital topographic data collected in September 1993 over a ∼500-km2 portion of K*lauea Volcano, Hawai'i, by the C-band (5.6-cm wavelength) topographic synthetic aperture radar (TOPSAR) airborne interferometric radar. Field surveys covering an ∼1-km2 area of the summit caldera and the distal end of an ∼8-m-thick 'a'* flow indicate that the 10-m spatial resolution TOPSAR data have a vertical accuracy of 1–2 m over a variety of volcanic surfaces. After conversion to a common datum, TOPSAR data agree favorably with a digital elevation model (DEM) produced by the U.S. Geological Survey (USGS), with the important exception of the region of the ongoing eruption (which postdates the USGS DEM). This DEM comparison gives us confidence that subtracting the USGS data from TOPSAR data will produce a reasonable estimate of the erupted volume as of September 1993. This subtraction produces dense rock equivalent (DRE) volumes of 392, 439, and 90×106 m3 for the Pu'u '*'*, K*pa'ianah*, and episode 50–53 stages of the eruption, respectively. These are 124, 89, and 94% of the volumes calculated by staff of the Hawaiian Volcano Observatory (HVO) but do not include lava of K*pa'ianah* and episodes 50–53 that flowed into the ocean and are thus invisible to TOPSAR. Accounting for this lava increases the TOPSAR volumes to 124, 159, and 129% of the HVO volumes. Including the ±2-m uncertainty derived from the field surveys produces TOPSAR-derived volumes for the eruption as a whole that range between 81 and 125% of the USGS-derived values. The vesicularity- and ocean-corrected TOPSAR volumes yield volumetric eruption rates of 4.5, 4.5, and 2.7 m3/s for the three stages of the eruption, which compare with HVO-derived values of 3.6, 2.8, and 2.1 m3/s, respectively. Our analysis shows that care must be taken when vertically registering the TOPSAR and USGS DEMs to a common datum because C-band TOPSAR penetrates only partially into thick forest and therefore produces a DEM within the tree canopy, whereas the USGS DEM is adjusted for vegetation. Received: 28 April 1998 / Accepted: 1 February 1999  相似文献   

18.
 The Alban Hills, a Quaternary volcanic center lying west of the central Apennines, 15–25 km southeast of Rome, last erupted 19 ka and has produced approximately 290 km3 of eruptive deposits since the inception of volcanism at 580 ka. Earthquakes of moderate intensity have been generated there at least since the Roman age. Modern observations show that intermittent periods of swarm activity originate primarily beneath the youngest features, the phreatomagmatic craters on the west side of the volcano. Results from seismic tomography allow identification of a low-velocity region, perhaps still hot or partially molten, more than 6 km beneath the youngest craters and a high-velocity region, probably a solidified magma body, beneath the older central volcanic construct. Thirty centimeters of uplift measured by releveling supports the contention that high levels of seismicity during the 1980s and 1990s resulted from accumulation of magma beneath these craters. The volume of magma accumulation and the amount of maximum uplift was probably at least 40×106 m3 and 40 cm, respectively. Comparison of newer levelings with those completed in 1891 and 1927 suggests earlier episodes of uplift. The magma chamber beneath the western Alban Hills is probably responsible for much of the past 200 ka of eruptive activity, is still receiving intermittent batches of magma, and is, therefore, continuing to generate modest levels of volcanic unrest. Bending of overburden is the most likely cause of the persistent earthquakes, which generally have hypocenters above the 6-km-deep top of the magma reservoir. In this view, the most recent uplift and seismicity are probably characteristic and not precursors of more intense activity. Received: 15 April 1997 / Accepted: 9 August 1997  相似文献   

19.
The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich (up to 5 wt.% H2O), crystal-poor (~10 vol.% Fo90 olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.% microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 105–4 × 106 Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western México, were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt.  相似文献   

20.
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   

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