Eruption Site Research Articles

Horizontal ground deformation patterns and magma storage during the Puu Oo eruption of Kilauea volcano, Hawaii: episodes 22?42

Horizontal ground deformation measurements were made repeatedly with an electronic distance meter near the Puu Oo eruption site approximately perpendicular to Kilauea's east rift zone (ERZ) before and after eruptive episodes 22–42. Line lengths gradually extended during repose periods and rapidly contracted about the same amount following eruptions. The repeated extension and contraction of the measured lines are best explained by the elastic response of the country rock to the addition and subsequent eruption of magma from a local reservoir. The deformation patterns are modeled to constrain the geometry and location of the local reservoir near Puu Oo. The observed deformation is consistent with deformation patterns that would be produced by the expansion of a shallow, steeply dipping dike just uprift of Puu Oo striking parallel to the trend of the ERZ. The modeled dike is centered about 800 m uprift of Puu Oo. Its top is at a depth of 0.4 km, its bottom at about 2.9 km, and the length is about 1.6 km; the dike strikes N65° E and dips at about 87°SE. The model indicates that the dike expanded by 11 cm during repose periods, for an average volumetric expansion of nearly 500 000 m3. The volume of magma added to the dike during repose periods was variable but correlates positively with the volume of erupted lava of the subsequent eruption and represents about 8% of the new lava extruded. Dike geometry and expansion values are used to estimate the pressure increase near the eruption site due to the accumulation of magma during repose periods. On average, vent pressures increased by about 0.38 MPa during the repose periods, one-third of the pressure increase at the summit. The model indicates that the dikelike body below Puu Oo grew in volume from 3 million cubic meters (Mm3) to about 10–12 Mm3 during the series of eruptions. The width of this body was probably about 2.5–3.0 m. No net long-term deformation was detected along the measured deformation lines.

New structural limits on magma chamber locations at the Valley of Ten Thousand Smokes, Katmai National Park, Alaska

New structural data from the Novarupta basin, Katmai National Park, Alaska, site of the largest volcanic eruption of this century (1912), provide limits for the location of magma chambers associated with this eruption. To investigate the subsurface structure of the 1912 vents, and to support an interdisciplinary study of this young volcanic system, a project of geologic mapping of surficial and bedrock structures in the vent region of the 1912 eruption has been undertaken. Landslide scarps, arcuate grabens, a monoclinal fold, and truncated ridges circumscribe the Novarupta basin, marking the inferred outer rim of the vent. A set of radial fissures crosses the southern margin of the basin, striking {approximately}140{degree}, subparallel to the dominant bedrock joint set. These fissures and joints, along with the local plate-motion vector and the inferred regional stress orientation, are consistent with a feeder dike propagating from a reservoir beneath Trident volcano to the eruptive vent.

Occurrence of recurrent herpes labialis in an adult Swedish population

In a general adult Swedish population recurrent herpes labialis (RHL) was found in 3.1% of 20,333 people aged 15 years and more. When a 2-year history of RHL was considered, the total prevalence was 17.4%. There was a slight predominance among women. Only slight differences were found between various age groups. In a fraction of the population studied, constituting 2771 people, interviews concerning cases of RHL were carried out. Thus, for example, most individuals referred to only one herpes episode a year; the most prevalent site of eruption was an area comprising both the vermilion border and the skin, and most of the lesions showed healing times of 5-8 days. The prevalence of RHL was influenced by some tobacco habits. Thus, the prevalence was significantly lower among smokers, and especially among pipe smokers, than among people with no tobacco habit at all. Snuff dipping did not seem to influence the prevalence values.

Implications of historical eruptive-vent migration on the northeast rift zone of Mauna Loa Volcano, Hawaii

Research Article| July 01, 1990 Implications of historical eruptive-vent migration on the northeast rift zone of Mauna Loa Volcano, Hawaii John P. Lockwood John P. Lockwood 1U.S. Geological Survey, P.O. Box 51, Hawaii Volcanoes National Park, Hawaii 96718 Search for other works by this author on: GSW Google Scholar Author and Article Information John P. Lockwood 1U.S. Geological Survey, P.O. Box 51, Hawaii Volcanoes National Park, Hawaii 96718 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (7): 611–613. https://doi.org/10.1130/0091-7613(1990)018<0611:IOHEVM>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John P. Lockwood; Implications of historical eruptive-vent migration on the northeast rift zone of Mauna Loa Volcano, Hawaii. Geology 1990;; 18 (7): 611–613. doi: https://doi.org/10.1130/0091-7613(1990)018<0611:IOHEVM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Five times within the past 138 yr (1852, 1855-1856, 1880-1881, 1942, and 1984), lava flows from vents on the northeast rift zone of Mauna Loa Volcano have reached within a few kilometres of Hilo (the largest city on the Island of Hawaii). Most lavas erupted on this rift zone in historical time have traveled northeastward (toward Hilo), because their eruptive vents have been concentrated north of the rift zone's broad topographic axis. However, with few exceptions each successive historical eruption on the northeast rift zone has occurred farther southeast than the preceding one. Had the 1984 eruptive vents (the most southeasterly yet) opened less than 200 m farther southeast, the bulk of the 1984 lavas would have flowed away from Hilo. If this historical vent-migration pattern continues, the next eruption on the northeast rift zone could send lavas to the southeast, toward less populated areas.The historical Mauna Loa vent-migration patterns mimic the southeastern "younging" of the Hawaiian-Emperor volcanic chain and may be cryptically related to northwestward movement of the Pacific plate. Systematic temporal-spatial vent-migration patterns may characterize eruptive activity at other volcanoes with flank activity and should be considered as an aid to long-term prediction of eruption sites. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

New geological investigations in eastern Disko: redeposited volcanoclastic sediments with rhyolite from the Nordfjord Member

During field work in 1988 in eastern Disko we encountered redeposited volcanoclastic sediments with acid rocks at several localities between Rensdyrdalen and Charles Polaris Dal within lava sequences from the Nordfjord Member of the Maligât Formation. Some of the sediments were only observed during helicopter reconnaissance of steep inaccessible walls and corries, while other sites provided detailed information and samples through field visits. Fig.1 shows the area with newly discovered acid volcanic rocks in eastern Disko. Localities 1 and 2 denote localities investigated in some detail. In north-western Disko peraluminous rhyolite tuffs and conglomerates with almandine-bearing rhyolite blocks occur as widespread but minor deposits in the Nordfjord Member (Pedersen, 1977; Hansen &amp; Pedersen, 1985; Pedersen &amp; Pedersen, 1987). The acid rocks contain graphite and scarce sediment xenoliths and they are associated with contaminated basalts, andesites and dacites, same af which carry nativc iran. The distribution af airfall rhyolite tuff shows that the aciel rocks were erupted somewhere in north-western Disko or in the present shallow sea west af this arca. No eruption sites are exposed at present.

Structure and evolution of the Hengill‐Grensdalur Volcanic Complex, Iceland: Geology, geophysics, and seismic tomography

Recent geological and geophysical research indicates the presence of three volcanic systems in the Hengill‐Grensdalur area and progressive westerly migration of the accretionary plate boundary. The Hengill system comprises the active Hengill central volcano and a NNE trending fissure swarm that is the present locus of crustal accretion in the area. Extinct volcanic systems are identified with the mountain Hromundartindur and the Grensdalur area and associated NNE trending zones. The Grensdalur system was formerly the locus of accretion in the area. A widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems. A tomographic study of the upper 5 km of crust, using local earthquakes, imaged three bodies with velocities up to 15% higher than the average background velocities and volumes of several tens of cubic kilometers. One of these underlies the Grensdalur volcano, and a second underlies the Olkelduhals area within the Hromundartindur system. These are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them. The third high‐velocity body underlies the extinct basalt shield Husmuli, which is not associated with geothermal resources. That body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site. A low‐velocity body with a volume of a few cubic kilometers was imaged in the depth range 2–4 km beneath the northern part of the presently active Hengill central volcano. This volume may contain partial melt and represent the heat source, or part of the heat source, fueling the Hengill field. Heat balance calculations show that subsurface magmatism continued in the extinct Grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the Hengill system.

Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers

The Siqueiros transform fault system, which offsets the East Pacific Rise between 8°20′N–8°30′N, has been mapped with the Sea MARC II sonar system and is found to consist of four intra-transform spreading centers and five strike-slip faults. The bathymetric and side-looking sonar data define the total width of the transform domain to be ≈20km. The transform domain includes prominent topographic features that are related to either seafloor spreading processes at the short spreading centers or shearing along the bounding faults. The spreading axes and the seafloor on the flanks of each small spreading center comprise morphological and structural features which suggest that the two western spreading centers are older than the eastern spreading centers. Structural data for the Clipperton, Orozco and Siqueiros transforms, indicate that the relative plate motion geometry of the Pacific-Cocos plate boundary has been stable for the past ≈1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is ≈500 000 years old and parallels the present EPR abyssal hill trend (350°) we conclude that a small change in plate motion was not the cause for intra-transform spreading center development in Siqueiros. We suggest that the impetus for the development of intra-transform spreading centers along the Siqueiros transform system was provided by the interaction of small melt anomalies in the mantle (SMAM) with deepseated, throughgoing lithospheric fractures within the shear zone. Initially, eruption sites may have been preferentially located along strike-slip faults and/or along cross-faults that eventually developed into pull-apart basins. Spreading centers C and D in the eastern portion of Siqueiros are in this initial pull-apart stage. Continued intrusion and volcanism along a short ridge within a pull-apart basin may lead to the formation of a stable, small intra-transform spreading center that creates a narrow swath of ridge-parallel structures within the transform domain. The morphology and structure of the axes and flanks of spreading centers A and B in the western and central portion of Siqueiros reflect this type of evolution and suggest that magmatism associated with these intra-transform spreading centers has been active for the past ≈0.5–1.0 Ma.

Autoreactive Cytotoxic T Lymphocytes in Pemphigus and Pemphigoid

In order to know whether effector cells are capable of recognizing the epidermal targets in pemphigus vulgaris (PV) and bullous pemphigoid (BP), non-adherent cells (NAC) of peripheral blood from 27 primary PV, 19 BP patients and 12 healthy volunteers were used in cytotoxic tests. Autologous, allogenic and murine epidermal keratinocytes as well as autologous and allogenic fibroblasts were employed as targets. Total esterase activity of serine proteinases was measured in supernatants of samples and the results obtained were compared with the data received in 51Cr-release assay. These two techniques yielded similar results. The effector cells of PV and BP patients were found to lyse the epidermal target cells without presensitization in vitro. Epidermal cells from involved and uninvolved skin of PV patients, as well as from perilesional skin of BP patients, were highly sensitive to cytotoxic effect on the part of effector cells. The NAC activity in BP was higher than in PV. On the one hand, the obtained data suggest that PV and BP patients develop autoreactive cytotoxic T lymphocytes, sensitized to pemphigus and pemphigoid antigens, respectively, and on the other hand that epidermocytes in pemphigus and pemphigoid patients turned to be more fragile compared to normal human epidermal keratinocytes. It was supposed that cell-mediated cytotoxic reactions are probably related to enhanced proteolytic activity in the site of bullous eruption.

Geology and petrology of the Lower Tertiary plateau basalts of the Scoresby Sund region, East Greenland

Geology and petrology of the Lower Tertiary plateau basalts of the Scoresby Sund region, East Greenland

"Nature of local magma storage zones and geometry of conduit systems below basaltic eruption sites: Pu'u 'O 'o, Kilauea East Rift, Hawaii, Example""

The fluid dynamics of the well‐documented eruptive episodes at Pu'u 'O 'o, Kilauea (Wolfe et al., 1987) are used to investigate quantitatively the size and shape of the shallow conduit system beneath the vent. Cooling calculations are employed to study the long‐term survival of conduits and the consequences of multiple dike injection events. We find that the subvent conduit must have a planar geometry at depths greater than a few tens to at most a few hundreds of meters, with a width of less than a few meters, a length of the order of 100 m, and a height of the order of 1 km. This structure is clearly the residue of the preemption dikes. Although such a feature can be widened somewhat by repeated dike emplacement events, there is no evidence to suggest that a larger and much more equant magma reservoir should develop at shallow depth. The extensive degassing often occurring after single eruptions or between repeated eruptions, previously thought to imply the presence of a large equidimensional shallow magma chamber, can be readily explained by the volume of magma in a dikelike subvent planar magma storage zone. On the basis of these observations, which we infer to apply commonly to basaltic eruption sites, we suggest that the terms “chamber” or “reservoir” be used with caution, because they most often connote a relatively large equant magma body, distinct from the feeder dike, lying just below the vent. We find that the subvent region has the same basic geometric characteristics as the parental dike, and we propose that it be referred to as a “planar magma storage zone.”

The influence of loading by lavas on the siting of volcanic eruption vents on Mt Etna

Until now it has been considered that the positioning of magma conduits, dykes and eruption sites on Mt Etna is controlled by stresses set up by regional tectonism. It is also likely that gravitational stresses have some influence on the subsurface path taken by Etnean magmas prior to eruption. An additional effect discussed here is based mainly on convincing field evidence gathered over a number of years. It is proposed that fracturing of the surface and changes in the stress pattern due to loading by recent lava flows may strongly influence the siting of eruptions. Detailed field observations are presented, showing that cracks and fumaroles have consistently appeared round the edges of recent lava flows, and that these cracks are sometimes later used by erupting lavas. Eruption statistics indicate that since 1970, 50% of new eruptive vents have appeared within 80 m of a recent flow edge, and field measurements show that downward flexure is still occurring at the edges of lava flows more than sixteen years old. Simple theoretical modelling indicates that this effect is strongest at the surface, with gravitational and regional tectonic control of eruption sites dominating at progressively deeper levels.

The Agnew nickel deposit, Western Australia; Part II, Sulfide geochemistry, with emphasis on the platinum-group elements

Nickel sulfide mineralization in the Agnew deposit comprises matrix or net-textured ores, primary in situ massive sulfides, structurally remobilized massive sulfides, and low-grade disseminated or suifides. Matrix and massive sulfides are also present at the laterally correlative Rocky's Reward deposit 2 km to the north. Systematic compositional contrasts are noted between the different ore types when analyses are normalized to abundances in 100 percent sulfides. Matrix ores are enriched in Ni, Co, Cu, Pt, Pd, and Au but depleted in Os, Ir, Ru, and Rh relative to adjacent massive ores, both at the Agnew deposit and at Rocky's Reward. Cloud sulfides are similar to matrix sulfides in Os, Ir, Ru, and Rh but are enriched in Ni, Co, Cu, Pt, Pd, and Au relative to both massive and matrix ores. Overall abundances of all the above elements are slightly lower at Rocky's Reward than at Agnew, and both deposits are depleted in these elements relative to ores at Kambalda. Remobilized and in situ massive sulfides are identical in composition, implying that remobilization occurred by tectonic, not hydrothermal, processes. Massive sulfides could not have originated by metamorphic upgrading of matrix ore.The nickel content of olivine within the mineralized flow at Agnew is strongly depleted relative to typical komatiitic olivines, Olivine in the cloud sulfide zone is depleted to a lesser extent. Quantitative modeling shows that the observed olivine Mg/Fe ratios and nickel contents, as well as the Ni contents of the Agnew sulfides, formed by extensive preemplacement extraction of olivine and sulfide liquid from a primitive komatiite parent. Olivine and sulfides in the cloud zone show less extensive differentiation in a model.Compositional contrasts between the Agnew and Rocky's Reward deposits are attributed to lateral differentiation within lava flows, since Agnew is closer to the site of eruption. The difference in Os, Ir, Ru, and Rh concentrations between matrix and massive ore cannot be the result of in situ fractionation of the sulfide liquid or of hydrothermal remobilization. The different ore types must have formed from discrete batches of sulfide liquid which segregated under different fugacities of sulfur and/or oxygen.

Age and Sr isotopic signature of the Catoctin volcanic province: Implications for subcrustal mantle evolution

Variations in chemical and Sr isotopic signatures of a series of basaltic flows have been correlated with newly mapped stratigraphic sections in the upper Precambrian Catoctin flood basalts in the central Appalachians. Element mobility through fluid interaction is commonly observed in flows with a well-developed fabric, but some samples with more massive textures (commonly in the cores of thicker flows) still retain relict clinopyroxene and igneous textures and show only minor element redistribution. Five whole-rock samples with relict igneous textures and a clinopyroxene mineral separate yield an isochron with an age of 570 +/- 36 Ma (2sigma) and a strontium initial ratio of 0.7035 +/- 1. This age is consistent with recent paleontological evidence for a latest Precambrian time for Catoctin volcanism and, along with isotopic age determinations of 680-710 Ma for other rift-related magmatism, suggests either a two-stage or a very long, protracted episode of continental rifting. Paleogeographic reconstruction indicates that the original site of eruption for the lavas must have been at least 56 km to the east, in the same geographic location from which the Mesozoic Appalachian tholeiite terrane evolved. Variations in Sr isotopic signatures and differences in major-element abundances between these two sequences suggest mantle heterogeneitiesmore » that must have developed beneath the eastern North American craton during the interim 400 m.y.« less

The Rotokawa geothermal system, New Zealand; an active epithermal gold-depositing environment

The Rotokawa geothermal system is less than 20,000 yrs old and is situated in a sequence of Pleistocene and Recent volcanic rocks and underlying Mesozoic graywacke basement in the Taupo volcanic zone, a region dominated by extensional tectonics. At lease 12 different hydrothermal explosition breccias are preserved in surface outcrops, the largest eruptive site being a crater of 1.5 km in diameter which formed about 6,060 years ago and is presently occupied by an acid lake (Lake Rotokawa) with a pH approximately 2. Significant bedded sulfur deposits which formed in an earlier crater lake exist beneath the present lake. The deep fluids are dilute chloride (0.022 mole/kg) waters having higher reduced sulfur concentrations (H 2 S approximately 7X10 (super -3) mole/kg) than any previously studied active geothermal system. A maximum temperature of 320 degrees C has been measured in the system. Water-rock interaction has given rise to a zone of propylitic alteration at depth which is overlain by a region of advanced and intermediate argillic alteration. Extensive boiling occurs at depth an sulfide minerals such as sphalerite, galena, chalcopyrite, and argentite occur sporadically with ubiquitous pyrite. Argentiferous alunite occurs in the advanced argillic alteration zone. clasts of silicified lake beds in the hydrothermal explosion breccias commonly contain pyrite and carry subeconomic gold values of up to 0.1 g/metric ton. Thermal waters discharging on the northeast shore of the lake are depositing gold ore grade arsenic and antimony sulfide-rich muds which also contain high concentrations of tungsten, thallium, mercury, silver, and gallium. This surface deposit is estimated to contain about 250 kg (8,000 oz) of gold and has been deposited during the past 1,800 yrs. Since the formation 6,060 yrs ago of the large hydrothermal explosion crater in which Lake Rotokawa is now situated, about 12.0 million oz. (370 metric tons) of gold may have been transported into the environment beneath the crater. It is estimated that 1.7 to 3.3 million oz. of gold may have been deposited in the upper 300 to 400 m underlying Lake Rotokawa, from hydrothermal fluids initially saturated with gold.

Tectonic controls on magma genesis and evolution in the northwestern United States

Field, chronologic, chemical, and isotopic data for late Cenozoic basaltic rocks from the northwestern United States illustrate the relationship between crustal structure and tectonic forces in controlling the genesis and evolution of continental volcanism. In the northwestern U.S., the first major episode of basaltic volcanism was triggered by crustal rifting in a “back-arc” environment, east of the westward-migrating volcanic arc created by the subduction of the Juan-de-Fuca plate beneath the North American plate. Rifting and volcanism were concentrated by pre-existing zones of crustal weakness associated with boundaries between the old Archean core of the continent and newly accreted terranes. Basalts erupted during this time (Columbia River, Steens Mountain) show evidence of significant fractionation histories including contamination by crust of varying age depending on the crustal structure at the eruption site. Presumably this reflects ponding and stagnation of primary magmas in the crust or at the crust-mantle interface due to their encounter with thick crust, not yet extended and still containing its low-density, easily fusible component. Continued rifting of this crust, and modification of its composition through extraction of rhyolitic partial melts and deposition of the fractionation products from primary basaltic melts, coupled with a shift in stress orientation roughly 10.5 Ma ago, allowed relatively unfractionated and uncontaminated magmas to begin reaching the surface. In the western part of the region (Oregon Plateau), these magmas tapped a mantle source similar to that which produced most of the ocean island basalts of the northern hemisphere. To the east (Snake River Plain), however, the mantle sampled by basaltic volcanism has isotopic characteristics suggesting it has preserved a record of incompatible element enrichment processes associated with the formation of the overlying Archean crustal section some 2.6 Ga ago.

Cretaceous paleogeography of southern South America

During Berriasian-Hauterivian times the Andean regions north of 38°S and south of 42°S were occupied by two magmatic island arcs and two back-arc marine basins, the Andean and Austral Basins. At the same time the Chubut Basin of central Patagonia was being filled with continental sediments. In northern Argentina and Bolivia, the Aimara Basin began to develop with deposition of continental sediments intercalated with basaltic rocks. In northeastern Argentina, Paraguay, Uruguay and southern Brazil, a series of basins formed in Precambrian-Paleozoic basement were the site of continental sedimentation and eruption of basic flows. In Barremian-Cenomanian times the uplift of the Cordillera Principal of Argentina and Chile began and a general regression occurred in the Andean and Austral Basins. Continental sedimentation prevailed in the Chubut Basin of central Patagonia, as well as in all other basins of central-northern, and eastern Argentina and southern Brazil. In Bolivia a shallow marine transgression occurred during Cenomanian time. And in the Santos Basin of southern Brazil a proto-oceanic stage is represented by Aptian evaporites. In (Turonian-Coniacian) Santonian-Campanian times most of the Andean area was uplifted and thick volcanic and volcanoclastic sequences were accumulated in a series of restricted continental basins along the Andes of central and northern Chile. In the easternmost areas, south of 33°S, red continental sediments were deposited. In the Austral Basin marine sedimentation was restricted to the southern part. Most of the Parana Basin was emergent, although in the northern areas continental sedimentation continued. In northern Argentina and Bolivia continental sediments with interbedded basalts and shallow marine deposits were accumulated. The Atlantic basins of southern Brazil were characterized by marine sedimentation, and the Salado Basin was the site of continental deposition. In Maastrichtian time sedimentation was discontinued in the continental basins of west-central Argentina and central and northern Chile, although volcanism was still active. The marine area of the Austral Basin became more restricted. A large region of northern Patagonia, northeastern Argentina and Bolivia were covered by a shallow sea, and some restricted marine basins were developed on the coast of central Chile. Marine sedimentation was also continuous in the Atlantic basins of southern Brazil and central Argentina. Continental deposition was restricted to central Patagonia and the intracratonic Parana Basin of southern Brazil. There was a correlation between diastrophism, magmatic activity and global cycles of sea level changes. Contemporary regression existed in areas, such as the Andean Basin, where tectonic vertical movements were greater than eustatic changes of sea level. The Cretaceous flora is mostly known from Patagonia. Lower Cretaceous invertebrates are mostly known from west-central Argentina, whereas Upper Cretaceous invertebrates are almost exclusively known from southern Patagonia. Vertebrates are better known from continental Upper Cretaceous strata of central-northern Patagonia and southern Brazil. The Cretaceous climate north of 43°S was characterized by warm and extremely arid conditions. South of this arid belt was an area with temperate and humid climate.

Variations in tilt rate and harmonic tremor amplitude during the January–August 1983 east rift eruptions of Kilauea Volcano, Hawaii

During January–August 1983, a network of telemetered tiltmeters and seismometers recorded detailed temporal changes associated with seven major eruptive phases along the east rift of Kilauea Volcano, Hawaii. Each eruptive phase was accompanied by subsidence of the summit region and followed by reinflation of the summit to approximately the same level before renewal of eruptive activity. The cyclic summit tilt pattern and the absence of measurable tilt changes near the eruptive site suggest that conditions in the summit region controlled the timing of the last six eruptive phases. The rate of summit subsidence progressively increased from one eruptive phase to the next during the last six phases; the amplitude of harmonic tremor increased during the last four phases. The increases in subsidence rate and in tremor amplitude suggest that frequent periods of magma movement have reduced the flow resistance of the conduit system between the summit and the rift zone.

Rabaul Caldera, Papua New Guinea: Volcanic hazards, surveillance, and eruption contingency planning

Planning for volcanic contingencies at the Rabaul caldera complex is a difficult task, requiring urgent attention in view of the results of volcano surveillance and historical eruption frequency. Historical eruption periodicities have ranged from about 2 to 59 years; the most recent eruptions were in 1937–1943 and caused more than 500 fatalities in May 1937. Surveillance since 1971 has revealed uplift of part of the caldera floor and increasing seismicity in a ring structure believed to define the caldera bounding fault or faults. These phenomena are interpreted as the prelude to a possible eruption. Potential eruption sites are all close to Rabaul town and outlying residential areas, and include the volcanoes of Sulphur Creek (last active about 1850), Vulcan and Tavurvur (1937–1943), and Rabalanakaia, but new sites should also be considered in relation to the results of volcano surveillance. A wide range of volcanic hazards accompanying different scales of eruption must be considered for the Rabaul area. The most likely types of eruption may be similar to the relatively small events that have been witnessed over the last century whose hazards included air-fall tephra, pumice rafts, pyroclastic flows and surges, volcanic earthquakes, volcanic-gas discharges, tsunamis, lightning strikes, and the fall of mud-rain, torrential run-off, and mudflows as a result of accompanying rain storms. Other potential eruption types of much larger scale could involve cauldron subsidence which could cause destructive tsunamis and could be accompanied by explosive activity of major proportions. Air-fall tephra is considered to be one of the main hazards from a future eruption, and its effects in and around Rabaul are primary influences in the choice of evacuation routes and points of refuge. Hence, theoretical tephra dispersal patterns have been considered in some detail. Air-fall tephra deposition from Rabaul eruption clouds at heights of less than about 4 km is determined by a biannual change in wind direction from southeast in May to October/November (trade winds) to northwest during the monsoon from December to March/April. In contrast, clouds that rise higher than about 4 km are blown by east winds all year round. Another principal hazard is pyroclastic flows and surges, which took place during the 1937 Vulcan eruption, but have until recently been overlooked, although flows or surges (or both) are clearly distinguishable on eruption photographs. The lack of suitable evacuation routes and problems in choosing safe and reasonably accessible refuge points compound the difficulties in preparing a practicable contingency plan. However, in small—moderate eruptions the safest areas around Rabaul are considered to be at the Duke York Islands, about 30 km to the east, and at Cape Gazelle, about 25 km to the southeast. Seaborne evacuation to main ports in central New Britain or on the neighbouring islands of New Ireland and Bougainville, or overland evacuation using a southwards-expanded road system, would be necessary in the event of forewarning of a major eruption.

Degassing-induced crystallization of basaltic magma and effects on lava rheology

During the north-east rift eruption of Mauna Loa volcano, Hawaii, on 25 March–14 April 1984 (Fig. 1), microphenocryst contents of erupted lava increased from 0.5 to 30% without concurrent change in either bulk magma composition or eruption temperature (1,140 ± 3 °C). The crystallization of the microphenocrysts is interpreted here as being due to undercooling of the magma 20–30 °C below its liquidas; the undercooling probably resulted from separation and release of volatiles as the magma migrated 12 km from the primary summit reservoir to the eruption site on the north-east rift zone. Such crystallization of magma during an eruption has not been documented previously. The undercooling and crystallization increased the effective viscosity of the magma, leading to decreased eruption rates and stagnation of the lava flow.

An estimate of gas emissions and magmatic gas content from Kilauea volcano

Emission rates of CO 2 have been measured at Kilauea volcano, Hawaii, in the east-rift eruptive plume and CO 2 and SO 2 have been measured in the plume from the noneruptive fumaroles in the summit caldera. These data yield an estimate of the loading of Kilauean eruptive gases to the atmosphere and suggest that such estimates may be inferred directly from measured lava volumes. These data, combined with other chemical and geologic data, suggest that magma arrives at the shallow summit reservoir containing (wt.%) 0.32% H 2O, 0.32% CO 2 and 0.09% S. Magma is rapidly degassed of most of its CO 2 in the shallow reservoir before transport to the eruption site. Because this summit degassing yields a magma saturated and in equilibrium with volatile species and because transport of the magma to the eruption site occurs in a zone no shallower than the summit reservoir, we suggest that eruptive gases from Kilauea characteristically should be one of two types: a ‘primary’ gas from fresh magma derived directly from the mantle and a carbon-depleted gas from magma stored in the summit reservoir.