Tephra Eruptions Research Articles

Stratigraphy and paleontology of Pleistocene submarine volcanic‐sedimentary sequences at the northern Kermadec Islands

Early Pleistocene marine volcanic/sedimentary sequences on Raoul Island and adjacent islets of the northern Kermadec Islands, southwest Pacific, are included in the Herald Group (new) and constituent Boat Cove Formation, Chanter Formation (new) and Dayrell Formation (new). They are the oldest known shallow‐marine deposits in the Kermadec Islands region. The following history is inferred from their stratigraphy, structural relationships and paleontology. (1) Development of a large stratovolcano formed of basaltic andesite lava flows, hyaloclastite breccia and sandstone and epiclastic deposits (Boat Cove Formation). Shallow marine biotas colonised lava, boulder and sandy gravel substrata on the flanks of the volcano, and hermatypic coral patchreefs formed locally. (2) Removal of parts of the upper flanks of the volcano by marine erosion, caldera formation or flank collapse, followed by further eruptions of submarine basaltic andesite lava and tephra from parasitic vents (Chanter Formation). (3) Local cessation of volcanism followed by subsidence of the volcano, and accumulation of an upwards deepening coral patchreef and epiclastic volcaniclastic to detrital bioclastic and coralline algal‐dominated sedimentary sequence (Dayrell Formation).

Fine resolution palynology of Erua Swamp, Tongaririo, New Zealand, since the Taupo Tephra eruption of c. 1718 B.P.

Fine resolution pollen analysis of a core from Erua Swamp shows that prior to the Taupo eruption of c. 1718 B.P., the site bore a dryland vegetation type on river flats. Patchy Nothofagusl Phyllocladus forest on the flats was destroyed by the eruption and replaced by Gleichenia‐restionad swamp vegetation with abundant Halocarpus. Regional forest during the period from after the eruption to c. 650–560 B.P. was mixed podocarp, dominated by Dacrydium cupressinum and Prumnopitys taxifolia. A period of widespread and sustained anthropogenic destruction by fire of forest commenced c. 650–560 B.P.

Fine resolution palynology of Gibsons’ Swamp, central North Island, New Zealand, since c. 13 000 B.P.

Fine resolution pollen analysis shows that the late‐glacial (c. 13 000–10 700 B.P.) vegetation of the Ohakune‐Horopito area was dominated by Prumnopitys taxifolia, indicating a cooler and probably drier climate than the present. Around 10 700 B.P., Dacrydium cupressinum replaced Prumnopitys taxifolia as the forest dominant, and tree ferns and hardwood trees expanded, suggesting a change to wanner or wetter conditions. Around 5800–6300 B.P., Dacrydium cupressinum and tree ferns declined, Prumnopitys taxifolia regained some of its former dominance, and hardwood species continued to expand, suggesting a change to more variable conditions. Immediately following the Taupo Tephra eruption of 1718 B.P., Libocedrus bidwillii expanded at Gibsons’ Swamp. The eruption may have facilitated a regional expansion of this species which was apparently already underway as a result of a climate change to stormier and cooler conditions prior to the eruption. Extensive logging for podo‐carps in Ohakune‐Horopito after AD 1850 resulted in an increase in the abundance of Weinmannia racemosa.

Changes in Whangaehu river lahar characteristics during the 1995 eruption sequence, Ruapehu volcano, New Zealand

During the 1995 Ruapehu eruptive sequence multiple lahars occurred in the Whangaehu river, which drains Ruapehu's Crater Lake. During the earlier phreatic and phreatomagmatic eruptions, lahars were generated by expulsion of waters from the Lake, but once the lake had emptied, lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra. Four types of lahars occurred during the eruptive sequence: (1) Initial snow-slurry lahars, composed of granular snow and ice incorporated by eruptively expelled Crater Lake waters which left behind frozen deposits with 2.5–20% clastic sediment. (2) Large dilute lahars, generated as the volumes of ejected lake water increased and removed much of the readily available snow. At least one third of the pre-eruption Crater Lake volume was expelled during one day producing the largest lahars of the series. These lahars were hyperconcentrated flows for up to 84 km from source, leaving extensive deposits along the channel margins. (3) Concentrated lahars; smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined. These lahars were able to maintain high sediment concentrations, measured at 46–52% by volume suspended sediment at 42 km from source. Their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins. (4) Remobilised tephra lahars, generated following the two largest tephra eruptions of the sequence. Seasonal snowpack was covered by water-saturated tephra. Warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra, producing several lahars in catchments draining eastern Ruapehu.

Impact of Ruapehu ash fall on soil and pasture nutrient status 1. October 1995 eruptions

Eruptions from Ruapehu volcano on 11 and 14 October 1995 covered large portions of the central and eastern North Island, New Zealand, with tephra, including c. 17 000 km2 of land in primary production. The thickness and distribution of the tephra was mapped and it was chemically characterised in order to predict its effects on soil fertility. Tephra from the two eruptions contained 3.0 and 0.7 % by weight sulphur (S), respectively, 67–55 % of which was in an elemental form (S0), with the remainder in the form of soluble sulphate. The S0 in the tephras was derived from the former, hydrothermal Crater Lake system in the active vent of Ruapehu. Sulphate was derived from Crater Lake water and gases erupted with the tephra. Field and glasshouse investigations showed that the tephra significantly raised soil sulphate levels. The initial elemental S component was very fine grained (65–99% < 125 μm diameter) and oxidised rapidly causing depression of soil pH. In a transect of northern Rangitikei soil and pasture samples, S concentrations increased with increasing tephra thickness and remained above normal levels for at least 8 months after tephra fall. Pasture Se concentrations also rose briefly but returned to more normal levels for these soils after 5 months. Other nutrient elements were deposited in useful amounts only under heavy tephra falls. Toxic elements were minor components of the tephra sampled. As Crater Lake reforms following eruption episodes, future tephra eruptions from Ruapehu can be expected to have similar agronomic effects.

Volcanic history of Macauley Island, Kermadec Ridge, New Zealand

Abstract Macauley Island (3 km2) is the tiny emergent part of the large submarine Macauley volcano (c. 380 km2at the 900 m isobath) on the Kermadec Ridge. It is composed mainly of arc tholeiite basalts, with a single interbedded dacite tephra. The oldest rocks seen are subaerial aa flows (North Cliff Lavas), overlain by basaltic tephra deposits (Boulder Beach Formation). Continued eruption of thin basalt flows (Annexation Lavas) built a large shield volcano, at least 150 m above sea level, with a crater in the vicinity of what is now Mt Haszard. A large eruption of dacite tephra (Sandy Bay Tephra) caused collapse of the flanks of the submarine volcano, to form a caldera immediately northwest of the present island. Renewed basaltic volcanism produced scoria cones, flows, and tephra (Haszard Formation), and the final stage of this eruptive phase was associated with the collapse of the northwest edge of the island into the caldera. The freshness of the exposed rocks on Macauley Island indicates a late Quater...

Discharge rates of fallout tephra and frequency of plinian eruptions during the last 400,000 years in the southern Northeast Japan Arc

Tephrochronology is one of the most useful tools for reconstructing the record of past explosive eruptions of the last several hundred thousand years. In order to estimate discharge rates of fallout tephra and frequency of plinian eruptions (including sub-plinian eruptions) in the south Northeast Japan Arc, tephra eruptions have been identified by cataloging all known plinian and sub-plinian deposits. One hundred and fifty-two plinian and sub-plinian deposits derived from twelve polygenetic volcanoes, from Asama volcano in the south to Adatara volcano in the north, are identified. The volume of the largest plinian deposit is 25.9 km 3 (DRE: 6.2 km 3 ), and 41% of the deposits have volumes >1 km 3 . Most volcanoes have one or two periods characterized by repeated plinian eruptions, named in this study as a ‘plinian stage’. The lengths of plinian stages for these volcanoes are variable, i.e. from 20,000 to 270,000 years. The frequency of plinian eruptions during the plinian stage varies from 0.04 times per 1000 years to 1.3 times per 1000 years. In contrast, discharge rates do not vary so greatly, and average 0.78-0.04 (DRE: 0.19-0.01) km 3 per 1000 years.

A 6500-year-old history of tephra deposition recorded in the sediments of Lake Tutira, eastern North Island, New Zealand

Abstract Lake Tutira is a small lake formed as a result of a landslide about 6500 radiocarbon years BP. It is situated 100 km east, and downwind, of the central North Island volcanoes and has provided a trap for tephras which have erupted since the mid-Holocene. Detailed studies of cores from the lake and adjacent swamps have revealed 23 tephra layers, which represent tephra deposition into Lake Tutira every 300 years on average. Sixteen of these layers are macroscopic and seven are microscopic, occurring as concentrations of volcanic glass. The tephra layers mark rhyolitic and andesitic tephra eruptions from Taupo, Okataina, Egmont, and Tongariro volcanic centres. They were identified using their ferromagnesian mineralogy, major element compositions of glass shards, and their stratigraphic position, particularly with respect to radiocarbon dates. The Lake Tutira tephra record is the most detailed record yet found in eastern North Island, and the detection of microscopic tephra layers indicates the value of lake sediments and swamps as traps for distal tephras.

Seismic evolution of the 1989–1990 eruption sequence of Redoubt Volcano, Alaska

Redoubt Volcano in south-central Alaska erupted between December 1989 and June 1990 in a sequence of events characterized by large tephra eruptions, pyroclastic flows, lahars and debris flows, and episodes of dome growth. The eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater. Notable features of the eruption seismicity include : (1) small long-period events beginning in September 1989 which increased slowly in number during November and early December; (2) an intense swarm of long-period events which preceded the initial eruptions on December 14 by 23 hours; (3) shallow swarms (0 to 3 km) of volcano-tectonic events following each eruption on December 15; (4) a persistent cluster of deep (6 to 10 km) volcano-tectonic earthquakes initiated by the eruptions on December 15, which continued throughout and beyond the eruption; (5) an intense swarm of long-period events which preceded the eruptions on January 2; and (6) nine additional intervals of increased long-period seismicity each of which preceded a tephra eruption. Hypocenters of volcano-tectonic earthquakes suggest the presence of a magma source region at 6–10 km depth. Earthquakes at these depths were initiated by the tephra eruptions on December 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths. The locations and time-history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989–1990 eruption. This source region appears to be connected to the surface by a narrow pipe-like conduit as inferred from the hypocenters of volcano-tectonic earthquakes. Concentrations of shallow volcano-tectonic earthquakes followed each of the tephra eruptions on December 15; these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths. This shallow zone was the source area of the majority of long-period seismicity through the remainder of the eruption. The long-period seismicity likely reflects the pressurization of the shallow portions of the magmatic system.

Statistical forecasting of repetitious dome failures during the waning eruption of Redoubt Volcano, Alaska, February–April 1990

The waning phase of the 1989–1990 eruption of Redoubt Volcano in the Cook Inlet region of south-central Alaska comprised a quasi-regular pattern of repetitious dome growth and destruction that lasted from February 15 to late April 1990. The dome failures produced ash plumes hazardous to airline traffic. In response to this hazard, the Alaska Volcano Observatory sought to forecast these ash-producing events using two approaches. One approach built on early successes in issuing warnings before major eruptions on December 14, 1989 and January 2, 1990. These warnings were based largely on changes in seismic activity related to the occurrence of precursory swarms of long-period seismic events. The search for precursory swarms of long-period seismicity was continued through the waning phase of the eruption and led to warnings before tephra eruptions on March 23 and April 6. The observed regularity of dome failures after February 15 suggested that a statistical forecasting method based on a constant-rate failure model might also be successful. The first statistical forecast was issued on March 16 after seven events had occurred, at an average interval of 4.5 days. At this time, the interval between dome failures abruptly lengthened. Accordingly, the forecast was unsuccessful and further forecasting was suspended until the regularity of subsequent failures could be confirmed. Statistical forecasting resumed on April 12, after four dome failure episodes separated by an average of 7.8 days. One dome failure (April 15) was successfully forecast using a 70% confidence window, and a second event (April 21) was narrowly missed before the end of the activity. The cessation of dome failures after April 21 resulted in a concluding false alarm. Although forecasting success during the eruption was limited, retrospective analysis shows that early and consistent application of the statistical method using a constant-rate failure model and a 90% confidence window could have yielded five successful forecasts and two false alarms; no events would have been missed. On closer examination, the intervals between successive dome failures are not uniform but tend to increase with time. This increase attests to the continuous, slowly decreasing supply of magma to the surface vent during the waning phase of the eruption. The domes formed in a precarious position in a breach in the summit crater rim where they were susceptible to gravitational collapse. The instability of the February 15–April 21 domes relative to the earlier domes is attributed to reaming the lip of the vent by a laterally directed explosion during the major dome-destroying eruption of February 15, a process which would leave a less secure foundation for subsequent domes.

Precursory swarms of long-period events at Redoubt Volcano (1989–1990), Alaska: Their origin and use as a forecasting tool

During the eruption of Redoubt Volcano from December 1989 through April 1990, the Alaska Volcano Observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long-period (LP) seismic events (dominant period of about 0.5 s). The initial eruption on December 14 occurred after 23 years of quiescence and was heralded by a 23-hour swarm of LP events that ended abruptly with the eruption. After a series of vent-clearing explosions over the next few days, dome growth began on December 21. Another swarm, with LP events similar to those of the first, began on the 26th and ended in a major tephra eruption on January 2. Eruptions continued over the next two weeks and then ceased until February 15, when a large eruption initiated a long phase of repetitive dome-building and dome-destroying episodes that continued into April. Warnings were issued before the major events on December 14 and January 2, but as the eruptive sequence continued after January 2, the energy of the swarms decreased and forecasting became more difficult. A significant but less intense swarm preceded the February 15 eruption, which was not forecast. This eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until March 4, when the first of three new stations was installed within 3 km of the active vent. From March 4 to the end of the sequence on April 21, there were eight eruptions, six of which were preceded by detectable swarms of LP events. Although weak, these swarms provided the basis for warnings issued before the eruptions on March 23 and April 6. The initial swarm on December 13 had the following features: (1) short duration (23 hours); (2) a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm, followed by a decline of activity during the 5 hours preceding the eruption; (3) a magnitude range from −0.4 to 1.6; (4) nearly identical LP signatures with a dominant period near 0.5 s; (5) dilatational first motions everywhere; and (6) a stationary source location at a depth of 1.4 km beneath the crater. This occurrence of long-period events suggests a model involving the interaction of magma with groundwater in which magmatic gases, steam and water drive a fixed conduit at a stationary point throughout the swarm. The initiation of that sequence of events is analogous to the failure of a pressure-relief valve connecting a lower, supercharged magma-dominated reservoir to a shallow hydrothermal system. A three-dimensional model of a vibrating fluid-filled crack recently developed by Chouet is found to be compatible with the seismic data and yields the following parameters for the LP source: crack length, 280–380 m; crack width, 140–190 m; crack thickness, 0.05–0.20 m; crack stiffness, 100–200; sound speed of fluid, 0.8–1.3 km/s; compressional-wave speed of rock, 5.1 km/s; density ratio of fluid to rock, ≈0.4; and ratio of bulk modulus of fluid to rigidity of rock, 0.03–0.07. The fluid-filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0.4 to 40 bar. Such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack.

Impact of hydro‐isostatic holocene sea‐level change on the geologic context of Island archaeological sites, Northern Ha'apai group, Kingdom of Tonga

AbstractArchaeological sites in the northern Ha'apai Group of central Tonga occur on small islands within the uplifted forearc belt of the Tonga‐Kermadec arc‐trench system. The present inland positions of occupation sites that probably once occupied coastal settings imply significant expansion of some island shorelines during late Holocene time (ca. 3250 B.P. to present). Geologic processes leading potentially to enlargement of the islands include continuing forearc uplift, eustatic or glacio‐hydro‐isostatic fall in sea level following a mid‐Holocene highstand, and progressive accretion of beach ridges to island coasts, with or without changes in relative sea level. Radiometric dates for uplifted coral terraces in Tonga indicate that forearc uplift has been negligible during Holocene time. By contrast, theoretical considerations, regional analysis of shoreline indicators throughout the South Pacific, and limited empirical data from Tonga itself all imply that regional sea level has declined locally by 1–2 m since a mid‐Holocene highstand (ca. 6000‐3000 B.P.), which was a hydro‐isostatic response to transfer of water mass from Pleistocene ice caps to the ocean basins. Emergence of originally coastal sites is thus expected since initial settlement of the islands by Lapita peoples. Accretionary coastal flats composed of multiple beach ridges are 250–500 m wide on favorable leeward shores and the flanks of sand cays, but some presently unknown proportion of this incremental island growth may have occurred prior to the post‐mid‐Holocene decline in relative sea level. Ash falls from tephra eruptions at Tongan volcanoes also modified island environments through Quaternary time. Evidence for significant change in the configuration and morphology of islands in Ha'apai during the period of human settlement highlights the need for systematic interdisciplinary archaeological and geological research in the study of Pacific prehistory. © 1994 John Wiley &amp; Sons, Inc.

Eruption-Controlled Epiclastic Sedimentation in a Devonian Trench-Slope Basin: Evidence from Sandstone Petrofacies, Klamath Mountains, California

ABSTRACT The Devonian Gazelle Formation comprises sparsely fossiliferous shale, siltstone, siliceous mudstone, volcaniclastic sandstone, and conglomerate whose cumulative thickness is at least 1.25 km. Two end-member sand compositions combined locally to form a third petrofacies. The volcaniclastic petrofacies was derived from an undissected magmatic arc, whereas the chert-mudrock petrofacies reflects derivation from melangee of the underlying accretionary prism. The stratigraphically restricted mixed petrofacies is compositionally discrete and records thorough mixing of these two compositional end members. Data from sandstone petrography are consistent with independent geologic evidence indicating that the Gazelle Formation represents the fill of a Devonian trench-slope basin that for ed above an east-dipping subduction zone. Our interpretation of the Balaklala Rhyolite as the source of the volcaniclastic petrofacies requires proximity of the Yreka and Eastern Klamath terranes during the Early Devonian, and indicates that the eastern Klamath province has behaved as a single crustal block (sensu lato) ever since. Field study and petrographic analysis indicate that the delivery of coarse terrigenous detritus to the basin was controlled principally by episodic eruptions of rhyolitic tephra in the arc source and also by one discernible episode of change in relative sea level. This study indicates that caution must be used when invoking changes in relative sea level to explain the stratigraphic distribution of epiclastic turbidites in island-arc settings. Identification of compositionally and stratigraphically discrete mixed petrofacies in other ancient orogens may permit similar inferences regarding controls on stratigraphic development that have operated on poorly fossiliferous, immature convergent margins.

The Platanar-Aguas Zarcas volcanic centers, Costa Rica: spatial-temporal association of Quaternary calc-alkaline and alkaline volcanism

The Platanar volcanic center is dominated by a calc-alkaline, basalt-andesite-dacite-rhyolite magma series with unusual LREE enrichment. Adjacent and overlapping the calc-alkaline rocks are the most alkaline basalts found along the volcanic front of Central America. These basalts are mafic, LIL- and LREE-enriched transitional to alkaline basalts. Several are found on the north flank of Platanar in the Aguas Zarcas region, where there are nine cinder cones and a few isolated flows. However, they are also found in isolated lava outcrops at least as far south as Porvenir volcano along the volcanic front. The addition of mafic alkaline magmas with high La/Yb and low Ba/La into the Platanar magma chamber or chambers may contribute to the LREE-enriched character of the Platanar basaltic andesites and andesites. At Platanar the field and geochemical evidence suggest mixing between calc-alkaline and alkaline magmas, a process that has probably occurred throughout the development of the Cordillera Central of Costa Rica. The presence of negative Ce anomalies in several of the calc-alkaline lavas also make the Platanar complex very unusual compared to the rest of the Central American volcanic front. In the center of the Platanar complex is the Chocosuela caldera, an apparent remnant of an avalanche caldera created by the collapse in the Middle Pleistocene of an ancestral stratovolcano toward the NNW in a directed blast-type eruption. Rhyolite is present as pumice lapilli in pyroclastic flow deposits outside the caldera rim. Whole lapilli analyses span the daciterhyolite range. The previous eruption of high silica tephra as pyroclastic flows, the current long dormant period and the repeated occurrence of earthquake swarms on the flanks of the Platanar complex make it a candidate for volcanic hazard mapping, detailed geological mapping and emergency planning.

The pre-Taupo eruption (c.AD 130) forest of the Benneydale-Pureora district, central North Island, New Zealand

Abstract The composition and structure of three small areas of west Taupo forest, blasted down and buried during the c. AD 130 Taupo tephra eruption, are revealed by analysis of plant macrofossils, including leaves, seeds and wood. Data from these three sites and from the recently described Pureora buried forest were then used to interpret the pre-eruption vegetation along a 20 kilometre transect, from Pureora westward to Benneydale. All the stands were on flat or undulating terrain and dominated by podocarps, ranging from rimu (Dacrydium cupressinum)-tanekaha. (Phyllocladus trichomanoides) at Pureora to rimu at Benneydale. Broadleaved trees increased westward; on lower hillslopes in the Benneydale buried forest, northern rata (Metrosideros robusta) was an important canopy component. An amelioration of climate with decreasing altitude westward was indicated by a gradual change from prominence of horopito (Pseudowintera colorata) at Pureora to P. axillaris at Benneydale, and the presence of rewarewa (Knigh...

Age, Extent and Climatic Significance of the c. 3400 BP Aniakchak Tephra, Western Alaska, USA

Tephra deposits at Whitefish Lake and Cape Espenberg on the northernmost Seward Peninsula of western Alaska were derived from the Aniakchak Caldera on the Alaska Peninsula more than 1500 km to the south. Radiocarbon dates on the climactic, caldera-forming Aniakchak eruption and on proximal and distal tephra indicate the Aniakchak tephra was deposited about 3435 ± 40 BP (3614-3815 cal. BP), and forms an isochronous marker horizon across much of western Alaska. The Aniakchak tephra eruption and several other targe eruptions during the seventeenth century BC may have had widespread effects on climate.

Correlation of the Holocene Jarvis Creek, Tangle Lakes, Cantwell, and Hayes Tephras in South-Central and Central Alaska

The geochemistry, petrography, and distribution of the Jarvis Creek Ash (Péwé, 1965, 1975a) indicate that this tephra from the lower Delta River area of central Alaska is correlative with vol volcanic ash from sites in south-central Alaska near Tangle Lakes (upper Delta River area) and the Cantwell ash from Hayes volcano found in the upper Nenana River area (Riehle et al., 1990). Volcanic glass compositions of distal Jarvis Creek and Tangle Lakes tephra samples are compositionally restricted, while several discrete glass populations are present in some samples are compositionally collected nearer Hayes volcano. These correlations extend the known distribution of Hayes volcano tephras across the Alaska Range and into central Alaska, a distance of more than 650 km. New geochronologic data for the Jarvis Creek Ash suggest it was deposited ca. 3660 ± 125 yr B.P., consistent with previous age estimates of tephra eruptions at the Hayes volcano. The name “Jarvis Creek Ash” has well-established priority with respect to “Cantwell ash” or other local names for this tephra layer from the Hayes volcano.

The September 1988 intracaldera avalanche and eruption at Fernandina volcano, Galapagos Islands

During 14–16 September 1988, a large intracaldera avalanche and an eruption of basaltic tephra and lava at Fernandina volcano, Galapagos, produced the most profound changes within the caldera since its collapse in 1968. A swarm of eight earthquakes (m b 4.7–5.5) occurred in a 14 h period on 24 February 1988 at Fernandina, and two more earthquakes of this size followed on 15 April and 20 May, respectively. On 14 September 1988, another earthquake (m b 4.6) preceded a complex series of events. A debris avalanche was generated by the failure of a fault-bounded segment of the east caldera wall, approximately 2 km long and 300 m wide. The avalanche deposit is up to 250 m thick and has an approximate volume of 0.9 km3. The avalanche rapidly displaced a preexisting lake from the southeast end of the caldera floor to the northwest end, where the water washed up against the lower part of the caldera wall, then gradually seeped into the avalanche deposit and was completely gone by mid-January 1989. An eruption began in the caldera within about 1–2 h of the earthquake, producing a vigorous tephra plume for about 12 h, then lava flows during the next two days. The eruption ended late on 16 September. Most of the eruptive activity was from vents on the caldera floor near the base of the new avalanche scar. Unequivocal relative timing of events is difficult to determine, but seismic records suggest that the avalanche may have occurred 1.6 h after the earthquake, and field relations show that lava was clearly erupted after the avalanche was emplaced. The most likely sequence of events seems to be that the 1988 feeder dike intruded upward into the east caldera wall, dislocated the unstable wall block, and triggered the avalanche. The avalanche immediately exposed the newly emplaced dike and initiated the eruption. The exact cause of the earthquakes is unknown.

Tephra studies in New Zealand: an historical review

The development of tephra studies in New Zealand may be divided into four main periods: Period 1, late 19th century to late 1920s; Period 2, late 1920s to early 1950s; Period 3, early 1950s to 1973; Period 4, 1973 to late 1980s. The important events and advances that characterise each of these periods, and their causes and influences, are described with reference to contemporary scientists and their publications. Period 1: determination by dendrochronol ogy of first numerical of a prehistorical eruptive (Burrell Lapilli); first isopach map (Tarawera Tephra). Period 2: first tephra mapping in central North Island (for soil survey). Period 3: first use of I4C dating; establishment of late Quaternary tephrostratigraphic framework by 'hand-over-hand' mapping in central North Island and Taranaki; initial development of tephra 'fingerprinting' using laboratory methods; application of tephrochronol ogy to many disciplines. Period 4: revision and refinement of proximal stratigraphy, particularly in central TVZ calderas and on Mayor Island; extension of tephra mapping to distal regions, on and offshore, and to older deposits; advances in tephra correlation and dating methods; new tephrochronological applications; revolutionary studies of pyroclastic deposits for determining nature and effects of eruptions (physical volcanology and petrology); renewed awareness of volcanic hazards associated with tephra eruptions. The advances relate to indigenous, external, and 'individualistic' factors. They generally parallelled overseas trends but in some topics preceded or lagged behind them. Tephra studies, or tephrology, may be regarded as having come of age early in the 1980s, about 100 years after the first tephrostratigraphic studies in New Zealand.

Effects of forest land management on erosion and revegetation after the eruption of Mount St. Helens

AbstractThe 1980 eruption of Mount St. Helens covered soils with a tephra blanket and killed the forest tree cover in a 550 km2 area. After the eruption, rates of sheetwash and rill erosion, and plant cover were measured on tephra‐covered hillslopes which had been subject to three land‐management practices: grass seeding; scarification, and salvage logging. On rapidly‐eroding hillslopes subject to grass seeding, limited plant covers were established only after erosion had declined sharply. Logging of trees downed by the eruption and scarification of previously logged surfaces slowed erosion, although the effect was small because erosion rates had already slowed substantially by the time these two practices were implemented. The factors controlling erosion, revegetation, and their relative timing at Mount St. Helens are similar to those following explosive volcanic eruptions elsewhere, suggesting that grass seeding is not likely to be effective at slowing erosion following most tephra eruptions, and that early mechanical disturbance could be an effective erosion‐control measure. The results also indicate that even without deliberate conservation measures, processes which mechanically disturb a surface layer of low hydraulic conductivity (such as frost‐action or trampling) can radically reduce runoff and erosion before revegetation has an important effect.