Christopher J. Nye

Magmas in collision: Rethinking chemical zonation in silicic magmas

The heterogeneity of eruptions attributed to protracted fractionation in subvolcanic magma chambers may instead result from chamber recharge. The case of mafic magma intruding andesitic slush is recognized as giving rise to hybrid effusive eruptions such as at Unzen volcano, Japan, and Soufriere Hills volcano, Montserrat. We propose that zoned andesite-rhyolite explosive eruptions are the complement to mafic recharge, where the resident magma is also andesitic but the intruding magma is silicic.

The reawakening of Alaska's Augustine volcano

Augustine volcano, in south central Alaska, ended a 20-year period of repose on 11 January 2006 with 13 explosive eruptions in 20 days. Explosive activity shifted to a quieter effusion of lava in early February, forming a new summit lava dome and two short, blocky lava flows by late March (Figure 1). The eruption was heralded by eight months of increasing seismicity, deformation, gas emission, and small phreatic eruptions, the latter consisting of explosions of steam and debris caused by heating and expansion of groundwater due to an underlying heat source.

Petrology, geochemistry, and age of the Spurr volcanic complex, eastern Aleutian arc

The Spurr volcanic complex (SVC) is a calc-alkaline, medium-K, sequence of andesites erupted over the last 250000 years by the eastern-most currently active volcanic center in the Aleutian arc. The ancestral Mt. Spurr was built mostly of andesites of uniform composition (58%–60% SiO2), although andesite production was episodically interrupted by the introduction of new batches of more mafic magma. Near the end of the Pleistocene the ancestral Mt. Spurr underwent avalanche caldera formation, resulting in the production of a volcanic debris avalanche with overlying ashflows. Immediately afterward, a large dome (the present Mt. Spurr) formed in the caldera. Both the ash flows and dome are made of acid andesite more silicic (60%–63% SiO2) than any analyzed lavas from the ancestral Mt. Spurr, yet contain olivine and amphibole xenocrysts derived from more mafic magma. The mafic magma (53%–57% SiO2) erupted during and after dome emplacement from a separate vent only 3 km away. Hybrid block-and-ash flows and lavas were also produced. The vents for the silicic and mafic lavas are in the center and in the breach of the 5-by-6-km horseshoe-shaped caldera, respectively, and are less than 4 km apart. Late Holocene eruptive activity is restricted to Crater Peak, and magmas continue to be relatively mafic. SVC lavas are plag ±ol+cpx±opx+mt bearing. All postcaldera units contain small amounts of high-Al2O3, high-alkali amphibole, and proto-Crater Peak and Crater Peak lavas contain abundant pyroxenite and anorthosite clots presumably derived from an immediately preexisting magma chamber. Ranges of mineral chemistries within individual samples are often nearly as large as ranges of mineral chemistries throughout the SVC suite, suggesting that magma mixing is common. Elevated Sr, Pb, and O isotope ratios and trace-element systematics incompatible with fractional crystallization suggest that a significant amount of continental crust from the upper plate has been assimilated by SVC magmas during their evolution.

Geochemistry of the 1989-1990 eruption of redoubt volcano: Part I. Whole-rock major- and trace-element chemistry

Abstract The 1989–1990 eruption of Redoubt Volcano produced medium-K calc-alkaline andesite and dacite of limited compositional range (58.2–63.4% SiO 2 ) and entrained quenched andesitic inclusions (55% SiO 2 ) which bear chemical similarities to the rest of the ejecta. The earliest (December 15) magmas are pumiceous, often compositionally banded, and the majority is relatively mafic ( 2 ). The most silicic magmas of the eruption are the late December to early January domes (up to 63.4% SiO 2 ). Subsequent magmas formed domes and rare pumices which converge on 60% SiO 2 . Chemical variations among ejecta comprise tight, linear, two-component arrays for all elements for which the analytical uncertainty is much less than the compositional range. The two-component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent. Additionally, the felsic endmember cannot be easily related to the mafic endmember by normal high-temperature igneous processes (e.g., the silicic endmember has higher Zr yet lower Hf than the mafic endmember). Also relative enrichments of highly incompatible elements are dramatically different across the arrays. The mixing event must have preceded eruption by a significant, yet unspecified amount of time because groundmass glass compositions are homogeneous for all post-December samples (Swanson et al., 1994-this volume), in spite of the whole-rock chemical diversity. This implies time for additional crystallization after the mixing event. Swanson et al. (1994-this volume) discuss evidence for a potentially different mixing event recorded only in December 15 magmas. Cognate cumulate xenoliths composed of pl+cpx+opx+hb+mt+melt were recovered from January and April deposits. These blocks differ from local batholithic country rock in their low concentrations of incompatible elements (e.g., Rb vs 20–90 ppm, Ba vs 300–2000 ppm) and low SiO 2 ( 60 wt.%). They have Mg, Cr, Ni, Sc, and V contents higher than the andesites, but lower than Redoubt basalts and basaltic andesites. Thus, they may be crystallization products of andesites, but do not represent the cumulate residue of basalt fractionation. The xenoliths were probably derived from a shallow or intermediate crustal chamber.

Geochemistry of the 1989–1990 eruption of redoubt volcano: Part II. Evidence from mineral and glass chemistry

Abstract Early stages (December 1989) of the 1989–1990 eruption of Redoubt Volcano produced two distinct lavas. Both lavas are high-silica andesites with a narrow range of bulk composition (58–64 wt.%) and similar mineralogies (phenocrysts of plagioclase, hornblende, augite, hypersthene and FeTi oxides in a groundmass of the same phases plus glass). The two lavas are distinguished by groundmass glass compositions, one is dacitic and the other rhyolitic. Sharp boundaries between the two glasses in compositionally banded pumices, lack of extensive coronas on hornblende phenocrysts, and seismic data suggest that a magma-mixing event immediately preceeded the eruption in December 1989. Textural disequilibrium in the phenocrysts suggests both magmas (dacitic and rhyolitic glasses) had a mixing history prior to their interaction and eruption in 1989. Sievey plagioclase and overgrowths of magnetite on ilmenite are textures that are at least consistent with magma mixing. The presence of two hornblende compositions (one a high-Al pargasitic hornblende and one a low-Al magnesiohornblende) in both the dacitic and rhyolitic groundmasses indicates a mixing event to yield these two amphibole populations prior to the magma mixing in December 1989. The pargasitic hornblende and the presence of Ca-rich overgrowths in the sievey zones of the plagioclase together indicate at least one component of this earlier mixing event was a mafic magma, either a basalt or a basaltic andesite. Eruptions in 1990 produced only andesite with a rhyolitic groundmass glass. Glass compositions in the 1990 andesite are identical to the rhyolitic glass in the 1989 andesite. Cognate xenoliths from the magma chamber (or conduit) are also found in the 1990 lavas. Magma mixing probably triggered the eruption in 1989. The eruption ended when this rather viscous (rhyolitic groundmass glass, magma capable of entraining sidewall xenoliths) magma stabalized within the conduit.

Postglacial eruption history of Redoubt Volcano, Alaska

Abstract Volcaniclastic deposits preserved in valleys on the flanks of Redoubt Volcano comprise a record of the volcanos postglacial eruption history. The oldest and largest deposit is the Harriet Point debris avalanche, emplaced more than 10,500 yr B.P. This debris avalanche travelled more than 30 km down the Redoubt Creek valley to Cook Inlet. About 3600 yr B.P., a massive slope failure of Redoubt Volcano produced at least two lahars that travelled 30 km down the Crescent River valley (Riehle et al., 1981). A series of smaller eruptions between ca. 3600-1800 yr B.P. generated additional lahars and floods that affected the upper Crescent River valley. A pyroclastic fan on the south flank of Redoubt Volcano probably also formed during this time interval. Sometime between 1000 and 300 yr B.P., hydrothermally altered debris collapsed from the summit edifice, and produced a large lahar that travelled more than 30 km down the Drift River valley. At least 5–6 eruptions in the last 250–300 years have produced lahars and floods large enough to impact the site of the Drift River Terminal on the lower Drift River fan. If the eruptive pattern of the last several centuries continues, another eruption is likely sometime in the next 25–100 years. The Holocene eruptions produced calc-alkaline high-silica andesite and dacite, although quenched andesite and basaltic inclusions record the presence of more mafic magmas. Chemical discontinuities indicate that small, chemically discrete batches of magma fed individual eruptions. Progressive enrichments of highly incompatible trace elements presumably reflect crustal contamination of Holocene magmas.

August 2008 Eruption of Kasatochi Volcano, Aleutian Islands, Alaska—Resetting an Island Landscape

Abstract Kasatochi Island, the subaerial portion of a small volcano in the western Aleutian volcanic arc, erupted on 7–8 August 2008. Pyroclastic flows and surges swept the island repeatedly and buried most of it and the near-shore zone in decimeters to tens of meters of deposits. Several key seabird rookeries in taluses were rendered useless. The eruption lasted for about 24 hours and included two initial explosive pulses and pauses over a 6-hr period that produced ash-poor eruption clouds, a 10-hr period of continuous ash-rich emissions initiated by an explosive pulse and punctuated by two others, and a final 8-hr period of waning ash emissions. The deposits of the eruption include a basal muddy tephra that probably reflects initial eruptions through the shallow crater lake, a sequence of pumiceous and lithic-rich pyroclastic deposits produced by flow, surge, and fall processes during a period of energetic explosive eruption, and a fine-grained upper mantle of pyroclastic-fall and -surge deposits that probably reflects the waning eruptive stage as lake and ground water again gained access to the erupting magma. An eruption with similar impact on the islands environment had not occurred for at least several centuries. Since the 2008 eruption, the volcano has remained quiet other than emission of volcanic gases. Erosion and deposition are rapidly altering slopes and beaches.

Late Pleistocene and Holocene Caldera-Forming Eruptions of Okmok Caldera, Aleutian Islands, Alaska

Okmok volcano, in the central Aleutian arc, Alaska, produced two caldera-forming eruptions within the last ∼12,000 years. This study describes the stratigraphy, composition, and petrology of those two eruptions. Both eruptions initially produced small volumes of felsic magmas, followed by voluminous andesite and basaltic andesite. The Okmok I eruption produced >30 km 3 DRE of material on Umnak Island, and Okmok II ∼15 km 3 . However, a significant proportion of material not accounted for here was deposited into the oceans during both events. The Okmok I pyroclastic flow deposits contain evidence for interaction with snow/ice, particularly along the northern flanks of the caldera. Although both Okmok I and II eruptions involved a phreatomagmatic component, the accumulation of a large volume (>15km 3 ) of volatile-rich, mafic-intermediate magma in the shallow crust may provide the driving force for the catastrophic eruptions. Agglutinate deposits associated with Okmok II indicate energetic lava fountaining simultaneous with caldera-collapse, similar to other descriptions of mafic-intermediate caldera-forming deposits such as in the New Hebrides.

The geomorphology of an Aleutian Volcano following a major eruption; the 7-8 August 2008 eruption of Kasatochi Volcano, Alaska, and its aftermath.

Abstract Analysis of satellite images of Kasatochi volcano and field studies in 2008 and 2009 have shown that within about one year of the 7–8 August 2008 eruption, significant geomorphic changes associated with surface and coastal erosion have occurred. Gully erosion has removed 300,000 to 600,000 m3 of mostly fine-grained volcanic sediment from the flanks of the volcano and much of this has reached the ocean. Sediment yield estimates from two representative drainage basins on the south and west flanks of the volcano, with drainage areas of 0.7 and 0.5 km2, are about 104 m3 km−2 yr−1 and are comparable to sediment yields documented at other volcanoes affected by recent eruptive activity. Estimates of the retreat of coastal cliffs also made from analysis of satellite images indicate average annual erosion rates of 80 to 140 m yr−1. If such rates persist it could take 3–5 years for wave erosion to reach the pre-eruption coastline, which was extended seaward about 400 m by the accumulation of erupted volcanic material. As of 13 September 2009, the date of the most recent satellite image of the island, the total volume of material eroded by wave action was about 106 m3. We did not investigate the distribution of volcanic sediment in the near shore ocean around Kasatochi Island, but it appears that erosion and sediment dispersal in the nearshore environment will be greatest during large storms when the combination of high waves and rainfall runoff are most likely to coincide.

The geyser bight geothermal area, Umnak Island, Alaska

The Geyser Bight geothermal area contains one of the hottest and most extensive areas of thermal springs in Alaska, and is the only site in the state with geysers. Heat for the geothermal system is derived from crustal magma associated with Mt. Recheshnoi volcano. Successive injections of magma have probably heated the crust to near its minimum melting point and produced the only high-SiO[sub 2] rhyolites in the oceanic part of the Aleutian arc. At least two hydrothermal reservoirs are postulated to underlie the geothermal area and have temperatures of 165 and 200 C, respectively, as estimated by geothermometry. Sulfate-water isotope geothermometers suggest a deeper reservoir with a temperature of 265 C. The thermal spring waters have relatively low concentrations of Cl (600 ppm) but are rich in B (60 ppm) and As (6 ppm). The As/Cl ratio is among the highest reported for geothermal waters. 41 refs., 12 figs., 8 tabs.