Explosive activity of the South Italian volcanoes during the past 80,000 years as determined by marine tephrochronology

The tectonic position of Aleutian arc volcanic centers and their magmatic differentiation trends (calc-alkaline or tholeiitic) appear to correlate. From 160°W to 175°E, the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates. The tholeiitic volcanoes are large, primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow, closed system differentiation. The calc-alkaline volcanoes are smaller, more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult. Differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers. The chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma, probably derived from mantle peridotite. Conditions for the derivation of the two trends through crystal fractionation support the proposed model. The tholeiitic magmas show characteristics (i.e., no hydrous phenocrysts, Fe enrichment trend, parallel REE patterns, vitrophyric and esites and dacites) consistent with low-pressure, high-temperature crystallization in large shallow magma chambers. The calc-alkaline magmas show characteristics (i.e., some hydrous phenocrysts, no Fe enrichment trend, nonparallel rare earth patterns, porphyritic lavas) consistent with higher pressure and lower temperature of crystallization than the tholeiitic series. Tertiary plutons also show both calc-alkaline and tholeiitic trends and appear to be chemically similar to the Quaternary volcanoes. The larger plutons are calc-alkaline and probably represent the extensive (1002 km ) magma chambers of the small calc-alkaline volcanoes. The small, shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series.

Tectonic controls on tholeiitic and calc-alkaline magmatism in the Aleutian Arc

The three-day eruption at Novarupta in 1912 consisted of three discrete episodes. Episode I began with plinian dispersal of rhyolitic fallout (Layer A) and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers. The plinian column was sustained throughout most of the interval of ash flow generation, in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite. Accordingly, plinian Layer B, which fell in unbroken continuity with purely rhyolitic Layer A, is zoned from >99% to ∼15% rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in Mageik Creek and the Valley of Ten Thousand Smokes (VTTS). Only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent, indicating that ignimbrite emplacement barely outlasted the plinian phase. On near-vent ridges, the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley-filling equivalents but exhibit evidence for turbulent deposition and recurrent scour. Episode II began after a break of a few hours and was dominated by plinian dispersal of dacitic Layers C and D, punctuated by minor proximal intraplinian flows and surges. After another break, dacitic Layers F and G resulted from a third plinian episode (III); intercalated with these proximally are thin intraplinian ignimbrites and several andesite-rich fall/flow layers. Both CD and FG were ejected from an inner vent <400 m wide (nested within that of Episode I), into which the rhyolitic lava dome (Novarupta) was still later extruded. Two finer-grained ash layers settled from composite regional dust clouds: Layer E, which accumulated during the D-F hiatus, includes a contribution from small contemporaneous ash flows; and Layer H settled after the main eruption was over. Both are distinct layers in and near the VTTS, but distally they merge with CD and FG, respectively; they are largely dacitic but include rhyolitic shards that erupted during Episode I and were kept aloft by atmospheric turbulence. Published models yield column heights of 23–26 km for A, 22–25 km for CD, and 17–23 km for FG; and peak mass eruption rates of 0.7–1x108, 0.6–2x108, and 0.2–0.4x108 kg s-1, respectively. Fallout volumes, adjusted to reflect calculated redistribution of rhyolitic glass shards, are 8.8 km3, 4.8 km3, and 3.4 km3 for Episodes I, II, and III. Microprobe analyses of glass show that as much as 0.4 km3 of rhyolitic glass shards from eruptive Episode I fell with CDE and 1.1 km3 with FGH. Most of the rhyolitic ash in the dacitic fallout layers fell far downwind (SE of the vent); near the rhyolite-dominated ignimbrite, however, nearly all of Layers E and H are dacitic, showing that the downwind rhyolitic ash is of ‘co-plinian’ rather than co-ignimbrite origin.

The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska.

Magma-mixing and the dynamics of withdrawal from stratified reservoirs

Stratigraphic relationships and tephrochronologic information for volcanic ash beds (4.0–0.1 m.y.) of the Western Interior allow the recognition of at least 68 different ash falls. The average reoccurrence rate, based on the succession of the isotopically dated ash beds, is near one eruption every 57,000 years. The ash beds were grouped into two chemical types, rhyolitic and dacitic; the main property that distinguishes dacitic from rhyolitic ashes is the larger Ca and Mg and smaller K content in dacitic ashes. Dacitic volcanic ashes were defined to have more than 0.55 wt % Ca. Rhyolitic ashes were subdivided further into two types, W- and G-type, based on the presence of biotite in the W-type and its absence in G-type. W-type rhyolitic ashes are chalky white, have less than 0.55 wt % Fe, have colorless glass shards of pumiceous habit. G-type rhyolitic ashes are light to medium gray, have from 0.55 to 2.0 wt % Fe in the glass shards, lack biotite, have colorless, platy, bubble wall, and bubble junction glass shards. Dacitic ashes are light gray to grayish brown, have more than 0.55 wt % Fe and Ca, and have a complex mixture of colorless to light brown shards, including pumiceous, fibrous, chunky, and bubble wall and bubble junction types. W-type rhyolitic ashes possibly formed from magmas that were cooler (695° to 830°C) and thus more viscous than the hotter (870° to 1000°C) magma progenitors of G-type ashes based on Fe-Ti oxide mineral compositions. Primary microphenocrysts in the ash beds comprise assemblages of the following: quartz, sanidine, plagioclase, biotite, amphibole, clinopyroxene, orthopyroxene, fayalite, zircon, apatite, allanite, chevkinite, sphene, magnetite, and ilmentite. Fayalite and chevkinite only have been found in G-type; biotite and sphene only have been found in W-type. Rhyolitic ashes are highly evolved and contain 74–79 wt % SiO2; dacitic ashes vary from 67–77 wt % SiO2. Al ranges from 10.5 to 14.5 wt % Al2O3, Mg ranges from 0.06 to 1.0 wt % MgO, and Fe (as Fe3O4) ranges from 0.50 to 4.1 wt %. Dacitic ash beds have the most (0.80–3.8 wt % as Fe3O4) and rhyolitic ashes have the least (0.50–1.9 wt % as Fe3O4). Ca varies from an unusually low amount (0.25 wt % CaO) in a few rhyolitic ashes to as much as 2.2% in some dacitic ashes. Rhyolitic glasses show larger Eu deficiencies compared to dacitic glasses on rare-earth element diagrams. W-type rhyolitic glasses have moderately large La to Sm ratios and have La contents from about 30 to 100 times chondrite values; G-type rhyolitic glasses also have moderately large La to Sm ratios and contain somewhat more La than do the W-type rhyolitic ashes, generally from 100 to 300 times chondrites amounts.

K.F. Scheidegger

Papers

Compositions of deep-sea ash layers derived from north pacific volcanic arcs: variations in time and space

Compositional heterogeneity of distal tephra deposits from the 1912 eruption of Novarupta, Alaska