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Makushin reported activity

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EVENT SPECIFIC INFORMATION

Event Name : Makushin Driftwood Pumice

Start: 7740 (± 60 Years) Years BP C-14 (raw)
Stop: 5070 (± 160 Years) Years BP C-14 (raw)

Tephrafall: BibCard BibCard
Eruption Type:Explosive
Eruption Product: dacite BibCard
MaxVEI: 4 BibCard
ChemYes
Otherfelsic

Description: From Bean (1999): "It is found in thicknesses of over a meter in Driftwood and Waterfall Valleys but is generally 15-20 cm thick in the Lava Ramp area and thins to approximately 2 cm thick in the Dutch Harbor area (Fig 5.2 [in original text]). It is easily recognized as a yellow, blocky pumice containing up to ~10% black glassy lithics. Proximally this unit may be underlain by a thin, fine, yellow ash (YA). Dates from under this deposit indicate that it is younger than ~7740 14C yr B.P. An age date of 5070 14C yr B.P. places an upper limit on its age although its stratigraphic position suggests that it is much closer in age to the lower limiting ages. Thick soils and tephras under this deposit however preclude the possibility that it was deposited shortly following the Holocene caldera forming eruptions. This deposit is distinct in the field and has a unique geochemistry; analyses are reported in Appendices 1 and 2 [of original text]."

"Several cm of gray ashy silt commonly separate the CFE2 from a thin 2 cm yellow pumice unit containing small glassy lithics. The pumice unit can be found in most distal sites and has been geochemically correlated with proximal deposits of the Driftwood Pumice. The pumice unit is overlain by several tens of cm of black and gray fine grained tephras and sandy to silty organic soils. A thick (.5 m) vegetative mat also containing black to gray tephras generally caps these deposits."

From Lerner (2010): "The Driftwood Pumice (Fig. 1 [in original text]) contains multiple types of juvenile and accidental ejecta. Juvenile pumice is tan, highly vesicular, and low-density. Juvenile scoria is vesicular with grayish-black glass, and is denser and more crystal-rich than the pumice. Accidental ejecta included vitric and a variety of non-vitric lithics."

"The bottom of the Driftwood Pumice deposit (PL) is a relatively thin horizon of vitric lapillistone (76 wt% lapilli, 24 wt% ash) consisting of well-sorted, light brown-beige pumice fragments (avg. dia. ~4 mm) and abundant smaller lithics (22 wt%). The accidental lithic ejecta consist almost entirely of vitric material. There is sharp inverse grading at the very base of the layer, as well as at the top where PL grades into PM."

"The main body of the pumice deposit (PM) consists of a much thicker horizon of large, light brown-beige pumice with sparse, smaller lithic inclusions (5%) that are mostly vitric (60%). The horizon is a moderately well-sorted, generally ungraded, vitric lapillistone (92% lapilli, 8% ash), with an average grain size of ~11 mm. Pumice bombs are commonly larger than 9 cm across."

"PM abruptly transitions to a thinner, darker yellow-brown upper horizon (PU) of vitric lapillistone (75% lapilli, 25% ash) consisting of a mixture of light brown pumice, dense, dark-gray to black scoria, and abundant, non-vitric lithics. The PU horizon is very poorly sorted, with great variability in grain size (6-14 mm) between sample locations. Scoria (~12 mm) is larger than pumice (~5 mm) and lithic fragments (~3 mm), with scoriaceous bombs reaching up to 10 cm. The upper contact of PU is poorly defined, with the capping ash (Mcap) filling gaps between the uppermost clasts."

"Mcap is a relatively thick, dark brown-grey ash layer that caps the deposit. It is a well-sorted vitric tuff (99% ash, 1% lapilli) consisting of scoria, free plagioclase crystals, mafic minerals, and lithics. The upper contact of the ash is often reworked and is overlain with soil, marking the end of the eruptive sequence."

"For all Driftwood Pumice horizons, the axis of maximum deposition is to the northeast, though the axis of maximum deposition for each stratigraphic horizon rotates counterclockwise by ~20° through the course of the eruption. PL has a limited dispersal area that uniformly thins, reaching a zero-line ~13 km from the Makushin summit at its furthest extent. The main PM horizon is over 35 cm thick at the zero line for PL and reaches thicknesses of ~15-20 cm over 25 km away in the towns of Dutch Harbor and Unalaska. The upper PU horizon has a distribution area similar to PL, though thins more sporadically."

"Using the isopach maps and the exponential thinning model of Pyle (1989), the total volume of material erupted is calculated to be 0.3 -0.9 km3. The volume of the Mcap ash was not included in this calculation, making it possible that the volume of erupted material exceeded 1 km 3. These estimates give the Driftwood Bay eruption a Newhall and Self (1982) Volcanic Explosivity Index of 4-5, which would correspond to a plinian eruption with a column height of 10-25 km and a duration of ≥ 12 hours - an eruption on the scale of the 1980 Mt. St. Helens eruption."

"Pumice from PL and PM display little variation in overall mineral abundances and rock texture. Pumice from both horizons is vitrophyric and mineral-poor, with plagioclase (2%), clinopyroxene (<1%), orthopyroxene (<1%), and Fe-Ti oxide (1%) phenocrysts and glomerocrysts set in a vesicle-rich matrix of hyalopilitic-holohyaline glass. Very little olivine is present in PM, and none occurs in PL. Scoria from the upper PU horizon are texturally similar to pumice from the other horizons, but is more mineral-rich, with relatively abundant plagioclase (5-8%), clinopyroxene (1%), orthopyroxene (2%), Fe-Ti oxides (1-5%), and scarce olivine (<<1%) phenocrysts. Fe-sulfides and apatite are trace minerals present in all horizons. The matrix glass is dacitic, ranging from 67-69 wt% SiO2in PL and PM, and becoming more mafic in PU (64-67 wt% SiO2) (Fig. 3 [in original text])."

The Global database on large magnitude explosive volcanic eruptions (LaMEVE; 2017) reports a magnitude of 4, bulk eruptive volume of 0.100 cubic km and a dense rock equivalent eruptive volume of 0.040 cubic km for the eruption.

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