The ~400 yr B.P. eruption of Half Cone, a post-caldera composite cone within Aniakchak caldera, Alaska Peninsula

Aniakchak volcano is a historically active caldera located on the central Alaska Peninsula.
The largest eruption from Aniakchak since the ~3,400 yr B.P. caldera-forming eruption
occurred ~400 yr B.P. from Half Cone volcano, an intracaldera composite cone on the
northwest floor of the Aniakchak caldera that was largely destroyed by the eruption. The
~400 yr B.P. eruption produced a widely dispersed pumice fall deposit known as the
Pink and Brown Pumice. Following small phreatomagmatic explosions, a buoyant Plinian
eruption column combined with southwesterly winds dispersed ~1.3 km3 of crystal-poor
dacite (66.1–67.1% SiO2) Pink Pumice at least 70 km to the northeast from Half Cone (~0.05
km3 dense rock equivalent; DRE). Fluctuations in the diameters of pyroclasts and accidental
lithics in the Pink Pumice indicate at least two cycles of waxing and waning mass flux at the
Half Cone vent. This vent produced an eruption column that twice expanded and gained
altitudes of ~15–20 km before weakening to lower altitudes. Brown Pumice scoria (58.2–
66.9% SiO2) as well as compositionally banded pyroclasts at the top of the Pink Pumice
indicate that both dacite magma and an increasing amount of low-SiO2 (58.2–60.5%
SiO2) andesite magma were erupted simultaneously during the transition to the Brown
Pumice phase of the eruption. The reversely graded Brown Pumice fall deposit records
an escalating Plinian column dominated by low-SiO2 Brown Pumice scoria that reached
altitudes of ~20–24 km and led to the emplacement of least ~3.5 km3 of fall deposits up
to at least 230 km to the northeast (~1 km3 DRE). Over time, the Brown Pumice eruption
column repeatedly experienced partial collapse that ultimately produced thick pyroclastic
density current deposits, most of which were confined to within the caldera. Lithic-rich
agglutinate and spatter exposed in 60-m-thick deposits atop the severed flanks of Half
Cone and within ~2 km of Half Cone were emplaced at the end of the Brown Pumice
phase. Agglutinate deposits range from 58.6 to.64.8% SiO2, which generally falls in the
compositional range between Brown and Pink Pumice compositional endmembers. Most
of the Half Cone edifice was destroyed by the end of the Brown Pumice phase. The ~0.1
km3 crystal-rich dacitic Cobweb lava flow (64.8–65.8% SiO2) filled a basin left behind by
the destruction of Half Cone as a series of radiating lobes. Subsequently, a small andesitic
tuff cone (62.2–62.8% SiO2) formed over the Cobweb lava flow vent. In all, we estimate
that at least ~5.4 km3 of tephra and ~0.1 km3 of lava erupted during the ~400 yr B.P.
eruption, yielding a total magmatic volume (DRE) of ~1.3 km3. Titanomagnetite-ilmenite
pairs in Pink and Brown Pumice samples record similar equilibrium temperature ranges
(944–997 °C and 959–985 °C, respectively) but different fO2 conditions—Pink Pumice
pairs plot between NNO and NNO +0.5, Brown Pumice pairs plot below the NNO buffer.
Titanomagnetite-ilmenite pairs in Brown Pumice agglutinate record a wider range of
temperatures than either Pink or Brown Pumice samples (899–1018 °C) but also show
two populations of fO2—one that overlaps the Pink Pumice array at higher fO2 and one
that overlaps the Brown Pumice array at lower fO2. Titanomagnetite-ilmenite pairs from
the Cobweb lava flow have the largest fO2 range (NNO -0.5 to NNO +0.5), although
most pairs overlap Brown Pumice samples at lower fO2 conditions near NNO -0.5. Pairs
in Cobweb lava samples record temperatures from 837 to 1054 °C, which is the largest
temperature range recorded in deposits emplaced during any phase of the ~400 yr B.P.
eruption. Geothermometry results of titanomagnetite-ilmenite pairs in ≤3,400 yr B.P.
samples erupted from Aniakchak volcano record a similar temperature range and the
presence of two fO2 arrays as the ~400 yr B.P. samples, which implies the existence of
two magma regions of the mush column; each the product of slightly different evolution. In
addition, results from in situ compositional analyses of plagioclase suggest that the ~400
yr B.P. eruption may have been initiated, at least in part, by intrusion of basaltic magma,
which ascended from the lower crust into the shallow subvolcanic magma mush column
prior to and during eruption. The bimodal distribution of whole-rock compositions and the
two plagioclase populations in the low-SiO2 Brown Pumice—one defined by An40–An60
cores and one defined by An79–An95 cores—is consistent with an abbreviated period of
mixing between intruding basalt and resident dacite mush prior to eruption. Progressive
mixing between mafic and felsic magmas during and after the eruption likely produced
the subsequently erupted Cobweb lava flow, which has an intermediate composition
with abundant mineral disequilibria. Aniakchak volcano continues to show episodic signs
of unrest, suggesting that eruptions will occur in the future.