Description: From Derkachev and others (2018): "The ~ 64.5 ka SR4 tephra has a distinct basaltic andesite glass composition and was found in three cores on the Shirshov Ridge (Fig. 1) [in text]. Comparison of our glass data with that of the Alaska Volcano Observatory Geochemical Database of whole rock compositions (Cameron et al. 2014; Nye et al. 2018) shows that SR4 glass compositions are very similar to basaltic andesites of Okmok volcano (Umnak Island, Eastern Aleutians) (Figs. 6 and 7) [in text]. Additional support for this correlation comes from the discovery of a geochemically similar, but significantly younger (~ 12 ka), tephra in core U1343 at the eastern margin of the Aleutian basin, which is closer to Okmok (Fig.1 [in text]; Aoki et al. 2012). Peculiar features of Okmok rocks include predominantly mafic compositions, a common presence of olivine in the mineral assemblages, and relatively low Ba/Nb and La/Nb ratios (Figs. 6 and 7) [in text].
As the products of the volcano demonstrate similar compositions during its history (Finney et al. 2008; Larsen et al. 2013), it is possible that the ~ 64.5 SR4 tephra was also a product of Okmok. The SR4 tephra forms a visible 4-5-cm-thick layer in three cores on the Shirshov Ridge. If the source of SR4 is Okmok volcano, then only justified isopach of 4 cm is an NW-SE elongated ellipse. This isopach embraces an area of 487,400 km2. Accordingly, the single-isopach approach (Legros 2000) yields a minimum ashfall volume of 72 km3, which corresponds to VEI 6 (Newhall and Self 1982). Adopting a density for basaltic ash of 0.8 g/cm3 (Kutterolf et al. 2008b), and a basalt density of 3.0 g/cm3, we obtain a total tephra mass of 5.8 × 10 4Mt and DRE volume of 19 km3, which corresponds to an eruption magnitude of 6.8 (Pyle 1995; Mason et al. 2004). These are only minimum estimates of the eruption parameters, as only few thickness measurements are available and no proximal deposits are known.
In the absence of well-documented proximal deposits, detailed reconstruction of possible mechanisms of this large eruption is not possible. The estimated volume of SR4 tephra of 20 km3 DRE is, however, comparable to caldera-forming eruptions of Okmok that occurred over the past 12 ka and produced 30 km3 (Okmok I) and 15 km3 (Okmok II) DRE of basaltic material (Larsen et al. 2007). Both the Okmok I and Okmok II eruptions involved phreatomagmatic components. Larsen et al. (2007) thus concluded that interaction of magma with surface water and ice may trigger catastrophic eruptions of mafic magma at Okmok caldera, in addition to large amount of magmatic volatiles accumulated in magma chamber prior to the eruption. We suggest that the mid-Pleistocene Okmok eruption, which provisionally produced SR4 tephra, had a genesis similar to such recent eruptions. Because this older eruption occurred during the last glaciation, an interaction of voluminous basaltic magma with a thick ice cap in Okmok caldera is a plausible mechanism to facilitate the high explosivity of this eruption (Larsen et al. 2007)."