Description: From Fierstein (2007): "From the vent at an elevation of 2,010 m, two of the flows extend 6 km to termini as low as 300-350 m in Martin Creek; we call these the "south lobes". One of the youngest flows bifurcates to the northeast; we call this the "Y-flow". The rim of the ice-filled crater on the East Summit is strewn with coarse scoriae and dense glassy blocks of phreatomagmatic ejecta, deposited upon the surfaces of thin lava flows exposed locally. Compositional data for one such ejecta block is used here to represent the "East Summit" in the following correlations."
"Considerable overlap in the microprobe data between the different East Mageik lavas and tephras (Fig. 23 [in original text]) preclude firm correlations, but we tentatively suggest...that lower TiO2 of magnetites and lower CaO of glass correlate the "Y-flow" with the lower grey ash (∼3,600 14C years B.P.)..."
"The older grey ash, similar enough in thickness and color to the younger one that they are difficult to distinguish in the field, is only slightly coarser in the lower VTTS area (ash to 2 mm). In upper Windy Creek, however, they are clearly distinguishable. The older ash thickens and coarsens southward toward Mounts Mageik and Martin into a coarse lapilli fall more than 28 cm thick with largest clasts as big as 8 cm on top of pre-Holocene lava-flow benches at the northern foot of Mount Mageik (Fig. 8 [in original text]). From north to south, the unit coarsens from fine ash in the lower VTTS, to fine and medium ash along Windy Creek valley, to a slightly normally graded layer 5-to-10-cm thick with clasts as large as a centimeter in the upper reaches of the creek (Figs. 8, 9 [in original text]). There, the younger grey ash is still fine-grained and ∼1cm thick, but the older one coarsens to a sand+granule+lapilli fall very similar to (but slightly finer than) the "orange dacite lapilli fall" described below. Crystal proportions and micro- probe analyses of glass and Fe-Ti oxides were essential in distinguishing these fall layers. Even where both young ash layers are grey and fine-grained, their mineral proportions are distinctive, as the opx/cpx ratio of ∼2:1 in the lower ash is an order of magnitude smaller than that of the upper (∼20:1). Microprobe glass data also clearly distinguish the two, as the lower grey ash has much more silicic glass (∼74-78 wt.% SiO2) than the upper (59-67 wt.% SiO2; Fig.7 [in original text])."
"Preserved sections are too few to permit construction of well-constrained isopachs and isopleths, but the data do indicate a fairly narrow northeasterly dispersal (clasts as large as 4-8 cm are found near the foot of Mount Griggs; Fig. 8 [in original text]) and a source vent at either Mount Martin or East Mageik (the only Holocene vent on the Mount Mageik multi-vent stratovolcano). Glass and magnetite microprobe data for the lower grey tephra are similar to those for some of the youngest lava flows from both Martin and Mageik (Figs. 7, 10 [in original text]); although neither data set conclusively discriminates between those sources, the glass data favor Mount Mageik. The large pumice clasts (5 to 8 cm) and thick fallout (>28 cm) so close to the Mageik vent also favors that edifice. Moreover, a younger tephra (less widely preserved, discussed in the next section) is linked to the youngest cone-building phase of Mount Martin, and we are aware of no other eruption of Martin young enough to have produced this "lower grey ash". Nine radiocarbon dates for soils above and below this fall layer bracket its deposition between ∼3,200 and 3,800 14C years (Table 2 [in original text]); stratigraphy and evaluation of the 14C dates favors∼3,600 14C years B.P."