An important component is slightly scoriaceous, dark gray, high-Al basalt found scattered throughout the deposit near the volcano. This is interpreted to be juvenile and the intrusion of the mafic magma into the main dacite magma body may have helped trigger the eruption. The basaltic scoria also occurs as discrete clots within dacitic pumice clasts.
The near-source deposit shows inverse grading. The single fall unit appears to be the result of a continuous gas blast which grew steadily in intensity before ending abruptly with no evidence of a waning phase. The plinian pumice deposit is overlain by a thick section of ash-fall deposits localized near the volcano. This ash-fall totals up to 20 m thick near the vent (Rose, 1972b) and, in an area of approimately 150 km² west and southwest of the volcano, reaches thicknesses of up to 185 cm. The boundary between the deposit of the main plinian event and the overlying units is sharp; it is marked by 4 to 5 cm of extremely lithic-rich ash mixed with typical white dacitic pumice of the main phase. The overlying units are 2 to 10--cm-thick sequences of alternating very fine, gray, vitric-rich ash (sometimes with accretionary lapilli), and lithic-rich layers. They formed during the six-month period of intermittent activity which followed the cataclysmic eruption (Sapper, 1905), and during the renewed eruptive history which marked the birth of Santiaguito Dome in 1922 (Rose, 1972b).
On the slopes of the volcano, the 1902 deposit was reported by Sapper (1905) to reach thicknesses of 30 m; he also noted the trees were totally covered with ash in some instances. The present work casts doubt on this thickness although compaction must have taken place. The combined plinian and post-plinian deposits only total 23 m. A maximum thickness of no greater than 2.2 m for the plinian pumice is most probable; 2.7 km NW of the vent the plinian layer is only 125 cm thick, yet totally preserved. At distances of about 30 km the deposit is still more than 1 m thick on the axis of dispersal.
The plinian deposit is a single fall unit. It resulted from a continuous gas blast, with an essentially constant wind direction and a gradually increasing column height. The exception to this simple form occurs where the ash appears to have been deposited during a rain storm; an area centered at San Marcos shows extensive water-reworking. The presence of irregular bedding, concentration of lithic fragments, and removal of large pumice clasts supports this conclusion.
(Williams and Self, 1983)