Page 196 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 171

Figure 18 Alacra Ł n Ignimbrite type section at the eastern graben caldera margin. (a) Panora-
mic view to the north of the Alacra Ł n Ignimbrite on the eastern shoulder of the graben caldera
of Bolan ì os, indicating the two main deposits that compose this section, the lower surge deposits
and the major ignimbrite with a thick basal co-ignimbrite lithic-lag breccia. Also indicated is
a rhyolitic lava dome that post-dates the Alacra Ł n Ignimbrite. (b) General view of the sequence,
with the underlying surge deposits sequence (white, layered deposits), and the overlying major
ignimbrite. (c) Lower surge deposits with ballistic blocks. (d) Detail of a ballistic lithic indicat-
ing a trajectory from west (left) to east (right), con¢rming that the graben caldera of Bolan ì os
was the source for these deposits. (e) Coarse co-ignimbrite lithic-lag breccia at the base of Ala-
cra Ł n Ignimbrite with lithics of underlying lavas up to 1.5 m in diameter. (f) Closer view of co-
ignimbrite lithic-lag breccia of Alacra Ł n Ignimbrite, showing a matrix that is
composed of pumice and ash (hammer for scale).

degassed magma that remained in the sub-graben caldera magma chamber after the
explosive ignimbrite-forming eruptions, as occurs with the ring-lava domes in a
typical caldera cycle, such as in the Valles caldera in New Mexico (Heiken et al.,
1990).