Component Analysis
Tephra samples contain essential juvenile, (meaning new magma), accessory (older volcanic materials), and accidental (subvolcanic basement fragments) components. In juvenile components, fragments of glass, lava, and crystals vary in proportion in a complex fashion that is dependent on the magma composition and temperature as well as the mode of ejection and transport. Glass is often vesiculated and forms pumice or scoria. The three tephra components (glass, crystals, and lithic fragments) can be easily recognized with assistance of a hand lens or microscope. An example from Walker and Croasdale (1972) shows vertical and lateral changes of pyroclast constituents for the Fogo A tephra sampled southeast of Lake Fogo at Sao Miguel in the Azores (Fig. A.18).
An analysis of tephra components is especially important for identifying samples from deposits that have major nonjuvenile contributions. Abundant accidental and accessory lithic fragments are indicative of eruptions that have fractured and excavated rocks from around the magma conduit, as is the case for vent-opening and hydrovolcanic eruptions. A careful count of lithic-fragment abundances for the scoria cone at Lathrop Wells (Fig. A.12) showed the relative abundance of lithic fragments in pyroclastic deposits from hydrovolcanic phases (Fig. A.19). In addition, Fig. A.19 illustrates the relative increases of crystals in pyroclastic surge samples from the tuff ring and fine ashes from
Fig. A.12
Stratigraphic section of the Lathrop Wells, Nevada, scoria cone,
showing sampled intervals.
(Adapted from Wohletz, 1986.)
Fig. A.13
Plot of sorting vs median diameter for samples from the scoria cone described in Fig. A.12.
Fig. A.14
Plot median diameter (Mdf ) and wt% ash <1 mm vs the scoria cone
stratigraphy (shown in Fig. A.12). The peaks in median diameter and ash abundance
for the hydrovolcanic samples are unlike those for the Strombolian
samples, which are products of magmatic eruption.
Fig. A.15
Plots of the size-frequency distribution for a
sample of a planar surge bed from Crater
Elegante in Sonora, Mexico. The upper plot
shows a cubic spline curve fit to the data
points (
modeled distribution (solid curve) made
by adding three subpopulations of SFT
form (dashed curves).
(Adapted from Wohletz et al ., 1989.)
the early and late cone samples, which were interpreted as surge deposits. This increase in crystal concentration is a typical feature of surge-emplaced tephra.