GEOLOGY 235 (2015) LAB 5
PYROCLASTIC MAGMATISM
Introduction:
Silicic volcanism is common on the continents, and the subject of this week’s lab. Most
volcanism on the continents is related to either subduction or rifting; however, much of
the recent volcanism in the western U. S. is not clearly related to either tectonic process.
One school of thought is that silicic volcanism in "fundamentally basaltic" in the sense
that the heat energy and mass flux necessary to produce it are mainly derived from
basaltic magma which migrates upward from the mantle and into the upper crust. In
terms of volume most rhyolitic volcanic centers are "bimodal", in other words, they are
composed mainly of basalt and rhyolite, with lesser amounts of intermediate composition
rocks.
Magma viscosity (which is a function of melt composition and temperature), as discussed
in lecture, is a key control on the nature of volcanic eruption style. Silicic magmas are
extremely viscous due to the highly polymerized nature of high-silicate melts. They are
also relatively rich in volatile components relative to more mafic magmas since volatiles
are fractionated into the liquid during crystallization of anhydrous minerals like
plagioclase, olivine, pyroxenes, and opaque oxides.
Due to their high volatile content and high viscosity silicic eruptions are often explosive,
giving rise to pyroclastic deposits. Pyroclastic rocks have a wide array of textures
resulting from the highly variable nature of their eruption, temperature, transport, and
deposition. Hopefully this lab exercise will give you a feel for their variability, and an
understanding of some of the key factors controlling it.
Silicic magmas can also erupt as flows but displaying different characteristics from mafic
flows due to their higher viscosities. Steep sided domes and short blocky flows are the
most commonly observed forms. Textures developed in lava flows are usually distinct
from those produced by pyroclastic eruption, but are similar enough that larger scale
(field) relations are often necessary to distinguish a tuff from a lava flow.
This lab is divided into three sections. In the first a suite of samples from the Long Valley
Caldera is provided. This includes several samples of the Bishop Tuff that illustrate the
general character of ash flows (ignimbrite). The chemistry of these rocks is typical of
continental margin silicic volcanism. A diagram illustrating the textural zones developed
in welded tuffs are also provided (Fig. 1). Use the Bishop Tuff samples to get familiar
with the features of welded ash flow tuffs.
In the second section there is a suite of andesitic ash-flow tuffs (nuee ardente deposits)
from Martinique. They are included to stress that not all pyroclastic deposits are rhyolitic.
In the third section there are some samples of silicic lava. You should learn to distinguish
their textural features from ash flow textures.
Long Valley Caldera and the Bishop Tuff, California
Silicic pyroclastic eruptions are the largest of terrestrial eruptions often producing several
hundred cubic kilometers of tephra in a matter of days to weeks. The venting of such
huge quantities of magma is usually accompanied by collapse of the roof of the magma
chamber thus producing a caldera. The Bishop Tuff, erupted 700,000 years ago, provides
an excellent example of a large (roughly 200 km3) ash flow tuff. Since ash flow tuffs
represent "inverted versions" of rapidly evacuated silicic magma chambers they can tell
us things about compositional and physical gradients that existed in those chambers. The
Bishop Tuff is probably the most thoroughly studied ash flow in the world. Detailed
study has shown that the Bishop Tuff magma chamber was thermally zoned from 720 to
790o C. The observed mineral assemblage, trace element, and volatile content also
change systematically from top to bottom.
Silicic Lava Flows and Domes
Silicic lava flows lose most of their volatiles on eruption and move as a very sluggish
fluid with yield strength. Their outer carapace is usually quenched to a glass but, if large,
the inner portion of the flow devitrifies. The basal and upper portions of the flow show
variable degrees of self-brecciation or flow breccia. During lava flow, banding usually
develops. Flow banding is more continuous than the "streaky banding" that characterizes
welded ash flows.
ASSIGNMENT
In the process of studying these rocks in hand sample and in thin section, please complete
a full petrographic report on CA-10 (include in petrographic report, labeled ignimbrite
diagram, and the answers in your lab hand-in).
1.a) General Features of Silicic Tuffs
Bishop Tuff, California. Name all the samples (hint: use the terms provided in Fig. 1).
Provide a detailed name that describes the degree of welding, whether it’s glassy and/or if
there is devitrification observed.
CA-12:
. Note this sample is vitrophyric composed of
compressed pumice fragments, ash (glassy shards), crystals, and lithic fragments. The
pumice lumps and shards are defined by very fine-grained dust and iron oxides that coat
their surfaces. Also notice that many of the phenocrysts are broken. This probably was
caused by the force of eruption.
CA-16B:
. This portion of the tuff stayed hot long enough
after eruption for devitrification to occur. Most of the pumice and glass shards have
converted to very fine-grained intergrowths of alkali feldspar, quartz, and iron oxides.
Sketch an example of a devitrification texture.
CA-8:
. Note that the pumice fragments show less
compaction than in the first two samples. What is the phenocryst assemblage?
CA-16F:
biotite and clay/calcite alteration
CA-14:
. Note the late stage oxidation of
.
1b) Answer the questions for each sample below.
RTG: This rhyolite is glassy and displays flow banding. What defines the bands?
Examine the hand samples of glassy lava and the thin sections.
Question: In one hand sample there are round grey ‘balls’, they are spherulites. How
could they form?
RTD: This rhyolite is a devitrified equivalent to RTG above. Examine the hand samples
of the glassy lava (RTG) and the devitrified equivalent (RTD).
Question: Which would weather more easily and therefore be less likely to be preserved
in the rock record?
CA-10: This rhyolite is from a dome at Long Valley, California.
Question: What is the phenocryst assemblage?
Figure 1: Cross section through a pyroclastic flow deposit.
On the diagram below, place CA-12, CA-16B, CA-8, CA-16F and CA-14 in the most
likely locations on the diagram below. Hand in your diagrams with your lab with a
BRIEF explanation as to your rock sample placement.
2. Andesitic Pyroclastic and Lahar Deposits
Lahars are volcanic mudflows that are emplaced at room temperature. They form the
rock agglomerate that is typically polymict with heterogeneous unsupported clasts (up to
several feet in diameter) in a finer, unsorted, matrix. Deposits formed by nuee ardentes
(glowing cloud) or glowing avalanche eruptions are poorly sorted (gas-charged) material,
emplaced at a moderately high temperature (but less than lava flows). During deposition
the lava clasts continue to effervesce releasing hot gases which help cushion the
avalanche deposit as it moves down slope at speeds in excess of 100 km/hr. Nuee
ardentes of the Pelean type are fairly small eruptions, often following stream valleys on
the sides of the volcano. The rock type produced is agglomerate similar to lahar except
that the clasts are >90X of one rock type (they may be up to 40 ft long!). These deposits
have degassing pipes through which the fines have been expelled along with the gas from
the effervescing rock. Later infilling with other detritus "fossilizes" this feature. This is
the definitive feature distinguishing nuee ardentes from lahars but it may be difficult to
find examples of it.
"The Killer Rock" of Martinique, Lesser Antilles
M-10, M-11 and M-16:
These are clasts of vesicular andesite (are they all the same rock?) from the 1902 eruption
of Mt Pele which devastated St Pierre, killing 30,000 people (only 2 survived). There
were many individual nuee ardentes during the 1902 eruption; these particular samples
come from R. Claire, just north of St Pierre. In thin section, note that this material is not
pumiceous. The vesicles are not round bubbles of glass; instead they are irregular,
elongate, and very probably interconnected. Thus the rock is very porous and the
enclosed gas was able to escape during eruption (with catastrophic results).
Exercise:
Provide a brief description that outlines the key features of each sample and indicate how
they are different. How might understanding these rock types and textures help to
interpret the threat level of volcanoes that are in proximity to major population centers
throughout the world? Also, where might you expect these types of eruptions to occur
elsewhere in the world? Provide two potential locations and a reasoning statement.