Name: _______________________________
MID-TERM EXAMINATION
50 minutes, closed book; no need for calculators. Total possible: 100 points plus 10 pts extra
credit.
Definitions (short answer) [2 pts each, total 16 pts + 2 EC]
Using a few words or a short phrase, define each of the following terms:
(1) Melt [2 pts]
Molten (liquid) igneous material (i.e., not the crystals) (L01)
[Note that magma = melt ± crystals ± vapor]
(2) Aphyric [2 pts]
Lacks phenocrysts (L02)
(3) Obsidian [2 pts]
Volcanic glass, usually rhyolitic (L05)
(4) Vitrophyre [2 pts]
A volcanic rock containing crystals in a matrix of glass, as in the glassy, densely welded portion
of an ignimbrite cooling unit (L08)
(5) Flow foliation [2 pts]
Layering or banding of magma produced during flow and emplacement of a dome, lava flow, or
shallow intrusion (L05)
(6) Compaction foliation [2 pts]
A planar preferred orientation produced by flattening (including of pumice lumps) during
compaction of a pyroclastic rock (L08)
(7) Fiamme [2 pts]
Discs of compacted glass in a densely welded tuff formed by flattened pumice that has
distinctive, frayed, flame-like terminations (L08)
(8) Resurgent dome [2 pts]
A structural dome formed inside a caldera shortly after collapse (L10)
Extra credit
(9) Rheoignimbrite [2 EC]
Densely welded ignimbrite that flowed after initial emplacement (L09)
subtotal this page [16 + 2 EC]
Geosciences 470R/570R
Spring 2015
[running total 16 + 2 EC]
Volcanology: Physical Processes and Petrologic Applications
12 noon Friday Feb. 20
p. 1
Name: _______________________________
MID-TERM EXAMINATION
Physical properties and volatiles (short answer) [3 pts each, total 9 pts]
What physical property most explains why silicic lavas tend to form steep-sided lava flows and
domes, intermediate lavas tend to form composite volcanoes, and mafic lavas tend to form shield
volcanoes? [3 pts]
Viscosity is the fundamental control on morphology of volcanic landforms (L01); viscosity
generally increases from mafic to silicic compositions, though there are secondary factors such
as volatile composition and content and alkali content (L02)
Why do lavas and pumice lumps in ash-flow tuffs rarely have >60 volume percent phenocrysts?
[3 pts]
Viscosity increases greatly with volume fraction solids (e.g., vol % phenocrysts);
~60% phenocrysts causes a magma to be at the limiting viscosity for it to be mobile enough to
migrate and erupt (L02). Note that the phenocryst content is “locked in” at the time of eruption,
as there is negligible growth of phenocrysts during eruption and cooling
Why do peralkaline magmas have low viscosities? [3 pts]
Peralkaline rocks have molecular ratio of total alkalis (K, Na) to alumina > 1 (L04); alkalis act
as network modifiers in peralkaline rocks, which serves to dramatically decrease their viscosity
relative to non-peralkaline rocks of comparable silica content; they also have abundant chlorine,
a volatile that also decreases viscosity (L02)
Volatiles in Volcanic Rocks (matching) [3 pts each; total 12 pts]
Match the volatile element or compound with its characteristics.
Volatile
1. Water (H2O)
2. Chlorine (Cl)
3. Sulfur (S)
4. Carbon
dioxide (CO2)
Characteristics
A. Solubility is a function of oxidation state and the
presence or absence of certain minor phenocrysts;
generally has greatest abundance in mafic magmas
but eruptions from intermediate magmas tend to
have a greater impact on climate
B. Tends to be the dominant volatile species in
mafic magmas; less soluble in magmas than water;
speciation depends on bulk melt composition
C. Tends to be the dominant volatile species in
rhyolitic magmas; speciation controlled by amount
of volatile present; drives phreatic eruptions
D. Volatile known for its abundance in peralkaline
magmas; in calc-alkaline magmatic hydrothermal
fluids, is an important ligand for complexing base
metals such as copper
subtotal this page [21]
Geosciences 470R/570R
Spring 2015
Put matching # here
3
4
1
2
[running total 37 + 2 EC]
Volcanology: Physical Processes and Petrologic Applications
12 noon Friday Feb. 20
p. 2
Name: _______________________________
MID-TERM EXAMINATION
Silicic domes and lava flows (sketches and short answer) [total 13 pts + 2 EC]
Sketch a half-cross section (one half of a section with a mirror of symmetry) through an idealized
subaerial silicic lava flow (e.g., McPhie et al., 1993, or Glass Mountain map of Metz and Bailey,
1993), and show the approximate relative distributions of pyroclastic rocks and the two zones
that are developed in the lava during emplacement, the brecciated facies and the coherent
(foliated, unbrecciated) facies [6 pts]
(L05)
Draw another half-cross section through a subaerial silicic lava flow, but this time show the
approximate relative distributions of three features developed during cooling:
crystalline/devitrified zone, glassy zone, and spherulitic ± lithophysal zone [6 pts]
(L05)
What is the physical cause for the zoning of primary recrystallization features developed during
cooling of silicic lava flows? [3 pts]
Rate of cooling--glass is quenched, spherulitic zone cooled at intermediate rate, and crystallized
zone cooled slowly (L05)
Extra credit [2 EC]
What process leads to formation of foreset beds of breccia at the snout of silicic lava flows, as
we saw at the Fish Creek public overlook area in the Superstition Mountains? [2 EC]
Tractor-tread-like advance of lava flows that causes blocks of the brecciated carapace to spill off
the snout of the advancing lobe of the flow (L05)
subtotal this page [15 + 2 EC]
Geosciences 470R/570R
Spring 2015
[running total 52 + 4 EC]
Volcanology: Physical Processes and Petrologic Applications
12 noon Friday Feb. 20
p. 3
Name: _______________________________
MID-TERM EXAMINATION
Pyroclastic transport processes (matching)
Match pyroclastic deposits with the associated transport processes. [3 pts each, total 15 pts]
Pyroclastic
deposit
1. Fall deposit
2. Welded fall
deposit
3. Pyroclastic
flow (block-andash flow deposit)
4. Pyroclastic
flow (ignimbrite
or ash-flow tuff)
5. Pyroclastic
surge deposit
Transport processes
Put matching # here
A. Transported by traction and favored by low
particle concentration and subhorizontal particle
trajectory
B. Transported by suspension and favored by low
particle concentration and subvertical particle
trajectory
C. Pyroclastic density current transported by massflow and commonly resulting from eruptive column
collapse or boil-over or fountaining of pumice
D. Pyroclastic density current transported by massflow and resulting from gravitational collapse or
from explosive collapse of a lava dome
E. Transported by suspension and favored by low
magma viscosities and proximity to eruptive vent
5
1
4
3
2
Ignimbrites and related rocks (short answer)
Standard ignimbrite flow unit [3 pts ea, total 9 pts]
The standard ignimbrite flow unit (above) displays key features that develop during
emplacement and settling of an ignimbrite. Based on study of ignimbrite deposits and
observation of emplacement of small pyroclastic flows, what processes are responsible for
(a) ash being located at the top in layer 3?
deposited from secondary plume that develops above pyroclastic flow, which settles on top
(b) the presence of lapilli pipes in layer 2?
upward escape of degassing of pumice and vitric shards, causing elutriation of ash during
emplacement and settling
(c) reverse coarse-tail grading of pumice fragments in layer 2?
preferential buoyant rise of low-density pumice fragments to top during transport (L08)
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Geosciences 470R/570R
Spring 2015
[running total 76 + 4 EC]
Volcanology: Physical Processes and Petrologic Applications
12 noon Friday Feb. 20
p. 4
Name: _______________________________
MID-TERM EXAMINATION
Ignimbrites, continued
Cooling units [total 4 pts]
The following two sketches show zonations observed in simple cooling units of ignimbrites. The
first sketch shows the features that develop during post-emplacement welding and compaction.
The second sketch shows the features that develop during cooling.
Use the sketches to explain briefly why vitrophyres occur near the base of ignimbrites. [4 pts]
Welded and glassy--enough weight from overlying tuff to remove pore space, but glass is
quenched by cold underlying ground surface, preventing devitrification (L08)
Pyroclastic surge deposits (short answer) [total 2 pts]
During a phreatomagmatic eruption, three facies are developed during emplacement of
pyroclastic surge deposits, from proximal waveform or sandwave facies, to intermediate massive
facies, to distal planar facies? What change in the flow conditions of the transporting fluid occurs
as it moves from proximal to distal positions? [2 pts]
Transition from turbulent to laminar flow conditions (L09)
subtotal this page [6]
[running total 82 + 4 EC]
Geosciences 470R/570R
Spring 2015
Volcanology: Physical Processes and Petrologic Applications
12 noon Friday Feb. 20
p. 5
Name: _______________________________
MID-TERM EXAMINATION
Scales of volcanoes and volcanologic processes (multiple choice) [total 15 pts + 6 EC]
Note: X = any digit, one to nine, e.g., “X00 years” means “hundreds of years,” and “X0,000 to
X00,000 years” means “tens to hundreds of thousands of years.” Place your answer on the line.
Three pts each for the correct answer; one pt if you are one category away from the right answer
(1) The diameter of calderas associated with large-volume silicic ignimbrites is generally
(a) X km; (b) X0 km; (c) X00 km; (d) X000 km [3 pts]
___b___ L10; also L02 and Hildreth (1981) reading
(2) The largest ash-flow tuffs on Earth have DRE (dense-rock equivalent) volumes of
approximately
(a) X00 km3; (b) X000 km3; (c) X0,000 km3; (d) X00,000 km3 [3 pts]
___b___ L10; also L02 and Hildreth (1981) reading
(3) Large silicic volcanic fields such as the San Juan volcanic field have typical lifetimes of
(a) X to X0 years; (b) X00 to X000 years; (c) X0,000 to X00,000 years; (d) X,000,000 to
X0,000,000 years; (e) X00,000,000 to X,000,000,000 years [3 pts]
___d___ L02; also L10 and Hildreth (1981) reading
(4) The time it takes to emplace a single cooling unit of a large-volume ignimbrite such as the
Bishop Tuff is
(a) X seconds to X minutes; (b) X hours to X days; (c) X months to X years; (d) X00 to X000
years, (e) X0,000 to X00,000 years [3 pts]
___b___ Reading, Wilson and Hildreth (1997) Bishop Tuff ~100 h and L10
(5) A large silicic dome complex such as Glass Mountain may have occasional eruptions for
(a) 0.X to X years; (b) X0 to X00 years; (c) X000 to X0,000 years; (d) X00,000 to X,000,000
years; (e) X0,000,000 to X00,000,000 years [3 pts]
___d___ L06 and L07; also Metz and Mahood (1985) and Hildreth (1981) readings; e.g.,
Glass Mountain example at Long Valley
Extra credit [6 EC]
(6) The explosive phase of eruption of a single silicic lava dome, such as one in the Mono-Inyo
craters chain, occurs within
(a) X seconds to X minutes; (b) X hours to X months; (c) X to X0 years; (d) X00 to X000 years
[3 EC]
___b___ L07; see Mono-Inyo Craters example
(7) Computational models indicate that a large ignimbrite compacts in
(a) X seconds to X minutes; (b) X hours to X0 hours; (c) X weeks to X0 years; (d) X00 to X000
years [3 EC]
___c___ “weeks to two or three years unless halted by devitrification” (L08)
subtotal this page [15 + 6 EC]
Geosciences 470R/570R
Spring 2015
[running total 97 + 10 EC]
Volcanology: Physical Processes and Petrologic Applications
12 noon Friday Feb. 20
p. 6
Name: _______________________________
MID-TERM EXAMINATION
Calderas (matching) [total 3 pts]
The three simplified stages of a caldera cycle are
A. Precollapse volcanism;
B. Ash-flow eruption and concurrent collapse, and
C. Resurgence and post-caldera deposition.
In such a simplified, three-stage, caldera cycle, which stage (A, B, or C) would you assign to:
 the silicic domes that occur on the southern side of Long Valley that are interpreted to be
responsible for hydrothermal activity of the last few hundred thousand years ___C____ ?
 the silicic lavas and pyroclastic rocks on the northwestern side of Long Valley at Glass
Mountain ____A___ ?, and
 the Bishop Tuff ignimbrite deposited near Long Valley ____B___ ? [1 pt each]
C, A, B.
L10; also Hildreth (1981), Wilson and Hildreth (1997) reading
subtotal this page [3]
[running total 100 + 10 EC]
Geosciences 470R/570R
Spring 2015
Volcanology: Physical Processes and Petrologic Applications
12 noon Friday Feb. 20
p. 7