GLG 103 Lab 5. Igneous Rocks | Questions
and Answers | 2026/2027 Updated | 100%
Correct - Arizona State University.
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
50 exam-style multiple-choice questions. Each question includes the correct answer and a complete rationale
for focused revision.
COVERAGE
This laboratory quiz assesses mastery of igneous rock classification, textures, magma composition and
viscosity, Bowen's Reaction Series, intrusive vs. extrusive environments, hand-sample identification of
common igneous rocks, and the relationship between igneous activity and plate tectonics. It emphasizes
practical application of the 2026/2027 GLG 103 lab manual and current geoscience literacy principles.
STUDY GUIDE
Recommended duration: 1 hour 15 minutes. Passing target: 70% (35/50).
HIGHP - Page 1 of 17
,1. A fine-grained volcanic rock contains 20% visible quartz, 55% plagioclase (An30), 15%
biotite, and 10% amphibole. According to IUGS classification, which rock name is correct?
[ ] A. Rhyolite
[ ] B. Dacite
[ ] C. Andesite
[ ] D. Basaltic andesite
CORRECT: B. Dacite
Dacite is defined by a fine-grained texture and a modal mineralogy with quartz between 20-60% and
plagioclase composition An10-50, with mafic minerals typically <20%. Rhyolite would require higher quartz
and alkali feldspar, andesite has less quartz and more mafic minerals, and basaltic andesite has plagioclase
more calcic (An50-70) and quartz <20%.
2. A phaneritic rock consists of approximately 60% olivine, 25% pyroxene, and 15%
plagioclase (An85). Which tectonic setting is most consistent with its origin?
[ ] A. Continental rift zone
[ ] B. Mid-ocean ridge
[ ] C. Subduction zone
[ ] D. Oceanic intraplate hotspot
CORRECT: B. Mid-ocean ridge
The rock is a peridotite (specifically lherzolite or harzburgite) with high olivine and low plagioclase, typical of
the upper mantle. Mid-ocean ridges are the primary setting where mantle peridotite is emplaced or exposed
via seafloor spreading. Continental rifts, subduction zones, and hotspots produce more evolved magmas or
different ultramafic assemblages.
3. Which texture indicates that a magma underwent two distinct cooling stages, with slow
cooling followed by rapid cooling?
[ ] A. Porphyritic
[ ] B. Glass
[ ] C. Aphanitic
[ ] D. Phaneritic
CORRECT: A. Porphyritic
Porphyritic texture features large crystals (phenocrysts) set in a fine-grained or glassy matrix (groundmass),
indicating slow cooling first (forming phenocrysts) then rapid cooling (forming groundmass). Glass indicates
extremely rapid cooling, aphanitic indicates rapid cooling only, and phaneritic indicates slow cooling only.
HIGHP - Page 2 of 17
, 4. A magma with 72 wt% SiO2 and 0.5 wt% H2O is most likely to produce which eruption
style?
[ ] A. Effusive lava fountains
[ ] B. Plinian eruption column
[ ] C. Surtseyan phreatomagmatic
[ ] D. Icelandic fissure eruption
CORRECT: B. Plinian eruption column
High silica content (72% SiO2) indicates rhyolitic magma, which is highly viscous and typically erupts
explosively as Plinian columns. Effusive and fissure eruptions are characteristic of low-viscosity basaltic
magma, and Surtseyan eruptions involve water-magma interaction but are not the primary style for dry
rhyolitic magma.
5. In Bowen's Reaction Series, which mineral pair represents the discontinuous and
continuous branches at the same temperature interval?
[ ] A. Olivine and Ca-rich plagioclase
[ ] B. Pyroxene and Na-rich plagioclase
[ ] C. Amphibole and biotite
[ ] D. Quartz and muscovite
CORRECT: A. Olivine and Ca-rich plagioclase
The discontinuous branch (olivine -> pyroxene -> amphibole -> biotite) and continuous branch (Ca-rich
plagioclase -> Na-rich plagioclase) operate simultaneously at the highest temperatures, with olivine and
Ca-rich plagioclase crystallizing first. Other pairs represent different temperature intervals or do not align on
the series.
6. A hand sample is dark gray, aphanitic, and contains visible vesicles. Which rock name
and environment are most likely?
[ ] A. Basalt; subaerial lava flow
[ ] B. Obsidian; subaerial lava flow
[ ] C. Pumice; pyroclastic fall
[ ] D. Scoria; subaqueous lava flow
CORRECT: A. Basalt; subaerial lava flow
Basalt is a dark, aphanitic volcanic rock; vesicles indicate trapped gas bubbles from subaerial lava flows.
Obsidian is glassy, pumice is highly vesicular and light-colored, and scoria is more vesicular and typically
associated with subaerial not subaqueous flows.
HIGHP - Page 3 of 17
and Answers | 2026/2027 Updated | 100%
Correct - Arizona State University.
Practice Question Bank & Answer Rationale
THIS PACK CONTAINS
50 exam-style multiple-choice questions. Each question includes the correct answer and a complete rationale
for focused revision.
COVERAGE
This laboratory quiz assesses mastery of igneous rock classification, textures, magma composition and
viscosity, Bowen's Reaction Series, intrusive vs. extrusive environments, hand-sample identification of
common igneous rocks, and the relationship between igneous activity and plate tectonics. It emphasizes
practical application of the 2026/2027 GLG 103 lab manual and current geoscience literacy principles.
STUDY GUIDE
Recommended duration: 1 hour 15 minutes. Passing target: 70% (35/50).
HIGHP - Page 1 of 17
,1. A fine-grained volcanic rock contains 20% visible quartz, 55% plagioclase (An30), 15%
biotite, and 10% amphibole. According to IUGS classification, which rock name is correct?
[ ] A. Rhyolite
[ ] B. Dacite
[ ] C. Andesite
[ ] D. Basaltic andesite
CORRECT: B. Dacite
Dacite is defined by a fine-grained texture and a modal mineralogy with quartz between 20-60% and
plagioclase composition An10-50, with mafic minerals typically <20%. Rhyolite would require higher quartz
and alkali feldspar, andesite has less quartz and more mafic minerals, and basaltic andesite has plagioclase
more calcic (An50-70) and quartz <20%.
2. A phaneritic rock consists of approximately 60% olivine, 25% pyroxene, and 15%
plagioclase (An85). Which tectonic setting is most consistent with its origin?
[ ] A. Continental rift zone
[ ] B. Mid-ocean ridge
[ ] C. Subduction zone
[ ] D. Oceanic intraplate hotspot
CORRECT: B. Mid-ocean ridge
The rock is a peridotite (specifically lherzolite or harzburgite) with high olivine and low plagioclase, typical of
the upper mantle. Mid-ocean ridges are the primary setting where mantle peridotite is emplaced or exposed
via seafloor spreading. Continental rifts, subduction zones, and hotspots produce more evolved magmas or
different ultramafic assemblages.
3. Which texture indicates that a magma underwent two distinct cooling stages, with slow
cooling followed by rapid cooling?
[ ] A. Porphyritic
[ ] B. Glass
[ ] C. Aphanitic
[ ] D. Phaneritic
CORRECT: A. Porphyritic
Porphyritic texture features large crystals (phenocrysts) set in a fine-grained or glassy matrix (groundmass),
indicating slow cooling first (forming phenocrysts) then rapid cooling (forming groundmass). Glass indicates
extremely rapid cooling, aphanitic indicates rapid cooling only, and phaneritic indicates slow cooling only.
HIGHP - Page 2 of 17
, 4. A magma with 72 wt% SiO2 and 0.5 wt% H2O is most likely to produce which eruption
style?
[ ] A. Effusive lava fountains
[ ] B. Plinian eruption column
[ ] C. Surtseyan phreatomagmatic
[ ] D. Icelandic fissure eruption
CORRECT: B. Plinian eruption column
High silica content (72% SiO2) indicates rhyolitic magma, which is highly viscous and typically erupts
explosively as Plinian columns. Effusive and fissure eruptions are characteristic of low-viscosity basaltic
magma, and Surtseyan eruptions involve water-magma interaction but are not the primary style for dry
rhyolitic magma.
5. In Bowen's Reaction Series, which mineral pair represents the discontinuous and
continuous branches at the same temperature interval?
[ ] A. Olivine and Ca-rich plagioclase
[ ] B. Pyroxene and Na-rich plagioclase
[ ] C. Amphibole and biotite
[ ] D. Quartz and muscovite
CORRECT: A. Olivine and Ca-rich plagioclase
The discontinuous branch (olivine -> pyroxene -> amphibole -> biotite) and continuous branch (Ca-rich
plagioclase -> Na-rich plagioclase) operate simultaneously at the highest temperatures, with olivine and
Ca-rich plagioclase crystallizing first. Other pairs represent different temperature intervals or do not align on
the series.
6. A hand sample is dark gray, aphanitic, and contains visible vesicles. Which rock name
and environment are most likely?
[ ] A. Basalt; subaerial lava flow
[ ] B. Obsidian; subaerial lava flow
[ ] C. Pumice; pyroclastic fall
[ ] D. Scoria; subaqueous lava flow
CORRECT: A. Basalt; subaerial lava flow
Basalt is a dark, aphanitic volcanic rock; vesicles indicate trapped gas bubbles from subaerial lava flows.
Obsidian is glassy, pumice is highly vesicular and light-colored, and scoria is more vesicular and typically
associated with subaerial not subaqueous flows.
HIGHP - Page 3 of 17
