GLG 103 Lab 6. Volcanic Hazards |
Questions and Answers | 2026/2027 Updated
| 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 lab exam assesses mastery of volcanic hazards, including eruption styles, lava and pyroclastic products,
volcanic gases and ash fall, pyroclastic density currents, lahars, debris flows, sector collapse, and hazard
assessment/monitoring. It emphasizes quantitative reasoning, map and data interpretation, and emergency
response decision-making consistent with USGS frameworks and ASU GLG 103 Lab 6 (2026/2027).
STUDY GUIDE
Recommended duration: 2.5 hours. Passing target: 90% (A+).
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,1. A volcano's edifice is built of alternating lava flows and unconsolidated pyroclastic
layers. Following a moderate eruption, the summit area shows a 200-m-long fracture.
Which hazard is most likely to increase dramatically during the next rainfall event?
[ ] A. Phreatic explosion
[ ] B. Lahar
[ ] C. Lava dome collapse
[ ] D. Volcanic lightning
CORRECT: B. Lahar
Unconsolidated pyroclastic material on steep slopes becomes easily mobilized by water, generating lahars.
Phreatic explosions require water-magma interaction, not rainfall. Dome collapse relates to lava domes, and
volcanic lightning occurs during ash plumes, not rainfall-triggered flows.
2. Which monitoring signal best indicates that a volcano is transitioning from unrest to an
imminent explosive eruption?
[ ] A. Deep long-period earthquakes at 30 km depth
[ ] B. Gradual inflation over months without seismicity
[ ] C. Swarms of shallow volcano-tectonic earthquakes plus rapid SO2 increase
[ ] D. Steady low SO2 emissions with no deformation
CORRECT: C. Swarms of shallow volcano-tectonic earthquakes plus rapid SO2 increase
Shallow VT swarms and rapid SO2 increase reflect magma ascent and depressurization. Deep long-period
events may indicate deep magma movement but not imminent eruption. Gradual inflation alone is
ambiguous, and steady low gas suggests quiescence.
3. A pyroclastic density current (PDC) travels down a valley. Which factor most strongly
controls whether it will transform into a lahar?
[ ] A. The temperature of the PDC
[ ] B. The amount of water available along the flow path
[ ] C. The grain size of the ash
[ ] D. The presence of a lava dome
CORRECT: B. The amount of water available along the flow path
PDCs can mix with water (from rivers, snow, or rain) to become lahars. Temperature, grain size, and dome
presence influence PDC behavior but not directly its transformation into a lahar.
4. Which statement correctly distinguishes a debris avalanche from a lahar?
[ ] A. Debris avalanches are water-saturated; lahars are dry.
[ ] B. Debris avalanches originate from sector collapse; lahars are water-saturated flows.
[ ] C. Lahars occur only on shield volcanoes; debris avalanches only on stratovolcanoes.
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, [ ] D. Debris avalanches are always hot; lahars are always cold.
CORRECT: B. Debris avalanches originate from sector collapse; lahars are water-saturated flows.
Debris avalanches result from large-scale sector collapse and are typically dry, whereas lahars are
water-saturated mixtures of volcanic debris. The other options mischaracterize water content, volcano type,
or temperature.
5. During a VEI 4 eruption, ash fall is forecast. Which atmospheric condition would most
increase ash loading on roofs downwind?
[ ] A. Strong vertical wind shear
[ ] B. High humidity causing ash aggregation
[ ] C. Dry, calm conditions
[ ] D. Low-altitude temperature inversion
CORRECT: B. High humidity causing ash aggregation
High humidity promotes ash aggregation, forming larger particles that fall faster and accumulate heavily on
roofs. Wind shear disperses ash, calm conditions reduce deposition, and inversions can trap ash aloft.
6. A lahar hazard map shows a 1% annual exceedance probability zone. What does this
imply for a town located within that zone?
[ ] A. The town will be destroyed within 100 years.
[ ] B. There is a 1 in 100 chance of a lahar affecting the town in any given year.
[ ] C. The lahar will occur exactly once every 100 years.
[ ] D. The zone is safe because the probability is low.
CORRECT: B. There is a 1 in 100 chance of a lahar affecting the town in any given year.
A 1% annual exceedance probability means a 1 in 100 chance per year. It does not guarantee destruction
within 100 years or a regular 100-year cycle, and low probability does not mean safe.
7. Which volcanic gas is most useful for remote sensing of magma ascent because it is
emitted early from ascending magma?
[ ] A. CO2
[ ] B. SO2
[ ] C. H2S
[ ] D. HCl
CORRECT: A. CO2
CO2 is the first gas to exsolve from magma at depth, making it a key indicator of early magma ascent. SO2,
H2S, and HCl exsolve at shallower levels and are useful but less indicative of deep ascent.
HIGHP - Page 3 of 17
Questions and Answers | 2026/2027 Updated
| 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 lab exam assesses mastery of volcanic hazards, including eruption styles, lava and pyroclastic products,
volcanic gases and ash fall, pyroclastic density currents, lahars, debris flows, sector collapse, and hazard
assessment/monitoring. It emphasizes quantitative reasoning, map and data interpretation, and emergency
response decision-making consistent with USGS frameworks and ASU GLG 103 Lab 6 (2026/2027).
STUDY GUIDE
Recommended duration: 2.5 hours. Passing target: 90% (A+).
HIGHP - Page 1 of 17
,1. A volcano's edifice is built of alternating lava flows and unconsolidated pyroclastic
layers. Following a moderate eruption, the summit area shows a 200-m-long fracture.
Which hazard is most likely to increase dramatically during the next rainfall event?
[ ] A. Phreatic explosion
[ ] B. Lahar
[ ] C. Lava dome collapse
[ ] D. Volcanic lightning
CORRECT: B. Lahar
Unconsolidated pyroclastic material on steep slopes becomes easily mobilized by water, generating lahars.
Phreatic explosions require water-magma interaction, not rainfall. Dome collapse relates to lava domes, and
volcanic lightning occurs during ash plumes, not rainfall-triggered flows.
2. Which monitoring signal best indicates that a volcano is transitioning from unrest to an
imminent explosive eruption?
[ ] A. Deep long-period earthquakes at 30 km depth
[ ] B. Gradual inflation over months without seismicity
[ ] C. Swarms of shallow volcano-tectonic earthquakes plus rapid SO2 increase
[ ] D. Steady low SO2 emissions with no deformation
CORRECT: C. Swarms of shallow volcano-tectonic earthquakes plus rapid SO2 increase
Shallow VT swarms and rapid SO2 increase reflect magma ascent and depressurization. Deep long-period
events may indicate deep magma movement but not imminent eruption. Gradual inflation alone is
ambiguous, and steady low gas suggests quiescence.
3. A pyroclastic density current (PDC) travels down a valley. Which factor most strongly
controls whether it will transform into a lahar?
[ ] A. The temperature of the PDC
[ ] B. The amount of water available along the flow path
[ ] C. The grain size of the ash
[ ] D. The presence of a lava dome
CORRECT: B. The amount of water available along the flow path
PDCs can mix with water (from rivers, snow, or rain) to become lahars. Temperature, grain size, and dome
presence influence PDC behavior but not directly its transformation into a lahar.
4. Which statement correctly distinguishes a debris avalanche from a lahar?
[ ] A. Debris avalanches are water-saturated; lahars are dry.
[ ] B. Debris avalanches originate from sector collapse; lahars are water-saturated flows.
[ ] C. Lahars occur only on shield volcanoes; debris avalanches only on stratovolcanoes.
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, [ ] D. Debris avalanches are always hot; lahars are always cold.
CORRECT: B. Debris avalanches originate from sector collapse; lahars are water-saturated flows.
Debris avalanches result from large-scale sector collapse and are typically dry, whereas lahars are
water-saturated mixtures of volcanic debris. The other options mischaracterize water content, volcano type,
or temperature.
5. During a VEI 4 eruption, ash fall is forecast. Which atmospheric condition would most
increase ash loading on roofs downwind?
[ ] A. Strong vertical wind shear
[ ] B. High humidity causing ash aggregation
[ ] C. Dry, calm conditions
[ ] D. Low-altitude temperature inversion
CORRECT: B. High humidity causing ash aggregation
High humidity promotes ash aggregation, forming larger particles that fall faster and accumulate heavily on
roofs. Wind shear disperses ash, calm conditions reduce deposition, and inversions can trap ash aloft.
6. A lahar hazard map shows a 1% annual exceedance probability zone. What does this
imply for a town located within that zone?
[ ] A. The town will be destroyed within 100 years.
[ ] B. There is a 1 in 100 chance of a lahar affecting the town in any given year.
[ ] C. The lahar will occur exactly once every 100 years.
[ ] D. The zone is safe because the probability is low.
CORRECT: B. There is a 1 in 100 chance of a lahar affecting the town in any given year.
A 1% annual exceedance probability means a 1 in 100 chance per year. It does not guarantee destruction
within 100 years or a regular 100-year cycle, and low probability does not mean safe.
7. Which volcanic gas is most useful for remote sensing of magma ascent because it is
emitted early from ascending magma?
[ ] A. CO2
[ ] B. SO2
[ ] C. H2S
[ ] D. HCl
CORRECT: A. CO2
CO2 is the first gas to exsolve from magma at depth, making it a key indicator of early magma ascent. SO2,
H2S, and HCl exsolve at shallower levels and are useful but less indicative of deep ascent.
HIGHP - Page 3 of 17
