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Difficulty: HardNecessary Assumption

Geologist: Subglacial volcanic calderas beneath the West Antarctic Ice Sheet are capped by dense ice shelves that exert immense hydrostatic pressure, which generally suppresses explosive eruptions. However, geological sediment cores from the mid-Holocene reveal periodic, catastrophic explosive eruptions from these subglacial vents that coincided precisely with brief intervals of regional ice sheet thinning. Because the reduction in ice volume during these intervals substantially reduced hydrostatic confining pressure, this rapid unloading must have been what triggered the sudden shift to explosive volcanism.

Which one of the following is an assumption on which the geologist's argument depends?

  1. A
    Any significant reduction in hydrostatic confining pressure over a subglacial caldera inevitably precipitates an explosive volcanic eruption.
  2. B
    Subglacial explosive eruptions cannot occur in regions where overlying ice sheets maintain a stable thickness over centuries.
  3. The rapid thinning of the regional ice sheet during the mid-Holocene intervals was not primarily caused by geothermal heat emitted by the subglacial volcanoes prior to their eruptions.Answer
  4. D
    Hydrostatic confining pressure from overlying ice is the single most important factor governing the eruptive behavior of all terrestrial volcanoes.
  5. E
    Sediment core records from the mid-Holocene provide a more reliable indicator of eruption magnitude than do modern seismic monitoring techniques.

Answer

The argument depends on the assumption that the rapid thinning of the regional ice sheet was not primarily caused by geothermal heat emitted by the subglacial volcanoes prior to their eruptions.
The author concludes that regional ice thinning caused the shift to explosive eruptions based on a temporal correlation between the two phenomena. For ice unloading to be the initiating trigger, the thinning must not have been merely a byproduct or consequence of the volcanic eruption sequence already occurring. Negating the statement that the thinning was not caused by geothermal heat from the volcanoes means that pre-eruptive volcanic heating melted the ice, rendering the thinning an effect rather than the trigger. Because negating this statement collapses the causal argument, it is an indispensable assumption.

Step-by-Step Solution

1
Deconstruct the argument into its core premises and conclusion.
Premise: Subglacial ice exerts hydrostatic pressure that suppresses explosive eruptions. Premise: Mid-Holocene core data show explosive eruptions coincided with brief intervals of ice sheet thinning. Premise: Thinning reduces hydrostatic pressure. Conclusion: The reduction in pressure (unloading) triggered the shift to explosive volcanism.
Identifying the exact structural relationship between evidence and the author's causal claim is necessary to spot unstated gaps.
2
Identify potential alternative causal explanations or reverse causality gaps.
The author infers that ice thinning caused the volcanic eruptions simply because they coincided. A fatal vulnerability is reverse causality: if the volcano itself melted the ice before erupting, thinning was an effect rather than the cause.
Necessary assumptions in causal arguments frequently rule out reverse causation or confounding third variables.
3
Apply the Negation Test to the candidate assumption.
Negating the statement yields: 'The rapid thinning was primarily caused by geothermal heat emitted by the subglacial volcanoes prior to their eruptions.' If this is true, the volcanic activity was already underway and caused the thinning, destroying the claim that ice unloading triggered the eruptions.
A true necessary assumption, when logically negated, must completely shatter the validity of the conclusion.

Key Concept

Necessary Assumption (Defender Archetype / Ruling Out Reverse Causation)

Alternative Method

Apply the Negation Technique: Negate each choice and test whether the argument's conclusion becomes logically impossible. The statement that shatters the argument upon negation is the necessary assumption.
Estimated Time:1m 30s
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Topics

Necessary Assumption

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