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Zorluk: OrtaNecessary Assumption

Geologist: Basaltic volcanic eruptions typically produce low-viscosity lava flows that pose minimal explosive hazard, whereas rhyolitic eruptions feature highly viscous lava that traps volatile gases, frequently generating explosive eruptions. A newly analyzed ancient deposit in the Karoo Basin consists of finely fragmented basaltic pyroclastic tephra rather than continuous lava sheets, proving that a violent explosive eruption of basaltic magma occurred there. Therefore, volatile gas content, rather than magma viscosity, must have been the primary determining factor in whether this Karoo Basin eruption was explosive.

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

  1. A
    Any volcanic eruption involving magma with a high concentration of dissolved volatile gases will inevitably result in an explosive tephra deposit.
  2. The basaltic magma involved in the Karoo Basin eruption was not significantly more viscous than typical basaltic magma.Cevap
  3. C
    Magma viscosity is never a contributing factor in determining the explosiveness of volcanic eruptions.
  4. D
    Continuous basaltic lava sheets are preserved in the geological record more frequently than fragmented pyroclastic deposits.
  5. E
    The volatile gas concentration in rhyolitic magma is consistently higher than that found in any basaltic magma.

Cevap

The argument depends on the assumption that the basaltic magma involved in the Karoo Basin eruption was not significantly more viscous than typical basaltic magma.
The conclusion asserts that volatile gas content, rather than magma viscosity, was the primary factor behind the explosive Karoo Basin eruption. This inference is based on the premise that basaltic magma is typically low in viscosity. However, for this inference to hold, the specific magma in this eruption must not have been an anomaly with high viscosity. The correct answer defends the argument by ruling out this possibility: if the magma were indeed significantly more viscous than typical basaltic magma, viscosity could have been the primary driver of the explosion, directly refuting the author's conclusion.

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1
Analyze the premises and the conclusion of the argument.
Premises: (1) Typical basaltic magma is low-viscosity and non-explosive, while rhyolitic magma is high-viscosity and explosive. (2) The Karoo Basin deposit demonstrates an explosive basaltic eruption. Conclusion: Volatile gas content, rather than magma viscosity, was the primary factor determining why this eruption exploded.
Identifying the structural components highlights the logical gap between the general properties of basaltic magma and the specific magma erupted at Karoo Basin.
2
Identify the unstated assumption bridging the premises to the conclusion.
The author concludes that viscosity could not have been the deciding factor by presuming that the Karoo Basin basalt followed the typical rule of low viscosity. If the Karoo Basin basalt happened to be unusually viscous, viscosity could still have been the primary cause of the explosion.
A necessary assumption must protect against alternative explanations that would allow viscosity to remain the deciding factor.
3
Apply the Negation Test to confirm necessity.
Negating the statement gives: 'The basaltic magma involved in the Karoo Basin eruption was significantly more viscous than typical basaltic magma.' If this is true, the explosion could have been caused by high viscosity, destroying the conclusion that gas content rather than viscosity was the primary determinant.
A statement whose logical negation fatally weakens the conclusion is strictly necessary for the argument to hold.

Anahtar Kavram

Necessary Assumption and the Negation Technique
Tahmini Süre:1m 15s
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Konular

Necessary Assumption

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