Discussion
Assertion: a porphyritic texture records two stages of cooling. Reason: large crystals require time to grow and a fine groundmass requires rapid chilling.
Answer: A
Correct answer: both the assertion and the reason are true and the reason explains the assertion
Both are true and the reason supplies the mechanism. Large crystals require a long period at slowly falling temperature to grow, and a fine or glassy groundmass requires the remaining melt to be chilled quickly - two conditions that cannot occur at once. The rock therefore records a magma that began crystallising at depth and was then erupted or intruded to a shallow level, which is exactly what a porphyry is.
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