“Falls” of this substance can contaminate drinking water when its Stokes number becomes large and it decouples from the surrounding gas. For 10 points each:
[10e] Name this component of tephra whose particles are less than 2 millimeters in diameter. Volcanoes that eject these fine rocks into the stratosphere can cause a volcanic winter.
ANSWER: volcanic ash [or ashes; accept ashfall]
[10h] The next-smallest component of tephra consists of these particles that are flattened into lens-shaped fiamme (“fee-AH-may”) on the surface of welded tuff, also called ignimbrite. These particles range between 2 and 60 millimeters in diameter.
ANSWER: lapilli [or lapillus]
[10m] Tephrochronology, which uses tephra to measure the dates of past volcanic eruptions, is most accurate when the tephra is dominated by this rock due to its high potassium content. A type of magma named for this extrusive equivalent of granite is the most viscous due to its high silica content.
ANSWER: rhyolite [accept rhyolitic magma]
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