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Scientists still disagree on which condition sets the break point.","posed_since":"","precise":"Determine the fragmentation criterion for vesiculating silicic magma: a critical porosity (about 0.6 to 0.8), a critical bubble overpressure relative to melt tensile strength, or a critical strain rate exceeding the inverse Maxwell relaxation time of the melt, including the effects of permeability and crystals. 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