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thermoclastic (insolation weathering) Process of physical weathering whereby the stresses set up when a rock is alternately heated and cooled become sufficient to cause failure. It may bring about surface spalling (splintering) because of the low conductivity of rock, and may be particularly effective on polymineralic rocks because of varying coefficients of expansion. Some observational and experimental evidence suggests that the process may be less effective than was believed formerly. See also EXFOLIATION.