Sudden, violent expulsion of coal from one or more pillars, accompanied by loud reports and earth tremors. Bumps occur in coal mines where a strong, thick, massive sandstone roof rests directly on the coal with no cushioning layer of shale between. The breaking of this strong roof as the seam is mined causes violent bumps and the crushing and bursting of pillars left for support. There are two distinctive types of bumps: (1) pressure bumps, which appear to be due to the unit loading of a pillar being too great for its bearing strength, and where the coal roof and floor are strong, the pillar is ruptured suddenly and with violence; and (2) shock bumps, which are thought to be due to the breaking of thick, massive, rigid strata somewhere above the coalbed, which causes a great hammerlike blow to be given to the immediate roof, which it transmits as a shock wave to the coal pillar or pillars.