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CORE DEFINITIONS LOOP (repeat this 3–5x) Weathering = breakdown of rock at or near Earth’s surface. Erosion = removal of rock by water, wind, glaciers, gravity. Mechanical weathering = physical breakdown of rock. Chemical weathering = decomposition by chemical change. Frost action = water expands 9 percent when frozen. Soil = mix of sand, clay, humus from weathered rock. A horizon = leaching, darkest, most humus. B horizon = accumulation. C horizon = broken bedrock.