Marble weathers at a faster rate than granite. Over a period of 1,000 years, granite will lose 10 micrometers from its surface while, during the same period of time in the same warm, humid conditions, marble will lose 200 micrometers from its surface. Both stones weather faster in warm, humid climates than in cold climates.
Warmth and Humidity
Warm, humid climates have a greater effect on weathering rates than cool climates. The reason for rapid warm climate weathering has to do with humidity. Aqueous solutions, such as fog, dew and air moisture, seep into the cracks and crevices within marble and granite surfaces. In warm environments these moist environments invite bacteria and mold. Mold and bacteria increase weathering rates through their life cycles and activity. These growths emit chemicals that slowly eat at a rock's integrity. Once granite and marble have begun to deteriorate their rate of degradation increases rapidly.
Cool climates also present challenges to the integrity of marble and granite. Freeze and thaw cycles help to create cracks and crevices in stone surfaces, helping to open the rock face to moisture. Moisture trapped within cracks and crevices expands and contracts with temperature changes and effectively widens the previously existing cracks and crevices. Freeze and thaw cycles also cause the ground to heave, forcing rocks above the soil line. A larger portion of the rock's surface area is then exposed to climatic conditions and its propensity for degradation increases.
Moisture is the element most responsible for weathering marble and granite. Hot, dry climates, as well as consistently cool, dry climates, present little danger to rock and stone. When marble weathers its particles are easily brushed from the rock's surface. The particles are gritty and semi-hollow. Complete degradation can take hundreds of years. When granite weathers it breaks down into its two major components: quartz and feldspar. After extensive water-induced weathering, feldspar is reduced to clay and ions. Ions leach into soil and clay becomes part of the soil's mass. Quartz has greater weathering resistance than feldspar and will remain as smaller pieces of stone until it is weathered down into sand grain sized particles. This process can take thousands of years, depending on the prevailing climate conditions and size of the original stone.
Weathering is both a chemically and physically induced process. Chemicals, like acid rain, can weaken rock, but it takes physical pressure to exploit the weakened areas and crack separate particles from the larger rock surface. Similarly, climate can weaken granite and marble integrity, but it takes physical force to fully degrade stone.
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