Young children often learn that one of the most obvious distinctions between living and nonliving things is the capacity to grow. They soon discover, however, that growth is hardly exclusive to living organisms. Several non-living things – such as crystals, icicles and even dust bunnies – grow by the continuous accumulation of the same material out of which they are made.
Crystals grow through a process known as “nucleation,” which splits into two categories – assisted and unassisted. The nature of the core, or nucleus, upon which the crystal builds serves as the distinguishing factor. Unassisted nucleation begins with a molecule of the same substance which comprises the rest of the crystal, while assisted nucleation begins with foreign matter. Crystals grow when a nucleus attracts more of the same substance from a solution until the crystal and the solvent reach an equilibrium point.
Stalactites and Stalagmites
Stalactites and stalagmites abound in limestone caves and come from the continuous deposition of calcium carbonate, the main component of limestone. Slightly acidic water containing carbon dioxide dissolves the limestone in these caves. Gravity carries this solution downward in slow drips. Calcium carbonate precipitates either on the tip of the stalactite or stalagmite, depending on the point where air causes sufficient evaporation in the drop of solution.
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Icicles and Glaciers
Icicles grow from snow-melt flowing off ledges which refreezes when ambient temperature goes below the freezing point. They grow progressively as layer upon layer of water flows down and freezes. Glaciers, on the other hand, grow from the continuous compression of falling snow. Because snow accumulation in winter surpasses mass losses from melting in some polar and alpine regions, some glaciers grow slowly but continuously through the years.
Dust bunnies form when pet and human hair, dead skin cells, clothing fiber and lint, mold spores, pollen, soil, pillow feathers, dust from micrometeorites and other minor debris accumulate in still air pockets – such as the space under furniture – or corners of air vents. Electrostatic forces, as well as fiber entanglements, hold them together. If kept unmolested by air circulation, dust bunnies keep growing until exceeding the amount of matter static electricity can keep together.