Earth Science: Changing Earth – Weathering
Craggy mountains, rounded hills, wide plateaus, and endless coasts—the landscapes of the Earth are almost as varied as the outside forces that have shaped them.
The current form of the Earth is only a moment in its geological history. Weathering, erosion, and precipitation are constantly changing the appearance of our world.
Weathering
Degradation due to weathering is part of the natural cycle of rocks and a fundamental prerequisite for soil formation. Even the most massive mountains cannot withstand this unremitting force for long.
As soon as rocks reach the surface of the Earth they are exposed to physical and chemial processes, which change and ultimately destroy them. The effects of weathering on rocks depend on the climate and the type of rock. Pressure and temperature variations play a large role in physical weathering by causing mechanical destruction of rocks.
Stones and rocks expand during the day under the influence of solar radiation, then contract again as they cool during the night. This constant fluctuation loosens the rock structure, and over time rock fragments chip off. Large boulders may split open along internal fracture lines; small stones disintegrate into grit.
Changes in temperature together with moisture lead to “frost weathering,” especially in high mountainous areas and near polar regions. Water, when it freezes, increases in volume by about one-tenth. Thus, the ice that forms in the crevices of the rocks have the potential to crack them open. In arid regions, salt takes the role of ice.
When saline water evaporates, the salt crystallizes outward, leading to a volume increase that cracks the rocks. In contrast to mechanical weathering, rocks are totally disintegrated during the process of chemical weathering (decomposition). Rocks are gradually corroded by water and the dissolved salts, acids, and gases it contains.
If organisms, especially microorganisms, are involved in rock disintegration—through chemical or mechanical means—it is called biological weathering.
Acids excreted by lichens, for instance, attack the crystal matrix of minerals and thus destroy the structure of rocks. The process is rather less delicate when boring clams, sponges, or worms penetrate rocks along rocky shores, or when the roots of plants force their way up into rock formations, effectively cracking open the rocks with their growth (known as root fracture).
LIMESTONE CAVES
Limestone and dolomite are rarely corroded by pure water; however, water with carbon dioxide (CO2) dissolved in it will produce carbonic acid. This reacts with the calcium carbonate in limestone and dissolves it.
Such carbonic acid weathering, especially in calcareous regions, leads to the formation of rugged rocks, sinkholes, caves, and subterranean watercourses. This is known as karst landscape, a name derived from the limestone plateau east of the Gulf of Trieste.