Have you ever wondered what makes the low, rounded mountains of the American Northeast different from the tall, rugged Himalayas? The main answer is time. Over millions of years, the rock and earth constituting young mountains like Everest is broken down through the process of weathering and erosion until these orogenic giants are transformed into comparative hills like the much older Appellation Mountains.
Mechanical weathering occurs when materials break up via outside forces that do not alter the composition of the material. Through mechanical forces, geologic formations like cliff faces or monoliths simply fragment into smaller and smaller pieces. The smaller the fragments become, the greater the surface area exposed to the forces of chemical weathering. As a direct result of this process of fragmentation, the material also becomes more susceptible to the forces of erosion. It is through the varying forces of erosion that these broken-down fragments of material are transported either from the mountain top to the valley or sometimes even halfway across the globe.
One of the more common and easily recognized forms of mechanical weathering is frost wedging. Frost wedging occurs as water or moisture invades the pore space or crevices within the rock. As the temperature cools to the freezing point, the moisture or water expands with great force, mechanically breaking up the material.
Another common form of mechanical weathering occurs as a result of biological activity. Similar in process to frost wedging, root wedging, for example, breaks up geologic materials as tiny new plant growths work their way into cracks and crevasses. As the roots grow, they apply tremendous force on the fractured rock until it breaks and falls away. Human endeavors like strip mining or excavation are another, more extreme example of mechanical weathering through biological activity.
Chemical weathering occurs as rock and other materials are broken down and altered at a microscopic and even molecular level. Weathering catalysts like oxygen react with rocks and minerals, causing chemical reactions that change the material’s composition.
Water and Chemical Weathering
Water in its pure form is not a chemical catalyst for weathering, but it is the most important component of chemical weathering as it carries catalysts like oxygen within it. Consider the rusting of a great iron bridge beam. Over time and unprotected exposure to the tiny chemical catalysts within water’s varied solutions, those iron beams can become fragile and week. Iron deposits in rock formations react in a similar manner. As these formations react with oxygen in water, they break down, weakening the internal structure of the rock itself until it too begins to fragment.