Plant Growth and Mechanical Weathering
- Mechanical weathering is a process by which physical forces render rock into smaller materials. A common example is the action of water in the high country: It seeps into seams in a boulder or rockface, then freezes with the chill temperatures of night. As it freezes, it expands, exerting a subtle but intense pressure on the crevice. With the warmth of day the ice melts and the water trickles deeper to repeat the process. Over time, this "frost-wedging" can pry off chunks of rock, littering the ground before a great slab with unconsolidated rubble.
- One major effect of mechanical weathering for plant growth is the creation or modification of habitat. Specific vegetation communities prosper in some of the environments shaped by weathering. For example, various species of buckwheat might colonize the so-called littoral soil of a high ridge littered with rock debris. A great skirt of talus or scree at the base of a cliff, the stones derived from weathering and then transported downslope by gravity (a process of denudation called mass wasting), might play host to thickets of alder. Ecological succession proceeds in any such environment: Over time, as more organisms colonize it, the talus field can build up organic material and deeper soil, possibly supporting a forest down the line.
- Mechanical weathering also plays a major role in the development of soil, which obviously has profound influence on vegetation. Along with contributing nutrients, the weathering of bedrock helps define the texture of a given soil. As Tom L. McKnight notes in "Physical Geography: A Landscape Appreciation," sandstone tends to weather into relatively large bits, which makes a coarse-textured soil through which oxygen and water can pass easily. Contrast this with a rock type like shale with its more minuscule weathering products, which contributes to a very fine soil with minuscule pores more impeditive to water and oxygen. Different kinds of plants flourish better in one or another kind of soil -- some preferring well-drained substrates, others more waterlogged situations.
- Plant growth itself can be an agent of weathering. This influence is sometimes classed as mechanical weathering, or differentiated as "biological weathering." The roots of plants can, like the repetitive cycles of freezing and thawing water, exert a powerful physical force even on rock. A juniper seedling that has managed to grow in the crevice of a cliff can, as it grows and expands its root network, pry apart the rock where it is perched. Other examples of biological weathering are the leaching of nutrients from rock by lichen and the movement of rock materials in soil by burrowing creatures, which can make them more or less available to weathering agents.