Vegetation and Weathering and Erosion

Before we begin this section on biological weathering (that is, the role that vegetation plays in rock weathering), let's briefly describe the broad types of weathering, physical/mechanical and chemical weathering.  NOTE:  More detail and a more complete description of these processes are found later in the introduction pages, but for now, here's a brief definition:  Physical weathering is the breaking up and destruction of rocks into smaller chunks...  Chemical weathering is the decomposition of rocks by surface processes that change the chemical composition of the original material.

What role does vegetation play in rock weathering?

Okay.  Let's start our discussion of the role of plants in rock weathering...  First of all, keep in mind that vegetation plays a role in both physical/mechanical and chemical weathering:  

Dissolution:   Dissolution (a type of chemical weathering) occurs when minerals are dissolved by reactions with acids in the environment (acids such as carbonic acid in water; humic acid in soil, and man-made acids, such as sulfuric acid in rain).  Vegetative bacteria which secrete acid solutions speed chemical weathering.  Generally speaking, because chemical weathering is enhanced by the presence of water, the wetter (and warmer) a climate, the more chemical weathering is occurring.

Root Pry:  Plants and plant roots also tend to pull rock apart (a form of mechanical weathering).  Roots follow nooks and crannies along in the subsurface and, as they get older, expand.  Root expansion pulls and pries apart rock.

These roots are "lifting" (breaking apart) the rock in the image above prying apart the rocks in the image to the right...

Root Throw:  Vegetation can also weather rock as the tree or bush (or any type of vegetation) is "yanked" out of the ground by forces of wind or by merely falling over after it has died.  Root systems tend to penetrate below the surface and attach themselves to and wrap around rocks in the sub-surface.  When the plant falls over, the roots, still clinging to the rocks, literally "throw" up and move around soil and surface material.  Though John and I have noticed that saguaro cacti tend to "snap" off near their base and NOT yank up a bunch of soil, certain plants are very rapid "weatherers" of soil and rock and do rapidly tear away and break up rock.  You may think that "root throw" is a form of erosion due to the "transport" of the material...  This is true -- root throw overlaps in both weathering (break-down of material) and erosion (transport of material), and could have been described in the section below, "What role does vegetation play in erosion"?

Root throw:  Notice how the roots of this tree have "yanked" a ball of soil and rock out of the surface?

What role does vegetation play in erosion?

Plants provide a protective cover on the landscape and slow soil erosion:

-- plants get in the way and slow down water as it drains off the land

-- plants literally soak up water (further slowing runoff)

-- plant roots hold the soil in place, keeping it from being washed away

-- plants weaken the impact of a raindrops hitting the ground, slowing the ability of the rain to induce erosion

Image shows a river bank (on right side of river) that is reinforced (stabilized) with vegetation.  Without the mat of vegetation, the material would probably get washed downstream.