Note: This program first aired on February 9, 2013.
The movement of the plates that makeup the earth’s crust causes earthquakes, yields volcanoes and ultimately results in orogeny, or the uplift of mountains. In the story we tell ourselves about the world, these forces are cast in dramatic roles (whether or not they really deserve them is for a different day). Volcanoes and earth quakes are exciting (if also dangerous), mountains soar above us, the alpine realm virtually synonymous with adventure. Today though, we turn away from these easy to tell tales, to explore the other side of the coin. Remember, in the universe for every action, there is an equal and opposite reaction. And while we are easily distracted and entertained by the expansive spectacle of orogeny, you need to be aware that there is another force at work in the universe. A subversive force, one that constantly and endlessly eats away at the world as we know it, seeking equilibrium. This leveling energy is most commonly known and experienced as erosion.
In physics there is a concept called entropy, which states that systems will tend from states of order to states of disorder, if isolated from additional inputs that would decrease the disorder. Basically put—without input of energy, things gradually fall apart. One way to define a living being is that it is anti entropy, by being alive the life form is using a constant input of energy to combat the tendency towards disorder of its molecules and atoms. Death is when entropy finally wins.
Erosion is entropy in the geologic frame of reference. Mountains may rise up, rocks may form bed rock, but little by little they will be broken down and, given enough time, flattened and brought back down to earth. The mechanism of erosion is called weathering, and can happen either mechanically or chemically. The easiest one to think about is mechanical weathering, we all understand about the physical break down of material. Big rocks turn into small rocks, small rocks turn into pebbles, pebbles turn into sand, sand turns into…well you get the idea. If no new rocks formed at mid ocean ridges and volcanoes, the earth would just be covered with dust.
The number one agent of this mechanical break down of rocks is water, and the fact that we are on the water planet goes a long way towards explaining why we have all this erosion going on all the time. Water can carry scouring particles, which grind away on the surface of bed rock, and dissolve soluble minerals in the crystalline matrix of rocks, weakening them. Water can physically undermine rock structures, and waves can transmit hundreds if not thousands of pounds of pressure per square foot upon impact. And this is all liquid water, frozen water in the form of glaciers, is responsible for huge amounts of erosion over entire landscape regions. On a smaller scale, the freeze and thaw cycle of physical weathering, where liquid water seeps into cracks in rocks, and then expands as it freezes, splitting the rock open, is responsible for much of the break down of the granite we have here in Maine. In fact, here in Maine, between the recent glaciation and the freeze thaw cycles that are inevitable in winter, most of the erosion of rock we see around us is physical.
Equally, if not more important world wide is the process of chemical weathering. The chemical weathering of rocks results mainly from weak acids that form in water. This is primarily carbonic acid, which results from the carbon dioxide naturally found in the atmosphere mixing with surface water. Chemical weathering tends to run faster in warm humid environments, (one reason we have less of it occurring here). In chemical weathering the particles of rock tend to get smaller, just like in mechanical weathering, but they also often are changed chemically as well. The residual minerals that result from chemical weathering are often forms of clay, which as a former amateur potter I find very interesting. And while we have lots of clay here in Maine, it is formed from a totally different process, and isn’t at all related to the clay we get from the southern US, clay that results from the chemical weathering of rocks.
We’ve all heard the expression “rust never sleeps”, well, erosion doesn’t either. We will talk more about erosion in the coming weeks, but in the mean time understand that the forces of mountain building and erosion are in a tenuous balance, the landscape we see around us at any given time is a snap shot of the tension between these two forces. Orogeny and erosion teeter totter back and forth over millions of years, but without them both, the world would be a very different looking place.
References:
The business: From Prof. Stephen Nelson at Tulane University, from an introductory Earth Science class: http://www.tulane.edu/~sanelson/geol111/weathering.htm
