GEOSC 10
Geology of the National Parks

Chemical Weathering

Chemical Weathering

 

Chemical processes break down rocks, and some of the material dissolves in water and washes away as part of a great, slow cycle.  Here’s a short introduction from Dr. Alley at the Bear Meadows National Natural Landmark, not far from Penn State’s University Park campus. Bear Meadows is one of the few natural wetlands in central Pennsylvania, and has a lot of blueberries and bears as well as interesting geology.

Video: Weathering at Bear Meadows (:49 sec)

Click here for a transcript of the Bear Meadows video.

I'm here at Bear Meadows in central PA, one of the few natural wetlands in the area, and Sinking Creek comes out of here. It's a nice trout stream, and you can probably see that the water is just a little bit brownish from tannic acids that are picked up in the wetland, and those acids help the water break down the rocks. Even without them, CO2 in the water makes a weak acid which breaks down the rock. And so what you can't see in here are pieces of rocks that are the raw materials for shells that eventually will go down subduction zones. And sometime 100 million years or more in the future might make a new mountain to be hosting a wonderful place like Bear Meadows.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0. Filmed by Cindy Alley.

Chemical changes are often more interesting and more complex than physical ones. There is a great range of possible changes, and you must know a lot of chemistry to really appreciate all of them. In general, weak acids are the most important. (Strong acids would be most important, except nature doesn’t make large quantities of them!) Rainwater picks up carbon dioxide from the air and becomes a weak acid called carbonic acid. In soils, water may pick up more carbon dioxide plus organic acids from decaying organic material, becoming a slightly stronger but still-weak acid.

When acid attacks a rock, the results depend on what minerals are present, how warm, wet, and acidic the conditions are, and a few other things you don’t need to worry about. We can sketch some general patterns. Suppose we start with granite, a silica-rich rock that forms in many continental and island-arc settings. Granite is fairly common and contains a lot of the commonest elements in the Earth’s crust, so learning about granite gives you insights into weathering of other things. Don’t obsess about learning the details of the minerals we discuss; start by looking for the big picture.

In the image below, you see Penn State graduate Matt Spencer in front of white granite that was intruded into dark metamorphic rock, along Trail Ridge Road in Rocky Mountain National Park. The granite has weathered faster than the metamorphic rock in this environment, so the granite remains only where it is protected by the overhanging metamorphic rock. (These vaguely mushroom-shaped features are called "hoodoos", by the way.)

Video: White Granite Intruded into Metamorphic Rock (5min, 6 sec)

Click here for the transcript of the granite and metamorphic rock video.

So we are on Trail Ridge Road in Rocky Mountain National Park and you're looking at Matt Spencer, who's a Penn State grad and became a great professor of geology, standing next to some really interesting outcrops. And on top is this dark metamorphic rock and below it is a granite that was intruded into a crack in the metamorphics and then froze there. So a fascinating-looking place, just to go visit if you're ever up on Trail Ridge Road, I strongly recommend it.

Here is the metamorphic rock viewed a little closer and if you know often that metamorphic rocks are sort of folded and you can follow the folds in the layers of the rock here if you look carefully. And so that is on there and then if we go and look just next to the metamorphic rock you can see the contact where the metamorphic rock meets the granite. The metamorphic rock was probably hot when the granite was squeezed in, but the granite was hotter, and so the granite froze faster, cooled faster, and got smaller crystals right next to the metamorphic rock. And so you can see the chilled margin of that granite when it got squirted in there. All right, so then we can go back and see Matt standing next to this. In this environment, the granite weathers a little faster so it sort of saved where it's being protected by the metamorphic on top. Let's now zoom in and look at granite. This is actually a different granite from down the hill but a lot of granites look very similar. That is my index finger at the bottom sticking in there and we'll zoom in some more and you'll notice that the granite is made of grains. Some of those grains are little gray ones with the arrows pointed at them. They are a mineral called quartz. The chemical formula of quartz is SiO2, sometimes it's called silica. The Si stands for silicon, the O is oxygen. When the granite weathers, the quartz basically just stays as quartz and makes quartz sand. The arrows are now pointing at feldspars. There's actually two different feldspars in here but they're very similar. Feldspars include these silica, the SiO2. They all have aluminum and some of them have calcium, Ca, or sodium, Na, or potassium K. You do not need to memorize this. The formulas of the feldspars are KAlSi3O8 and then sort of a range between the calcium one and the sodium one shown there on the bottom. When the feldspars weather, they make clay, and the clay tends to keep the potassium, K, and then a lot of the aluminum the Si and the O plus a little water and that calcium, Ca, and the sodium, Na, dissolve and wash away to the sea. There's a dark mineral. In this one it is biotite. In some it's a different mineral so we won't worry too much about that mineral. You don't have to memorize biotite. The dark mineral always has some silica and it almost always has some magnesium, Mg, and some iron, Fe, and maybe some other things. We have now named eight elements, oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium, and that order is the eight commonest elements in the crust of the earth. That's more than 99 percent of the crust of the Earth. When the granite weathers and the dark mineral breaks down it releases magnesium that washes to the sea and iron that rusts and stays in the soil. So granite breaks down to give sand, clay, and rust that stay as soil, usually with added worm poop or other formerly living things. And granite gives dissolved materials that wash to the sea where the calcium goes into shells, the sodium makes the sea salty, the Magnesium may go into shells or react with rocks. Eventually, the soil, the sand, and clay and rust washed to the sea and they go down subduction zones with the salty water and the shells and the rocks to melt under volcanoes and make new granite, but that takes a long time. At least a lot of millions of years and maybe hundreds of millions of years or longer.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

As shown in the close-up picture of a granite boulder below, granite usually is composed of four minerals: quartz (which is almost pure silica, with silica in turn composed of the elements silicon and oxygen), potassium feldspar and sodium-calcium feldspar (mostly silica, with a little aluminum replacing some of the silicon, and potassium, sodium or calcium added for balance), and a dark silica-bearing mineral containing iron and magnesium (often a dark mica called biotite). The eight elements named in this paragraph make up almost 99% of the atoms in the rocks of the crust of the Earth. (Helping living things survive and running our economy requires many other elements that are quite rare in rocks, one reason that geologists are hired to find valuable, rare things and help mine them.)

Close-up view of a part of a granite boulder.
Close-up view of a part of a granite boulder, also in Rocky Mountain National Park, with Dr. Alley's index finger for scale. The large crystal just above and right of the finger is a feldspar (there are two different kinds of feldspar here, but they look similar). The small gray spots are quartz, and the darker spots are biotite mica. The different mineral grains are identified by Dr. Alley in the video just above, beginning at about 2:00 minutes.
Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

When granite interacts with carbonic acid, several things happen. Typically, for most of the minerals in most environments:

  • Iron (Fe) rusts. It picks up water and oxygen and remains in the soil as little pieces of rust.
  • The aluminum (Al), potassium (K), and silica (SiO2) from the feldspars and from the dark mineral rearrange into new minerals, called clays, that also include some water.
  • The calcium (Ca), sodium (Na), and magnesium (Mg) dissolve in water and wash away.
  • Most of the quartz (silica as a mineral) sits there almost unchanged as quartz sand (a little of it may dissolve and wash away, but most stays).

One can write a sort of equation:

Granite → rust + clay + (dissolved-and-washed-away Ca + Na + Mg) + quartz sand

The rust, sand, and clay left behind, plus a little organic material often including worm poop, become the indispensable layer we know as soil. (And, if you have ever tried to drive a car on soft soil during a rainstorm and had your tires sink in and get stuck, you may call the soil “mud”, possibly with some bad words added.)

The calcium and silica that dissolve and wash into the ocean are used by sea creatures to make shells, the dissolved magnesium washed into the ocean often ends up reacting with hot rocks at spreading ridges to make new minerals in the seafloor or goes into some of the shells, and the dissolved sodium accumulates in the ocean to make it salty. (Eventually, the ocean loses some salt, often by the salty water getting trapped in spaces in sea-floor sediments and going down subduction zones to feed volcanoes; evaporation of water in restricted basins also may cause deposition of some salt.)

You should recognize that this is a very general description of what happens; were it this easy, there would not be hundreds of soil scientists working to understand this important layer in which most of our food grows. In general, the hotter and wetter the climate, the more stuff is removed—rust and quartz sand can be dissolved in some tropical soils, leaving aluminum compounds that we mine for use in making aluminum. In dryland soils, calcium and magnesium may be left behind forming special desert soils, or sodium may be left behind forming salty soils in which little or nothing will grow.

You also should recognize that the “chunks” in soil – rust, clay, sand, and organic materials – can be carried away by streams or wind, or glaciers, but as chunks rather than invisible dissolved materials. We discuss this loss of chunks in the next sections. If chunks are carried away more rapidly than new ones are formed, the soil will thin, and we will find it difficult to grow food to feed ourselves (this is what Teddy Roosevelt worried about in the quote at the start of this Module). The chunks eventually are carried to the oceans and deposited as sediment on the seafloor, together with a lot of shells.

Recycling

Granite may form beneath a volcano in a subduction zone. We have just seen that the granite then will begin to break down, making dissolved things and chunks. Eventually, the chunks are carried to the sea, by rivers and glaciers and wind (we will study this transport soon), while the dissolved things also go to the sea where they are turned into shells or other things. Sediment consisting of these chunks and shells, with some of the salty water in the spaces, is then taken down subduction zones to feed volcanoes that make granite. Some of the shells even contain a little carbon, and some dead things containing carbon are buried in the sediments, and some of this carbon is taken down subduction zones and supplies carbon dioxide to the volcanoes with water, helping make carbonic acid that weathers the granite.

If this looks like a cycle, it is! The Earth really does cycle, and recycle, everything! But, going around this loop once takes at least millions of years, and may take a lot longer than that, issues we'll discuss later.