Physical weathering

In physical weathering, rocks break down into smaller pieces without a chemical change or reaction. However, these processes still relate to properties of minerals and rocks as chemicals. Here are some examples:

Figure 1. Exfoliation in Yosemite National Park. Photo by author.

• Exfoliation: When an igneous rock such as granite cools below the surface of the earth, it is under high pressure and its atomic structure is compressed as a result. As erosion removes weight above this rock, the space between atoms can expand, which creates cracks in the rock that are approximately parallel to the surface. The layers created this way can then slide off as slabs — sometimes very large ones. This makes relatively smooth and often rounded rock faces on mountains; good examples are Stone Mountain, GA and the many domes of Yosemite National Park, CA.
• Crystal wedging: Rocks can develop cracks from a variety of causes, such as: exfoliation as discussed above, the pressures of continental motions that make parallel sets of cracks called “joint sets,” or the simple layering of sedimentary rocks. When water gets into these rocks, two processes might pry the cracks open:

Figure 2. Rocks broken by frequent freeze-thaw cycles on top of Pikes Peak, CO. Photo by author.

Figure 3. Gypsum crystals forming by evaporation as water seeps out of cracks near Death Valley, CA, with knife for scale. Photo by author.

• Ice wedging: As every chemistry class discusses, hydrogen bonding makes water increase its volume when it freezes. This expansion opens the cracks in rocks, and freeze-thaw cycles allow the water to get farther into the crack and repeat the process. You may already discuss this as part of the process that makes road potholes in winter in cold locations.
• Mineral crystal wedging: Groundwater flows through rocks and dissolves out soluble ions. In dry climates, when this solution reaches the surface, the water evaporates and the ions move into crystal structures to make solids that can pry the rock apart where they form. Common examples are the formation of crystals of evaporite minerals such as halite (NaCl) or gypsum (CaSO₄ ∙ 2H₂O).

The important result of physical weathering is that breaking a rock into smaller pieces quickly increases the surface area exposed to chemical weathering. Of course this type of change is already taught in chemistry class, as the effect of crushing or powdering a reactant on the reaction rate. We’ll see this process fits in with several other weathering reaction rate factors discussed below.

Chemical weathering

Chemical weathering occurs when reactions change reactant minerals into new compounds that are the mineral products. There are several major processes at work, and some examples are given below.

• Carbonation: Rainwater falling through the air picks up CO₂, making a dilute carbonic acid solution, even in normal rain water (not “acid rain”): CO₂ (g) + H₂O (l) → H₂CO₃ (aq). If the rainwater soaks into the ground, it may pick up more CO₂ from bacteria in the soil, so the carbonic acid is more concentrated.When this water reaches limestone or marble (both made of calcite, CaCO₃) the carbonic acid can dissolve the rock, in addition to neutralizing the acid.
  
  

  H2CO3 (aq)	+	CaCO3 (s)	→	Ca2+ (aq)	+	2HCO31-(aq)
  rainwater	+	calcite		ions in solution

  
  So the rock slowly washes away in the rain and groundwater. This means areas underlain by rocks made of calcite like limestone and marble may weather to make valleys, while surrounding rocks remain to make higher elevations. Weathering of these rocks also can make sinkholes and caves.
  
  

  Figure 4. Where limestone has dissolved to make an opening in Mammoth Cave, KY. National Park Service public domain image.

• Oxidation: A very common reaction is iron ions from mafic minerals combining with oxygen from the atmosphere and water to make new minerals. This reaction makes the mineral hematite, which often gives red colors.
  
  

  4Fe3+

  +

  3O2

  →

  2Fe2O3

  
  When water attaches in the crystal structure, we get limonite, which is a brown flaky rusty mineral with a sometimes variable formula.

  4FeO

  +

  3H2O

  +

  O2

  →

  2Fe2O3 ⋅ 3H2O

  Figure 5. Volcanic black basalt, made largely of iron-bearing mafic silicate minerals, has weathered to red as the iron has oxidized in a warm, wet climate. Maui, Hawaii. Photo by author.

• Hydrolysis: This is a reaction with water (not just dissolving). A common example:
  
  

  2KAlSi3O8	+	2H+	+	9H2O	→	2K+	+	Al2Si2O5(OH)4	+	4H4SiO4
  potassium feldspar	+	hydrogen ion	+	water		potassium ions(soluble)	+	kaolinite (clay)	+	silica (in solution)

  
  As a result of hydrolysis reactions, mafic minerals produce Mg and Fe oxides, perhaps with OH- attached, as well as silica in solution
  :
  

  4FeSiO3	+	O2	+	H2O	→	4FeO(OH)	+	4SiO2
  pyroxene	+	oxygen	+	water		geothite	+	silica

  MgFeSiO4	+	2H2O	→	Mg(OH)2	+	4H2SiO3	+	FeO
  olivine						silica		oxidizes as above

Therefore, chemical weathering creates a range of products. These may include clay minerals, iron and aluminum oxides (Fe₂O₃, Al_2O3 and others), as well as colloidal silica, usually attached to clay minerals in soil. Products may also include element ions that are nutrients for life, such as Na⁺, Ca²⁺, K⁺, Mg²⁺, and CO₃^2-. Other possible product types include residual material that is largely chemically inert, especially quartz sand and pebbles (though sometimes appearing as larger pieces). Note also that weathering products combine: clay + rusty brown iron oxides make brown or red soil (with organic material such as soil bacteria mixed in).

Figure 6. This rock (metabasalt) doesn’t normally have layers like this, but layers are forming and peeling off as feldspars weather to clay minerals. Due to the clay’s crystal structure, the new minerals take up more volume than the original feldspar, so the minerals no longer fit, forcing layers to come off. A pen is placed on top of the rock for scale. Photo by author.

The nutrient ions mentioned above are of particular note: these ions are carried by water to plants, which use them to grow. Animals and humans then eat the plants, thus ingesting the nutrients that once were in the rocks. There would be no life on earth if these nutrients weren’t released from the rocks, so the chemical weathering processes are vital for life to exist! Water also carries the ions from rocks to the ocean, thus making the ocean salty.

As noted above, weathering of rocks also produces soil, which covers the bedrock in most places. We will continue to concentrate on rock chemistry in this article, but soil chemistry is important and could be the subject of another entire series of articles on its own. Briefly, most soils have lots of organic material mixed in with their weathered mineral material, especially at the top. As rain sinks into soil, it washes some materials deeper into the soil profile, so layers with varying chemistries develop. At a greater depth — but before reaching hard unweathered rock below — one finds weathered bedrock that can crumble in one’s hand.

Soil will generally reflect the chemistry of the underlying bedrock when the soil is thin or newly forming, such as when a glacier retreats and exposes the bedrock to weathering in the air. As time progresses, however, the nature and chemistry of the soil is strongly influenced by the climate, and so becomes more uniform in an ecosystem with less regard to the rock chemistry. Also, soil varies with topography, tending to be thin on top of hills and mountains, while accumulating to thicker depths in valleys. The National Resources Conservation Service provides a poster of various soil types, and related soil information that may be of interest to teachers.

Of note for teaching chemistry, we find that all of the usual factors affecting the rate of chemical reactions also affect the rate of chemical weathering of minerals and rocks. These are summarized in Table 1 and can be used to recognize that effects we might see in the lab are also occurring in nature.

Recall that in earlier articles in this series, we have seen earth processes such as plate tectonics act as a refinery for earth materials, separating and concentrating substances. This also happens by weathering, and can be useful to people. Here are two examples:

• Physical separation: After breaking down rocks, erosion carries them away, sorting them in the process. One example is when rivers carry quartz sand — left after many other minerals have weathered away — to the ocean; later, ocean currents move the sand around to make beaches. Rivers also deposit high-density materials, such as gold, in places where the water is not moving fast enough to support the heavier particles. This makes “placer deposits” that can be mined for the concentrated minerals.
• Chemical separation: One example of a chemical reaction is the precipitation of iron ore nodules. This can occur when groundwater picks up CO₂, becomes acidic, and then dissolves iron ions out of the rocks. This groundwater can flow downhill to a valley containing calcite (CaCO₃, in limestone or marble), where the acid is neutralized. With this change in pH, the iron ions can no longer remain in solution, and are deposited out as rounded chunks of iron oxide called nodules. These can also form if the groundwater reaches the surface as a pond, where the CO₂ can de-gas from the solution. Humans can then further refine these nodules to get metallic iron. In warm and wet tropical weathering conditions, many ions react chemically and then are leached out of soils by the flow of groundwater. This leaves hydrous aluminum oxide in a multi-mineral deposit called bauxite. Since the weathering processes have concentrated the aluminum ions, this bauxite ore is the major component used in producing pure aluminum.

What is the visible result of all this weathering? It changes the rocks around you into the landscape you see. Among other factors, the landforms around you are shaped by the nature of those rocks and the weathering that occurs on them. So, as mentioned at the start, the geology around your school and town is also the chemistry of the earth around you, and that affects how things weather to make your local landscape. That local chemistry can be very different depending on where you live — each color in Figure 7 represents a different rock unit and the variety is very obvious. This is the “chemical stockroom” under your feet!

Figure 7. Geologic map of conterminous U.S., showing the many varied rock units that may be visible in some places and under the soil as bedrock in other places. You can find more insights about this map from this USGS resource. Image source.

Students can investigate the chemistry of your local area using the directions and information in the activity that accompanies this article. They can first use online geologic maps to find the rocks under your school. Then, using information in the activity, students can determine the basic chemistry of those rocks, and whether those rocks are likely to be resistant or nonresistant to weathering. Finally, they can look for landforms in the region that might be influenced by the rock weathering — that is, the nature of the earth’s chemicals under their feet — and also be able to see those landforms as they travel in your area. With some luck, you will see some “aha!” light bulbs come on when your students see the chemistry that is all around them.

I hope this series of articles has helped chemistry teachers see there is literally a whole world of chemistry visible when looking at the materials and processes of the earth. This can help broaden the application of chemistry beyond the lab, meet some earth science standards for NGSS (if applicable to your program), and generally grow students’ appreciation for earth processes going on around all of us.

Photo credit:
(article cover) Scott/bigstockphotos.com