Types of Chemical Weathering

TYPES OF CHEMICAL WEATHERING

Chemical weathering is the major driving force for weathering in warmer and more humid parts of the world. Minerals can get degraded with natural substances that react with them. Such processes can literally turn a mineral into a soluble ionic compound that can get washed away by running water. This only works where the rock is exposed in some way, but you can see how this could accelerate over time. The smaller a piece of rock the more surface area it has to work on. This means that the process will just speed up as pieces of rock get smaller.

Higher temperatures are best for chemical weathering. The best case scenario is for chemical and mechanical weathering to happen along with it. This means that you will get more chemical weathering if there is mechanical weathering to break rocks up into smaller pieces for greater surface area to volume ratios.

One chemical reaction involves carbonic acid, which is water and carbon dioxide mixing in clouds. This is what makes rain somewhat acidic. Carbonic acid causes hydrolysis, which means that water mixes with a substance to change its chemistry. Carbonic acid can react with aluminum and silicon-containing feldspars to make molecules that are involved in clay. In other words, hydrolysis takes silicates in feldspar to make clay, metal ions, and ordinary silica. This clay creates fine sediment that goes into making sedimentary rock, such as limestone.

Dissolution is essentially hydrolysis that dissolves bedrock minerals and allows the resultant metal ions to remain in water. Certain minerals, such as carbonates and evaporites, are more likely to do this process. More acidic water will turn rocks into dissolved substances much more rapidly. Biological agents can release organic acidic substance onto rocks that accelerate the process of dissolution of rock. Humid and wet areas will also have accelerated dissolution.

Remember the Bowen Reaction Series that turned basaltic rock into rocks with more and more silica in it in magma, and that leads to crystals in the rock as it cools? This reaction series indicates that those things at the top will crystallize first and those that will crystallize last on the bottom at lower temperatures as magma rises. The olivine

and pyroxene near the top will chemically weather faster than those at the bottom. This means that quartz is the least likely to chemically weather.

This fact is described by the Goldich Dissolution series, which is essentially the Bowen Reaction Series in reverse. The idea is to demonstrate that olivine and pyroxene are rarely seen in highly weathered rock because they dissolve so readily into water-soluble ions that simply wash away. You might see this phenomenon when you see sinkholes or caves in bedrock that has a lot of carbonate crystals in it. These sinkholes are created by dissolved substances and are called Karst topography. Figure 35 shows the different types of karst topography features you can get:

Figure 35.

Oxidation is a common chemical phenomenon in geology. You've seen this when iron oxidizes to form rust. Iron binds with oxygen to make iron oxide, which is red or rustcolored in nature. If there is a lot of iron in the rock, you'll see it deeper within the rock. Sediment where water and oxygen can get in more easily might have an oxide in it. Like