Chemical Weathering - Another important type of weathering that happens on the Earth’s surface is chemical weathering. Chemical weathering is different than mechanical weathering because with this type of weathering, rock is changed, not just in size of pieces, but changed in composition. This means that one type of mineral changes into a different mineral. The reason chemical weathering happens is that most minerals form at high pressure or high temperatures, deep within the Earth.
When rocks reach the Earth’s surface, they are now at very low temperatures and pressures. This is a very different environment from the one in which they formed. The environment at Earth’s surface is so different that these minerals are no longer stable. That’s where chemical weathering begins. Minerals formed deep within the Earth must change to minerals that are stable at Earth’s surface. Chemical weathering is important because it starts the process of changing solid rock into the soil we need to grow food and for the plants we need for our clothing and medicine. The way that chemical weathering works is through chemical reactions that cause changes in the rock.
There are many types of chemical weathering because there are many agents of chemical weathering. You probably remember that mechanical weathering is caused by several agents, such as water, wind, ice,and gravity. Well, water is also an agent of chemical weathering, so that makes it a double agent! Two other important agents of chemical weathering are carbon dioxide and oxygen. We will talk about each of these one at a time.
The minerals that make up most of the Earth’s crust are called silicate minerals. These minerals are mostly made of just eight elements; oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K) and sodium (Na). When chemical weathering occurs, the elements that make up the minerals react to form new minerals. The minerals that form at the lowest temperatures and pressures (closest to the situation at the Earth’s surface) are the most stable while minerals that form from very hot magmas or at very high pressures are the least stable. The elements sodium, calcium, potassium and magnesium actually dissolve easily in water. Iron reacts with oxygen, which leaves atoms of silicon, oxygen and aluminum to combine to form new minerals, like clay minerals.
Water is an amazing molecule. It has a very simple chemical formula, H2O, which means it is made of just two hydrogen atoms bonded to one oxygen atom. Even though it is simple to remember, water is pretty remarkable in terms of all the things it can do. Water is an excellent solvent. The way that a water molecule joins together allows water to attract lots of other elements, separate them from their compounds and dissolve them. Water is such a good solvent that some types of rock can actually completely dissolve in water. Other minerals change by adding water into their structure.
Hydrolysis is a chemical reaction between a mineral and water. When this reaction takes place, water itself separates into ions. These ions grab onto other ions, dissolving them in water. As the dissolved elements are carried away, we say that these elements have been leached. Through hydrolysis, a mineral like potassium feldspar is changed into a clay mineral. Once clay minerals have formed, they are stable at the Earth’s surface.
Carbon dioxide (CO2) combines with water as raindrops fall through the air in our atmosphere. This makes a weak acid, called carbonic acid. This happens so often that carbonic acid is a very common, weak acid found in nature. This acid works to dissolve rock. It also slowly changes the paint on a new car or eats away at sculptures and monuments. The normal situation can be made worse when we add pollutants to the air. Any time we burn any fossil fuel, it adds nitrous oxide to the air. When we burn coal rich in sulfur, it adds sulfur dioxide to the air. As nitrous oxide and sulfur dioxide react with water, it forms nitric acid and sulfuric acid. These are the two main components of acid rain. Acid rain accelerates chemical weathering.
Oxidation is the type of chemical reaction that happens when oxygen reacts with elements at the Earth’s surface. Oxygen is very strongly chemically reactive. The type of oxidation that you are probably most familiar with produces rust when iron reacts with oxygen. Many minerals are rich in iron. They break down as the iron oxidizes, forming new compounds. Iron oxide produces the red color in soils. Chemical weathering can also be contributed to by plants and animals. As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration.
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