Weathering is the process that breaks down rocks into smaller pieces. The rate at which rock weathers depends on certain factors. Learn how factors, such as exposure to the atmosphere, the composition of rock and climate affect the rate of weathering.
Weathering is the mechanical or chemical process by which rocks are broken down. The weathering that happens to rocks can be understood by relating it to the digestion of a chocolate chip cookie. When you bite into and chew a chocolate chip cookie, your teeth mechanically break the cookie down into smaller pieces. Your mouth also produces saliva, which contains enzymes that chemically mix with the cookie pieces, breaking them down even further.
The physical elements in nature, such as wind, water, cold and heat, physically chip away at rocks, just like your teeth chipped away at your cookie. Chemical reactions involving water, acids, and certain atmospheric gases mix with the minerals within rocks and break them down, much like saliva acting on your cookie. Now that you have a better understanding of what weathering means, let’s take a look at factors that affect the rate at which weathering happens.
Exposure to the Atmosphere
One factor that influences the rate at which rocks will weather is their degree of exposure to the atmosphere. Rocks that are covered by ground are relatively protected from the environmental elements that tend to mechanically weather rocks, such as wind, water and day-night temperature fluctuations.
As rocks get uncovered and become more exposed to the atmosphere, cracks within the rocks widen due to the expansion of ice that freezes within the cracks and plant roots that grow into the cracks and break apart the rocks. These widened cracks provide more surface area where chemical weathering can occur, further accelerating the weathering rate.
Composition of Rock
Another factor that affects the rate of weathering is the composition of rock. Rocks seem like a pretty basic structure, but in reality, we see that most rocks are composed of a variety of minerals. Minerals are solid substances found within rocks that have their own distinct chemical composition. Minerals that are most reactive when mixed with water, oxygen or other elements will weather more rapidly.
For example, some rocks contain the mineral iron. Iron reacts with oxygen to form iron oxide, which you know as rust. If you have ever had a rusty car, you know that rust is not very strong. In fact, with very little pressure, you could poke your finger through a patch of rust. We see the same thing happens in rocks. The presence of iron in rocks causes them to weather more quickly and break apart more easily.
Some minerals are softer than others, and therefore rocks that contain these minerals will weather more rapidly. For example, the mineral calcite, which is commonly found in limestone, is soft and particularly vulnerable to chemical weathering. When exposed to carbonic acid, which can form when carbon dioxide mixes with rainwater, calcite dissolves. This type of chemical weathering is often seen with cave formation. On the other hand, the mineral quartz is very hard and is slow to weather.
We also see some rocks are more porous than other rocks, meaning the rocks have more pores or holes throughout their structure. If a rock is very porous, the amount of surface area exposed to atmospheric elements is increased and weathering is accelerated.
Climate is another factor that plays a significant role in the rate of weathering. Climate is mostly based on the temperature and rainfall of a region. Water is an important element in weathering, so we see that wetter climates are prone to faster weathering.
Temperature is also important. In a lab, chemists will often use heat to speed the rate of a chemical reaction. Just as in the lab, we see that warmer climates accelerate chemical weathering. Therefore, warm, humid environments, such as the tropics, tend to experience higher rates of weathering than cold, dry climates. For example, if you built a wall of limestone in the middle of a dry desert, it would stay there for a long time. But, if you put the same size limestone wall on a tropical island, the stone would be worn away relatively quickly due to the accelerating rate of weathering.
Let’s review. Weathering is the mechanical or chemical process by which rocks are broken down. One of the factors that affects the rate of weathering is exposure to the atmosphere. Rocks that are fully exposed to the atmosphere and environmental elements, such as wind, water and temperature fluctuations, will weather more rapidly than those covered by ground.
Another factor that affects the rate of weathering is the composition of rock. Rocks are composed of minerals, which are solid substances found within rocks that have their own distinct chemical composition. Minerals that are most reactive when mixed with water, oxygen and other elements will weather more rapidly, as seen in rocks that contain iron. Rocks that contain softer minerals, such as calcite, and those that are more porous, will also weather more rapidly.
Climate is also a factor that plays a significant role in the rate of weathering. Warm, humid climates, such as the tropics, tend to experience higher rates or weathering, than cold, dry climates.
You’ll have the ability to do the following after this lesson:
- Define weathering
- Describe chemical and mechanical factors that affect the rate of weathering