Weathering is the breakdown of rocks and minerals at or near the Earth’s surface. It is caused by chemical and physical interactions with air, water, and living organisms. The variety of agents responsible for weathering means that the rate and extent of weathering is highly variable and depends on multiple processes and phenomena including the chemical composition of rocks, the water cycle, uplift from tectonic processes, and regional biodiversity. Over thousands to many millions of years, the weathering and erosion of rocks affects Earth’s surface features (that is, its topography), soil, nutrients on the land and in the ocean, and the composition of the atmosphere, which in turn affects global climate and ecosystems.
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For the classroom:
Global Change Infographic
Weathering is an essential part of How the Earth System Works. Click the image on the left to open the Understanding Global Change Infographic. Locate the weathering icon and identify other Earth system processes and phenomena that cause changes to, or are affected by, the weathering of rocks.
What is weathering?
The weathering of rocks occurs through both physical and chemical processes. Physical weathering is the breakdown of rock into smaller pieces without altering the chemical composition of the rock. For example, physical weathering can happen as the temperature changes, causing rocks to expand as they warm and contract as they cool, resulting in cracks that lead to the breakdown of the rock. Additional causes of physical weathering include the freezing and expansion of water in rock cracks, and tree roots that dig into rocks that can split them apart.
Chemical weathering occurs when the breakdown of rock results from chemical change in the rock, or the when the rock is dissolved away. The rate of these chemical reactions is affected by climatic conditions such as precipitation and temperature, with water and warmer temperatures increasing the rate. Plant growth, aside from physically breaking up rocks, can also change the environmental chemistry (for example, increase acidity) contributing to chemical weathering.
Over thousands to many millions of years, the weathering of silicate rocks on land (rocks made of minerals that contain the element silica) is an important part of the carbon cycle. Over long-time scales, significant amounts of carbon dioxide (a greenhouse gas) are removed from the atmosphere when rainwater (H2O) mixes with CO2 to form carbonic acid (H2CO3). This weak acid reacts with rocks, breaking them down, resulting in the transport of carbon via rivers to the ocean, where it ultimately becomes buried in ocean sediments to become limestone rock. In contrast, the weathering of limestone by carbonic acid releases carbon dioxide into the atmosphere, but there is no net removal of CO2 from the atmosphere as happens with the weathering of silicate rocks. The burning of fossil fuels returns carbon to the atmosphere (as CO2) at a rate that is hundreds to thousands of times faster than it took to bury. This rate is so high that even though the warming produced by the increased CO2 increases the rate of weathering of silicate rocks, which draws down atmospheric CO2, is not nearly enough to offset the increase in carbon dioxide added to the atmosphere by human activities. Visit the carbon and rock cycle pages to learn more about how weathering is connected to other Earth system processes.
The weathering of rocks affects how much and what kind of rock is eroded by water, ice, wind, and gravity, which can affect soil formation, and where various sedimentary rocks form over thousands to millions of years. Additionally, regional soil quality, nutrient levels (especially nitrogen and phosphorus levels), are dependent on the type of rock that is weathered, which in turn affects local biodiversity. Ultimately, over millions of years, geologic changes in the uplift of land, driven by plate tectonics, alter the geographic location and amount of rock exposed for weathering.
Earth system model about weathering
The Earth system model below includes some of the processes and phenomena related to the carbon cycle. These processes operate at various rates and on different spatial and temporal scales. For example, carbon is transferred among plants and animals over relatively short time periods (hours-weeks), but the human extraction and burning of fossil fuels has altered the carbon cycle over decades, while weathering and volcanism affect the carbon cycle over millions of years. Can you think of additional cause and effect relationships between the parts of the carbon cycle and other processes in the Earth system?
Explore the Earth System
Click the bolded terms (e.g. erosion, rock cycle, and carbon cycle) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you.