Weathering

Weathering is a chemical and physical process in which rocks fragment or break into smaller sedimentary rocks. It plays a significant role in the creation and deposition of minerals in the soil. It is also involved in the cycling of elements such as oxygen, carbon, and hydrogen and of vital compounds, including water and carbon dioxide.

Basic Principles

Weathering is a geological process involving the chemical or physical breakdown of rocks into smaller pieces or minerals. In some cases, weathering leads to the formation of new minerals that compose the Earth's soil. In other cases, the process creates smaller particles, such as sand and dust.

The rock particles produced out of this breakdown are known as sedimentary rocks. Once those particles are formed, they may be transported to another location by water, wind, or other natural process through erosion. When those particles cement together to form new rocks, the new stones are known as clastic rocks.

Two other basic types of rock are subject to weathering. The first is igneous rock, produced as molten rock deep beneath the Earth's crust. The magma flows outward, and in some cases, it simply pools in small pockets; in other cases, it is ejected more violently through volcanic eruptions. In both situations, the magma cools to form igneous rocks.

The second basic type of rock is metamorphic. This rock was originally igneous or sedimentary, but under heat or extreme pressure (such as when the Earth's tectonic plates contact each other), it “transforms” in composition to reestablish equilibrium or balance.

Weathering leads to the formation of many different types of soil at the Earth's surface. Several factors contribute to these variations, including climate, the physical and chemical environment, the type of basic rock undergoing weathering, and the time that transpires. Weathering is one of the most significant contributors to the formation of Earth's many landscapes, shaping mountains and forming plains and plateaus.

Background and History

Before the eighteenth century, prevailing scientific thought held that the biblical flood formed the Earth's surface features, including its mountains, valleys, and plains. Most scientists believed that even the rocks and soil had been carried and then deposited by such a cataclysmic event. However, in the late eighteenth century, Scottish scientist James Hutton considered the founder of modern geology, developed an alternative point of view.

In observing the remnants of flooding that occurred in the rivers near his home, Hutton noticed a large volume of sand, pebbles, and other rocks. He then theorized that instead of through a great biblical flood, the rocks and soil that appeared on the Earth's crust (and on the mountains and hills) had been deposited through millions of years of flooding, mountain erosion, and other natural processes. The sediment that covers the Earth is deposited in an ongoing, lengthy process of recycling and erosion. Hutton's concept was later dubbed uniformitarianism.Weathering plays an essential role in another of Hutton's concepts, in which each type of rock (metamorphic, igneous, and clastic) goes through a cycle whereby it is formed, moved to the Earth's surface, broken down, and returned to the Earth's interior; here, Hutton added, the process restarts. Weathering is the primary mechanism by which the minerals and compounds contained in the soil and rocks are prepared for this recycling process. Hutton's concepts of the rock cycle and uniformitarianism led to the notion that Earth's history is far more expansive than Hutton's predecessors believed. With the Earth's geologic time dating to 4.6 billion years ago (a figure now accepted), much more can be learned about how the planet's geological and geodynamic processes have evolved.

Types of Weathering

Weathering occurs in two general forms. The first is physical weathering, in which a rock, for example, which has long been situated beneath the great weight of other rocks or under glaciers and bodies of water, expands when the pressure of those elements is removed. If preexisting fractures (or joints) exist in these rocks, the relief of this pressure can cause the rock to split along those cracks. Physical weathering also occurs when ice and other crystals within the rock push outward, when plants grow into fractures in the rock, when animals burrow through them, and when a rock is subjected to extreme heat.

The second type of weathering is chemical. In this manifestation of weathering, compounds and elements surrounding the rock react with the minerals contained within the rock, breaking down the stone. For example, carbon dioxide in the atmosphere may be captured in falling rain. When that rainwater reaches a rock, it causes the less stable minerals on the rock's surface to react to the carbon, resulting in the corrosive substance carbonic acid. In other situations, acidic compounds (such as sulfuric acid), resulting from the combination of oxygen and certain minerals found in the soil or vegetation, are formed and transported through rivers and streams. These compounds strip and break down rocks.

Weathering can have several effects on a rock's appearance. In some rocks, weathering is evident on an external layer, but inner layers remain unaltered but exposed. In other examples, a rock is broken down by sloughing away small fragments from its exterior. Furthermore, some weathering occurs from within, in which water, ice, and minerals entering the rock's fractures break apart the rock into cube-like pieces.

Environmental and Structural Factors

Weathering occurs in a variety of forms because of many contributing factors. One of the key elements in weathering is the rock's structure. Chemical weathering typically occurs when unstable minerals on the rock's surface come into contact with the corrosive element. Some minerals, however, are less susceptible to weathering than others. Quartz, for example, is highly resilient to chemical and physical weathering. Limestone, however, comprises calcite, a mineral that is more easily weathered. Studies show that limestone can be broken down even by algae and seaweed.

Also playing an important role in the degrees to which weathering occurs is the rock's immediate environment. For example, some minerals are more susceptible to weathering in a warm environment. Sodium, according to one study, shows a higher rate of weathering in warmer climates than in cooler temperatures. Warmer climates also foster vegetation growth, which is important in weathering. Furthermore, certain compounds and elements in an environment are factors in weathering. Iron, for example, typically reacts to free oxygen when exposed, causing a weathering process called oxidation. Meanwhile, when calcite is submerged in water, it quickly dissolves.

The Role of Water in Weathering

Water plays a significant role in the weathering and erosion processes. Regarding physical weathering, water that seeps into fractures and cracks in a given rock may freeze and expand, breaking the rock into smaller pieces. High water pressure can also contribute to the physical weathering of rocks, pushing apart fractured rocks and separating them from the main body. In chemical weathering, compounds and minerals flowing in the water can bond with the minerals on the rock's surface, drawing them away for erosion.

Two important hydrological factors can either foster or inhibit the weathering process: the rate at which water flows and the length of time the rock is submerged underwater. Weathering is likely to occur at greater rates when the water flow is slow because the minerals contained in the water have more time to bond with and remove unstable surface minerals. In a study of granite material, scientists concluded that, as fluid residence (the period in which the rock is submerged in the water) continues for ten or more years, the amount of chemical weathering that will occur is significant when compared with rocks that have shorter periods of fluid residence.

Computer Modeling

When studying the geological phenomenon of weathering, scientists look for common characteristics and trends by employing computer models to analyze patterns within different environmental conditions. For example, in one study, statistical data were collated from several sedimentary rock formations in shallow-water environments. Using these data, scientists created a computer model that simulated sedimentary rock formation and deformation according to high and low sea-level conditions. Because of the complexity of studying clastic rock formation and composition, computer models are proving to be an increasingly reliable tool for scientists. In the twenty-first century, Artificial Intelligence has been increasingly employed to analyze the data provided by computer models. 

Stratigraphy

When clastic rocks are carried to their ultimate destination, they frequently cement together in basins to form large, solid slabs that, in time, become layered. Studying these layers, a scientific approach known as stratigraphy, helps scientists find clues about the myriad geological changes that have occurred in and since the Earth's prehistory. For example, stratigraphic analysis of different clastic basins can yield information about the geodynamic activity during a particular era or epoch.

One stratigraphic study of sites in Nevada and Estonia revealed several radioactive minerals (namely, strontium and a carbon isotope) deposited in those two regions more than 400 million years ago. An analysis of the curves within those layers suggested that the minerals came from weathered source rocks pushed outward during a continental collision nearly 50 million years earlier. In this case, a stratigraphic study of these regions revealed that weathering plays a significant role in recycling and transporting minerals from their basic sources.

Lithology

One of the more useful ways to study weathering is by analyzing the sedimentary rocks that result from weathering. A microscopic and chemical analysis of the minerals found in soil, sand, and clay can reveal much about the geochemical characteristics (lithology) of the rocks from which the particles are separated. Scientists studying lithology can, therefore, assess how weathering occurs in specific minerals and environments. Lithology also can reveal the basic rocks from which weathered particles were formed and composed. For example, a hydrochemical analysis of soil and sediment samples from hundreds of sites in Japan revealed large volumes of silicate (known to undergo high weathering). Scientists concluded that silicate, coupled with water runoff, plays a dominant role in the geological processes in this region. This study of Japan's overall lithology reveals much about the overall process of weathering.

Relevant Organizations and Institutions

Weathering has a wide range of implications for those interested in studying the Earth's past and those seeking prime locations for exploratory drilling and mining. The following groups and institutions are interested in the study of weathering.

The United States (US) government plays a significant role in the study of weathering. For example, the US Geological Survey (USGS) studies weathering in a wide range of environments. Scientists within the USGS have authored several scholarly papers on chemical and mechanical weathering and its effects (including monitoring weathering rates in certain regions). Meanwhile, the National Science Foundation provides funding for scientists working in geological weathering.

Universities and their faculties and research staff play key roles in studying weathering processes. The University of California system and the University of Tennessee are among the myriad institutions whose Geology and Earth Science departments offer weathering and sedimentary geology courses. At many of these universities, the leading theories and research in this field are developed and investigated.

Energy companies always seek new oil, coal, and gas deposits. Such findings are usually unearthed in or near clastic rock basins. For this reason, the petroleum industry often calls upon full-time geophysicists, mineralogists, and geodynamics consultants to help them find such deposits.

Implications and Future Prospects

The study of weathering has several implications for the overall study of Earth's geodynamic and geological processes. Sedimentary rocks produced by weathering provide many clues to geodynamic processes, including volcanism and tectonics (the geodynamic processes surrounding the Earth's outer crust). Scientists who seek to learn about climate change may also look to the strata of clastic rock dating to understand the conditions during prehistoric periods that featured significant climate shifts. These samples might not have been available had the weathering process not broken down the basic rocks that contained such evidence.

Weathering studies have been greatly aided by the technologies available in the twenty-first century. Computers capable of collating large volumes of data and creating two- and three-dimensional models based on this data are helping scientists better understand how weathering occurs under a wide range of environmental conditions. Furthermore, the Internet allows scientists worldwide to quickly share data on samples found in various basins and collectively analyze such data.

Principal Terms

chemical weathering: a form of weathering in which rocks are broken down through chemical interactions between surface minerals and water and acidic compounds

clastic rock: a type of rock formed by the cementing of sedimentary rocks

erosion: a process in which weathered rock particles are transported by water, wind, and other natural means

igneous rock: a type of rock created from cooling magma beneath the Earth's surface

physical weathering: a form of weathering in which rocks are broken down by environmental conditions, crystal growth, or interaction with animals or vegetation

rock cycle: a process in which all three basic types of rock are formed, brought to the Earth's surface, weathered, and returned to the Earth's interior in a cyclical pattern

sedimentary rock: a type of rock that breaks from a larger igneous, metamorphic, or clastic rock

stratigraphy: the study of layered clastic rocks, revealing the type of sedimentation and weathering that has occurred in time

uniformitarianism: a theoretical concept that sedimentation occurs because of events transpiring through Earth's history instead of through a single, biblical flood

weathering: a geological process in which igneous, metamorphic, or sedimentary rock is broken into smaller particles

Bibliography

Atkinson, David. Weathering, Slopes, and Landforms. London: Hodder Education, 2004.

Beaulieu, E., et al. “Modeling of Water-Rock Interaction in the MacKenzie Basin: Competition Between Sulfuric and Carbonic Acids.” Chemical Geology, vol. 289, nos. 1/2, 2011, pp. 114-123.

Bland, Will, and David Rolls. Weathering: An Introduction to the Scientific Principles. London: Hodder Education, 1998.

Calcaterra, D., and M. Parise. Weathering as a Predisposing Factor to Slope Movements." Engineering Geology Special Publication, vol. 23. London: Geological Society of London, 2010.

Dagdelenler, Gulseren, Ebru A. Sezer, and Candan Gokceoglu. “Some Non-linear Models to Predict the Weathering Degrees of a Granitic Rock from Physical and Mechanical Parameters.” Expert Systems with Applications, vol. 38, no. 6, 2011, pp. 7476-7485.

Horbe, Adriana M. C., and R. R. Anand. “Bauxite on Igneous Rocks from Amazonia and Southwestern of Australia: Implication for Weathering Process.” Journal for Geochemical Exploration, vol. 111, nos. 1/2, 2011, pp. 1-12.

Maher, K. “The Dependence of Chemical Weathering Rates on Fluid Residence Time.” Earth and Planetary Science Letters, vol. 294, nos. 1/2, 2010, pp. 101-110.

Richards, Cory. “Weathering.” National Geographic Education, 24 Apr. 2024, education.nationalgeographic.org/resource/weathering. Accessed 27 July 2024.

“Weathering.” Understanding Global Change, ugc.berkeley.edu/background-content/weathering. Accessed 27 July 2024.

“Weathering.” British Geological Survey, www.bgs.ac.uk/discovering-geology/geological-processes/weathering. Accessed 27 July 2024.