Chemical weathering results from chemical changes to minerals that become unstable when they are exposed to surface conditions. The kinds of changes that take place are highly specific to the mineral and the environmental conditions. Some minerals, like quartz, are virtually unaffected by chemical weathering, while others, like feldspar, are easily altered. In general, the degree of chemical weathering is greatest in warm and wet climates, and least in cold and dry climates.
As with mechanical weathering, water is the most important agent in chemical weathering. The important characteristics of surface conditions that lead to chemical weathering are the presence of water (in the air and on the ground surface), the abundance of oxygen, and the presence of carbon dioxide, which produces weak carbonic acid when combined with water. That process, which is fundamental to most chemical weathering, can be shown as follows:
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H2O + CO2 ↔ H2CO3 then H2CO3 ↔ H+ + HCO3− water + carbon dioxide ↔ carbonic acid then carbonic acid ↔ dissolved hydrogen ions + dissolved bicarbonate ions |
Yikes! Chemical formulas
Lots of people seize up when they are asked to read chemical or mathematical formulas. It’s OK, you don’t necessarily have to! If you don’t like the formulas just read the text underneath them. In time you may get used to reading the formulas.
The double-ended arrow “↔ ” indicates that the reaction can go either way, but for our purposes these reactions are going towards the right.
Here we have water (e.g., as rain) plus carbon dioxide in the atmosphere, combining to create carbonic acid. Then carbonic acid dissociates (comes apart) to form hydrogen and bicarbonate ions. The amount of CO2 in the air is enough to make weak carbonic acid. There is typically much more CO2 in the soil, so water that percolates through the soil can become more acidic. In either case, this acidic water is a critical to chemical weathering.
In some types of chemical weathering the original mineral becomes altered to a different mineral. For example, feldspar is altered—by hydrolysis—to form clay minerals plus some ions in solution. In other cases the minerals dissolve completely, and their components go into solution. For example, calcite (CaCO3) is soluble in acidic solutions.
The hydrolysis of feldspar can be written like this:
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CaAl2Si2O8 + H2CO3 + ½O2 ↔ Al2Si2O5(OH)4 + Ca2+ + CO32− plagioclase feldspar + carbonic acid ↔ kaolinite + dissolved calcium ions + dissolved carbonate ions |
This reaction shows calcium-bearing plagioclase feldspar, but similar reactions could also be written for sodium or potassium feldspars. In this case, we end up with the mineral kaolinite, along with calcium and carbonate ions in solution. Those ions can eventually combine (probably in the ocean) to form the mineral calcite. The hydrolysis of feldspar to clay is illustrated in Figure 6.2.1, which shows two images of the same granitic rock, a recently broken fresh surface on the left and a clay-altered weathered surface on the right. Other silicate minerals can also go through hydrolysis, although the end results will be a little different. For example, pyroxene can be converted to the clay minerals chlorite or smectite, and olivine can be converted to the clay mineral serpentine.
Oxidation is another very important chemical weathering process. The oxidation of the iron in a ferromagnesian silicate starts with the dissolution of the iron. For olivine, the process looks like this, where olivine in the presence of carbonic acid is converted to dissolved iron, carbonate, and silicic acid:
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Fe2SiO4+ 4H2CO3 ↔ 2Fe2+ + 4HCO3− + H4SiO4 olivine + (carbonic acid) ↔ dissolved iron ions + dissolved carbonate ions + dissolved silicic acid |
But in the presence of oxygen and carbonic acid, the dissolved iron is then quickly converted to the mineral hematite:
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2Fe2+ + 4HCO3− + ½ O2 + 2H2O ↔ Fe2O3 + 4H2CO3 dissolved iron ions + dissolved bicarbonate ions + oxygen + water ↔ hematite + carbonic acid |
The equation shown here is for olivine, but it could apply to almost any other ferromagnesian silicate, including pyroxene, amphibole, or biotite. Iron in the sulfide minerals (e.g., pyrite) can also be oxidized in this way. And the mineral hematite is not the only possible end result, as there is a wide range of iron oxide minerals that can form in this way. The results of this process are illustrated in Figure 6.2.2, which shows a granitic rock in which some of the biotite and amphibole have been altered to form the iron oxide mineral limonite.
A special type of oxidation takes place in areas where the rocks have elevated levels of sulfide minerals, especially pyrite (FeS2). Pyrite reacts with water and oxygen to form sulfuric acid, as follows:
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2FeS2 + 7O2 + 2H2O ↔ 2Fe2+ H2SO4 + 2H+ pyrite + oxygen + water ↔ dissloved iron ions + sulfuric acid + dissolved hydrogen ions |
The runoff from areas where this process is taking place is known as acid rock drainage (ARD), and even a rock with 1% or 2% pyrite can produce significant ARD. Some of the worst examples of ARD are at metal mine sites, especially where pyrite-bearing rock and waste material have been mined from deep underground and then piled up and left exposed to water and oxygen.
At many ARD sites, the pH of the runoff water is less than 4 (very acidic). Under these conditions, metals such as copper, zinc, and lead are quite soluble, and this can lead to toxicity for aquatic and other organisms. For many years, the river downstream from the Mt. Washington Mine, pictured above, had so much dissolved copper in it that it was toxic to salmon. Remediation work has since been carried out at the mine and the situation has improved.
The hydrolysis of feldspar and other silicate minerals and the oxidation of iron in ferromagnesian silicates all serve to create rocks that are softer and weaker than they were to begin with, and thus more susceptible to mechanical weathering.
The weathering reactions that we’ve discussed so far involved the transformation of one mineral to another mineral (e.g., feldspar to clay), and the release of some ions in solution (e.g., Ca2+ or Fe2+). Some weathering processes involve the complete dissolution of a mineral. Calcite, for example, will dissolve in weak acid, to produce calcium and bicarbonate ions. The equation is as follows:
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CaCO3 + H+ + HCO3− ↔ Ca2+ + 2HCO3− calcite + dissolved hydrogen ions + dissolved bicarbonate ions ↔ dissolved calcium ions + dissolved bicarbonate ions |
Calcite is the major component of limestone (typically more than 95%), and under surface conditions, limestone can dissolve completely, as shown in Figure 6.2.4. Limestone also dissolves at relatively shallow depths underground, forming limestone caves. This is discussed in more detail in Chapter 14, where we look at groundwater.
There are many sinkholes throughout Arizona, and at least seven documented around the city of Sedona, but one of the most well-known is Devil’s Kitchen Sink. Collapsing in the late 1880’s, Devil’s Kitchen Sink has become a sought after tourist destination.
Sedona is known for it’s dramatic exposures of red sandstone that formed during the Permian time period.
Though many sinkholes occur in karst topography, the outcrops in this area contain no limestone! The explanation comes from underlying bedrock. Over 400 feet beneath the red sandstones exposed at the surface is the Redwall Limestone, a Mississippian aged limestone that is known for its extensive cave systems throughout the state.
Approximately 75 million years ago, long after the deposition and lithification of the rock layers, plate tectonics uplifted the entire area. This caused the rocks fracture, as well as to be exposed to weathering processes. As groundwater worked its way down fractures through the insoluble sandstones, it eventually made it to the Redwall limestone, carving out vast caverns. Over time the overlying rock layers began to collapse into the caverns, essentially forming a chimney of collapsed material upward until it eventually broke the surface and formed the sinkhole.
Online Resources: Lindberg, P.A., 2010, Sedona Sinkholes and Groundwater Flow: The Geologic History of Their Evolution, Coconino and Yavapai Counties, Arizona: Arizona Geological Survey Contributed Report CR-10-C, 67 p. and 1 map plate
http://repository.azgs.az.gov/uri_gin/azgs/dlio/1034
Lindberg, P.A., 2010, Geologic Report of the Devils Kitchen sinkhole, Sedona, Arizona: Arizona Geological Survey Contributed Report CR-10-B, 19 p.
***See 6.9 for Text and Media Attributions
What is Geology?“Geology” comes from the Greek roots “geo” (earth) and “logy” (field of study), so it literally translates into the Study of Earth. While initial geologic observations were limited to materials found at the surface and processes that could be directly observed, it has evolved over time to encapsulate the study of Earth as a system. In the present day, geology encompasses the physical structure of the Earth, its change over time, and the dynamic processes that act on it. Geology is a science, which means it is both a body of knowledge about the natural world, as well as a process. That process, called scientific inquiry, studies the Earth through systematic observation, experimentation, and reasoning based on evidence.
After carefully reading this chapter and completing exercises within it, you should be able to:
Geology is an umbrella science that encompasses all biological, chemical, and physical processes that act on the planet and make up our world. In the same way that the human body can be divided into systems (circulatory, digestive, endocrine, etc.), the Earth can be described as a complex entity that encompasses many different systems all acting on one another. There are 5 main systems, or spheres, found on Earth: geosphere, biosphere, hydrosphere, atmosphere, and cryosphere.
The geosphere encompasses all physical material that makes up the interior and surface of the Earth. This includes all rocks and minerals, but is also includes the active processes that link these rocks and minerals together. While many tend to focus on the three types of rocks: igneous, sedimentary, and metamorphic, the forces of plate tectonics and weathering actively change rocks from one type to another. This system is referred to as the Rock Cycle. This is the focus of most physical geology courses and will be the main framework for this text. Because the geosphere describes the physical structure of the Earth, it is integral to all other spheres as well.
The biosphere is made up of all living things, as well as the ecosystems that support that life. Obviously, life can be found in many different places, from the tops of mountains to the bottoms of oceans, in wet and dry places, in cold and hot environments. The biosphere therefore overlaps and interacts with all other spheres.
The Earth has been described as the “Blue Marble” in space due to the presence of enormous amounts of water on its surface. All of the liquid water found on earth makes up the hydrosphere. As water flows on the Earth’s surface, it changes landscapes and nourishes life, and interacts with all other spheres.
All of the gases that circulate around and surround the earth are a part of the atmosphere. This includes the air we breathe, the gases that are responsible for global weather and climate, as well as the ones that burn up meteorites as they plummet toward the Earth’s surface. The atmosphere is further broken into layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
Cryo- comes from a Greek word meaning "frost." The cryosphere encompasses all of frozen water on the Earth. Typically found at high elevations and at the poles, ice can have dramatic impacts on landscapes (geosphere), provide sources of liquid water (hydrosphere), and support ecosystems for living things (biosphere).
While it is easier to look at each system independently, they are all integrated on the Earth. Each one is constantly acting and interacting with another on our dynamic planet.
H2O, or the water molecule, is constantly changing. It is the only substance on Earth that is present in all states of matter - solid, liquid, and gas. The cycling of water throughout the Earth, also called the hydrologic cycle, describes how water moves through various environments on Earth. The largest reservoir of water is in the ocean, and it exists there as a liquid. Through the process of evaporation, it becomes a gas. At that point, it may crystallize as a solid (snow or ice) and deposit on land. It could then melt, forming a liquid again, and run off the landforms, eternally shaping the Earth. It can be taken up into plants, aiding in photosynthesis, and released as a gas through the process of transpiration. The possibilities of pathways for water to travel on Earth are endless.
Backyard Geology: Spheres intersect for one of the 7 Natural Wonders of the World Arizona is known as the "Grand Canyon State" because of the dramatic landscape that unfolds at the intersection of brightly colored flat-lying sedimentary rocks and a dynamic river system. This landscape evolved over millions of years while the underlying rocks were uplifted by Plate Tectonics, and the Colorado River cut down through soft sedimentary rocks..
***See 1.5 for Text and Media Attributions
Geology is an umbrella science that encompasses all biological, chemical, and physical processes that act on the planet and make up our world. In the same way that the human body can be divided into systems (circulatory, digestive, endocrine, etc.), the Earth can be described as a complex entity that encompasses many different systems all acting on one another. There are 5 main systems, or spheres, found on Earth.
The geosphere encompasses all physical material that makes up the interior and surface of the Earth. This includes all rocks and minerals, but is also includes the processes that link these rocks and minerals together. This is the focus of most physical geology courses and will be the main framework for this text. Because the geosphere describes the physical structure of the Earth, it is integral to all other spheres as well.
The biosphere is made up of all living things, as well as the ecosystems that support that life. Obviously, life can be found in many different places, from the tops of mountains to the bottoms of oceans, in wet and dry places, in cold and hot environments. The biosphere therefore overlaps and interacts with all other spheres.
The Earth has been described as the “pale blue dot” in space due to the presence of enormous amounts of water on its surface. All of the liquid water found on earth makes up the hydrosphere. As water flows on the Earth’s surface, it changes landscapes and nourishes life, and interacts with all other spheres.
All of the gases that circulate around and surround the earth are a part of the atmosphere. This includes the air we breathe, the gases that are responsible for global weather and climate, as well as the ones that burn up meteorites as they plummet toward the Earth’s surface.
Cryosphere
Cryo- comes from a Greek word meaning "frost." The cryosphere encompasses all of frozen water on the Earth. Typically found at high elevations and at the poles, ice can have dramatic impacts on landscapes (geosphere), provide sources of liquid water (hydrosphere), and support ecosystems for living things (biosphere).
Geology is an umbrella science that encompasses all biological, chemical, and physical processes that act on the planet and make up our world. In the same way that the human body can be divided into systems (circulatory, digestive, endocrine, etc.), the Earth can be described as a complex entity that encompasses many different systems all acting on one another. There are 5 main systems, or spheres, found on Earth.
The geosphere encompasses all physical material that makes up the interior and surface of the Earth. This includes all rocks and minerals, but is also includes the processes that link these rocks and minerals together. This is the focus of most physical geology courses and will be the main framework for this text. Because the geosphere describes the physical structure of the Earth, it is integral to all other spheres as well.
The biosphere is made up of all living things, as well as the ecosystems that support that life. Obviously, life can be found in many different places, from the tops of mountains to the bottoms of oceans, in wet and dry places, in cold and hot environments. The biosphere therefore overlaps and interacts with all other spheres.
The Earth has been described as the “pale blue dot” in space due to the presence of enormous amounts of water on its surface. All of the liquid water found on earth makes up the hydrosphere. As water flows on the Earth’s surface, it changes landscapes and nourishes life, and interacts with all other spheres.
All of the gases that circulate around and surround the earth are a part of the atmosphere. This includes the air we breathe, the gases that are responsible for global weather and climate, as well as the ones that burn up meteorites as they plummet toward the Earth’s surface.
Cryo- comes from a Greek word meaning "frost." The cryosphere encompasses all of frozen water on the Earth. Typically found at high elevations and at the poles, ice can have dramatic impacts on landscapes (geosphere), provide sources of liquid water (hydrosphere), and support ecosystems for living things (biosphere).
A geologic period of time 251-297 million years ago.
landscape underlain by limestone which has been eroded by dissolution, producing ridges, towers, fissures, sinkholes and other characteristic landforms.
A geologic time period 359-299 million years old.
An organic or chemical sedimentary rock that is primarily composed of calcium carbonate (CaCO3). Limestone is a subgroup of rocks that includes chalk, coquina, and fossiliferous limestone.
The solidification of loose sediment materials as solid sedimentary rock through compacting pressures and cementation.
Physical Geology: An Arizona Perspective Copyright © 2022 by Merry Wilson is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
