How Does Chemical Weathering Work?

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Basalt Weathering Rind

Lava nibbled by groundwater
Chemical Weathering Gallery From stop 20 of the California Subduction Transect. Photo (c) 2006 Andrew Alden, licensed to (fair use policy)

Chemical weathering can dissolve rock or change its composition. In some cases, chemical weathering attacks and transforms the minerals in bedrock from primary minerals to surface minerals. The two main processes in chemical weathering of igneous rocks are hydrolysis (which yields clays plus dissolved ions from plagioclase and alkali feldspar) and oxidation (which produces the iron oxides hematite and goethite from the other primary minerals).

In this photo, you can see chemical weathering in the process of altering this lava cobble into surface minerals. Over time, groundwater acts upon rock like this basaltic lava from the Sierra Nevada. The weathering rind (the discolored strip around the outside of the rock) displays an inner white layer where the basalt's minerals are beginning to break down and an outer red layer where new clay and iron minerals are formed.

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Chemical Weathering and Joints

Rounded in the ground
Chemical Weathering Gallery From stop 18 of the California Subduction Transect. Photo (c) 2006 Andrew Alden, licensed to (fair use policy)

Joints and fractures create blocks with exposed corners. These corners become rounded as they are weathered by water and other chemicals. Over time, the rocks become smooth ovals, like a square bar of soap after repeated use.

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Differential Weathering

Gritty survivors
Chemical Weathering Gallery From stop 20 of the California Subduction Transect. Photo (c) 2006 Andrew Alden, licensed to (fair use policy)

Chemicals attack the major rock-forming minerals of igneous and metamorphic rocks. The first rocks to show visible weathering are those that are least stable at the Earth's surface. 

In this picture of a weathered piece of basalt, you can see crystals that are revealed as less stable rocks are weathered away.

Olivine is the least stable mineral in the basalt pictured here. As a result, it has weathered faster than the other elements. Olivine is followed by pyroxenes plus calcic plagioclase, then amphiboles plus sodic plagioclase, then biotite plus albite, then alkali feldspar, then muscovite and finally quartz. Chemical weathering turns these into surface minerals.

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Holes in the ground
Chemical Weathering Gallery. Photo courtesy jimvangundy of Flickr under Creative Commons license

Limestone, like the bedrock shown here in West Virginia, tends to dissolve in groundwater, creating sinkholes with caves beneath them. 

Both rainwater and soil water contain dissolved carbon dioxide, which creates a very dilute solution of carbonic acid. The acid attacks the calcite that makes up limestone and turns it into calcium ions and bicarbonate ions, both of which enter the water and flow away. This dissolution reaction is also sometimes referred to as carbonation.

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Hydration Weathering of Obsidian

Glass prone to frosting
Chemical Weathering Gallery. Photo (c) 2007 Andrew Alden, licensed to (fair use policy)

Being a glass, when obsidian is exposed to water it is chemically altered to become the more stable hydrated mineral perlite

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Marble Sugaring

Chemical Weathering Gallery. Photo (c) 2004 Andrew Alden, licensed to (fair use policy)

Calcite grains in marble start to dissolve in rainwater, giving it a sugary texture. (click to see full size)

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Oxidation in Ultramafic Rocks

Rusty in a roadcut
Chemical Weathering Gallery. Photo (c) 2009 Andrew Alden, licensed to (fair use policy)

Rocks like peridotite are especially prone to oxidation, forming rusty weathering rinds (edges) only a few years after exposure to the air in moist climates.

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Oxidation of Sulfides

Dissolved into rust and acid
Chemical Weathering Gallery. Photo (c) 2009 Andrew Alden, licensed to (fair use policy)

The sulfide mineral pyrite in this roadcut in California's Klamath Mountains turns to red-brown iron oxides and sulfuric acid when exposed to air.

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Palagonite Formation

Steam alteration of fresh lava
Chemical Weathering Gallery. Photo (c) 2011 Andrew Alden, licensed to (fair use policy

Lava erupting into shallow water or groundwater may be rapidly altered by steam to become palagonite. The palagonite may range from a thin skin to a thick rind. Further chemical weathering causes the palagonite to degrades to clay.

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Spheroidal Weathering of Basalt

Onion-skin weathering
Chemical Weathering Gallery. Photo (c) 2005 Andrew Alden, licensed to (fair use policy)

Some rocks weather in spherical layers. This process, called spheroidal weathering, affects many bodies of solid rock or large blocks. It's also called onion-skin or concentric weathering.

In this basalt outcrop, groundwater penetrates along joints and fractures, loosening and decaying the rock layer by layer. As the process progresses, the surface of weathering grows more and more rounded. Spheroidal weathering resembles the exfoliation that occurs on a larger scale in plutonic rocks. That process, however, is mechanical rather than chemical.

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Spheroidal Weathering in Mudstone

Affects soft rocks too
Chemical Weathering Gallery. Photo (c) 2010 Andrew Alden, licensed to (fair use policy)

Spheroidal weathering affects this massive mudstone in a bluff above the Eel River in northern California. It may also be called concentric weathering.