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Chemical Weathering
What Is Chemical Weathering?
Chemical
weathering can change the composition and shape of rocks
by Andrew Alden
There are three
types of weathering which affect rock: physical, biological, and chemical.
Chemical
weathering, also known as decomposition or decay, is the breakdown of
rock by chemical mechanisms.
How Chemical Weathering Happens
Chemical weathering
does not break rocks into smaller fragments through wind, water, and ice
(that's physical weathering).
Nor does it break
rocks apart through the action of plants or animals (that's biological
weathering).
Instead, it changes
the chemical composition of the rock, usually through carbonation, hydration,
hydrolysis or oxidation.
Chemical weathering
alters the composition of the rock material toward surface minerals,
such as clays.
It attacks minerals
that are relatively unstable in surface conditions, such as the primary
minerals of igneous rocks like
basalt, granite or peridotite.
It can also occur
in sedimentary and metamorphic rocks
and is an element of corrosion or
chemical erosion.
Water is especially
effective at introducing chemically active agents by way of fractures and
causing rocks to crumble piecemeal.
Water may also
loosen thin shells of material (in spheroidal weathering). Chemical
weathering may include shallow, low-temperature alteration.
Let's take a look
at the four main types of chemical weathering that were mentioned earlier. It
should be noted that these are not the only forms, just the most common.
Carbonation
Carbonation occurs
when rain, which is naturally slightly acidic due to atmospheric
carbon dioxide (CO2), combines with a calcium carbonate
(CaCO3), such as limestone or chalk.
The interaction
forms calcium bicarbonate, or Ca(HCO3)2. Rain has a normal pH level
of 5.0-5.5, which alone is acidic enough to cause a chemical reaction.
Acid rain,
which is unnaturally acidic from atmospheric pollution, has a pH level of 4 (a
lower number indicates greater acidity while a higher number indicates greater
basicity).
Carbonation,
sometimes referred to as dissolution, is the driving force behind the
sinkholes, caverns and underground rivers of karst
topography.
Hydration
Hydration occurs
when water reacts with an anhydrous mineral,
creating a new mineral. The water is added to the crystalline structure of a
mineral, which forms a hydrate.
Anhydrite, which
means "waterless stone," is a calcium sulfate (CaSO4) that
is usually found in underground settings. When exposed to water near the
surface, it quickly becomes gypsum,
the softest mineral on the Mohs hardness
scale.
Hydrolysis
Hydrolysis is the
opposite of hydration; in this case, water breaks down the chemical bonds of a
mineral instead of creating a new mineral. It is a decomposition
reaction.
The name makes this
one particularly easy to remember: The prefix "hydro-" means water,
while the suffix "-lysis"
means decomposition, breakdown or separation.
Oxidation
Oxidation refers to
the reaction of oxygen with metal elements in a rock, forming oxides.
An easily
recognizable example of this is rust. Iron (steel) reacts easily with
oxygen, turning into reddish-brown iron oxides.
This reaction is
responsible for the red surface of Mars and
the red color of hematite and magnetite, two other common oxides.
Andrew
Alden
Professional
geologist, writer, photographer, and geological tour guide
Thirty-seven
years of experience writing about geological subjects
Six
years as a research guide with U.S. Geological Survey (USGS)
Experience
Andrew
Alden is a former writer for ThoughtCo who contributed hundreds of
articles for more than 17 years. Andrew works as a geologist, writer, editor,
and photographer. He has written on geological subjects since 1981 and
participates actively in his field. For example, Andrew spent six years as a
research guide with the U.S.
Geological Survey, leading excursions on both land land and at sea. And
since 1992, he has hosted the earthquakes conference for the online discussion
platform, The Well, which began as a dialogue between the
writers and readers of the Whole Earth Review.
In
addition, Andrew is a longtime member of the member of the Geological Society of
America — an international society that serves members in
academia, government, and industry; and the American Geophysical Union — a community of
earth and space scientists that advances the power of science to ensure a
sustainable future.
Andrew
lives in Oakland, California; and though he writes about the whole planet
and beyond, Andrew finds his own city full of interest too and blogs about its
geology.
Education
Andrew
Alden holds a bachelor's (B.A.) degree in Earth Science from the
University of New Hampshire, College of Engineering and Physical Sciences, in
Durham, N.H.
Awards
and Publications
Andrew Alden on Earthquakes (The Well Group,
Inc., 2011)
Assessment of River — Floodplain Aquifer Interactions (Environmental
and Engineering Geoscience, 1997)
Andrew
Alden on Hosting (The Well Group, Inc., 1995)
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and Dotdash
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