Weathering Notes
Weathering, erosion, and deposition act together in a cycle
that wears down and builds up Earth’s surface. The
weathering, erosion, and deposition that geologists
observe today also shaped Earth’s surface millions of
years ago.
There are two types of weathering: 1. Mechanical weathering 2.
Chemical weathering. (Both types of weathering act slowly, but
over time they break down even the biggest, hardest rocks.)
Weathering is the result of the physical and chemical alteration of
rock and mineral material; the resultant products might or might
not be transported.
Mechanical Weathering (physical weathering): the type of
weathering in which rock is physically broken into smaller pieces.
These smaller pieces have the same composition as the rock they
came from.
*Causes of mechanical weathering include: freezing and thawing,
Exfoliation-release of pressure, plant growth, actions of animals,
abrasion, and thermal expansion
1. Freezing: Ice Wedging; Frost Wedging: water seeps into cracks in
rocks and then freezes when temperature drops. Water expands when
it freezes. Ice therefore acts like a wedge that forces things apart.
Wedges of ice in rocks widen and deepen cracks. When the ice melts,
the water seeps deeper into the cracks. With repeated freezing and
thawing, the cracks slowly expand until pieces of rock break off.
2. Abrasion: refers to the grinding away of rock by rock particles carried
by water, ice, wind, or gravity (ex. River rocks rounded because they
have been tumbled in the riverbed by fast-moving water for many
years; rocks shaped by blowing sand (ventifacts); rocks grind against
each other in rock slides, which creates smaller and smaller rock
fragments.
3. Exfoliation: As overlying rock is removed by uplift and erosion, the
pressure on the rock is reduced. As the pressure is reduced, the rock
expands in volume and long, curved cracks develop parallel to the rocks
surface. In this way, an outcrop “sheds” layers of rock. Exfoliation can
often be observed in granite outcrops. (Granite forms underground, so
it forms under a great deal of pressure from the rock above. A granite
pluton 15 km underground forms at 5, 000 times the pressure at Earth’s
surface. As the granite is pushed toward the surface and the overlying
rock is weathered away, the pressure on the rock is reduced and the
granite exfoliates.)
4. Thermal stress weathering: results from the expansion and
contraction of rock, caused by temperature changes. For example,
heating of rocks by sunlight or fires can cause expansion of their
constituent minerals. As some minerals expand more than others,
temperature changes set up differential stresses that eventually cause
the rock to crack apart. Because the outer surface of a rock is often
warmer or colder than the more protected inner portions, some rocks
may weather by exfoliation- peeling away of outer layers. (desertslarge temperature changes- hot in the day and cold at night; forest
fires- intense localized heat can rapidly expand a boulder
5. Plant growth: Plants send their roots into existing cracks in rocks.
As the plant grows, the force of the expanding root becomes so strong
that the crack widens. Eventually, the entire rock can split apart. (ex.
Grass and roots in the cracks of sidewalks and streets)
6. Actions of animals: animals burrow through the soil and move soil
particles around. This exposes fresh surfaces to continued weathering.
Any animal that burrows can cause mechanical weathering. Ants,
worms, mice, coyotes, and rabbits are just some of the animals that
contribute to weathering.
Chemical Weathering: the process that breaks down rock as a
result of chemical reactions.
*Causes of chemical weathering include: dissolution, hydrolysis,
and oxidation, living organisms, carbon dioxide
1. Dissolution: soluble rocks and minerals dissolve in acidic waters
(acid precipitation): Rain, sleet, or snow that contain a high
concentration of acids. Over the past 150 years, people have been
burning large amounts of coal, oil, and gas for energy. Burning these
fuels can pollute the air with sulfur, carbon, and nitrogen compounds.
2.
3.
4.
5.
Such compounds react chemically with the water vapor in clouds,
forming acids. These acids mix with raindrops and fall as acid rain.
Acid rain causes very rapid chemical weathering of rock. Small
amounts of sulfuric and nitric acids from from natural sources, such as
volcanoes, can make precipitation acidic.
Hydrolysis: (hydro- means “water” and lysis- means “separation”)
usually means the cleavage of chemical bonds by the addition of water.
(ex. feldspars altered to clay; groundwater comes in contact with
limestone, dissolving forms of limestone forms karst features, such as
caverns.)
Oxidation: a chemical reaction in which an element, such as iron,
combines with oxygen to form an oxide. This common form of chemical
weathering is what causes rust. (The oxygen in the ari is reacting with
the iron in old cars, aluminum cans, or even a bike creating the rusting
of iron.)
Biological Weathering: living organisms, such as a plant’s roots
growing, produce weak acids that slowly dissolve rock around the
roots. Lichens- plantlike organisms that grow on rocks- also produce
weak acids that chemically weather rock.
Carbon Dioxide: another gas found in air, carbon dioxide, also
causes chemical weathering; carbon dioxide dissolves in rainwater and
in water that sinks through air pockets in the soil. The result is a weak
acid called carbonic acid. Carbonic acid easily weathers rocks such as
marble and limestone.
Rate of Weathering: The rate at which a rock weathers.
* The most important factors that determine the rate at which
weathering occurs are the type of rock and the climate.
Type of Rock:(Differential Weathering): The minerals that make up the
rock determine how fast it weathers. Rock made of minerals that do not
dissolve easily in water weathers slowly. Rock made of minerals that dissolve
easily in water weathers faster. Some rock weathers more easily because it is
permeable. Permeable means that a material is full of tiny, connected air
spaces that allow water to seep through it. Permeable rock weathers
chemically at a fast rate. As water seeps through he spaces in the rock, it
dissolves and removes material broken down by weathering. Also, the more
surface area that is exposed to weathering, the faster the rock will be worn
down. A large rock has a large surface area, but a large rock also has a large
volume. Because of the large rock’s volume, the large rock will take a long
time to wear down. If a large rock is broken down into smaller fragments,
weathering of the rock happens much more quickly. The rate of weathering
increases because a smaller rock has more surface area to volume than a
larger rock has. So, more of a smaller rock is exposed to the weathering
process.
Climate: Climate refers to the average weather conditions in an area. Both
chemical and mechanical weathering occur faster in wet climates. Rainfall
provides the water needed for chemical changes as well as for freezing and
thawing. Chemical reactions occur faster at higher temperatures. That is why
chemical weathering occurs more quickly where the climate is both hot and
wet. (granite is a very hard rock and weathers very slowly in cool climates
and is often used as building stone, but in hot and wet climates granite
weathers more rapidly and eventually crumbles apart.) Elevation also effects
the rate of weathering because rocks at peaks of a mountain weather faster
because they are exposed to more ice, rain, and wind.