WEATHERING

Weathering is the process of breakdown 3. Temperature

of rocks at the Earth’s surface, by the Temperature changes can also
action of water, ice, acids, salts, plants, contribute to weathering through the
animals, gravity and changing process called thermal stress. During
temperatures. There are three types of thermal stress, rock tend to expand
weathering: physical weathering, with heat and contract with low
chemical weathering, and biological temperature. As this happens
weathering. repeatedly, the structure of the rock
weakens and over time crumbles.
Agents of Weathering
4. Plants
1. Water
Plants also served as agents of
Either in liquid or solid form is one of
weathering. Its contribution take
the agents of mechanical
place when the seed of a tree being
weathering. In liquid form, it seeped
spread in the environment sprout in
into cracks and crevices of rocks and
soil that has collected in a cracked
when the temperature dropped, it
rock. As the roots grow, they widen
freezes and definitely will expand in
the cracks, eventually breaking the
the form of ice. The ice then works
rock into pieces. Over time, trees can
as a wedge which slowly widens the
break apart even large rocks. Even
cracks and splits the rock. When the
small plants, such as mosses, can
ice melts, the liquid water performs
enlarge tiny cracks as they grow.
the act of erosion by carrying away
the tiny rock fragments lost in the 5. Animals
split.
Animals that tunnel underground,
2. Salt such as moles and prairie dogs, also
work to break apart rock and soil.
In the process of haloclasty, salts
Other animals dig and trample rock
served as an agent of weathering.
aboveground, causing rock to slowly
Saltwater sometimes gets into the
crumble.
cracks and pores of rock. When it
evaporates, salt crystals are left
behind and grow in the cracks and
pores which caused pressure on the
rock and slowly break it apart.
 the rock, causing them to split
further open→ during the day the
Types of Weathering
ice melts and the water seeps
A. Physical Weathering deeper into the cracks → at
night, the water freezes again. 3.
Physical weathering is caused by the 3. Exfoliation. It can happen as
effects of changing temperatures on cracks develop parallel to the
rocks, causing the rock to break apart. land surface as a consequence of
The process is sometimes assisted by the reduction in pressure during
water. It happens especially in places uplift and erosion. It occurs
where there is little soil and few plants typically in upland areas where
grow, such as in mountain regions and there are exposures of uniform
hot deserts. It occurs either through coarsely crystalline igneous
repeated melting and freezing of water rocks. The following are the
(mountains and tundra) or through process of exfoliation.
expansion and contraction of the surface
layer of rocks that are baked by the sun The rock mass at depth is under high
(hot deserts). There are two main types pressure from underlying rocks. It
of physical weathering: tends to be uniform and lack
fractures. → As progressive erosion
1. Abrasion. It occurs when rocks occurs, the rock mass is subjected to
surface is frequently exposed to progressively lower pressure of
water, wind and gravity. overlying rocks which leads to
2. Freeze-thaw. It occurs when tension in directions at right angles to
water continually seeps into the land surface → this tension is
cracks, freezes and expands, relieved by formation of cracks which
eventually breaking the rock follow the land surface. They are
apart. It occurs in mountainous relatively flat on plateaus but can be
regions like the Alps or steep on the flanks of mountains
Snowdonia. It occurs through the which are called exfoliation domes
following process: → once the crack is developed;
water enters and causes chemical
Rainwater or snow- melt collects weathering leading to the formation
in cracks in the rocks→ at night of new low- density minerals. This
the temperature drops and the enhances the cracks and
water freezes and expands→ the encourages slabs of rock to detach
increases in volume of the ice from the surface.
exerts pressure on the cracks in
 quartz and clay are the two of the
most common minerals in
B. Chemical Weathering sedimentary rocks.
Chemical weathering is caused by 3. Hydration – a type of chemical
rainwater reacting with the mineral weathering where water reacts
grains in rocks to form new minerals chemically with the rocks,
(clays) and soluble salts. These modifying its chemical structure.
reactions occur particularly when the Example: H2O (water) is added
water is slightly acidic. These chemical to CaSO4 (calcium sulfate) to
processes need water, and occur more create CaSO4 + 2H2O (calcium
rapidly at higher temperature, so warm, sulfate dihydrate). It changes
damp climates are best. Chemical from anhydrite to gypsum.
weathering (especially hydrolysis and 4. Oxidation – the breakdown of
oxidation) is the first stage in the rock by oxygen and water, often
production of soils. giving iron- rich rocks a rusty-
colored weathered surface.
There are different types of chemical
weathering, the most important are: C. Biological Weathering
1. Carbonation – carbon dioxide in Biological weathering of rocks occurs when
the air dissolves in rainwater and rocks are weakened by different biological
becomes weakly acidic. This agents like plants and animals. When plant
roots grow through rocks, it creates fracture
weak “carbonic acid” can dissolve and cracks that result eventually to rock
limestone as it seeps into cracks breakage. It can be classified into:
and cavities. Over many years,
solution of the rock can form 1. Biological Weathering by Physical Means.
Burrowing animals like shrews, moles and
spectacular cave systems. earthworms create holes on the ground by
2. Hydrolysis – the breakdown of excavation and move the rock fragments
rock by acidic water to produce to the surface. These fragments become
more exposed to other environmental
clay and soluble salts. Hydrolysis factors that can further enhance their
takes place when acid rain reacts weathering. Furthermore, humans also
with rock-forming minerals such indirectly contribute to biological
as feldspar to produce clay and weathering by different activities that
cause rocks to break.
salts that are removed in solution.
2. Biological Weathering by Chemical
The only common rock-forming Compounds. Some plants and animals
mineral that is not affected is also produced acidic substances that react
quartz, which is a chemically with the rock and cause its slow
disintegration.
resistant mineral. Therefore,