From the course of geology for engineers (CE-220,NED-UET). the slides contains weather and its agent with reference form the article

1. Syeda Hina Zainab NED-University CE-220 2019-2020
WEATHERING & ITS AGENTS
Geology

2. Weathering is the process of loosening, dissolving and wearing away of the Earth's surface. It occurs at
or near the Earth’s surface and produce changes to the landscape that influences surface and surface
topography and landform developments..
.
Following are the types of weathering:
• Mechanical weathering
• Chemical weathering
• Biological weathering
Types of Weathering
Weathering
Weathering example
No rock on Earth is hard enough to resist the forces of weathering .These processes carved landmarks such as the Grand
Canyon, in the U.S. state of Arizona. This massive canyon is 446 kilometers (277 miles) long, as much as 29 kilometers
(18 miles) wide, and 1,600 meters (1 mile) deep.
A picture of Grand Canyon Mountain

3. Agents of Weathering
Weathering agents varies from types of weathering. It shows how the different types of weathering have different agents:
a. 1.Water & Ice:
• Water, in either liquid or solid form, is often a key agent of mechanical weathering. For instance,
liquid water can seep into cracks and crevices in rock. If temperatures drop low enough, the water
will freeze.
• When water freezes, it expands. The ice then works as a wedge. It slowly widens the cracks and
splits the rock. When ice melts, liquid water performs the act of erosion by carrying away the tiny
rock fragments lost in the split.
• This specific process (the freeze-thaw cycle) is called
frost weathering or cry fracturing
Weathering due to water and ice
a. Mechanical Weathering

4. Mechanical Weathering
2.temperature & pressure:
• Temperature changes can also contribute to mechanical weathering in a process called thermal stress.
Changes in temperature cause rock to expand (with heat) and contract (with cold).
As this happens repeatedly, the structure of the rock weakens. Over time, it crumbles
• Rocky desert landscapes are particularly vulnerable to thermal stress. The outer layer of desert rocks undergoes repeated stress as
the temperature changes from day to night. Eventually, outer layers flake off in thin sheets, a process called exfoliation.
• Exfoliation contributes to the formation of Barnhardt, one of the most dramatic features in landscapes formed by weathering.
Bornhardts are tall, domed, isolated rocks often found in tropical areas. Sugarloaf Mountain, an iconic landmark in Rio de Janeiro,
Brazil, is a bornhardt.
• Changes in pressure can also contribute to exfoliation due to weathering. The underlying rocks, released from overlying pressure, can
then expand. As the rock surface expands, it becomes vulnerable to fracturing in a process called sheeting.
3. Salt:
• Salt also works to weather rock in a process called haloclasty.
• Saltwater sometimes gets into the cracks and pores of rock.
• If the saltwater evaporates, salt crystals are left behind.
• As the crystals grow, they put pressure on the rock, slowly breaking it apart.
Weathering due to change in temperature & pressure
Honeycombing due to salt

5. mechanical Weathering
2. Plants:
• Plants can be agents of mechanical weathering. The seed of a tree may
sprout in soil that has collected in a cracked rock.
• As the roots grow, they widen the cracks, eventually breaking the rock into pieces.
• Over time, trees can break apart even large rocks. Even small plants, such as mosses, can enlarge tiny cracks as
they grow.
3. Animals:
• Animals that tunnel underground, such as moles and prairie dogs,
also work to break apart rock and soil.
• Other animals dig and trample rock aboveground, causing rock to slowly crumble.
Weathering due to plants
Animals as weathering agents

6. b. Chemical Weathering
a. Water:
• Water is an amazing molecule. It has a very simple chemical formula, H2O.
• It is made of just two hydrogen atoms bonded to one oxygen atom.
• Water is remarkable in terms of all the things it can do.
• Lots of things dissolve easily in water.
• Some types of rock can even completely dissolve in water.
• Other minerals change by adding water into their structure. Hydrolysis is the
name of the chemical reaction between a compound and water.
b.Carbon dioxide/ Acid Rain:
Carbon dioxide (CO2) combines with water as raindrops fall through the air. This makes a
weak acid, called carbonic acid. This happens so often that carbonic acid is a common, weak
acid found in nature. This acid works to dissolve rock. It eats away at sculptures and
monuments. While this is normal, more acids are made when we add pollutants to the air.
Any time we burn any fossil fuel, it adds nitrous oxide to the air. When we burn coal, rich in
sulfur, it adds sulfur dioxide to the air. As nitrous oxide and sulfur dioxide react with water,
they form nitric acid and sulfuric acid. These are the two main components of acid rain. Acid
rain accelerates chemical weathering.
Weathered rock in Walnut Canyon near
Flagstaff, Arizona.

7. 2.Chemical weathering
b. Oxygen
Oxygen strongly reacts with elements at the Earth’s surface.
This process is called oxidation. You are probably most familiar
with the rust that forms when iron reacts with oxygen.
Many minerals are rich in iron. Red rocks are full of iron oxides.
As iron changes into iron oxide, the iron oxides can also make red color
in soils.
d.Plants and animals:
also cause chemical weathering. As plant roots take in nutrients, they remove elements from the
minerals. This causes a chemical change in the rock.

8. c. Biological Weathering
a.Microorganisms:
• Wedging and burrowing by organisms like termites, earthworms, rodents, etc. help
in showing the new surfaces to chemical attack and helps in the penetration of air
and moisture.
• Bacteria, mosses, algae, and lichens frequently grow on rock surfaces, particularly
in humid areas.
• They form weak acids, which can convert some of the minerals to clay.
• Algae growth can deteriorate several rock types and make it more exposed to
weathering.
b.Humans:
• Humans also play an important role in biological weathering.
• Construction activities like road building, mining also causes weathering.
• Human beings by disturbing vegetation, ploughing, and cultivating soils, also help in
blending and producing new contacts between water, minerals, and air in the earth
materials.

9. REFERENCES
• 1. https://www.nationalgeographic.org/encyclopedia/weathering/#:~:text=1%2F13-
,Weathering%20describes%20the%20breaking%20down%20or%20dissolving%20of%20rocks%20
• 2. Stone in Architecture, Erhard M.winker (pp 111-141)
• 3. 1966 Published by Elsevier B.V.
• 4. CK-12, Dana Desonie, Ph.D