Exogenic processes are external processes that occur at or
near the surface of Earth. Exogenic processes are part of the
rock cycle. They are responsible for transforming rock into
sediment. Exogenic processes include degradation processes (
weathering, mass wasting, erosion, and transportation)and
aggradation processes (deposition).
3. Mechanical Weathering
Mechanical weathering or disintegration is the breaking up
of large rocks into smaller fragments without changing the
rock’s mineral composition.
There are phyical processes that occur in nature that break
rocks into smaller pieces. These include frost wedging(frost
weathering), insolation weathering (thermal stress
unloading (pressure release), and biological activity.
4. Frost wedging or frost weathering occurs in regions where
temperature fluctuates above and below freezing point,
resulting in a freeze-thaw cycle.
Insolation weathering or sometimes called thermal stress
results from the expansion and contraction of rocks caused by
temperature change. The coeffecient of expansion differs from
each mineral in the rock, resulting into different stresses that
may cause the rock to crack apart. The outer surface of the rock
surface is ogten warmer or colder that the inner portions, which
ma cause the peeling away of outer layers. The process of
“peeling off” is called exfoliation.
5. Unloading or pressure release occurs when the overlaying
rock is eroded away, causing the outer rock to expand more
than the rock below. This expansion may separate the outer
rock from the rock body. When erosion removes the overlaying
materials, the presence on the igneous rocks is released,
causing the outer portion to expand. Continued weathering
causues sheets of rocks to break away. This exfoliation is due to
the pressure released, ofetn referred to as sheeting.
6. Chemical Weathering
Chemical weathering decomposes rocks through chemical
range. The processes involved in chemical weathering include
Oxygen dissolved in water will oxidize some materials.
Reddish-brown rust will appear on the surface of iron-rich
minerals which easily crumbles and weakens the rock.
7. 2. Hydrolysis
Water is perhaps the most important agent of chemical
weathering. It affects silicate. For example, in the case of
ilivine, a siliciate, and water:
Mg₂Si₄₍s₎ + 4H₂O₍ɩ₎ Mg(OH)₂₍ₐq₎ + H₄₍ₐq₎
olivine water magnesium silicic acid in
The reaction results in the dissolution of the original mineral.
8. 3. Carbonation and Solution
Carbon dioxide (CO₂) dissolves in water to form carbonic
acid (H₂CO₃) and reacts with carbonate rocks (CaCO₃) to form a
soluble product (calcium bicarbonate).
CO₂₍g₎ + H₂O H₂CO₃₍ₐq₎
carbon dioxide water carbonic acid
CaCO₃ + H₂CO₃ Ca(HCO₃)₂₍ₐq₎
calcium carbonate carbonic acid calcium bicarbonate
9. 4. Biological Action
Some plants and animals may create chemical
weathering by releasing chelating and audifying compounds
that react with some minerals in rocks. Decaying remains of
dead plants in soil may form organic acids and, when dissolved
in water cause chemical weathering.
10. Mass Wasting
The mass movement of rocks, soil, and regolith is often
referred to as mass wasting. It is the step that follows
weathering and is also a degradation process. The driving force
of mass wasting is gravity. Although gravity is the controlling
force, there are other factors that influence or trigger the down
slope movement of materials with water, such as over
steepening of slopes beyond the angle of repose, removal of
anchoring vegetation, and ground vibration from earthquakes.
11. Types os Mass Wasting
1. Rock and Debris Falls
Rock falls occur when a piece of rock or mass of rocks
become dislodged and makes free-fall along a steep cliff.
Debris fall is similar to a rock fall, except that it involves a
mixture of soil regolith, vegetation and rocks. At the base of
cliff is an accumulation of fallen materials called talus.
12. 2. Land Slides
Landslides are another type of mass wasting. They are
sudden fast movement of cohesive mass of soil, rock, or
regolith. They occur in two types: translation and rotational
slides. Translational slides involve he movement of a mass of
materials along a well-defined surface, such as bedding
plane, foliation surface, or joint surface. Rotational slides or
slumps occur when the descending materials move en-masse
along a concave, upward curved surface.
13. 3. Flows
Flows may be slurry flows or granular flows. Slurry flows
consist of a mixture of rocks and/or regolith with 20% to 40%
of water. They are considered as water-saturated flows.
Granular flows contain 0 to 20% water. They are not saturated
14. Slurry flows may be subdivided further on the basis of
Solifluction – the flow rate is around 1 cm/yr and occurs in
areas saturated with water
Debris flows – the velocities range from 1 m/yr to 100 m/h;
these result from heavy rains
Mudflows – these are high velocity mixture of sediment and
water. The velocities are greater than 1 km/h; these result
from heavy rain and can travel along distances
15. Granular flows may be subdivided further based on velocity:
Creep – the slow movement of regolith down a slope,
observed in bent tress and fences
Earhtflows – the velocities range from less than 1 cm/yr to
less than 100 m/h
Grain flows – the velocities range from 9 to 100 m/h to less
than 100 k/h; they are relatively dry material like sand dunes
Debris avalanche – these are very high velocity flows (more
than 100 km/h) of large volume of mixtures of rocks and
16. Human and Mass Wasting
Vegetation helps hold soil, loos rocks, and regolith,
through the roots of plants. Plant debris like leaves, and
protect the soil. Observing practices, such as selective
logging, reforestation, planting trees on steeper slopes and
along streams and ricers, and not removing or burning
debris, may reduce mass movement.
17. Oversteepening slopes may be due to quarrying, open-pit
mining, home building on steep slopes, highway construction
(road cuts) and adding weight to slopes with housing
construction. Reducing the slope may reduce mass movement.
Thus, it can be done through proper grading or building
terraces, reinforcing the base of the slope, and reducing the
load in the slope.
Adding miosture to slope through lawn watering, leakage and
draining in swimming pool, and water reservoir leakage also
induces mass movement. Water serves as lubricant in mass
movement. Reducing water in a slope makes the slope more
18. Erosion and Transportation
Agents of Erosion
Weathered materials are removed from the original
site and transported away by natural agents. These
agents derive the energy from the sun through the water
cycle and through differential heating.
19. There are different types of water erosion that affect
1. Splash erosion – Raindrops cause tiny particles of soil to
be detached and to move out.
2. Sheet erosion – Raindrops break apart the soil structure
which moves down the slope as water flows overland as
3. Gully erosion – After heavy rain, water flows in narrow
channels, eroding gullies into great depth.
20. 4. Valley erosion – Continuous water flow alongside land and
move downward, which deepens a valley.
5. Bank erosion – Continuous flow of water wears out stones
along the bank of streams and rivers.
6. Coastline erosion – Waves from the ocean and seas crash
against the shore, pounding the rocks into pebbles and
reducing the pebbles to sand. Sand is also removed from the
beaches, which changes the coastline.
7. Seaside cliff erosion – The battering of ocean waves erodes
seaside cliffs, forming hales that create caves. Continuous
pounding of waves may create an arch that may fall, leaving
nothing but rock columns called sea stocks.
A glacier is a thick large mass of ice formed hundreds or
thousands of years mostly in remote areas like in the poles or in
high mountains. They move very slowly.
Glaciers are capable of great erosion. They can erode land
through plucking and abrasion. Plucking is the process wherein
fractured bedrocks are incorporated into the ice. This occurs
when ice water freezes. It expands, loosens, and pulls out the
fragments, becoming part of the glacier.
Recall that wind is formed by differences in pressure due
to differential heating of the atmosphere by the sun. Wind
carries dust, sand, and volcanic ash from one place to
another. In dry areas, strong winds wears away soft rocks,
and also polishes rocks and cliffs until they are smooth.
Wind can also erode materials until nothing left. In the
desert, wind flows sand to form sand dunes.
Solution – Materials are described in water and carried
along by water.
Suspention – The suspended particles are carried by a
medium (air, water, or ice).
Traction – Particles move by rolling, sliding, and shuffling
along eroded surface. These movements occur in all
Saltation – Particles move from the surface to the medium
quick repeated cycles. The repeated cycle has enough
to detach new particles.
24. Water and Landforms
Whenever stream velocity decreases, it deposits the
sediments it carries. The material deposit of a stream is called
alluvium. Some may be temporary because the materials may
be carried again and eventually carried to the ocean. Materials
like sand and gravel may be deposited in a longer lifespan as
deltas, alluvial fans, flood plains, and levees.
25. Deltas are formed when river loses energy as i flows in to
an area of slow-moving water, such as a lake or sea. When
the river meets the sea, clay particles coagulate and settle in
the sabed (flocculation).
Alluvial fans are formed when a stream reaches a flat area
(called piedmont) or gently sloping plain. When it velocity
slows, it loses its capacity to carry its sedimant material and
deposits them at the junction of the hill and piedmont,
forming fan-shaped deposits.
26. Flood plain is a flat wide expanse of alluvium covering flat
area prone to flooding. A sheet of slits is deposited after
successive flood, increasing the richness of the soil as well as its
Levees are fromed by successive floods over many years.
When a river overflows its back, its velocity decreases, leaving
coarse sediments deposited to form a small, natural
imbankment alongside the channel. As the water spreads out,
fine sediments are deposited over the valley, forming gentle
slopes of levees.
27. Glaciers and Landforms
Glaciers, as they move across the land, pick up and
transport huge load of debris. Ultimately, these materials are
deposited when glaciers melt. In general, all unsorted
of rock formed directly by the ice are called glacial till.
Moraines are layers or ridges of till. A long pile of rocky
material at the edge of a glacier is called lateral moraine while
at the middle of the glacier is called medial moraine.
28. Esker is a winding ridge of sand and gravel deposited under
a glacier by water melting from the ice.
Drumlins are streamlined asymmetrical hills composed of
till. They may occur in clusters as drumlin fields.
Kames are steep-sided hills. Likes eskers, they are composed
of sand and gravel.
29. Wind and Landforms
Loess is the accumulated blanket of silt carried by wind in
suspension and deposited over broad areas. When a loess is
by a streams in road buliding, it tends to maintain a vertical cliff.
Sand dunes are deposits of coarse materials in the shape of hills
or ridges. Dune is a general term to descride any mound or
of windblown sand rising to various heights up to 50 m. Dunes
are found in deserts or above low-lying coasts where sand is
constantly renewed by onshore winds flowing across the sandy