3. UNIT 8 – I. BASIC CONCEPTS
Weathering is the process of alteration and fragmentation of rocks by
the joined action of the atmosphere, water and living organisms.
What is wheathering?
It is a static process: it does not involve movement.
Where does it take place?
4. There are two main classes of weathering processes:
a) Physical weathering: It’s the breakdown of rocks and minerals
into small-sized particles through physical forces.
b) Chemical weathering: It’s the breakdown of rocks through
chemical reactions causing changes in their mineral composition.
Living things can affect the rocks both physically (e.g. roots) and
chemically (lichens) through their activity. We can consider them as a
third type of weathering, called biological weathering.
e.g. Freeze-thaw weathering or frost wedging in high mountain areas.
e.g.: Hydrolysis, Oxidation, Carbonation and Dissolution.
e.g.: Thermal stress weathering in deserts.
5. Water flows into cracks.The freeze-thaw cycle is repeated, and
eventually the rock cracks.
When the water freezes, it expands.
The ice acts like a wedge.
Mechanical
weathering
Freeze-thaw weathering is
typical of mountainous regions
where temperature fluctuates
below and above the freezing
point. Water in the rock cracks,
freezes and melts repeatedly
overtime, acting as a wedge and
causing them to break.
Freeze – thaw weathering:
As a result of this process, great piles of
rocky fragments called scree are
formed.
6. Hydrolysis of granite
Granite
Gravel
Hydrolysis processes take place when slightly acid rainwater
dissolves some minerals like feldspars and transforms them into
clay minerals. Granite is a hard and resistant rock, but becomes
weakened and disintegrated when feldspars are broken down by
hydrolysis and quartz crystals remain as loose debris,
unaffected.
7. There are some factors which can affect the processes of weathering.
In general, weathering is more efficient when climatic conditions are
extreme.
•Hot and wet climate conditions enhance chemical
weathering. Why is this?
•Low or changing temperatures and wet conditions
promote physical weathering.
•Weathering processes are typically slower in dry
regions, whether cold or hot.
8. What is erosion?
What is transportation?
What is sedimentation?
It is the removal of weathered materials due to the action
of water, wind, ice and gravity. It is a dynamic process that
involves movement and wears rocks away.
Eroded materials are carried away by gravity, wind,
water or ice.
Deposition and accumulation of transported materials in
sedimentary basins (depressions in the surface of the land or in the
ocean floor). This happens when the agent loses energy.
9. Landscape shaping
Some landscapes are fascinating: river canyons, coastal cliffs, huge sand
dunes, waterfalls… The responsible agents for these breathtaking reliefs
are called external geological agents.
The geological agents are liquid water, ice, and air when they move:
•Wind
•Glaciers
•Surface runoff
•Rivers
•Groundwater
•Oceans
All these agents are powered by the Sun or the Earth’s gravity. The sun
heats the air and water and causes the wind and oceanic currents. Gravity
forces water and ice to move towards lower areas.
11. Landscape shaping takes place through three different processes:
a) Erosion: it is the removal of the weathered rocks away from the site
of weathering.
b) Transportation: eroded materials are carried away to another
place.
c) Deposition: it’s the laying down of the transported rock materials.
The resulting materials are called sediment.
12. Erosion, transportation, and deposition shape landscape creating
landforms (ex. valleys, cliffs, dunes, etc.). The relief of Earth’s surface is
the result of destructive processes tearing down the land, and
constructive processes building it up.
• If erosion exceeds deposition, part of the surface relief is
destroyed. Destructive processes are dominant and create
erosional landforms.
• If deposition exceeds erosion, new relieves are built up.
Constructive processes are dominant and create
depositional landforms.
14. UNIT 8 – II. TYPES OF LANDSCAPES
How does wind shape landscape? Aeolian landscapes
Wind is not a very powerful agent. Aeolian processes are only
important in arid environments with sparse vegetation and a large
supply of fine and unconsolidated (loose) sediment.
Wind removes and transports only fine particles from the soil surface,
leaving the larger ones. When the wind stops or reduces its speed,
suspended particles are deposited.
15. Aeolian erosional processes and landforms:
Process Landform
Deflation
(the wind removes fine, loose
particles from the ground)
Rocky deserts or
pavements (reg).
Abrasion
(suspended particles erode the
rocks close to the surface)
Mushroom-shaped rocks
16. Aeolian depositional processes and landforms:
Process Landform
Wind reduces its speed and
deposits sandy particles
Dunes (ex. Barchans) and
dune fields. Sandy deserts
or ergs.
The finest particles of dust
travel very long distances.
Loess
17. Gravel desert or
desert pavement
(Reg)
Deflation and sand
column
Mushroom-shaped
rocks created by
abrasion
Erg Barchan
Loess deposits are formed far away from deserts.
19. How do glaciers shape landscape? Glacial landscapes
Glaciers are moving ice masses. They form where there is more snowfall in winter
than melts during summer. There, snow accumulates over time, and turns slowly
into ice which starts moving downhill due to gravity.
They are only in polar regions or in high mountains.
Continental glaciers Alpine glaciers
Cirque
Tongue
20. Glacial erosional processes and landforms:
Near the mountain tops, ice causes intense mechanical weathering and
erosion, forming very steep relieves with vertical walls and sharp peaks
called horns.
21. CIRQUE
Tarn
Glacial erosional processes and landforms:
Cirques, where snowfall accumulates and turns into ice, are round bowl-
shaped depressions. When glaciers disappear, they are often occupied by
round lakes called tarns.
22. Glacial erosional processes and landforms:
Glacial tongues erode the bedrock forming characteristic U-shaped
valleys. Ice pulls chunks of rock up into the ice and scrape the underlying
bedrock due to abrasion.
Striations are
common in
rocks of glacial
areas.
23. Glacial depositional processes and landforms:
Glacial erosion produces much rock debris which is dragged and deposited
by glacial tongues forming moraines.
26. How do rainfall, surface runoff and temporary watercourses
shape landscape? Torrential landscapes
How do rivers shape landscape? Fluvial landscapes
They are the result of liquid water running off the surface!!
Liquid water is probably the most important geological agent in the world.
27. How do rainfall, surface runoff and temporary watercourses
shape landscape? Torrential landscapes.
In arid regions with scarce vegetation and sporadic but torrential showers, rainfall
intensity quickly exceeds soil infiltration capacity and important surface runoff is
produced, thus eroding and shaping landscape.
Raindrops cause mechanical
weathering and disintegrates
soil materials.
Torrential erosional processes and landforms:
28. Torrential erosional processes and landforms:
Surface runoff can erode and transport
big amounts or fertile soil: this is called
soil loss.
29. Sometimes runoff concentrates on certain flow paths and cuts a well defined channel on the
hillside, that grows from small gullies to ravines or deep gorges.
Torrential erosional processes and landforms:
GULLIES
32. Between gullies or ravines, there may be
tall and thin spires left, called fairy
chimneys.
Torrential erosional processes and landforms:
33. Torrential depositional processes and landforms:
Intense runoff transports a lot of soil particles which are deposited at the
end of ravines forming alluvial fans or alluvial cones. Alluvial fans are
high flood risk areas.
36. How do rivers shape landscape? Fluvial landscapes
UPPER COURSE
- Steep slope
-Fast flow
- Erosion is dominant
- V-shaped valleys
- Waterfalls and rapids
- Gorges and canyons
MIDDLE COURSE
- Gentler slope
- Lower velocity
- Erosion and deposition are
equally important.
-Transportation is dominant.
- Wide flat-floored valleys.
- Floodplains and terraces
- Meanders and oxbow lakes
LOWER COURSE
- Very gentle slope
- Very slow flow
- Deposition is dominant
- Very Wide floodplains.
- Deltas
In wetter regions, where rains are not torrential, but maintained and regular, water
forms rivers. Rivers are permanent watercourses, usually freshwater, flowing
towards another river, an ocean, or a lake.
37. The upper course:
The river flows fast down a steep slope. Erosion is dominant. Rivers cut a narrow
V-shaped valley. Waterfalls, rapids, gorges and canyons are common too.
39. The middle course:
The velocity of the water decreases as the slope gets gentler. Erosion and
deposition are equally important. Transportation is dominat. The river forms a
wider flat-floored valley, meanders, and a well-developed floodplain.
40. The middle course:
When the river erodes its bed deeper, it creates a new floodplain. The
“hanging” old floodplain is called a fluvial terrace.
42. The lower course:
The slope flattens out and the water flow gets slower. Deposition is dominant.
Valleys are very wide, with large floodplains. Most materials are deposited at the
mouth where a delta may appear.
44. How does groundwater shape landscape? Karst landscapes
Depending on soil characteristics and on rainfall intensity, a certain amount of
rainwater infiltrates the ground, thus becoming groundwater. Groundwater
fills the pores and grooves in the rocks forming aquifers.
45. When acidic water containing carbon dioxide gets into rock cracks, some minerals,
such as calcium carbonate or calcite (found in limestone) and gypsum can be
dissolved. The rock is chemically weathered.
Then many erosional landforms, such as caves and sinkholes, are formed.
Acidic water gets into the rock joints and
dissolves calcium carbonate, creating caves.
When the roof of a cave has been intensely
weathered, it collapses and a sinkhole
appears.
48. If the water discharges some of its dissolved carbon dioxide, then calcium
carbonate dissolved into water precipitates forming depositional features.
In caves, dripping water forms:
stalactites (conic deposits which grow downward from the cave ceiling)
stalagmites (which grow upward from the cave floor).
In karst landscapes,
disappearing streams and
reappearing springs are also
common.
53. How do oceans shape landscape? Coastal landscapes
Coastlines are very dynamic features, changing shape continually. In coastal
landscape, moving ocean water is the geological agent involved.
There are three types of ocean water motions, which affect coastal landscapes:
a) Waves: They are created by wind blowing the surface of the ocean. Waves
strike the shore repeatedly and disintegrate the rocks mechanically. They are also
very effective agents of erosion, transportation and deposition.
Destructive waves: If the backwash is
stronger than the swash, the dislodged
particles are removed (erosion).
Constructive waves: If the swash is
stronger than the backwash, deposition
occurs.
So they also transport materials along the shore.
54. b) Tides: They are the cyclic rise and fall of seawater due to the Moon’s and
Sun’s gravitational attraction. They intensify the action of waves: their
erosive power increases at high tide and deposition mainly takes place at low
tide.
c) Longshore currents: They are gradual movements of water parallel to the
shore. They appear when prevailing wind blows regularly at the same angle
to the coastline. They can transport many materials along the shore (beach
drift).
55. Coastal erosional processes and landforms:
Wave action over the most exposed parts of the shore shapes vertical rocky
walls called cliffs. Cliffs retreat progressively over time, giving rise to a
growing flat rocky surface called wave- cut platform.
This is the process:
56. Wave erosive action can also create other
amazing landforms such as arches,
caves and stacks.
Coastal erosional processes and landforms:
58. Depositional landforms:
Longshore currents also transport and deposit big amounts of sand. They can form
spits: extended stretches of beach material that project out to sea and are joined
to the mainland at one end.
Currents can also form tombolos: sand bars that connect a rocky island to the
mainland.
