any one that you learn geology

1. Chapter Two
Structural Geomorphology

2. Global geomorphology
• Geomorphology: The study of surface landforms,
processes and landscape evolution of the Earth.
• The surface of Earth is modified by a combination of
surface processes that shape landscapes, and geologic
processes that cause tectonic uplift and subsidence.
 Surface processes comprise;
• the action of water, wind, ice, fire, and living things
• the stability and rate of change of topography under the
force of gravity, and
• human alteration of the landscape.
 Many of these factors are strongly mediate by climate.

3. Cont’d
• Geologic processes include:
 the uplift of mountain ranges,
 the growth of volcanoes,
isostatic changes in land surface elevation
the formation of deep sedimentary basins
where the surface of Earth drops and is filled
with material eroded from other parts of the
landscape.
• The Earth surface and its topography therefore
are an intersection of climatic, hydrologic, and
biologic action with geologic processes.

4. Cont’d
• The broad-scale topographies of Earth illustrate
this intersection of surface and subsurface action.
• Mountain belts are uplifted due to geologic
processes.
• Denudation of these high uplifted regions
produces sediment that is transported and
deposited elsewhere within the landscape or off
the coast.
• On progressively smaller scales, similar ideas
apply, where individual landforms evolve in
response to the balance of additive processes
(uplift and deposition) and subtractive processes
(subsidence and erosion).

5. Cont’d
 Geomorphically, relevant processes generally fall into;
1) the production of regolith by weathering and erosion,
2) the transport of that material, and
3) its eventual deposition.
 Although there is a general movement of material
from uplands to lowlands, erosion, transport, and
deposition often occur in closely spaced cycle all across
the landscape.
 Primary surface processes responsible for most
topographic features include wind, waves, chemical
dissolution, mass wasting, groundwater movement,
surface water flow, glaciations.
 Other more exotic geomorphic processes might include
freeze-thaw processes, salt-mediated action, or
extraterrestrial impact.

6. Processes and Landforms associated with
Plate margins
• What is plate tectonics
• The theory of plate tectonics proposes that the lithosphere is
divided into eight major plates (North American, South
American, Pacific, Nazca, Eurasian, African, Antarctic, and
Indian-Australian) and several smaller plates (e.g., Arabian,
Scotia, Juan de Fuca).
• These plates are mobile, moving in constant, slow motion
measured in rates of centimeters per year.
• The plate motion is driven by one or more of the following
mechanisms:
1. Convection -- heat transferred by movement of a fluid
(magma)
2. Conduction -- heat transfer by touching plates
3. Push-Pull Slab -- heavy slabs pull plates downward and
magma forced upward pushes plates to the surface (upwelling)

8. Plate Boundaries
• There are three general types of plate boundaries:
divergent, convergent, and transform.
1) Divergent
 Where tension forces are pulling the earth’s plates
apart. (where plates move away from each other)
2) Convergent
 Where compression forces push the earth’s plates
together (where plates crash into each other)
3) Transform:
 Where shearing forces cause the earth’s plates to slide
past each other. (where plates rub against each other in
opposite directions)
 Different Plate Boundaries Create Different Landforms
and Events:

9. 1) Landforms formed due to Divergent plate
boundaries
a) Two continental plates (CP) move away from
each other, stretching out the crust, until it
begins to break/fault.
• As crust is stretched wider, the valley drops
deeper.
• Eventually can lead to the creation of a new
body of water if low enough.

11. Cont’d
b) Two oceanic plates (OP) move away from
each other, allowing magma to rise up from
inside the Earth.
 The magma reaches the bottom of the
ocean, turns in to lava and cools (forming
new rock).
 This cycle continues constantly spreading the
sea floor and adding new material along this
chain of mountains.
 Sea Floor spreading occurs at these mid-
ocean ridges.

13. 2) Landforms formed due to Convergent
plate boundaries
• Convergent boundaries come in three
varieties depending upon the type of
lithosphere that is juxtaposed across a
subduction zone.
a) Oceanic Plate vs. Oceanic Plate
Convergence
b) Continental Plate vs. Continental Plate
Convergence
c) Oceanic Plate vs. Continental Plate
Convergence

14. a) Oceanic Plate vs. Oceanic Plate Convergence.
• The older of the two plates descends into the
subduction zone when plates of oceanic lithosphere
collide along a trench.
• The descending plate carries water-filled sediments
from the ocean floor downward into the mantle.
• The presence of water alters the physical and chemical
conditions necessary for melting and causes magma
to form. The magma rises up through the overriding
oceanic plate, reaching the surface as a volcano.
• As the volcano grows, it may rise above sea level to
form an island. Trenches often lie adjacent to chains
of islands (island arcs) formed by magma from the
subducted plate.

16. b) Continental Plate vs. Continental Plate
Convergence
continental plate is not dense enough to subduct.
As a result, the continental crust folds upward
(nowhere else to go) creating a chain of folded
mountains.
 The tallest mountains in the world were formed
(and continue to grow) as a result of continental
collision. The Himalayan mountains mark the
boundary between the Indian and Eurasian plates.

18. c) Oceanic Plate vs. Continental Plate Convergence
• As the more dense oceanic plate (OP) subducts
under the less dense continental plate (CP), it
pulls the front edge of the less dense plate down,
creating a deep ‘zone’.
• As a result, the deep-ocean trench landform is
formed.
• The more dense oceanic plate (OP) subducts under
the less dense continental plate (CP) and is driven
down in to the hot asthenosphere/mantle.

19. • Subducted plate melts due to extreme heat and
friction.
• Melted plate rises up through the crust, where it
reaches the surface and cools.
• Happens repeatedly to create large volcanoes.
• Convergence between these plates has resulted
in the formation of the Andes Mountains (the
second highest mountain range on Earth),
extensive volcanism, and widespread
earthquake activity. The largest earthquakes
are concentrated along subduction zones.

21. 3) Landforms associated with Transform
Plate Boundaries
• Transform plate boundaries are the
underappreciated members of the plate
tectonics machine.
• New lithosphere is not created at transform
boundaries, neither is old lithosphere
destroyed;
• consequently, these boundaries are sometimes
termed conservative plate boundaries.
• The San Andreas Fault, California, is a
transform boundary that separates the North
American and Pacific Plates.

22. Mineral Stability and Weathering
Mineral Stability Series
• minerals are most stable at the temperature and pressure at
which they form.
• In the case of the igneous rock minerals described in
Bowen's Reaction Series, the higher temperature minerals
(such as olivine, pyroxene, etc.), when exposed at the
surface, will be farthest from their comfort zone, and will
therefore chemically weather at a faster rate.
• Quartz, at the other end of Bowen's, is closer to its
preferred temperature and should therefore be more
stable (and it is). This is one reason why we find quartz
sand at the beach, instead of olivine sand.

24. What is weathering?
• Is decomposition and disintegration of rocks by chemical,
physical, and biological processes.
• Weathering breaks rocks into smaller pieces. It is the
effect of rainfall and temperature on rocks. Weathering
occurs in situ. This means the rocks stay in the same place
and are not moved.
• This is different from erosion. Erosion is when rocks are
moved around or hit by something moving so that they
break into smaller pieces.
Weathering

25. Rocks can be weathered in three ways:
1. Physical (Mechanical):
Large rocks broken into smaller fragments with no change
in composition frost wedging, exfoliation, root wedging,
running water and glacial abrasion, solar heating.
2. Chemical: chemical alteration or decomposition of rocks
and minerals.
Rocks dissolved - chemical and mineralogical composition
can be altered new minerals may form.
3. Biological Weathering: plants & animals
• Chemical (organic acids) and Physical (burrowing)

26. Factors that Influence Weathering
• Weathering of geologic material is determined
by:
• Rock type, structure
• Climate (Temperature, rainfall)
• Topography
• Time

28. LANDFORMS RESULTING FROM WEATHERING
• Some characteristic landforms resulting from
mechanical and chemical weathering include:
• Exfoliation (and exfoliation domes)
• Spheroidal weathering forms
• Talus slopes
• Karst landforms

29. Cont’d
• Two groups of weathering landforms are:
(1)large-scale cliffs and pillars and
(2) smaller-scale rock-basins, tafoni and
honeycombs.
Cliffs and pillars
• are associated with several rock types,
including limestones and sandstones.
• Throughout the world, sandstone cliffs and
pillars are distinctive features of sandstone
terrain.

30. Cont’d
• Rock-basins, tafoni, and honeycombs
• Virtually all exposed rock outcrops bear irregular
surfaces that seem to result from weathering.
• Flutes and runnels, pits and cavernous forms are
common on all rock types in all climates.
• Rock-basins, also called weathering pits, weather
pits or gnammas, are closed, circular, or oval
depressions, a few centimetres to several metres
wide, formed on flat or gently sloping surfaces of
limestones, granites, basalts, gneisses, and other
rock types.
• They are commonly flat-floored and steep-sided,
and no more than a metre or so deep

31. Weathering pit on Clach Bhàn

32. Cont’d
• Rainwater collecting in the basins may overflow
to produce spillways, and some basins may
contain incised spillways that lead to their being
permanently drained.
• Rock-basins start from small depressions in
which water collects after rainfall or snowmelt.
• The surrounding surfaces dry out, but the
depression stays moist or even holds a small pool
for long periods, so providing a focus for more
rapid weathering.
• In consequence, the rock-basin expands and
deepens.
• As rock-basins expand, they may coalesce to form
compound forms.

33. Cont’d
• Tafoni (singular tafone) are large weathering features
that take the form of hollows or cavities on a rock
surface.
• The origins of tafoni are complex.
• Salt action is the process commonly invoked in tafoni
formation, but researchers cannot agree whether the
salts promote selective chemical attack or whether they
promote physical weathering, the growing crystals
prising apart grains of rock.
• Both processes may operate, but not all tafoni contain
a significant quantity of salts.
• Once formed, tafoni are protected from rain wash and
may become the foci for salt accumulations and further
salt weathering.

34. Tafoni, northern Atacama desert, Chile.

35. Cont’d
• Honeycomb weathering is a term used to
describe numerous small pits or alveoli, no
more than a few centimetres wide and deep,
separated by an intricate network of narrow
walls and resembling a honeycomb.
• They are often thought of as a small scale
version of multiple tafoni.

36. Cont’d
 Overall susceptibility to weathering
• sedimentary rocks weather faster than
crystalline rocks (porosity).
• mafic crystalline rocks weather faster than
felsic ones (ionic vs. covalent)
• foliated metamorphic rocks may allow
enhanced weathering parallel to foliation
• limestone (soluble CaCO3) is less resistant
than silicate rocks in humid climate.

37. Slope Stability
Landforms associated with Slope

38. Slope Stability
• There are two main types of slope instability;
a) surface erosion and
b) landslides.
• Surface erosion is a water-driven process,
occurring largely during and after periods of
intense rainfall.
• landslides are gravity-driven (although they
are often exacerbated by heavy rainfall).

39. Landslide
• Landslides are rock or soil movement on slopes
due to gravity.
• Basically, the factors of landslide can be
categorized into two broad categories:
1) Natural Factors: Gravity, geology, slope, wave
action, heavy and prolonged rainfall, earthquake,
forest fire, and volcano.
2) Anthropogenic Factors:
• Inappropriate drainage system
• Cutting & deep excavations on slopes for
buildings, roads, canals & mining
• Change in slope/land use pattern, deforestation,
agricultural practices on steep slopes

40. Types of Landslides
 The landslide classification based on Varnes'
(1978) system has two terms:
a) the material type,
b) the type of movement.

41. Cont’d
 Based on Material Type
• Rock: is “a hard or firm mass that was intact and in its
natural place before the initiation of movement”.
• Soil: is “an aggregate of solid particles, generally of
minerals and rocks, that either was transported or was
formed by the weathering of rock in place. Gases or liquids
filling the pores of the soil form part of the soil”.
• Earth: “describes material in which 80% or more of the
particles are smaller than 2mm, the upper limit of sand
sized particles”.
• Mud: “describes material in which 80% or more of the
particles are smaller than 0.06mm, the upper limit of silt
sized particles”.
• Debris: “contains a significant proportion of coarse
material; 20% to 80% of the particles are larger than 2mm,
and the remainder are less than 2mm”.

42. Cont’d
 Based on types of movement
• fall,
• topple,
• Slide (translational and rotational)
• spread,
• flow.
 Combining the two terms gives classifications
such as: Rock fall, Rock topple, Debris slide,
Debris flow, Earth slide, Earth spread etc.

45. Cont’d
• Understanding and recognizing the differences
in slope form is key in potentially unstable
landform recognition.
• There are three major slope forms to be
observed when looking across the slope
(contour direction):
a) divergent (ridge top),
b) planar (straight), and
c) Convergent (spoon-shaped)

46. Cont’d
• Landslides can occur on any of these slope forms
but divergent slopes tend to be more stable than
convergent slopes because water and debris
spread out on a divergent slope whereas water and
debris concentrate on convergent slopes.
• Convergent slopes tend to lead into the stream
network, encouraging delivery of landslide debris
to the stream system.
• Planar slopes are generally less stable than
divergent slopes but more stable than convergent
slopes.
• In the vertical direction, ridge tops are convex
areas (bulging outward) and tend to be more
stable than planar (straight) mid-slopes and
concave areas (sloping inward)

47. The End
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