Download Geomorphology

Hamblin, W. K. and Christiansen, E. H. (2004) Earth’s Dynamic Systems (10th ed) Prentice Hall, New
Jersey
Strahler, A., and Strahler A., (2005) Physical Geography – Science and Systems of the Human
Environment, John Wiley, New York
Ritter, D. F., Kochel, R. C., and Miller, J. R. (1995) Process Geomorphology (3rd ed) WCB, Dubuque
Ahnert, F. (1996) Introduction to Geomorphology, Arnold, London
Introduction to Geomorphology – History and Key Concepts
Hamblin Chapter 1, 2; Briggs et al Chap 1; Ritter Chap 1, Ahnert Chap 1, 25
Geomorphology is the study of the Earth’s surface geological features (landscape) and the
processes which shape it.
Scope of Geomorphology
- Understanding the processes of erosion, weathering, transport and deposition
- Measuring rates at which such processes operate
- Quantitative analysis of the forms of the ground surface (morphometry) and of the
materials of which they are composed
- Applying geomorphological concepts and skills to address problems of natural hazards
Historical Development of Geomorphology
The era of descriptive models of landform evolution (up to 1960)
o William Morris Davis: Cycles of Erosion – he envisaged that in time, a landscape
would be eroded and eventually reduced to a flat plane, but due to the new
distribution of weight, crustal deformation would occur and mountains would be
built; erosion breaks down mountains, tectonics form new mountains
o Walther Penck: Waxing and Waning Slopes
o John T Hack: Steady State Landscapes (Dynamic Equilibrium) – contrary to Davis, he
said that landscapes reach a dynamic equilibrium with the climate of the time
The Modern Era 1
- Development of new techniques for measuring landform ages and long term process rates:
o Radiocarbon dating
o Cosmogenic isotopes, etc (ex: Carbon-14)
The Modern Era 2
- Integrating theory, process measurements, experiments into predictive methods of landfrm
evolution
o Early precedents by Gilbert (1887), Horton =(1932, 1945), Chorley, Strahler,
Kennedy, etc
o Modern work by Ahnert, Kirkby, Willgoose, MIT group, Slingerland, Howard, etc
The Modern Era 3
o Application of geomorphological techniques to environmental problems, ex:
groundwater contamination
o Development of predictive geomorphological techniques and application to risk
management: floods, slope failure, soil erosion, desertification, etc
Key Concepts in Geomorphology
- Dynamic Equilibrium - loosely defined as a situation where landscapes reach an active
balance between:
o The forces of erosion (wind, water, ice)
o And rates of transport (gravity)
o Acting on a resisting framework (rock structure)
For example, slopes developed on particular geological units produce angles which are in
balance with the prevailing climatic regime which controls mass movement, sheetwash, and river flow.
Continuous erosion would not change the slope angles as long as the erosion processes and their
climatic and tectonic controls remain constant. Landscapes which have reached some form of dynamic
equilibrium may be constantly changing at very slow rates. This, however, may also have sudden and
unforeseen consequences. The concept lends itself readily to systems analysis. Landforms and
processes are parts of a system in which energy and/or mass are continually added or removed. It was
further developed by Chorley (1962), Chorley and Kennedy (1971), Schumm (1977) and couched within
general System Theory.
-
Systems Analysis – a system is a collection of related components. Advantages of the
systems approach:
o Emphasizes relationship between processes and form (cause and effect relationship)
o Stresses multivariate nature of geomorphology
o Reveals process factors of inertia due to relict climates
The time factor is crucial to geomorphology. Three types of equilibrium can exist:
o Static equilibrium
o Steady state equilibrium
o Dynamic equilibrium
Driving Forces
Force is related to energy which drives processes that shape landforms. Energy is the capacity
for doing work.
Work = Fs (where F = force, s = distance)
Forms of energy: kinetic, potential, chemical energy.
Exogenic and endogenic geomorphological processes – exogenic (from the outside), endogenic
(from the inside)
1. Climate
Solar radiation drives
o Atmospheric engine
 Wind
 Precipitation
o Oceanic engine
 Ocean currents
2. Gravity
o
Controls rigour of
 Fluvial power
 Mass wasting
 Glaciation
 Tidal forces
o
 Groundwater
Sedimentation (the process by which loose material is transported elsewhere) and
isostacy
3. Internal Heat
o Thermal energy generated from
 Radioactive decay
 Friction caused by Earth tides
o Drives internal tectonic processes causing uplift and deformation
The Resisting Framework
The resisting force in geomorphology is implemented through two major geologic variables:
o Lithology
o Structure
Lithology
Diverse origins of rocks create lithologies at the surface that differ vastly in their chemical and
mineralogical compositions, textures, and internal strengths. Crustal composition by volume is 48%
Silicic (granite / gneiss / schist / granodiorite / diorite) and 43% Mafic (basalt / gabbro / amphibolites /
peridotite / dumite). Mafic rocks are denser than Silicic rocks.
Rocks and their % crustal volume
o Sands
1.7%
o Clays and shales
4.2%
o Carbonates
2.0%
o Granites, gneiss, crystalline schist
36.9%
o Granodiorite, diorite
11.2%
o Syenite
0.4%
o Basalt, gabbro, amphibolites, eclogite
42.5%
Sedimentary rocks are far more prevalent at the surface. They may range in magnitude, and
may be overlain by thick accumulations of sediment. The most likely lithological environment displaying
structural control is a sedimentary sequence with alternating resistant and non-resistant units.
Maltese geomorphology is a good example of this – resistant upper and lower coralline
limestones with weaker clays, sandstones, and marls in between.
Thresholds and Complex Response
Limits of equilibrium are defined by critical conditions called thresholds
o Extrinsic thresholds (ex: sediment movement in streams depends on river flow
rates)
o Intrinsic thresholds (ex: slope failure due to reduction in soil cohesive property not
storm intensity)
o Geomorphic threshold (abrupt changes may be a normal part of landscape
development, ex: cave collapse)
Complex response is a series of reactions that occur because thresholds are often reached…
Process linkage is when there is a process or event that may trigger driving forces in processes
within other systems (ie: the domino principle).
Earth’s Subsurface Structure
Hamblin & Christiansen Chapters 17-23; Ahnert Chapter 4
Earth’s Internal Structure
The whole planet is made up of a series of concentric shells, each with a distinct chemical
composition. The Earth’s interior is made up of several layers – crust, mantle and core. The crust and
mantle compose the lithosphere, asthenosphere, and mesosphere.
Continental crust largely consists of granitic rock. This is igneous in origin, and is light in colour.
Granite has a density of is 2.7 to 2.8g/cm3. It is mostly Oxygen, Aluminium, and Silicon (SIAL). Oceanic
crust largely consists of basaltic rock. It is plutonic (deep volcanic) in origin and dark in colour. The
density of basalt is 2.9g/cm3. It is rich in Oxygen, Silicon and Magnesium (SIMA). Continental crust is
normally much thicker than oceanic crust. At the end of a continent there is often a deep crustal root.
The area between the continental and oceanic crust is the Moho. Mantle rocks have a density of
3.3g/cm3.
Isostasy
Isostasy is the universal tendency of segments of Earth’s crust to establish a condition of
gravitational balance. Differences in both density and thickness can cause isostatic adjustments in
Earth’s crust. Deformation of the crust in one place brings about a change in another place. In this way,
continental blocks made up of the comparatively rigid outer layer of the crust ‘float’ on top of the more
mobile and denser material of the crust which is found at greater depths. The general shape and height
of the continental blocks changes dramatically over geological time. Our relatively short life span,
however, allows us to see very little of it.
Oceanic crust is more dense than Continental crust (2.9g/cm3 and 2.8g/cm3). To summarise:
Continental crust is thick and relatively low density granite; Ocean crust is thin but is made up of
relatively high-density basalt. Before comparing the thickness of continental and ocean crust, the
theory of isostasy has to be considered. Isostasy can be represented by a smple formula
Pressure = density x gravity x height
P = ρgh
If we use a bucket of water as a model, the theory of isostasy states that the pressure at the
bottom of the bucket is proportional to the height and density of the water in the bucket and to the
strength of gravity.
The theory of isostasy also explains why icebergs which are massive above water also extend
deeply into the water. Ice is made of water, but when water changes state from liquid to solid, it
expand, so when it freezes, it decreases its density.
The continental crust extends deep into the mantle to compensate for the height of things such
as mountain ranges.
Continental Drift
The theory of continental drift was proposed in 1915 by Alfred Wegener to explain the earth’s
history. The hypothesis was based on geological and paleontological (fossil) evidence. According to the
theory, a single continent, Pangaea, existed more than 200 million years ago. Pangaea was a large
landmass which broke apart 100 to 150 million years ago. The fragments became the continents.
Wegener’s theory did not consider how the continents drifted apart – he did not propose a mechanism
by which continental drift had occurred.
Between the continents, there can be seen a continuity of rocks and structures, as from South
America to Africa, ice sheets in Antarctica, and traces of ice sheet erosion in Australia and India; in India
there is evidence of massive ice sheets. A continuity of ancient coal beds can be seen from North
America to Eurasia. Coal forms from trees, of a lush tropical forest (Jurassic type); so along the line of
the coal beds, there must have been the equator.
During the Permian (225mya) there was Pangaea, in the Triassic (200mya), Laurasia and
Gondwanaland; in the Jurassic (135mya) more splits, in the Cretaceous (65mya), India’s fast movement
is evident
Wegener’s bold new hypothesis was ignored by most for more than 50 years. Many objected to
the idea that granitic crust could plough into basaltic crust.
Sea-floor spreading explains how the continents move and ocean basins form. It is based upon
various pieces of scientific evidence: deep sea soundings showing the existence and structure of midocean ridge systems; and magnetometer readings of ridge systems. During the time of Wegener, it was
believed that the bottom of the sea was flat and featureless. During the war, echo sounding revealed
that the sea was not featureless, but had topography that dwarfed anything that was above the sea.
Harry Hammond Hess, a professor of geology at Princeton University, was very influential in
setting the stage for the emerging plate-tectonics theory in the early 1960s. The mid-ocean ridge was
discovered in the mid-20th century from soundings of the deep sea-floor. It forms a continuous
mountain range girdling the earth; it is approximately 60,000km in length and approximately 3-4km
high. It has particular names at certain points – Central Indian Ridge, Pacific Antarctic Ridge, for
example.
Ridges are geologically active – volcanism, earthquakes and faulting. The ridge is characterized
by creases running across, these are called transform faults. Although ridge systems resemble mountain
chains, they are formed in different way. While mountain chains form by compression, ridge systems
form by tension; in fact they are moving apart, and as they do so, magma from the mantle spews out
and crushes up the crust. Plates moving together (mountains), plates moving apart (ridges).
Rift valleys, the result of sea-floor spreading, can also exist on land, such as in Africa. There are
fracture zones – lateral displacement of the ridges. The transform fault is between the two rift valleys
(they run perpendicular to the rift valley).
Ridge systems include two types of fault zones:
o Normal
 Along edges of rift valleys
 Relative movement is vertical
o Transform
 Offset the ridge axis
 Relative movement is horizontal
Ridge systems are characterized by magnetic anomalies. There are parallel bands of magnetic
highs and lows. During earth’s geological history, the poles reversed – the Earth’s magnetic field flipped.
This has in fact happened several times, although why this happens is unknown. The Earth’s magnetic
field is strongest at the poles. The force lines are invisible, but many organisms are sensitive to it, such
as birds that use it to navigate during migration.
Molten rock has iron particles within it; while it is liquid the iron particles will align themselves
with the Earth’s magnetic field. The rock will cool, and solidify. When it solidifies, the iron within it is all
aligned to the Earth’s magnetic field. If the Earth’s magnetic field changes and there is a subsequent
lava flow, this lava flow will have a different orientation – a reverse polarity.
The visible evidence of Earth’s magnetic field includes paleomagnetism. Magnetic minerals in
lava become fixed in place as it solidifies. This leaves a record of the direction of the earth’s magnetic
field. Since rocks can be dated, this allows for the construction of a geomagnetic polarity time scale.
Based upon the analysis of minerals in lava flows on land the Earth’s magnetic field has reversed
periodically during its 4.5billion year history.
There are a series of bands of normal polarity and reverse polarity running parallel to the rift
valleys. If there was a band that was normal on one side, it was normal on the other side – a mirror
image. This meant that they must have come from the same source. It is because of this that scientists
proved that plates are moving apart. Thierd Vanandel, in Understanding the Earth shows this process
very well.
In the 1960s, scientists proposed that magnetic anomaly stripes were evidence of sea floor
spreading. Magnetic anomalies can be used to calculate the rate of sea floor spreading: determine the
distance between parallel stripes, use dating techniques to determine the age of basalt flows on land.
The data indicate rates of 1 to 10cm per year. Due to this movement, the Atlantic Ocean is getting wider
every year, at a rate of about 5cm per year.
To summarize:
o Oceanic crust moves apart at ridge systems
o Crust becomes older with increasing distance from the ridge
o Sedimentary coverage of the crust increases with greater distance from the ridge
because the period of time over which the sediments accumulate increases with age
Global Plate Tectonics
The theory of plate tectonics was a revolutionary theory formulated in the 1960s to explain
continental drift and the geologic history of the Earth. Basic to the theory is the fact the Earth’s
diameter has remained stable for hundreds of millions of years and the idea that the Earth’s surface is
divided into plates.
Malta is found on the fracture zone between the African and Eurasian plates, but on the
boundary of the African plate, which is actually tipping underneath the Eurasian plate. There are areas
where plate boundaries are not so clear-cut, such as between the North American and Eurasian plates.
To understand the theory of plate tectonics, we will consider
o Subduction zones
o Seismic activity
o Lithospheric plates
o The opening and closing of
Subduction zones
These are regions where oceanic crust is being destroyed. The amount of old crustal material
destroyed by subduction zones equals the amount of new crustal material created by ridge systems.
These are zones of intense seismic activity. The Pacific plate is surrounded in part by the ‘Ring of Fire’, a
place of volcanic hotspots and earthquake activity. The Greek islands are also in a very active zone, as
are Indonesia, Papua New Guinea and New Zealand.
Much of the seismic activity in subduction zones is associated with deep-sea trenches (the
deepest parts of the ocean). Here oceanic crust slips beaneath continental crust and is destroyed. The
rate of destruction is approximately 15 to 45 cm/year. One of the deepest ocean trenches is 11km
deep. Seismic activity gives rise to earthquakes and volcanic activity, most of which occurs in the Pacific
Ocean – most of the deep sea trenches are in the Pacific Ocean.
The locations of the zones of sea-floor formation and destruction imply that the Atlantic Ocean
is gradually expanding, and the Pacific Ocean is gradually decreasing in size. However, the rates are not
identical because the production of new crust at some ridge systems in the Pacific Ocean counteracts
the destructive processes.
The theory proposes that the Earth’s surface is composed of a series of lithospheric plates, the
edges of which are defined by seismic activity. The plates consist of some combination of continental
and oceanic crust. The bottom boundaries extend downward through the entire lithosphere.
Plate boundaries occur at
o Mid-ocean ridges ex: Atlantic Ocean ridge (Iceland comes above the water); Pacific Ocean (New Zealand)
 Sites of tension, plates diverge
 Addition of new crust to trailing plate edge
o Subduction zones ex: West coast of North America
 Sites of compression, plates converge
 Destruction of old crust
o Transform faults ex: San Andreas fault
 Sites of lateral movement
 Crust is neither created nor destroyed
The rigid and brittle lithospheric plates overlie the molten and plastic asthenosphere. Currents
in the mantle move and form convection currents. When the current moves to the surface, it diverges.
As it moves up it pushes the plate up a bit, and then moves to either side and drags the plates apart. As
it drags them apart, some material spews out onto the surface, creating new plate. This is a constructive
margin. It is this process which Wegener did not propose. In some areas, where plates are meeting
each other, one plate dips down into the magma (being distilled, in a Benioff zone), and the other plate
overrides it. This is a destructive margin. Over it, volcanic island arcs will form. Japan is a volcanic
island arc, as are the Philippines.
Thermal convection produces slow moving currents in the underlying asthenosphere which
exert drag on the lithospheric plates setting them in motion.
Hot spots are other zones of volcanic activity on the sea-floor. They are located away from the
edges of plates and lead to the formation of island chains (NOT arcs – they are chains of volcanic
islands), such as the Hawaiian Islands. Hot spots were believed to be fixed, but it is now being proposed
that they do move. As the plate moves over the hot spot, it creates a series of volcanic peaks. In the
Hawaiian ridge, the hot spot is now right under the island of Hawaii, so Hawaii is the largest island in the
group – but not many people live there because it is highly volcanically active.
Based upon the theory of plate tectonics, ocean basins open and close. The Wilson Cycle
describes the stages in ocean basin development.
 Stage 1: the splintering of continental crust, ex: the East African rift valleys
 Stage 2: expansion and formation of shallow basin, ex: the Red Sea
 Stage 3: gradual separation into two distinct continents, ex: the Atlantic Ocean
 Stage 4: weakening of old crust from accumulated sediment and formation of a
subduction zone, ex: the Pacific Ocean
 Stage 5: gradual shrinking of the basin, ex: the Mediterranean
 Stage 6: colliding of the land masses, ex: the Himalayas