Some terms, definitions and concepts (which you will need in the module!)

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Some terms, definitions and concepts (which you
will need in the module!)

• Please note that these are summary notes and
  reminders and rarely go into detail. For this
  detail you will need one or other of
• Lecture notes and PowerPoints
• Recommended texts
• Dictionary of Physical Geography
• Properties of Materials - Electronic (POME)

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New Concepts

• By the end of this section you will know
• The basics about some basic terms
• How you can use this knowledge to help in the
  interpretation of landforms and landscapes.

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Index Note that you may need several slides to
cover a topic and that they may not be on
consecutive slides. A indicates a cross
reference is available

• Erosion

• Stress

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Erosion

• Erosion is the removal of material (solid rock or
  unlithified sediment) from the landscape.
• We can think of bedrock erosion on a widespread
  basis (e.g. by glaciers) or restricted (e.g.
  downcutting by a river)
• The same also applies to sediments
• To place erosion in a time framework we need to
  think of rates of erosion.

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Types of Erosion

• Several types of 'erosion' have been used by
  various authors and more widely
• Corrasion
• Corrosion

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Attrition(an aside)

• Attrition is used specifically to describe the
  removal of material on individual grains
• Such removal can be in aquatic, aeolian or
  (sub)glacial environments

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Erosion Censoring

• Erosion censoring is the evidence that you don't
  see in the landscape because of this removal -
  thus leaving imperfect and perhaps fragmentary
  evidence.
• It has been most often employed in a glacial
  erosion/deposition context
• (You'll not often find this term in textbooks but
  it's a useful idea - at least in remembering how
  landscapes develop.)

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Denudation

• This is a wider term in application than erosion
  and includes weathering as well as other
  erosional processes. Literally it means 'laying
  bare' the surface of the earth by removal of
  material. It can be considered as the opposite of
  tectonic (i.e. uplift) processes.
• Just as with erosion we need to consider
  denudation rates.

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Palimpsest

• Another term you'll not often find in textbooks
  on geomorphology.
• It comes from the Greek (palin, 'again' and
  psestos 'rubbed smooth') and was originally used
  to describe the cleaning and scraping of vellum
  parchment manuscripts so that it could be used
  again.
• The use in geomorphology fits in with erosion
  censoring and is the (physical) landscape that
  you see at any time. The term is also used in
  historical geography for the wider, physical and
  cultural landscape.

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Rates

• Rates are defined slightly differently according
  to context. Thus
• Rate of travel is a distance divided by time
  mph, metres per second are rates. Note that this
  could be instantaneously measured (e.g. by
  looking at a car speedo) or an average. Be
  careful about distinguishing this.
• Landslide rate (e.g) would be the number of
  landslides (in an area) per unit time. In this
  case we have discrete events. (ntensity is
  basically another term for 'rate'. Be careful
  about the length of time used as a basis for
  measurement.

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Fluxes

• Flux is a general scientific term meaning a
  quantity 'moving'.
• How it is used depends upon the concept. In
  geomorphology and sedimentology we can think of
  it as a mass (or volume) moved per unit time. The
  mass could be in kg or tonnes and the time in
  seconds or Ma (millions of years).
• It is vital to ensure that the units are
  appropriate and stated accordingly!

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Crucial questions to ask in geomorphologyespecia
lly when looking at a specific process

• How much?
• How fast?
• How frequently?
• Over what length of time?
• What recurrence interval?
• (which we cover next)

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Magnitude-frequency

• Having met ideas of how big (flux) and how fast
  (rate) we can combine these into the important
  concept of 'magnitude-frequency'.
• However, this is more than saying 'how big and
  how often' as it brings a statistical concept
  into play. We'll look at this in more detail
  later.

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Recurrence Interval (or return period)

• Usually applied to floods, but could be used for
  other phenomena - although having enough data to
  work it out is problem
• The expected frequency of occurrence (in years)
  of a discharge of a particular magnitude.
• Calculated from Tr (n 1)/m
• Where n is the total number of values in a series
  and m is the rank ordering (from 1 largest)

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Landscape interpretationHere's an example
What do you think is the oldest portion of the
landscape (why?) How rapidly do you think the
most recently added material was added?
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Ergodic principle or theorem

• Ergodic actually has a very specific meaning in
  probability theory but is used in several ways
  which are more guiding principles
• A system that eventually returns to its original
  condition - even if that is a long time - so
  that, roughly speaking, averages over time will
  suffice to explain the system. Alternatively
• A theory that attempts to explain macroscopic
  behaviour of matter from microscopic particle
  dynamics (as in thermodynamics). In
  geomorphology.

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Ergodic hypothesis(Andrew Goudie's entry in the
Dictionary of Physical geography)

• As used in geomorphology, suggests that under
  certain circumstances sampling in space can be
  equivalent to sampling through time.
  Geomorphologists have sometimes sought an
  understanding of landform evolution by placing
  such forms as regional valley-side slope profiles
  and drainage networks in assumed time sequences.
  The concept of the cycle of erosion was based to
  a large extent on ergodic assumptions, as was
  Darwin's model of coral reef development.
• Chorley et al. (1984, p. 33) point to certain
  dangers in ergodic reasoning landforms may be
  assembled into assumed time sequences simply to
  fit preconceived theories of denundation there
  is always a risk of circular argument and form
  variations may result from factors other than
  their position in time. 

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Energy

• Energy has various (complicated, thermodynamic)
  definitions - but just think of it as the means
  by which things get done. In fact, we usually
  push, pull or have things flow and 'stuff' moves.
  So we actually use 'forces' and 'stresses'
  more than we do energy per se.
• NB however, the different between potential
  energy and kinetic energy. (KE 1/2 mass x
  velocity2, and we can measure masses and
  velocities relatively easily.)

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Force

• A 'push' or, in Newton's formulation, a push (or
  a pull) is what gives a mass an acceleration
• In SI is the numerical equivalent to a mass
• (strictly, we should talk of a kgf or kilogramme
  force remember that force of gravity is less on
  the moon than earth)
• This follows from Newton's 2nd law force is
  proportional to the acceleration given to a body
  x the body's mass (Fµ a.m and thus Fk.a.m) so,
  if we give the value of 1 to each part we get a
  definition a force of 1 newton is produced when
  we accelerate a mass of 1kg by 1ms-2
• Incidentally, a force of 1N (newton) is 100g or
  0.01kg - about the mass of an apple!

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Stress

• Simply, stress is the ratio of Force/Area
• SI units are thus kg/m2
• Is also equivalent to pressure
• Stress directions give compressive, tensional,
  rotational
• Opposed stress couples
• Both normal to a plane normal stresses
• Both parallel to a plane shear stresses

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Friction

• The resistance to movement (or starting to move)
• It is inevitable in all systems
• Examples in geomorphology
• Stones in a scree slope keeping it at a high
  angle
• Differential friction related to wind/water
  velocity (Hjulstrom/Bagnold)
• Basal erosion of a glacier
• Side and bed drag in a stream
• There are specific ways of relating friction to
  the applied shear stress of the medium

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Dimensions and Units

• The main dimensions for measurement are
• Mass (M)
• Length (L) and
• Time (T)
• from these we can derive additional
  characteristics
• Area (L2), Volume (L3), Density (M/L2)
• Velocity (L/T), acceleration (L/T2)
• Units are the specific ways of measurement
• SI (Système Internationale mks
  metre-kilogramme-second)
• Imperial Units (fps foot-pound-second)

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Dimensionless Quantities

• This is not a contradiction in terms but just
  that they are ratios of dimensions where the
  dimensions used 'cancel out'.
• Consider a slope
• Remember a,h and o from
• Trigonometry so, sine a o/h , hence, if o1,
  h2 then o/h0.5 and sine 0.5 30
• Similarly, s are dimensionless a slope of 30
  is 50 (ie pretty steep!)
• We'll meet other dimensionless parameters as
• Reynold's number, Froude number, coefficient of
  friction (Manning's n isn't really dimensionless)

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Measurement

• The act of making a measurement is surprisingly
  complex as you need a standard unit for the
  corresponding dimension (involving usually, M, L
  T)
• A device to make the measurement
• An appreciation of accuracy and precision
• A means of recording the measurement (even paper
  and pencil)

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Accuracy and Precision

• Accuracy is how close the determined value is
  from the actual 'true' value.
• Precision is the spread of measurements about a
  central value (equivalent to the standard
  deviation)
• (For more on this see the powerpoint in geoskills)

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Conceptual models in geomorphology

• In the next few slides there is a little about
  different approaches and ways of viewing
  geomorphic systems.
• Again, these are reminders only and you should go
  to more explicit statements for details Ch 1 in
  Huggett and Ch 1 in Holden are good starts.

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Systems

• It's useful to think of the whole or parts of a
  landscape in terms of a system or sub-systems
• Kennedy (in DoPG) suggests three basic
  ingredients 'elements, states and relations
  between elements and states' but it is also
  necessary to consider 'boundary conditions' of
  the system of interest.
• Such boundaries need not be physical (e.g. basin
  watershed) but conceptual (e.g. considering only
  the ice part of a glacial system and neglecting
  the fluvial aspect - or v.v)
• This simplification is often useful in modeling
  the system, especially mathematically.

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Process-response models - 1

• Are where we look at the mechanisms in the
  defined system and what the mechanism does.
• This is simply 'cause and effect' but it needs a
  knowledge of the mechanism involved.
• A 'reverse' of this, seeing a landforms and
  assuming that a particular system has produced it
  can give rise to misinterpretation. This can be
  because there are two generating mechanisms
  possible (landform convergence)

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Process-response models -2

• If you think about it, my Materials-Processes-Geom
  etry aide memoire is really a form of p-r system.
  It asks you to consider the 'materials' under the
  specified 'process/mechanism' giving rise to a
  specific set of geometric results (landforms).

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Tectonic geomorphology

• Large scale endogenetic processes giving rise to
  mountain building, uplift etc. the uplifted
  masses are then subject to denudation.
• Faulting (at scales from 10s of km down to a few
  metres) can then be exploited bu subaerial
  processes.

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Climatic geomorphology

• Suggests the importance on climatic controls in
  our understanding of landscapes.
• In some areas, e.g. weathering, this has been
  especially important - although this has been
  rather overplayed.
• Peltier's ideas of 'morphometric regions'
  (morphoclimatic regions) are a good example of
  the significance. Beware, as from the lectures,
  that these divisions are much over-simplified.

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Process geomorphology

• A significant aim since the 1950s has been to
  explain landforms in terms of the processes seen
  (or believed) to operate.
• NB, I tend to distinguish between 'mechanisms'
  and 'processes' the latter is the operation of
  (perhaps several) mechanisms over time. Thus,
  e.g. 'glacial grinding processes' may involve one
  or another of the following micro-mechanisms
  indentation, scratching/abrasion,crack tip
  fracture, slip-stick. Hence, a 'process' is a
  rather shorthand term for something more
  complicated.

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A slight side-track to consider 'reductionism'

• Reductionism gets a bit of a bad press - usually
  from people who do not understand what it's
  about!
• If you want to see some more thoughts on this go
  to this PowerPoint lecture. For specifically
  ideas on reductionism start at slide 37 (NB, it's
  a 12MB file)

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Historical geomorphology

• Not the history of the subject but rather the
  emphasis given to the importance of time in
  explaining landscapes (especially as opposed the
  landforms)

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Denudation chronology

• A concept involved with historical geomorphology
  relating to the way in which landscapes develop
  over time. In particular is the idea that there
  are surfaces developed by various geomorphic
  agencies which can be seen, even in the present
  day landscape. This was a significant aspect in
  the UK between 1930 and 1960

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Bringing ideas together

• The geomorphological landscape is a compendium of
  events and features produced by energy
  relationships manifest over considerable time
  intervals and involving both very large and very
  small quantities of 'earth materials'.
• Because of this complexity we need to use a wide
  variety of tools and concepts to disentangle what
  we see in a landscape.