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

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)

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.

Stress

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

- 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.

Types of Erosion

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

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

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.)

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*.

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.

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*.

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!

Crucial questions to ask in geomorphology:especially when looking at a specific process

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

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.

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)

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?

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….

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.

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.)

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 F=k.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!

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

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

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)

o

a

a

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 o=1, h=2 then o/h=0.5 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)

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)

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)

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.

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.

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)

Process-response models -2

- If you think about it, my Materials-Processes-Geometry 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).

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.

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.

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.

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)

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)

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

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.

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