Lecture 6: Geomorphology

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Lecture 6 Geomorphology

• Questions
• What is geomorphology? What are the relationships
  between elevation, slope, relief, uplift,
  erosion, and isostasy?
• How do you measure the rates of geomorphic
  processes?
• What does geomorphology have to do with
  tectonics?
• Reading
• Grotzinger et al. chapters 16, 22

Basic principle Every feature of the landscape
is there for a reason. We just have to be smart
enough to figure out what the reason is.
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What is Geomorphology?

• Geomorphology is the study of landforms, i.e. the
  shape of the Earths surface. It attempts to
  explain why landscapes look as they do in terms
  of the structures, materials, processes, and
  history affecting regions.
• Geomorphology relates to all the other
  disciplines of geology in two directions
• Tectonics, petrology, geochemistry, stratigraphy,
  and climate determine the geomorphology of the
  earth and its regions by controlling the
  principal influences on landscape.
• Therefore evidence from observations of the
  landscape in turn constrain the tectonic,
  petrologic, geochemical, stratigraphic, and
  climatic history of the earth and its regions.

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Uses of geomorphology

• Consider how frequently we infer the geologic
  history of a region from observation of the
  landforms.
• We will see many examples on our field trip
• Tectonic motions create geomorphic features like
  fault scarps and grabens from observation of
  scarps and grabens we infer the sense of tectonic
  motions and something about their ages.
• Volcanic activity creates calderas from the form
  of the caldera we learn about the mechanism of
  eruption.
• Granite weathers to rounded jointstones from
  observation of the shape of boulders and outcrops
  we can quickly map granite plutons from the
  shape of these rocks we infer how they joint and
  how they chemically weather.
• Resistant and weak strata determine the shapes of
  cliffs from distant observations of cliff shapes
  and local knowledge of stratigraphy, we can map
  outcrops as far as the eye can see.
• Glacial processes create geomorphic expressions
  such as moraines from the position, form, and
  age of the moraines we learn about paleoclimate
  and the nature of glaciers.

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Geomorphology in the rock cycle

• Every part of the rock cycle that occurs at the
  Earths surface has geomorphic consequences

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Relevance of geomorphology

• Geomorphology is important because people live on
  landforms and their lives are affected (sometimes
  catastrophically) by geomorphic processes
• Slope determines whether soil accumulates and
  makes arable land
• Slope stability controls landslides
• Mountains drastically affect the weather
  rainshadows, monsoons
• This is also a two-way process Human action is
  one of the major processes of geomorphic
  evolution
• People have been building terraced hillsides for
  thousands of years
• People dam rivers, drain groundwater, engineer
  coastlines
• People plant or burn vegetation on a huge scale
• People are paving the world
• People are changing the climate

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Geomorphic Concepts

• Elevation height above sea level
• Slope spatial gradients in elevation
• Relief the contrast between minimum and maximum
  elevation in a region

How high is this mountain?

• Important a mountain is a feature of relief, not
  elevation (a high area of low relief is a
  plateau)
• Slope controls the local stability of hillsides
  and sediment transport
• Relief controls the regional erosion rate and
  sediment yield
• Elevation directly affects erosion and weathering
  only through temperature, however, high elevation
  and high relief are generally pretty
  well-correlated (with glaring exceptions, like
  Tibet and the Altiplano)

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Geomorphic Concepts

• Uplift/subsidence
• vertical motions of the crust (i.e., of material
  points)
• Accumulation/denudation
• vertical change in the position of the land
  surface with respect to material points in the
  bedrock.
• Important the net rate of change in elevation of
  the land surface is the sum of uplift/subsidence
  rate and accumulation/denudation rate.

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Geomorphic Concepts

• Isostasy
• The result of Archimedes principle of buoyancy
  acting on the height of the land surface in the
  limit of long timescale (fluid-like mantle below
  the depth of compensation) and long lengthscale
  (longer than the flexural wavelength of the
  lithosphere).
• The total mass per unit area above some depth of
  compensation (in the asthenosphere) should be
  globally constant.
• Areas that satisfy the principle of isostasy are
  called isostatically compensated.

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Geomorphic Concepts

• Variation in topography can be compensated
  through two end-member mechanisms differences in
  the thickness of layers or differences in the
  density of layers.
• Isostatic compensation through density
  differences is Pratt isostasy (in the pure form
  each layer is of constant thickness).
• Isostatic compensation through differences in the
  thickness of layers (where the layer densities
  are horizontally constant) is Airy isostasy.

Air 0
Air 0
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Geomorphic Concepts

• In reality, both mechanisms operate together
  neither the thickness nor the density of the
  crust is constant.
• However, since the density contrast between crust
  and mantle is larger than most internal density
  differences within either crust or mantle, the
  dominant mechanism of isostatic compensation is
  variations in crustal thickness, i.e. Airy
  isostasy.

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Geomorphic Concepts

• Items for speculation
• Why is the top of the ocean crust lower than the
  top of the continental crust?
• Why is Iceland above sea level?
• Are subduction zone trenches isostatically
  compensated?
• What controls how long it takes to achieve
  isostatic compensation?
• What controls the lengthscale over which isostasy
  operates?
• What do gravity anomalies have to do with
  isostasy?
• What happens when you put an ice-sheet on a
  continent? What happens when you take it off?

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Drainage networks and Catchment Areas

• By mapping local maxima (divides) in topography,
  natural terrains can always be divided, at all
  scales (from meters to 1000 km), into catchment
  areas, each exited by one principal drainage,
  into which surface runoff is channeled
• This is not a necessary property of any
  surfaceit is the result of processes that act to
  shape the landscape

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Geomorphic Concepts

• Fractal geometry
• the forces that shape landscapes are often
  scale-independent and lead to hierarchical
  regularity across scale, often with fractional
  scaling relations, hence fractals. The classic
  examples
• Length of a coastline coastlines get longer when
  measured with shorter rulers.
• Branching networks drainage channels come in all
  sizes, and join together to produce networks
  whose branching statistics are fractal.

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Process geomorphology

• Quantitative, physically based analysis of
  morphology in terms of endogenic and exogenic
  energy sources
• Basics of process geomorphology
• 1) Assume balance between forms and process
  (equilibrium and quasi-equilibrium)
• 2) Balance created and maintained by the
  interaction between energy states (kinetic and
  potential) force and resistance.
• 3) Changes in force-resistance balance may push
  the landscape and processes too far  thresholds
  of change exist  fundamental change of process
  and thus form.
• 4) Processes are linked with multiple levels of
  feedback.
• 5) Geomorphic analysis occurs at multiple spatial
  and temporal scales.

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Process geomorphology

• An example of a quantifiable process hillslope
  evolution
• What controls stability of a slope? Lithology and
  water, mostly

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Hillslope evolution qualitative approach

• Some rocks are resistant to erosion (they form
  cliffs), some are weak (they form slopes).
• Resistant and weak are qualitative terms, but
  useful for describing landscape evolution.

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Hillslope evolution quantitative approach

• In transport limited situations, where slope
  failure does not occur, evolution of scarps
  resembles solutions of the diffusion equation

• Physically, this claims that flux of material is
  proportional to slope gradient, and slope
  gradient changes due to flux of materiala
  diffusive process.
• Where the slope is concave down it is eroding.
  Where it is concave up it is aggrading.
• If you know the diffusivity of topography for a
  region, you can date fault scarps and terrace
  edges by the relaxation of their shape.
• However, once a slope reaches a steady profile,
  or where the limitation is not transport but
  slope stability, hillslopes propagate without
  change in shape, a wave equation

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Hillslope evolution quantitative approach

• When does a soil-covered slope fail and become a
  stream channel?
• A model for the thickness of soil cover on every
  part of a landscape can be developed by combining
  a criterion for failure of a soil layer with
  topography and hydrology.
• A Mohr-Coulomb failure criterion for a plane at
  the soil-rock interface, st C (sn - sp)tanf,
  can be written
• For given soil density and angle of internal
  friction, this gives the degree of saturation
  (height of water table) needed to make the slope
  unstable. Some slopes are stable even when
  saturated, some slopes are unstable even when
  dry.

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Hillslope evolution quantitative approach

• Failure model
• The failure criterion is coupled to a hydrologic
  model based on Darcy flow through the soil,
• This predicts the water level in the soil needed
  to drain rainfall q T is the transmissivity
  (integrated permeability) of the soil, a is the
  area uphill that drains through an element of
  width b, and sinq gives the hydraulic head.
• Coupling the above two equations predicts where
  the slopes will fail in each rainstorm. Knowing
  rain statistics, it predicts the overall
  evolution of a landscape, since failure removes
  soil and makes an open channel.
• The resulting rule for a/b is scale independent,
  and is an example of a system that will evolve a
  fractal branching network of channels.

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Feedbacks in geomorphology

• Feedback 1 Erosion is coupled to elevation, a
  negative feedback
• High elevation promotes rapid erosion through
  freeze-thaw processes (a rapid physical
  weathering mechanism), sparse vegetation (above
  the treeline, roots do not stabilize slopes),
  increased precipitation (orographic rainfall).
• There is also a general, though not perfect,
  correlation between high elevation and high slope
  and relief, which promotes physical weathering
  and sediment transport.
• Clearly erosion is one of the direct sources of
  changes in elevation, as well.
• Hence in the absence of tectonic
  uplift/subsidence, higher terrain will be lowered
  fastest, tending to eliminate high slopes and
  large relief differences.

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Feedbacks in geomorphology

• The idea that, in the absence of tectonic
  disturbance, the negative feedback between
  elevation and erosion tends to eliminate relief
  is the basis of W. M. Davis theory of landscape
  evolution

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Feedbacks in geomorphology

• Feedback 2 Elevation and erosion are coupled to
  climate
• Topography affects weather patterns e.g., rain
  shadow. More profoundly, the uplift of the
  Himalaya-Tibet system caused the onset of
  monsoonal circulation in south Asia.
• Climate affects erosion as well. This is clear
  in the case of glacial episodes when it gets
  cold enough, ice can become a very effective
  agent of erosion and sediment dispersal. On the
  other hand, warm temperatures promote faster
  chemical weathering. Higher rainfall always
  increases both chemical and physical weathering
  and erosion.

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Feedbacks in geomorphology

• Feedback 3 Erosion is coupled to uplift, a
  positive feedback
• Because of isostasy, removal of mass from the top
  of the crust causes it to rise. Loading of mass
  on top of the crust causes it to sink. Since
  isostasy operates over some finite regional size
  (flexural wavelength 100 km), it is the average
  mass of crust on that scale that determines
  uplift. Hence eroding of valleys can cause the
  intervening mountains to rise.

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Feedbacks in geomorphology

• Feedback 3
• There is evidence that this type of
  valley-incision denudation-uplift is raising the
  high Himalaya

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Global Synthesis of Erosion

• An example of a process geomorphology idea at the
  largest scale is an attempt at the
  parameterization of global erosion rates
• Given area of a river catchment (km2) and total
  sediment load of the river (Mg/yr), mean sediment
  yield (Mg/km2/yr) can be determined for the whole
  drainage. Given density of sediment this is
  equivalent to mean vertical erosion rate (knowing
  Mg/km3, we get km/yr) for the whole drainage

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Global Synthesis of Erosion

• If we have some idea what the relevant variables
  are, we can develop an empirical correlation from
  which the whole map of the earth can be filled in
  from measurements of the major rivers and a few
  tributaries.
• One such map is based on the correlation
• where E is sediment yield (Mg/km2/yr), p is
  rainfall of the rainiest month (mm), P is mean
  annual rainfall (mm), H is mean elevation of the
  catchment, and a is mean slope.
• This equation shows feedbacks 1 and 2
• E f(H,a) Elevation -gt Erosion -gt Change in
  elevation
• E f(p,P) Climate -gt Erosion
• It also shows some additional relations
• Episodic heavy rains have a larger effect the
  same total rain when steady
• Slope and elevation reinforce each other (E
  depends on their product)

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Global Synthesis of Erosion

• Since we know slope, elevation, and rainfall
  statistics everywhere, and can work our way up
  river drainages computing average sediment yield,
  the correlation of the measured rivers is turned
  into a global map of sediment yield/erosion rate.
• What are the major features of the resulting map?

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Geomorphology and Tectonics

• For young tectonic activity, elevation and relief
  are direct expressions of tectonic activity.
• For old stable terrains, elevation and relief
  become expressions of relative rates of erosion.
• Thus, in California, anticlines are hills or
  mountains, but in Pennsylvania, anticlines may
  just as well be valleys if the older strata
  exposed in anticlinal cores are easily eroded.
• Ancient tectonic features must be recognized by
  the relations of the rocks around them. Current
  tectonic activity can be monitored by seismology
  and geodesy. Everything in between depends on
  geomorphology.
• Geomorphic expression is by far the easiest way
  to locate faults at the surface, and far more
  precise (at the surface) than seismology.

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Geomorphology and Tectonics

• When the form of an original geomorphic feature
  is known, then the magnitude of tectonic
  deformation can be determined by measuring its
  current shape. Examples
• fault scarps start from nothing, so height of
  scarp gives magnitude of total dip-slip
  displacement.
• undisturbed drainages presumably go straight
  across faults lateral offset gives total
  strike-slip displacement.
• marine terraces start at sea-level, so height of
  wave-cut platform gives total uplift since
  abandonment of terrace.
• river terraces start with longitudinal profile of
  riverbed disturbances in shape and slope give
  total deformation and tilt.
• When, furthermore, the age of the geomorphic
  feature is also known, then the rate of tectonic
  deformation is determined as well.
• How do you date geomorphology? This is a
  different problem from dating rocks!

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Geomorphology and Tectonics

• Topographic profiles of uplifted marine terraces
  at Santa Cruz, CA, give two kinds of information
• Total vertical uplift from height of wave-cut
  platforms initially at sea level
• Relative deformation along shore from shape of
  initial horizontal markers
• What additional type of data would be useful here?

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Geomorphology and Tectonics

• Deformation of Ventura River terraces across
  syncline
• A surprising result, since transverse ranges are
  in compression and full of thrust faults, but you
  cant have anticlines without synclines in
  between! So here there is net uplift of
  terraces, but synclinal downwarping in the
  middle.
• No information on ratesthis study was done in
  1925 and terraces were not datable by any
  technique known then.

• A more up-to-date example terraces on Kali
  Gandaki river valley through Himalayan front.
  These terraces can now be dated (but note the
  lowest one).

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Measuring Geomorphic Rates

• We have several ways of measuring the rates of
  landscape evolution.
• Dating of geomorphic surfaces Much effort has
  been directed towards measuring the age of
  erosional surfaces (peneplains, terraces, etc.).
  using the exposure age of materials on that
  surface.
• Thermoluminescence or electron spin resonance
• 14C dating of organic matter in the soil
• Cosmogenic nuclides 10Be, 26Al, 36Cl
• Example clocking development of normal fault
  scarp in limestone

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Measuring Geomorphic Rates
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Geomorphic Rates

• Measuring uplift rates
• Instantaneous uplift can be measured directly by
  GPS or geodetic surveying methods in some cases.
• Uplift over longer timescale is measured by
  thermochronology rocks cool as they move towards
  the surface down a geothermal gradient. Various
  methods are sensitive to the time since the rock
  cooled through specific temperatures
• Fission tracks anneal above 240 C. Knowing U
  and Th content, counting of fission tracks gives
  a time since 240 C. Knowing the geothermal
  gradient converts this into a time since depth of
  6 km.
• He diffuses out of minerals quickly down to a
  closure temperature of 75 C. Knowing U and Th
  contents, Farley and co-workers have developed
  the ability to clock the time since apatite
  crystals passed through 2 km depth.
• Does thermochronology actually measure uplift
  rates (with respect to sea level) or erosion
  rates (motion of material points with respect to
  the land surface)?