basic principles of gm

8/2/2019 Basic Principles of GM

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Lecture 6: Geomorphology

• Questions

– What is geomorphology? What are the relationships

between elevation, slope, relief, uplift, erosion, andisostasy?

– 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 isthere for a reason. We just haveto be smart enough to figure outwhat the reason is.



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What is Geomorphology?

• Geomorphology is the study of landforms, i.e. theshape of the Earth’s surface. It attempts to explainwhy 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 andits 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 itsregions.



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Uses of geomorphology

• Consider how frequently we infer the geologic historyof 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 tectonicmotions 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 shapeof 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 canmap outcrops as far as the eye can see.

– Glacial processes create geomorphic expressions such as moraines; fromthe position, form, and age of the moraines we learn about paleoclimateand the nature of glaciers.



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Geomorphology in the rock cycle

• Every part of therock cycle thatoccurs at theEarth’s surface hasgeomorphicconsequences



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Relevance of geomorphology

• Geomorphology is important because people live on

landforms and their lives are affected (sometimescatastrophically) 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

• 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 sedimenttransport

– Relief controls the regional erosion rate and sediment yield – Elevation directly affects erosion and weathering only through

temperature, however, high elevation and high relief aregenerally pretty well-correlated (with glaring exceptions, likeTibet and the Altiplano)

• Elevation: height above sea level

• Slope: spatial gradients in

elevation• Relief: the contrast between

minimum and maximumelevation in a region

How high is this mountain?



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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 respectto material points in the bedrock.

• Important: the net rate of change in elevation of theland surface is the sum of uplift/subsidence rate and accumulation/denudation rate.

Uplift

Denudation elevation =

Uplift + DenudationElevation



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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 longlengthscale (longer than the flexural wavelength of thelithosphere).

– 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 islarger than most internal density differences within either crust or mantle, the dominant mechanism of isostatic compensation isvariations 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 thecontinental crust?

– Why is Iceland above sea level?

– Are subduction zone trenches isostatically compensated?

– What controls how long it takes to achieve isostaticcompensation?

– 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 localmaxima (divides) in

topography, naturalterrains can always bedivided, at all scales(from meters to 1000km), into catchment

areas, each exited by one principal drainage, intowhich surface runoff ischanneled

• This is not a necessary

property of anysurface…it is the resultof processes that act toshape 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 withfractional scaling relations, hence fractals. The classicexamples:

• Length of a coastline: coastlines get longer when measured with shorter rulers.

• Branching networks: drainage channels come in all sizes, and jointogether to produce networks whose branching statistics are fractal.



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“Process” geomorphology

• Quantitative, physically based analysis of morphologyin 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 betweenenergy states (kinetic and potential); force and resistance.

– 3) Changes in force-resistance balance may push thelandscape and processes too far: thresholds of changeexist: fundamental change of process and thus form.

– 4) Processes are linked with multiple levels of feedback.

– 5) Geomorphic analysis occurs at multiple spatial andtemporal scales.



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Process

geomorphology• An example of a

quantifiable process: hillslopeevolution

• What controlsstability of a

slope? Lithologyand water, mostly



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Hillslope evolution:

qualitative approach

Some rocks areresistant to erosion(they form cliffs),some are weak (theyform 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 notoccur, evolution of scarps

resembles solutions of thediffusion equation

h

t D

2h

x2

h

t C h

x

• Physically, this claims that flux of material is proportional toslope gradient, and slope gradient changes due to flux of material…a diffusive process.

• Where the slope is concave down it is eroding. Where it isconcave up it is aggrading.

• If you know the “diffusivity of topography” for a region,

you can date fault scarps and terrace edges by the relaxationof their shape.

• However, once a slope reaches a steady profile, or where thelimitation is not transport but slope stability, hillslopes propagate without change in shape, a wave equation:

ill l l i



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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 soilcover on every part of a landscapecan be developed by combining acriterion for failure of a soil layer with topography and hydrology.

– A Mohr-Coulomb failure criterion for a plane at the soil-rock interface, =C + (n - p)tanf, can be written

– For given soil density and angle of internal friction, this gives the degreeof saturation (height of water table)needed to make the slope unstable.Some slopes are stable even whensaturated, some slopes are unstable

even when dry.

h

z

s

w1

tan

tanf



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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 soilneeded to drain rainfall q; T is thetransmissivity (integrated permeability)

of the soil, a is the area uphill that drainsthrough an element of width b, and sin gives the hydraulic head.

– Coupling the above two equations predicts where the slopes will fail ineach rainstorm. Knowing rain statistics,

it predicts the overall evolution of alandscape, since failure removes soil andmakes an open channel.

– The resulting rule for a/b is scaleindependent, and is an example of asystem that will evolve a fractal

branching network of channels.

h

zq

T

a

bsin

QuickTime™ and a

Video decompressorare needed to s ee this picture.



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Feedbacks in geomorphology

• Feedback 1: Erosion is coupled to elevation, a negativefeedback – High elevation promotes rapid erosion through freeze-thaw processes (a

rapid physical weathering mechanism), sparse vegetation (above thetreeline, roots do not stabilize slopes), increased precipitation (orographicrainfall).

– There is also a general, though not perfect, correlation between high

elevation and high slope and relief, which promotes physical weatheringand 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 belowered 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 isthe basis of W. M. Davis’ theory of landscape evolution:



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• 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 theonset 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 theother hand, warm temperatures promote faster chemicalweathering. Higher rainfall always increases both chemical and

physical weathering and erosion.

F db k i h l



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• Feedback 3: Erosion is coupled to uplift, a positivefeedback

– Because of isostasy, removal of mass from the top of the crust causes it torise. Loading of mass on top of the crust causes it to sink. Since isostasyoperates over some finite regional size (flexural wavelength ~100 km), it isthe average mass of crust on that scale that determines uplift. Hence erodingof valleys can cause the intervening mountains to rise.

F db k i h l



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• Feedback 3

– There is evidence that this type of valley-incision denudation-

uplift is raising the high Himalaya:

Gl b l S th i f E i



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Global Synthesis of Erosion

• An example of a process geomorphology idea at the largest scale isan 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 wholedrainage. Given density of sediment this is equivalent to mean vertical erosionrate (knowing Mg/km3, we get km/yr) for the whole drainage




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• If we have some idea what the relevant variables are, we candevelop an empirical correlation from which the whole map of theearth 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 rainiestmonth (mm), P is mean annual rainfall (mm), H is mean elevationof the catchment, and a is mean slope.

• This equation shows feedbacks 1 and 2

– E = f ( H,a ); Elevation -> Erosion -> Change in elevation

– E = f ( p,P ); Climate -> 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)

log E 2.65 log( p2

/ P ) 0.46 log H tana 1.56




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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 aredirect expressions of tectonic activity.

• For old stable terrains, elevation and relief becomeexpressions of relative rates of erosion.

– Thus, in California, anticlines are hills or mountains, but inPennsylvania, 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 therelations of the rocks around them. Current tectonicactivity 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) thanseismology.



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• When the form of an original geomorphic feature is known, thenthe magnitude of tectonic deformation can be determined bymeasuring its current shape. Examples:

– fault scarps start from nothing, so height of scarp gives magnitude of totaldip-slip displacement.

– undisturbed drainages presumably go straight across faults; lateral offsetgives total strike-slip displacement.

– marine terraces start at sea-level, so height of wave-cut platform givestotal uplift since abandonment of terrace.

– river terraces start with longitudinal profile of riverbed; disturbances inshape and slope give total deformation and tilt.

• When, furthermore, the age of the geomorphic feature is alsoknown, then the rate of tectonic deformation is determined aswell.

– How do you date geomorphology? This is a different problem from datingrocks!




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• Topographic profiles of upliftedmarine 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 fromshape of initial horizontal markers

• What additional type of data would beuseful here?

G h l d T i



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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 thrustfaults, but you can’t have anticlines without synclines in between! So here there

is net uplift of terraces, but synclinal downwarping in the middle. – No information on rates…this 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 throughHimalayan front. These terraces can now be dated (but note the lowest one…).

M i G hi R



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Measuring Geomorphic Rates

• We have several ways of measuring the rates of landscapeevolution.

– Dating of geomorphic surfaces: Much effort has been directedtowards measuring the age of erosional surfaces (peneplains,terraces, etc.). using the exposure age of materials on thatsurface.

• Thermoluminescence or electron spin resonance

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C dating of organic matter in the soil• Cosmogenic nuclides: 10Be, 26Al, 36Cl

– Example: clocking development of normal fault scarp in limestone:

M i G hi R



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Measuring Geomorphic Rates

G hi R t



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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 thesurface down a geothermal gradient. Various methods aresensitive 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 thegeothermal 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 havedeveloped the ability to clock the time since apatite crystals passedthrough ~2 km depth.

• Does thermochronology actually measure uplift rates (with respect tosea level) or erosion rates (motion of material points with respect tothe land surface)?

basic principles of gm

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