Geomorphology, Lecture Two

Geomorphology, Lecture Two

Driving and Resistant Forces Driving forces > External (climate, gravity) > Internal (thermal energy from radio active decay, friction) and Resistant forces > Lithology

> Structure NEXT: Theories of landform evolution and the rise of modern geomorphology 1. Hutton and Lyells Uniformitarianism Articulated by James Hutton and Charles Lyell (~1785 1850s)

Landsurfaces shaped by imperceptivity slow, erosive action of water Present is the key to the past These ideas led the rock cycle Controversial at the time: I thought it was Noahs flood! (catastrophism) From Charles Lyell's Principles of Geology (second American edition, 1857), showing the origins of different rock types (the rock cycle).

2. G.K. Gilberts Delicate Balance Landsurfaces adjust to local geology and dominant processes Continuous erosion does not change slope angles, as long as process types and rates dont change Developed laws of landsurface development (incl. driving / resisting forces) Articulated in Report on the Geology of the Henry Mountains (1877) Gilbert, G. K. 1877, Report on the geology of the Henry Mountains [Utah]: U.S. Geog. and Geol.

Survey Rocky Mtn. Region, 100 p. 3. W.M. Davis Cycles of Erosion (~1890s) Viewed landform evolution as a systematic, predictable progression of landform change through time Following uplift, landscapes pass through youth, maturity, old age, peneplain (cycles of erosion)

Accepted in NA because many examples of stages were found Played down climate Assumed tectonic uplift, then seamlessly back to base-level (generalized uplift process too much) 4. Doubts about historical explanations 1960s 1970s

Doubts arise about the validity of historical explanations o Not much understanding of process o Not much empirical evidence

Now, a shift from descriptive to quantitative (math, statistics) Geomorphometry becomes part of analysis Increase in field measurements Plate tectonics theory refined and woven into geomorphology Now, an understanding of geomorphic response to different climates 5. Schumm: Different time intervals are critical 1960s 1990s

Equilibrium depends on the time interval examined Changes occur quickly-toimperceptibly slowly Quantitative 6. Today, Process Geomorphology Quantitative to analytical Applied, predictive, field-based, mega-to-micron in scope, uses geospatial technologies, multidisciplinary Process / form; form / process: How geomorphology works.

We now recognize the interconnectedness of systems. 6. Today, Process Geomorphology

Very connected to climatology Formed sub-disciplines (tectonic, Karst, coastal, etc.) Requires knowledge in chem., physics, comp. sci., geol., climatology Addresses how, when, what, where Uniformitarianism: Present is key to past? Problems? of Hutton & Lyells uniformitarianism: From: Schumm,1991

PROBLEMS with using modern conditions as a basis for explanation and extrapolation: > Time > Convergence > Divergence > Location > Singularity > Sensitivity Uniformitarianism: Present is key to past?

CAUTION #1: TIME -- some general problems: > Time period of data collection is usually too short: Conclusions extrapolated to longer time scales may result in error > HOW a landform changes (evolves) depends on the time frame over which observations are made. > Conclusion: Short time-span studies applied to long timespan problems are tenuous at best! Uniformitarianism: Present is key to past? CAUTION #1: TIME -- Some landforms are relict

(e.g., climate changes) Uniformitarianism: Present is key to past? CAUTION #1: TIME -- Organisms not the same as in the past; habitat change / evolution / migration No grasses >55 mybp 75 mil. grazers, compactors,

& wallowers in NA focused runoff Uniformitarianism: Present is key to past? CAUTION #2: CONVERGENCE -- Similarly shaped landforms can arise from different processes (different causes, similar effect) > caldera or impact crater or salt dome? > basalt flows or sediment deposits?

> plateau or river terrace? > U-shaped glacial valley or U-shaped tropical valley? > scarp (cliff) or landslide or fault or animal route? > sapping feature or hanging valley? > braided stream from uplift or braided stream from increased discharge? > folding or faulting? Uniformitarianism: Present is key to past? CAUTION #3: DIVERGENCE -- Similar processes

create different landforms (similar causes, different effect) > Same burn scar, different valley responses > Same thunderstorm, different valley responses > Increase in precip. creates different effect depending on climate: i) Eastern US rivers response: shift laterally; ii) Western US rivers response: vertically change

Uniformitarianism: Present is key to past? CAUTION #4: LOCATION Can you extrapolate from one location to another? > Stream A is eroding, 1 km away stream B is aggrading > Random occurrence of meteorological events > Pleistocene climate change had different effects N. America vs. Europe > Worker A is from Florida, Worker B is from Nepal > Worker A this is sedimentary rock, Worker B this is volcanic rock

Uniformitarianism: Present is key to past? CAUTION #5: SINGULARITY A characteristic that makes one thing different from another (even nearby landforms) > After explaining a landform at Site A, Dr. Smith tells you, Well, it isnt that way at Site A > Each landform has enough differences that it can be considered singular > You can predict based on a large sample, but not for

an individual > Singularity is the randomness or unexplained variation Uniformitarianism: Present is key to past? CAUTION #5: SENSITIVITY How a system responds to a minor change > How close is the system to a threshold? > A minor input (gentle rain) might cause major change > Example: A meander develops in a river, and one day

the river cuts off the meander Ponder this.. ..if the reproducibility of experiments is a requirement of scientific research, then does earth science even exist? (Schumm, 1991) NEXT: Articles

Thresholds & non-linear behavior (Phillips) Cascade effect & linked processes (Hewitt) Ergodicity (Savigear) Isostacy Elevation (ft.)

Nonlinear Behavior 1M ybp to today Nonlinear Behavior (Phillips 2003) Phillips lists nine sources of nonlinearity: 1. A threshold is exceeded 2. Storage effects 3. Saturation / depletion (too much or too little)!

NOTE: We will not cover Phillips 4 9. Nonlinear Behavior (Phillips 2003) Source #1: A threshold is exceeded 1. mass wasting (when shear stress overcomes shear strength), 2. earthquakes, 3. wxing rinds, 4. glacial stream blocking, 5. precip. intensity vs. infiltration, 6. particle entrainment 5

1 3 2 4 6

Nonlinear Behavior (Phillips 2003) Source #2: Storage effects 1. episodic mobility of sediment, 2. reservoir filling, 3. water stored in soil more important than rainfall intensity for flood frequency (in PA Troch et al., 1994). 2 1

Nonlinear Behavior (Phillips 2003) Source #3: Saturation / depletion (too much or too little)! 1. oxidation stops when soil is saturated, 2. biological systems (biosphere!) thrive or die depending on supply of water, sunlight, nutrients, 3. soil reaches a certain thickness where weathering slows significantly (surface moisture too high up in soil column) 1

Nonlinear Behavior (Phillips 2003) Why understanding nonlinear behavior is important: 1. Recognize that systems are COMPLICATED! 2. If cyclical, we can prepare for the next threshold breach 3. To make long-term predictions more reliable 4. To explain the unexplainable Cascade Effect & Linked Processes

(Hewitt 2006) Concept of disturbance regime The consequences of brief, highmagnitude episodes, such as fires, volcanic eruptions, debris flows, insect infestations, and rock avalanches are longterm, linked, and radiate outward like ripples in a pond! AND THEY ARE NON-LINEAR!!! Cascade Effect & Linked Processes (Hewitt 2006)

Rock avalanches main human impacts are.? 53 km long Lake Sarez in Tajikistan Cascade Effect & Linked Processes (Hewitt 2006) Rock avalanches some highlights: Effects can last 10s of thousands of years These landscapes are ALWAYS disturbed a disturbance regime!

In 2 3 minutes, can reduce up to of the slide mass into sand and silt sized material. Shock of sudden halting converts the material into a highly compacted matrix Travel 5 15 km from source slope Velocities over 250 km/hr (~155 mph)! Cascade Effect & Linked Processes (Hewitt 2006) Rock avalanches five landform-generating phases:

Phase Five: nick-point migrates upstream; odd misfit river shape from earlier channels Cascade Effect & Linked Processes (Hewitt 2006) E.g., volcanic eruption pyroclastic flows > downwind ash > debris

avalanche > mudflows > vegetation stripped > soil burned > rivers diverted > rivers dammed > mudflows from breached dams > subsequent mudflows dam lower order streams > internal hydrology disrupted (water through lava tubes) > deadfall in rivers > anthropogenic. Cascade Effect & Linked Processes (Hewitt 2006) E.g., forest fire

May-June 2002 the Schoonover / Hayman fires burned over 500 km2. fire > dead vegetation > channel cutting > flash flooding > debris flows > sedimentation of downstream meadows > choking of downstream streams/rivers > wildlife habitats removed/changed > Denvers water supply > tourism > recreation > insurance rates > mitigation > restoration Cascade Effect & Linked Processes (Hewitt 2006) E.g., insect infestation (e.g., mtn. pine bark beetle and spruce beetle; Douglas-fir tussock moths and Western spruce budworms) Ergodic Theorem (Savigear 1952) The cliffs from Pendine eastward have thus progressively lost contact with the sea Ergodicity: An alternative to long-term studies?

A N Ergodic Theorem (Savigear 1952) N A

Ergodic Theorem (Savigear 1952) Kirkbys (1984) take on Savigers model (only Kirkby accounted for climate change in his model). Ergodic Theorem (Savigear 1952) To understand landform evolution, we must look for different times by moving our place of observation

young old Ergodic Theorem (Savigear 1952) Ergodic Theorem (Savigear 1952) Isostacy = equal standing Isostacy is a state of equilibrium that exists between

the Earths ______ and _______. The crust adjusts to changes in mass. Isostacy RESTORES equilibrium, it does not UPSET equilibrium. As such, isostacy is a ______ feedback. Two types of adjustments: subsidence and rebound Isostacy is offset by: o o o o

Large amounts of sediment deposited OR eroded Glacial / post-glacial environments (ice sheets) Amount of water in oceans vs. on land (ice sheets) Volcanic deposits Isostacy

Gravity measurements suggest that the crust in the Hudson Bay region has another 100 meters still to rise (Hannah 2001). Isostacy Isostacy flexural isostacy

Gravity measurements suggest that the crust in the Hudson Bay region has another 100 meters still to rise (Hannah 2001). Isostacy Isostacy

In Nunavut, a Province in Canada Isostacy Raised beaches near St. Ignace, MI Copyright Louis Maher Isostacy a 1 mile-thick glacier above these islands melted only

a few thousand years ago

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