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Introduction to Geomorphology
I. INTRODUCTION
A. Geomorphology : The study of surface landforms, processes and landscape evolution of the Earth.
- Interdisciplinary Study: cross-over with scientific disciplines of sedimentology, soil science, geography, climatology, hydrology, glaciology, civil engineering and volcanology.
- Formal Subdisciplines of Study a. Fluvial Geomorphology b. Hillslope Geomorphology c. Glacial Geomorphology d. Tectonic Geomorphology e. Quantitative Geomorph (Civil Engineering) f. Coastal Geomorphology g. Desert Geomorphology h. Biogeomorphology / Landscape Ecology i. karst geomorphology (karst = study of cave processes / hydrology)
B. The Essential Ingredients of Geomorphology
- Focuses on continental landscapes, landforms, and processes a. Surface processes result in materials and landforms as products b. The landform/material products form a record of Earth surface history
- Landscape Dynamics a. Tectonic Uplift (continental building) vs. Erosion (continental destruction) (1) E.g. accretionary uplift of Coast Range vs. fluvial erosion / mass wasting b. Surface Deposition vs. Surface Erosion (1) E.g. sediment-filling of Willamette Valley vs. river transport of sediments to Pacific
- Driving Mechanisms a. Climate / Atmosphere (solar energy) b. Tectonics (geothermal energy) c. Gravity (fundamental / pervasive force in universe)
- Time-Averaged Processes a. Process Rates: fast vs. slow (1) e.g. catastrophic slope failure vs. rock weathering b. Time-Scaling Factors (1) seconds, minutes, hours, days, weeks (2) years, decades, millenia (3) 10,000 - 1,000,000 's of years
- System-based Science a. System - collection of components b. system components interact and affect one another
- Landforms
Class Exercise: let's collectively think of all the different types of landforms that we've seen in Oregon and the processes that form them??.... break into groups and make a list.
C. Classic Perspectives
a. Father of Modern Geologic Principle: James Hutton (1726-1797) Scottish geologist/physician, "Theory of the Earth"
(1) Uniformitarianism: the present is the key to the past,
(a) observable surface processes operating today, are assumed to have operated in the past, according to constant laws of nature
b. Early Geomorphologists
(1) Agassiz (1807-1873) French scientist, studied and popularized evidence for ancient glaciations of the Alps
(2) T.C. Chamberlin (1843-1928)- Harvard geomorphologist, worked on theories of continental ice sheets in North America, and past glacial advances.
(a) T.C. Chamberlin: multiple working hypotheses
(3) Westward expansion of U.S. and USGS Land Surveys
(a) J.W. Powell, first head of USGS, surveyed landscape and resources of much of the western US
i) Noted for expeditions down Colorado River and geological analysis of Grand Canyon stratigraphy
ii) First proposed the concept of "Base Level" for streams and rivers: the idea of erosional equilibrium.
(b) G.K. Gilbert (1843-1918): worked extensively on geomorphic processes in Utah i) identified ancient pluvial Lake Bonneville ii) discussed faulting/mountain building iii) developed the concepts of "equlibrium"
II. FUNDAMENTAL PRINCIPLES
A. Uniformitarianism and Geomorphology: detailed observation of modern surface processes can be used to interpret the origin, evolution and mode of occurrence of landforms.
- e.g. study of modern glaciers can lead to interpreting the nature and occurrence of ancient glacial systems by examining their products (moraine for e.g.).
B. Scope of surface geomorphic processes: includes a multitude of physical and chemical processes operating at the earth's surface to influence and modify the landscape.
- Rock weathering
- Mass wasting/erosion (gravity-driven)
- The work of running water (fluvial)
- Groundwater activity
- Wind action
- Glacial Activity/Moving Ice
- Wave action / Tide Action
- Tectonic Process a. Volcanism / earthquakes
The rate and magnitude of the geomorphic processes varies, and hence the rate and magnitude of landscape evolution varies spatially
C. Process-Response Models
- Process: action produced as a result of a force of change
- Physical and chemical surface processes operate in such a way as to initiate changes in the landscape.
a. If variable X changes in the process system, likewise a proportional change can be expected in the landscape.
- Based on empirical observation, it is possible to quantitively describe and model process-response relationships on the Earth's surface (cross-over to civil engineering).
D. The concept of Equilibrium Systems
- What is a system? A collection of related objects and the processes relating these objects. a. Isolated system: (1) no energy or matter leaks out of system & none comes in. (a) example - the Universe as a finite entity b. Closed System (1) energy may transfer into and out of the system, but not mass (a) e.g. the Earth as a whole is closed
c. Open system: (1) matter and energy may flow into and out of the system freely (a) most geomorphic systems are open i) energy and mass transfer functions (2) steady state System (a) (special type of open system) balance between input and out flow.... INPUT + OUTPUT _______________________________________________________________________________ Class Exercise in Systems.
Identify the following as isolated, open, or closed systems. Once you've identified them, cite your ideas / evidence as to why you answered the way you did.
(1) an open beaker of water on the lab bench (2) a covered beaker of water on the lab bench (3) this class room (4) the stretch of Willamette River adjacent to Independence (5) the ocean (6) the atmosphere you are breathing
Can you think of a steady state geologic system? List your ideas.
Is the Earth as a whole, truly a closded system as stated in the notes above? State your reasoning.
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- Equilibrium System : process-response balance between opposing forces such that any change in any of the controlling variables (independent variables) will necessitate a corresponding response in the dependent variables to maintain overall system equilibrium.
- The elements of the landscape adjust to changes in the process, forming a cause and effect relationship to maintain static equilibrium.
- Many Geomorphic systems are viewed as equilibrium systems
a. e.g. River systems: if a river is shifted out of equilibrium by increased sediment load, the river will adjust its geometry to carry the load relative to processes of erosion, transporation and deposition.
(1) G.K. Gilbert and Graded Stream Concept: Streams and rivers display an equilibrium state in that the velocity of the water is in equilibrium with the channel gradient.
(1) Climate (Exogenic Force: from without)- average weather conditions at any place over a long period of time.
(a) Climate and the sun
i) Driven by solar energy of sun, i.e heat a) Solar Energy Source = hydrogen fusion
ii) solar insolation variable around planet depending upon geometry and latitudinal position (highest at equatorial belt, lowest at poles)
iii) Solar energy transfered as heat in atmospheric/oceanic systems of the earth-----climate systems driven by the heat transfer of these systems (i.e. atmospheric and oceanic circulation patterns)
(b) Climate largely driven by heat transfer of suns energy about atmosphere and ocean waters
(c) Climate as a 1st order controlling factor, influences: i) rainfall/solar insolation of area ii) vegetative growth iii) style of weathering/erosion process iv) hydrologic processes (fluvial, glacial)
(2) Gravity as a controlling factor
(a) Force of attraction between the earth's center of mass and surface materials (sediment, soil, water) drives landscape evolution
(b) F = mg = "weigth" where m = mass of object, g = acceleration due to gravity i) g = acceleration of a falling object (e.g. sediment) due to gravitational force F, assumed to be constant at 980 cm/sec = 9.8 m/sec^2 (c) Gravity obviously influnces surface water flow, mass wasting/hillslope movement processes, serving as a driving force
i) Driving force for flowing water and ice
ii) Driving force for density-driven currents a) e.g. air flow / weather b) ocean currents c) convection in mantle / tectonics
(3) Internal Heat of the Earth (i.e. Tectonics)
(a) Internal Heat of Earth: supplied primarily by: i) radioactive decay with exothermic heat loss ii) frictional heat by earth tides and internal rock deformation
(b) How do we know the inside of the Earth is hot? i) volcanic eruptions ii) seismology iii) deep mines / wells
(c) Internal Heat Transfer
i) Mantle convection: physical movement of rock material as a heat transporting medium a) hot, deeper mantle rises as it is of < density b) cooler, shallower mantle sinks as it is of > density
(d) Internal heat transfer of the earth thought to be the driving mechanism of plate tectonics and plate motion i) oceanic spreading centers/volcanism ii) plate subduction and volcanic arcs iii) plate collision and rock uplift/deformation/mountain building
(3) Rock Structure generally forms zones of weakness upon which other surface processes can act to carve the landscape
F. Thresholds and Complex Response
- Threshold Concept : a. Within an equilibrium system, an equal state of disequilibrium also exists (1) results from severe imbalance within the system (2) system undergoes period of disequilibrium prior to re-establishment of equilibrium conditions
b. Geomorphic Thresholds: represent the limits of equilibrium or critical limits, as processes extend beyond thresholds, disequilibrium or response occurs.
(1) E.g. as thresholds for hillslope water moisture are exceeded, the system may respond in the form of slope failure or landslide.
(a) implies a critical threshold of moisture content beyond which slopes will fail
c. Extrinsic (external) vs. Intrinsic (internal) Thresholds (1) e.g. Extrinsic - (a) meteorite impact, (b) storm/rainfall event - flood discharge (2) e.g. Intrinsic - (a) roof collapse of a cave / sinkhole (b) channel cutoff of a meander loop
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SIMPLE THRESHOLD EXPERIMENT:
Determine the slope-angle threshold at which the following materials will be begin moving under the influence of gravity
(1) On a board, systematically place the following materials: (a) a puddle of water (b) a small pile of sand (c) a rock sample (a block of rock)
(2) For each of the materials, elevate one side of the board until the material starts moving. Record the critical angle at which the materials move in the table below _______________________________________________________________________________ Material Critical Angle Is there motion when slopeIs there motion when is less than critical? slope is greater than critical? _______________________________________________________________________________ Puddle of Water
Pile of Sand
Block of Rock _______________________________________________________________________________
Which materials are more sensitive to gravity-driven motion? Why?
Which materials are less sensitive to gravity-driven motion? Why?
On the Earth's surface, list mechanical (geological) methods by which the angle of slope could be increased, like in your experiment
- Time is an essential ingredient in any geologic process
a. In terms of geomorphic process, variable levels of time are required for desired products of change
(1) e.g. time scale variation between slow steady-state soil creep vs. instantaneous slope failure
- Cyclicity and Time
a. Geologic processes are by nature cyclic and repetitive over time.
b. Geologic cyclicity readily evident in geomorphic systems (1) e.g. Flood cyclicity of river basins
H. Constructional vs. Destructional Processes
- Constructional Landforms : those land units that have been or are being built (i.e. increasing in mass, height, or area)
a. Constructional Landforms created by mass redistribution
b. Examples (1) Tectonic (a) Volcanic Accumulation/Mountain Building (Orogeny) i) Cascades = volcanic ii) Coast Range = accretion (b) Fold/Fault Block Mountains (Orogeny) i) Steens Moutain (c) Isostatic Uplift of Land areas (2) Depositional (a) Loess regions (e.g. Palouse / SE Washington) (b) Dune fields (c) Glacial Terrain
- Destructive and/or Erosionally-Derived Landforms : those landforms that are derived by weathering and erosion (destruction)
a. Includes erosion of rock material and deposition of sediment
b. Examples
(1) Glacial scouring / valleys (e.g. Yosemite Valley) (2) Fluvial erosion / valleys (e.g. Grand Canyon) (3) Coastal cliffs
III. Exogenic vs. Endogenic Processes
A. Mass Balance: Exogenic vs. Endogenic Processes
- Exogenic Processes: destructive geomorphic processes that originate at or above the earth's surface a. Weathering-erosion-denudation processes (1) e.g. Chemical/Physical Rock Weathering (2) e.g. Rilling/Gullying/Fluvial Erosion (3) e.g. Glacial scouring/erosion
b. Theoretically: if exogenic processes were to operate on a landscape, unimpeded by opposing forces, there would be a tendency to reduce the landscape to a relatively flat, featureless surface with few topographic irregularities ("Peneplanation" concept)
(1) "Base Level" = theoretical surface of erosional equlibrium at which, the land surface will no longer be eroded.
(a) Ultimate baselevel: Sea level, theoretical end point of continental erosion.
c. Climate is an exogenic process that flucutates and upsets geomorphic equilibrium in the landscape.
- Endogenic Processes: internal processes within the earth that result in uplift and rejuvenation of the landscape
a. e.g. Tectonic Mountain Building Processes (1) Rock Folding, Faulting, Uplift (2) Epeirogeny b. Volcanism c. Endogenic Processes result in an influx of lithospheric mass and energy, rejuvenating the landscape and tipping geomorphic equilibrium out of balance
B. Endogenic Effects
- Diastrophism or Tectonism: Collective processes that deform the earth's crust
a. Epeirogeny: regional uplift or depression of the earth's crust over large areas with little internal deformation of original rock structure (broad, regional, gentle uplift)
b. Orogeny: relatively intense deformation of the crust to form structural mountains (folded, faulted, uplifted terrane).
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Class Exercise on Isostacy
Build a simple model of the Earth's crust and upper mantle (i.e. a lithospheric plate).
Materials needed: -6" x 4" x 2" blocks of styrofoam (a crustal plate) -a tank or tray of water (analogous to the upper mantle or asthenosphere) -a handful of pebbles, gravel, or other rock pieces
(1) Float the lithospheric plate (the block of styrofoam) in water. Note the relative position of the water line on the side of the "boat". (2) Load the lithospheric plate with several pebbles, carefully balancing them so the "boat" does not topple.
What happens to the Earth's crust when it is loaded with large volumes of sediment, ice, or tectonic blocks of rock? ... record your observations and analogies here...
(3) Incrementally erode your crustal plate by taking 1 pebble off the "boat" at a time.
Make a sequence of sketches and record your observations as to what happens to the Earth's crust as it is unloaded by erosion. Record them here....
(4) Provide a brief summary discussion regarding surface erosion, tectonic motion of the crust, and isostatic adjustment.
(5) Comment on how realistic it is to apply the "Davisian" peneplane concept to mountain ranges, i.e. is it very realistic to think that a mountain range could be eroded to a "peneplane". Explain your answer in terms of erosion and isostatic adjustment.
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- Uplift of earth's crust: creates potential energy that available for conversion to kinetic energy via exogenic geomorphic systems
a. In Comparison: Rates of crustal uplift are much higher than those of crustal denudation (a much slower process)
(1) E.g. calculations of vertical displacement rates based on dated events: (a) Surface Subsidence Rates Range to -1200 cm/1000 yr (b) Surface Uplift Rates Range up to +2400 cm/1000 yr (uplift). (2) Range of calculated denudation rates (based on river sediments) (a) Range 1 cm/1000 yr - 14 cm/1000 yr (b) Average ~ 3-4 cm /1000 yr i) High: Asia: 14 cm / 1000 yr
C. Mass Balance: Endogenic vs. Exogenic Processes
- Thus exists a balance between crustal uplift (endogenic) and crustal denudation (exogenic) in the form of "dynamic equilibrium"
- If rates of uplift far exceed rates of denudation, equilibrium threshold will be crossed and the geomorphic/landscape system will be thrown into disequilibrium
- e.g. climatic conditions could be such to trigger extensive erosion and denudation of the landscape, resulting in "de-loading" of the crust, thus promoting regional epeirogenic uplift.
- Equilibrium System: based on principles of mass balance and mass distribtution a. uplift: addition of mass to crustal region b. denudation: redistribution of mass out of region
IV. GEOMORPHOLOGY AND THE QUATERNARY PERIOD
A. Quaternary Period: Upper portion of the Cenozoic Era
- Quaternary divided into (1.6 Ma-Present): a. Pleistocene Epoch(1.6 Ma-10,000 Yr. B.P.) b. Holocene Epoch (10,000 Yr B.P.-Present)
Ma = Mega "ans" = Millions of Years; B.P. = Before Present
- We live in the (late) Holocene Epoch of the Quaternary Period
B. Quaternary Strata: Characteristics and Significance
- Relatively recent sedimentary and volcanic strata
- Amenable to absolute dating techniques (C-14, radiometric dating)
- Period known for its association with dramatic climatic fluctuations
