Introduction to Geomorphology

 

  • Text: Bloom, Arthur, 2004, Geomorphology: Chapters 1-4 (study guide)
  • Geomorphology From Space: Introduction: Regional Landforms Analysis and Geomorphological mapping (Optional)
wiki

Terms: geomorphology, landform, landscape, endogenic, exogenic, equilibrium, lag time, relict landscape, lithology, structure, catastrophism, uniformitarianism, dynamic equilibrium, steady state, isostatic, positive and negative feedback. relict landform, lag time, orogen, fluvial, aeolian, base level, graded stream

What is geomorphology?

  • Definition: (Geo, G. the Earth; Morph, G. Form, ology G. the science of) Geomorphology is the study of landscapes--It entails the systematic description of landforms and the analysis of the processes that create them. Geomorphologists are also concerned with understanding the function landforms and how landforms respond to changes in energy.  Because landforms and landscapes result from the combined effects of lithology, structure, and process, geomorphology draws upon nearly all fields of geology.
    • A Landform is an individual feature, such as a slope, valley or mountain
    • Landscape is the he combined effect of numerous landforms, such a mountainous or desert terrain.

What produces a landscape?

The three independent variables that control landscape are climate (precipitation and temperature), plate tectonics, and history (time). In the long term these factors control structure, lithology, and process, the three dependent variable that landscapes reflect most closely. A common way of viewing landscape variables is outlined below as the interplay between energy, process, and resisting framework.  How you approach a landscape will depend on the time frame you are considering. Energy, process, and sediment supply are important to a geologist studying a beach.  However, a geomorphologist studying the Rocky Mountains might focus on tectonic setting, climate, and process over the course of the last billion years.

1. Energy: Energy drives geomorphic change and is the force behind process. These energy sources (listed below) drive convection in both the atmosphere and the earth, and drive chemical reactions. Without energy we would not have the hydrologic cycle, weathering, or the tectonic activity that characterize earth dynamics.

  • Solar energy: Drives climate and exogenic processes
    • 2 cal/cm2-min reaches the outer atmosphere.
    • 30 to 14 % is absorbed into the system--depending on latitude
    • Solar energy and its interactions with the earth's surface, the atmosphere, and the hydrosphere determines climate, the mean weather condition experienced by a given region.
  • Geothermal energy: Drive Plate Tectonics  and endogenic processes
    • derived from decay of radioactive elements and residual heat
    • gradient ± 20-30°C/km (crust)
  • Gravity: Drives convection, which is important to everything
    • g= acceleration due to gravity = 980 cm/sec2 or 980 gals

2. Process: How energy is applied: The processes that shape the landscape fall into two broad categories.  Endogenic processes are largely driven by internal convection and adjustments caused by the redistribution of heat and mass.  Exogenic processes operate on the earth's surface and are driven by solar energy with the aid of water.

  • Endogenic processes: Processes that operate from within the earth.
    • Examples:
      • Tectonics processes: Plate tectonics (e.g. orogenic processes and rifting)
      • Volcanism: Intraplate hotspot activity (lava plateaus and volcanoes) arc volcanism, sea-floor spreading
      • Isostacy Animation
        • Epeirogenic processes: regional uplift and subsidence caused by mantle anomalies.
        • Isostastic processes: local subsidence and uplift caused by local loading and unloading
  • Exogenic processes : These processes operate on the earths surface.
    • Examples:
      • weathering and erosion
      • hydrologic cycle and related fluvial processes
      • glaciation
      • aeolian activity
      • biological activity and man (?)
      • waves
  • Extragenic (my term)--meteor impact
    • Importance:
      • Addition of water (?) from comets?
      • Impact structure

3. Resisting framework: lithology and structure

With the exception of delta plains, basin and similar deposition settings, most landscapes are largely denudational, created through the erosion of pre-existing features such as a plateaux, fold belts, sedimentary deposits, or plutons that provides a resisting framework that exogenic processes shape and reform to produce a landscape.  The two most important elements forming this framework are lithology and structure.

  • lithology affects how much energy or time is required to produce change.
  • Structure determines the grain of the topography (joints, fold patterns, layering, arrangement of rocks of varying resistance)

 

Importance of water

Water plays a key role in the earth's geomorphic processes in the following ways:

  • it is the principle agent of weathering: a catalyst for reactions and universal solvent
  • through the hydrologic cycle enables energy from the sun to be converted to potential and kinetic energy required to eroded mountains and valleys
  • beneath the earth water affects faulting the production of magma and metamorphic reactions.
  • circulates heat through the oceans and atmosphere -strongly influences climate

Important considerations

Scale: Typically a large feature such as a mountain massif is more persistent than a small rocky spire because more energy and time is required to modify it. Baker's classified of landforms by size (Table 1) is a useful display of this relationship.  However,  permanence in fact is a function both the size of the feature and its resistance to the processes acting on it. For example a glacial striation may be ground down within a day beneath a glacier, but once exposed may persist for a hundred years or more.  Likewise drumlins may persist for thousands of years once removed from the activity of flowing ice and glacial metwater streams.

Table 1. Classification of terrestrial landforms based on scale and their approximate persistence (After Baker, 1886)
Order
Approximate scale
Persistence (years)
Feature
1st Order
>10,000,000 km2 ~108-109 continents and ocean basins
2nd Order
~1,000,000 km2 ~108 Shields and provinces
3rd Order
~10,000 km2 ~107-108 orogens, uplifts
4th Order
~1,000 km2 ~107 intermontane basin

5th Order

1000 km² ~106 Major valley, large delta
6th Order
~100 km2 ~105-106 Individual mountain, large alluvial fan
7th Order
~10 km2 ~104-105 Hillslopes, stream channel, estuary, barrier island
8th Order
1 km2 ~103 small tributary channels, drumlin

9th Order
~m2 ~102 point bar , gully, glacial groove
10th Order
~cm2   ripples

 

Geologic history: (relaxation time of past events): Not all features are the product of current processes. Landscapes evolve over millions of years under ever changing conditions. Age, structure, resistance, climate  and past geologic events all play a role. Although glaciers retreated 12,000 years ago many glacial features persist. They have not yet been removed by today's processes. Glacial drumlins on the North Shore and cirques in the White Mountains are relict landforms, formed under a previous condition.  Lag time is the time required for a landform to change in response to changing conditions.  Moraines forming Cape Cod change rapidly in response to wave activity and rising sea level, whereas a glacially scoured rocky coast may take thousands of years.  As discussed above, such variation in lag time reflect the resistance of the landform and the nature and intensity of the processes acting upon it.

Equilibrium: (self-regulation of energy) Landforms adjust in response to available energy and mass input.  A storm introduces energy and mass to a drainage basin. In response streams will deep their channels and widen their banks to expend energy and accommodate increased flow.  In the summer, deposition changes the channel to accommodate lower flows and decreased energy inputs.  The stream therefore is self regulating, adjusting it's channel to handle the available input. A barrier island is another example of a self-regulating landform. By inlet migration and overwash  an inlet can migrate to keep up with rising sea level, unless inhibited by jetties and seawalls. Eventually a critical threshold is reached and a system experiences a rapid change to a new set of conditions.

Perception of equilibrium state is a function of time:

  1. Static Equilibrium: no perceived change
  2. Steady State Equilibrium: Fluctuation about a mean
  3. Dynamic Equilibrium: Fluctuation about a moving average

equilibrium

Figure 1. Two common view of equilibrium.

Feedback mechanism:

  • negative feedback: reduces or alters
  • positive feedback : enhances or exacerbates

Feedback mechanisms are extremely important in considering climatic changes and the processes that drive

Historic Trends in geomorphology

Catastrophism: A prominent concept in the 18th and 19th centuries, catastrophism considered landscapes to have an innate permanence changed only by catastrophic events. The theory was set aside with the acceptance of uniformitarianism but has resurfaced as we begin to understand the relationship between meteor impacts and mass extinctions.

Uniformitarianism and landscape evolution

Uniformitarianism is the principle that features on the earth form not by catastrophic events but evolve through the action of gradual processes observable today and occurring over a seemingly limitless period of time. The concept was the foundation that allowed others to investigate landscape evolution.  "The past is the key to the present" and "geologic time" all evolved from Hutton's doctrine of uniformitarianism.

  • The principal players in the evolution of uniformitarianism were:
    • James Hutton (1726-1797) Scottish naturalist: 1785 first formulated the law and published it in The Theory of the Earth.  For more on Hutton and to read his abstract first presented April 4, 1785, during a meeting the Royal Society of Edinburgh click here (Department of Geograpy-Geology,UW Marathon).
    • Playfair, (1748–1819) Scottish scientist and mathematician: 1802,published Illustrations of the Huttonian Theory of the Earth, a book elucidating Hutton's theory.
    • Sir Charles Lyell (1797-1875) British Lawyer and Geologist: Known of as the father of geology. Published Principles of Geology  in 1830 (cf. Darwin, 1859, Origin of the Species) in which he promulgated the theory to a broad audience.
  •  
  • Geologic exploration by post Civil War geologist in the western US: The post-civil war era was a time of geologic enlightenment.  Westward migration lead to the discovery and exploration of unique and expansive territories that needed to be inventoried and studied.  Investigations led to a blossoming of new ideas that outpaced the ensconced beliefs of European thinkers.  Even Swiss naturalist, Louis Agassiz, who developed the glacial theory, moved to the US to take position at Harvard and explore his new idea.  Grove Karl Gilbert and John Wesley Powell detailed the effects of streams and outlined the first geomorphic classifications of streams. These geologists laid the ground work for Davis' work on cycles of erosion and for future geomorphological investigations.
    • Grove Carl Gilbert (1843-1918) is the father of modern geomorphology. His greatest contribution was in fluvial geomorphology.   He introduced the concept of self-regulating equilibrium landforms, such as graded streams, described the structure of deltas and preformed the first quantitative studies of streamflow in flumes. He first described the laccoliths of the Henry Mountains, the block-faulted nature of the Basin and Range, and pluvial Lake Bonneville, located in the Great Salt Lake basin, Utah during the Pleistocene. [G. C. Gilbert, 1914, Report on the Geology of the Henry Mountains]
    • John Wesley Powell (1834-1902) is most famous for his excursion down the Colorado River. He developed a descriptive classification of streams, defined  the concept of base level, and elaborated on the progressive erosion of mountain ranges. 
    • Clarence   E, Dutton (1841-1912) studied in detail the features of the Colorado Plateau, pioneered seismology and evolved the modern concept of isostacy and its importance to geology. 
  • Continental Glaciation
    • Louis Agassiz (1807–1873) was a Swiss born zoologist who developed the modern theory of continental glaciation in his Etudes sur les Glaciers (1840).  In 1947 he accepted a position at Harvard where he could more openly pursue his interests in glacial studies.
  • Landscape evolution: Historical approaches
    • William Morris Davis (1850-1934): Cycles of erosion
    • Walther Penck (1888-1923): German Geomorphologist. The relative rates of processes (e.g. rate of uplift vs rate of denudation) controls landscape morphology (1894)
    • Lester King worked largely in South Africa. Published Canons of Landscape Evolution (1953) in which he discussed the roles of process, time, and structure in the formation of landscapes and evlolution of slopes.

Complex history of Landscapes

Rarely does a landscape represent a a single process, event, or progressive uninterrupted evolution of events, as inferred by Davis.  Like rocks landscapes are recycled, experiencing multiple events of uplift, erosion, denudation, subsidence, deposition, and deformation.  However, unlike a rock, a landscape retains aspects of the texture, structure and compost ion of the landscape before it.  The Southern Rocky Mountains are a good example.  They have been uplifted and eroded (peneplained) no less than three times.  Old faults are reactivated by new tectonic forces and older rocks are progressively unroofed adding their framework to the new landscape. The present Rockies are less than 5 million years old, but are cored by rocks nearly 2 billion years old. The flat upland surface of the southern Rockies is attributed to a previous peneplained surface that once lay close to sea level but now lies nearly 10,000 feet above it.

landscape
Figure 2.  The landscape cycle (c.f. the rock cycle).  Landscapes are a palimpsest of numerous events, each adding an aspect to to the present day terrain. 

Systems approach

Modern geomorphologist view a landform assemblage as an intricate system that can be studied by analyzing the variables or components that compose it.  The forces producing change (e.g. energy), the materials upon which the forces act (and their resistance to change), and the processes by which the change is produced are all considered. Many geomorphic system are inter-connected and often the mistake is make by treating them separately.
Megageomorphology

With the advent of air photography, satellite imagery and GIS geologists are able to view and analyze large scale (first and second order) features from an entirely new perspective. This approach has been particularly important in the exploration the Earth, Mars and other planets.

Uniformitarianism: Charles Lyell, Understanding Evolution, University of California Museum of Paleontology URL:http://evolution.berkeley.edu/evosite/history/uniformitar.shtml

Models of Landscape Evolution, Geography 423 Advanced Geomorphology, D.J. Sauchyn, University of Regina, CA, URL: http://uregina.ca/~sauchyn/geog423/models.html

Beyond the Plateau, a biography of Powell by the Smithsonian, URL: http://www.150.si.edu/chap3/plateau.htm

Vision of John Wesley Powell: NPR Radio audio URL:http://www.npr.org/programs/atc/features/2003/aug/water/part1.html

Puzzles and Discussions

Crossword Puzzle

Addtional questions related to text readings:

  1. Why does Bloom not title his book a Systematic Analysis of Landforms? Why does he stress "Late Cenozoic" landforms?
  2. How does Plate Tectonics affect climate?
  3. What is the relationship between CO2 and weathering and climate?
  4. Can you find and describe any examples of positive and negative feedbacks?

Bibliography

  • Baker, V.R., 1986, Introduction: Regional landforms analysis, in Short, N.M., and Blair, R.W., Jr., eds., Geomorphology from Space A global overview of regional Landforms. URL: http://disc.sci.gsfc.nasa.gov/geomorphology/GEO_1/GEO_CHAPTER_1.shtml
  • Bloom, Arthur. 1998, Geomorphology, A systematic analysis of Late Cenozoic landforms, (3rd edition): Prentice Hall, Upper Saddle River, N.B., 482 p.
  • Bourgeois, Joanne, 1998, Model Survey Geologist: G. K. Gilbert, GSA Today, p. 16-17. http://socrates.berkeley.edu/~geomorph/Bourgeois_GKG.pdf)
  • Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984, Geomorphology: Methuen and Co. Ltd., London, 605 p.
  • Easterbrook, Donald J., 1993, Surface Processes and Landforms: Macmillan Pub. Co., 520 p.
  • Gohn, Kathleen K., 2004, Celebrating 125 Years of the U.S. Geological Survey, U.S. Geological Survey, Reston, Virginia: 2004 (pdf)
  • Hart, M.G., 1986, Geomorphology pure and applied: George Allen And Unwin, Boston MA, 227 p.
  • Leopold, L.B., Wolman, M.G. and Miller, J.P., 1964, Fluvial processes in geomorphology. Freeman and Co., San Francisco, 522 p.
  • Mayer, Larry, 1990, Introduction to Quantitative Geomorphology: Prentice Hall, Englewood Cliffs, NJ, 380 p.
  • Morisawa, Marie, 1988, The Geological Society of America Bulletin and the development of quantitative geomorphology: GSA Bulletin, v. 100, p. 1016-1022.
  • Morisawa, Marie, 1985, Rivers: Longman Inc., New York, 222 p.
  • Pidwirny, Michael J., 1996-2001, Fundamentals of Physcial Geography, http://www.geog.ouc.bc.ca/physgeog/contents/4f.html
  • Ritter, D.F., Kochel, C.R., and Miller, J.R., 2002, Process Geomorphology (4rd Edition): McGraw Hill, 576 p.
  • Summerfield, M.A., 1991, Global Geomorphology. John Wiley and Sons, New York, NY, 536 p.
  • Thornbury, William D., 1969, Principles of Geomorphology (2nd edition): Wiley and Sons, New York 594 p.
  • Twidale, C.R., 2003, "Canons” revisited and reviewed: Lester King's views of landscape evolution considered 50 years later:
    Geological Society of America Bulletin 2003 115: 1155-1172

Historic Documents available on the Internet

Dutton, Capt. Clarence E, 1881, The physical geology of the Grand Canyon District, , Ordnance Corps. U.S.A. From Annual report of the United States Geological Survey to the Secretary of the Interior, 1880-81, pp. 47-166 (Available through Library of Congress) http://memory.loc.gov/cgi-bin/query/r?ammem/consrv:@field(DOCID+@lit(amrvgvg40)):

Powell, Capt. John Wesley,1895, Canyons of the Colorado it is (Available through Project Gutenberg) http://www.gutenberg.org/catalog/world/readfile?fk_files=16709&pageno=2.

___ 1875, Exploration of the Colorado River of the West and Its Tributaries Explored in 1869, 1870, 1871, and 1872 (Available through the Marriot Library Digital Collection) @http://content.lib.utah.edu/cdm4/document.php?CISOROOT=/Powell&CISOPTR=365&REC=1

  Lindley Hanson/Department of Geological Sciences/Salem State College/Geomorphology/GeoIndex/QkRef