Unit 37


Unit Overview

This unit, which introduces Part V: Sculpting the Surface, discusses primarily the process of erosion and its importance in the degradations and aggradations of landmasses. The main sections are as follows:

  • Landscapes and landforms
  • Gradation
  • Erosion and tectonics
  • Regional landscapes

A landform is defined as a single unit that forms part of the general topography of the Earth's surface. Therefore, a landscape contains an assemblage of landforms, and the landforms are the product of intervening processes.

Gravity is the major force driving gradation┬┐the leveling of a topographic surface. Gradation is comprised of both degradation and aggradation. These two processes operate together, with degradation wearing down a portion of a landmass and aggradation building up another portion. Degradational processes involve weathering, mass movements, and erosion. Weathering is the in situ disintegration of rocks, and it can involve mechanical, chemical, or biological processes. Mass movement is the spontaneous downslope movement of materials under the force of gravity (e.g., landslides). Erosion is the carrying of weathered and mass-moved material over long distances. Additional disintegration of materials occurs as materials are being eroded. Running water is the predominant agent of erosion across the globe. Other erosional agents include glaciers, winds, and coastal waves.

Degradational agents can also be aggradational agents, because what is removed from one location is deposited in another location. Erosion is not the only process acting on landmasses. Landmasses can be rejuvenated tectonically as they are worn down by erosion. Related to this condition is the principle of isostasy, where the removal of a large volume of rock (or other overburden) from an area of the crust leads to an upward rebound of that part of the landmass. Upward rebounding of the lithosphere compensates for downward erosion. Consequently, erosion is an indefinite and inescapable process.

Unit Objectives

  • To introduce three primary degradational processes: weathering, mass movements, and erosion
  • To focus attention on the aggradational processes that produce secondary landforms
  • To recognize the roles of degradation and aggradation in the formation and evolution of landscapes

Glossary of Key Terms

Aggradation The combination of processes that builds up the surface through the deposition of material that was removed from elsewhere by degradation; contributes to the lowering of relief by reducing the height differences between the high and low places in an area.
Degradation The combination of processes that wear down the landmasses; implies the lowering, reducing, and smoothing of those surfaces.
Erosion The long-distance carrying away of weathered rock material, and the associated processes whereby the Earth's surface is worn down.
Geologic time scale The standard timetable or chronicle of Earth history used by scientists; the sequential organization of geologic time units, whose dates continue to be refined.
Landform A single and typical unit that forms parts of the overall shape of the Earth's surface; also refers to a discrete product of a set of geomorphic processes.
Landscape An aggregation of landforms, often of the same type; also refers to the spatial expression of the processes that shaped those landforms.
Mass movement The spontaneous downslope movement of Earth materials under the force of gravity; materials involved move en masse - in bulk.
Primary landform A structure created by tectonic activity.
Secondary landform A landform that is the product of weathering and erosion.
Weathering The chemical alteration and physical disintegration of Earth materials by the action of air, water, and organisms; more specifically, the breakdown of rocks in situ, their disintegration and decomposition without distant removal of the products.

Unit Outline

  • Landscapes and landforms
    • Primary landforms are created by horizontal and vertical tectonic plate movement
    • Secondary landforms are created by erosion
    • A landform is a single unit, such as a mountain, a sinkhole, or a sand dune
    • A landscape is an aggregation of landforms
  • Gradation
    • Forces that work to flatten, or grade the landscape
      • erosion
      • weathering
      • gravity (avalanches, mudslides, landslides)
    • Degradation is the collective term for processes that wear down landmasses
    • Aggradation is the deposition of material, which can lower relief by filling in differences of high and low points in a landscape
    • Degradational processes and landscapes
      • weathering is the in situ breakdown of rocks
      • mass movement is the spontaneous downslope shift of material due to gravity
      • erosion involves long distance transport of the products of weathering and mass movement
        • water is the most effective erosional agent
        • glaciers are also erosional agents
        • wind is effective only in some areas as an erosional agent (sandy areas)
        • coastal waves have enormous impact on continental margin landscapes
        • chemical solution, such as when limestone is subjected to humid conditions and dissolves
    • Aggradational processes and landforms
      • streams, glaciers, wind, and waves also deposit materials
  • Erosion and tectonics
    • Downward erosion is compensated for by upward rebound of crust (isostatic principle)
  • Regional landscapes
    • Use criteria of regional physiography to map regional landscapes
      • climate
      • soil
      • vegetation
      • terrain

Review Questions

  1. Define the term degradation and give an example of this process.
  2. Name the three categories of degradational processes.
  3. List five of the most effective erosional agents.