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Unit 33


Unit Overview

This unit examines the theoretical processes initially responsible for plate tectonics as well as the ultimate effects of plate tectonics. The main sections are as follows:

  • Mechanism of crustal spreading
  • Evolution of continents
  • Isostasy

Plate movement is a confirmed lithospheric process; however, the mechanism driving plate movement is not completely understood. Plate movement is initiated by the formation of new crust from magma at midoceanic ridges, and this new crustal material diverges slowly towards the continental margins that border the ocean's basins. Several theories suggest that the large-scale mechanism responsible for the movement of lithospheric material involves convective cells that extend deep into the mantle.

Continents are assumed to have formed through the solidification of segments of the primitive crustal sphere. Continents can also grow in areal extent by accreting smaller bodies of crustal known as terranes. A phenomenon that controls the vertical extent of continental landmasses is isostasy┬┐a condition of equilibrium between floating landmasses and the asthenosphere beneath them. Tectonic and erosional forces cause constant isostatic adjustments, which, in turn, explain why erosional forces have not completely flattened all mountain ranges. In addition, the collision of drifting plates greatly affects isostasy, principally through the process of mountain building. The appearance and removal of massive ice sheets can also cause isostatic changes in the crust.

Unit Objectives

  • To briefly outline the mechanisms and processes that move lithospheric plates
  • To discuss the evolution of the Earth's continental landmasses
  • To discuss the concept of isostasy and relate it to the topography of the continents

Glossary of Key Terms

Accretion Process in which bodies of rock from another plate are attached to a given landmass.
Isostasy Derived from an ancient Greek term (isa the same; stasy to stand), the condition of vertical equilibrium between floating landmasses and the asthenosphere beneath them; this situation of sustained adjustment is maintained despite the forces that constantly operate to change the landmasses.
Suspect terrane A subregion of rocks possessing properties that sharply distinguish it from surrounding regional rocks; a terrane consisting of a "foreign" rock mass that is mismatched to its large-scale geologic setting.
Terrane A geological region of "consistent" rocks in terms of age, type, and structure; mismatched subregions can occur and are known as suspect terranes.

Unit Outline

  • Mechanism of crustal spreading
    • Sub lithospheric magma continuously spreads out of ridges, keeping them open
    • Some material subducted and reabsorbed by; asthenosphere
    • Entire mantle may be in motion
  • Evolution of continents
    • Crustal formation
      • landmasses appear to have about same volume as 2.5 billion years ago, crust has been recycled since then
      • continental shields relatively stable, margins more active with plate boundaries
    • Terranes and suspect terranes
      • a terrane is a region of consistent (similar) rocks
      • when there is a subregion of different rocks, they form a suspect terrane such as Wrangellia
  • Isostasy
    • A condition of equilibrium between floating landmasses and the asthenosphere below
    • Isotasy and erosion
      • as erosion removes load from a landmass, isostatic adjustment raises the rocks to compensate
      • as material is deposited (such as in a river delta) isostatic adjustment constantly lowers the level of the material
    • Isostasy and drifting plates
      • when plates collide, deformations occur
      • where mountains are built, trenches also exist to maintain isostatic equilibrium
    • Isostasy and regional landscapes
      • isostasy also active on continental plains - erosion by wind, water, ice removes material, and adjustment is required
      • plains and uplands affected by isostasy in phases - when erosion first takes place and sialic root is deep, more isostatic uplift; as more material eroded and root shorter, uplift slower
      • ice sheets cause isostatic sinking of crust, when glaciers retreat rapidly, it takes isostatic rebound a long time to fully raise the land back up to its original level
      • dams and crustal equilibrium
        • the weight of damned up water can cause isostatic sinking of the crust below it

Review Questions

  1. How does isostasy affect regional landscapes?
  2. Contrast terranes and suspect terranes.
  3. Draw a sketch of Airy's mountain root hypothesis, labeling sial and sima, using Fig. 33.7 as a guide.