Unit 29


Unit Overview

This unit examines the Earth's lithosphere and its context. The main sections are as follows:

  • Evidence of the Earth's internal structuring
  • The Earth's internal layers
  • The Earth's outer layer
  • The crustal surface

Seismic waves have provided scientists with clues about the Earth's internal structuring and composition. The internal layers consist of the solid inner core, the liquid outer core, the solid lower mantle, and the upper mantle.

The lithosphere is part of the Earth's outer layer and is comprised of the crust and the solid part of the upper mantle. The crustal surface has varying degrees of topographic relief; the continental shield (low relief) and orogenic belts (high relief) represent two extremes in relief. The atmosphere, biosphere, hydrosphere and cryosphere are responsible for gradational processes that wear away the lithosphere.

Unit Objectives

  • To outline the relevant properties of the Earth's five internal layers and to discuss some of the evidence leading to their discovery
  • To introduce the salient properties of the Earth's lithosphere, the nature of the crust, and the underlying mantle
  • To investigate the gradational processes that continually build as well as remove rock material at the Earth's surface, creating physical landscapes of great diversity

Glossary of Key Terms

Asthenosphere The soft plastic layer of the upper mantle that underlies the lithosphere, which is able to move over it.
Body waves A seismic wave that travels through the interior of the Earth; consists of two kinds - P waves and S waves.
Continental shield A large, stable, relatively flat expanse of very old rocks that may constitute one of the earliest "slabs" of solidification of the primeval Earth's molten crust into hard rocks; forms the geologic core of a continental landmass.
Earthquake A shaking and trembling of the Earth's surface caused by sudden releases of stresses that have been building slowly within the planetary crust.
Gradational processes A process that works to wear down the geologic materials that are built up on the Earth's landmasses.
Inner core The solid, most inner portion of the Earth, consisting mainly of nickel and iron.
Lithosphere The outermost shell of the solid Earth, lying immediately below the land surface and ocean floor (lithos means rock); composed of the Earth's thin crust together with the solid uppermost portion of the upper mantle that lies just below.
Lithospheric plate One of the fragmented, rigid segments of the lithosphere (also called a tectonic plate, which denotes its active mobile character); these segments or plates move in response to the plastic flow in the hot asthenosphere that lies just below the lithosphere.
Lower mantle The solid interior shell of the Earth that encloses the liquid outer core.
Mohorovicic discontinuity (Moho) The contact plane between the Earth's crust and the mantle that lies directly below it.
Orogenic belt A chain of linear mountain ranges.
Outer core The liquid shell that encloses the Earth's interior core, whose composition involves similar materials.
Relief The vertical distance between the highest and lowest elevations in a given area.
Seismic wave The pulses of energy generated by earthquakes that can pass through the entire planet.
Sial Derived from the chemical symbols for the minerals silicon and aluminum; refers to generally lighter-coloured rocks of the continents, which are dominated by granite.
Sima Derived from the chemical symbols for the minerals silicon and magnesium; refers to the generally-darker rocks of the ocean floors, which are dominated by basalt.
Upper mantle The viscous (syrup-like) interior shell of the Earth that encloses the solid lower mantle; the uppermost part of the upper mantle, however, is solid, and this zone, together with the crust that lies directly above it, is called the lithosphere.

Unit Outline

  • Evidence of the Earth's internal structure
    • Analysis of rocks, crust, magnetic fields, temperatures, and pressures
    • Earthquakes are the shaking and trembling of Earth's crust by releases of stress within the crust
      • seismic waves are pulses of energy generated by earthquakes
      • seismographs measure earthquake intensity
      • seismic reflection when waves bounce back off of a surface
      • seismic refraction when waves are bent by a surface
    • Types of seismic waves
      • surface (L) waves travel along Earth's crust
      • there are two types of body waves
        • P waves are compressional, or push waves: move objects parallel to their direction of movement
        • S waves are shear, or shake waves: move objects at right angles to their direction of movement
  • The earth's internal layers
    • P and S waves are measured up to 103 degrees from the earthquake's origin
    • Neither P nor S waves are measured from 103 to 142 degrees from the origin
    • From 142 to 180 degrees from the origin P waves are recorded
    • Evidence that there is a solid core that refracts seismic waves
    • Solid inner core
      • radius of only 1220 km (760 mi)
      • lies 5150 km (3200 mi) below sea level
    • Liquid outer core
      • lies 2900 km (1800 mi) below sea level
    • Solid lower mantle
      • believed to be composed of oxides of iron, magnesium, and silicon
    • Upper mantle
      • extends from base of crust to lower mantle
      • part of upper mantle is solid
      • solid upper mantle and crust together form the lithosphere
  • Earth's outer layer
    • Structural properties of the crust
      • Mohorovicic (Moho) discontinuity is a contact plane between the continental (or oceanic) crust and the mantle
      • continental crust contains lighter sial
      • oceanic crust contains the denser sima
    • The lithosphere
      • comprised of crust and uppermost solid mantle
      • the asthenosphere is the plastic-like transition zone between the lithosphere and the molten mantle; the lithosphere floats on it
    • Lithospheric plates
      • usually called tectonic plates
      • plates move in response to movement of asthenosphere
  • The crustal surface
    • Topographic relief
      • relief is vertical difference between the highest and lowest elevations of an area
      • low relief
        • continental shields - Laurentian (Canadian), Guyana (Venezuelan), Brazilian, Scandanavian, Siberian, Indian, African, Australian, and Antarctic Shield
    • Gradational processes
      • weathering includes the physical, chemical, and biological processes that break down rock
        • mass movements are generated by gravity
        • erosion is long-distance removal of weathered materials

Review Questions

  1. What are seismographs and what type of data do they record?
  2. List the differences between the different types of seismic waves (P, S, and L).
  3. Beginning with the innermost layer, describe each of the principal layers of the inner Earth, using Fig. 29.5 as a reference.