Chapter 13
The Lithosphere and the Tectonic
System
OVERVIEW
This chapter introduces the grand cycle of plate tectonics. This cycle explains how the
continents and ocean basins of the Earth’s surface slowly change as forces deep within
the Earth cause vast tectonic plates to converge, collide, split, and separate.

The solid Earth has a layered structure: a dense, metallic core surrounded by a
mantle of ultramafic rock which is covered by a crust with mafic rocks exposed on
the ocean floor and felsic rocks exposed on the continents.

Geologists use the term lithosphere to refer to the Earth’s outer layer of rigid, brittle
rocks which extend through the crust to the upper mantle. Beneath the lithosphere lies
the asthenosphere: the layer of soft, plastic rock in the mantle.

The history of the Earth can be subdivided into various time intervals using the
geologic time scale. Most of the landscape features of the Earth’s surface developed
during the Cenozoic Era which began sixty-five million years ago.

The major relief features of the Earth’s surface are the continents and ocean basins.

The continents contain young, dynamic alpine belts and old, stable continental
shields.

The ocean basins consist of extensive, smooth abyssal plains marked by long, narrow
midoceanic ridges.

Shallow continental shelves are found beneath the ocean next to continental shields,
while deep oceanic trenches are found adjacent to alpine belts.

The two basic tectonic processes are compression and extension.

The lithosphere is broken into six great plates and several lesser plates that move
relative to one another.

Spreading boundaries exist where plates move apart, converging boundaries where
they collide, and transform boundaries where they move past one another.

Spreading boundaries are marked by midoceanic ridges on the ocean floor and rift
valleys on continents. New ocean crust is formed along spreading boundaries.

Subduction occurs where continents meet the ocean floor along a converging
boundary and the denser rock of the ocean floor plunges beneath the continent. An
oceanic trench marks the subduction zone.

Subduction along continental margins leads to the formation of island arcs and
alpine belts as subducted ocean crust melts and rises to the surface in volcanoes, and
sediment from the ocean floor is folded and faulted as it accumulates on the
continental margin.

Plate movement is thought to be driven by convection currents in the plastic rock of
the asthenosphere.

The plate tectonics cycle ties together major relief features, volcanic and Earthquake
activity, and patterns of rock age and type at the Earth’s surface.
KEY TERMS
core
mantle
crust
continental crust
oceanic crust
lithosphere
asthenosphere
lithospheric plates
continental shields
continental suture
mountain arc
island arc
tectonics
subduction
plate tectonics
STUDY QUESTIONS
1. Describe the structure of the Earth’s interior and its crust.
2. What are the lithosphere and the asthenosphere?
3. List in order, from youngest to oldest, the names of the eras of the geologic time scale
since Precambrian Time. During which of these eras did the supercontinent of Pangea
exist? During which did most of the present landscape features of the continents
develop?
4. How do alpine belts and continental shields differ with respect to their topography,
rock type, and age?
5. Sketch a cross-section of an ocean basin with a passive continental margin on one
side and an active continental margin on the other. Label the midoceanic ridge, axial
rift, abyssal plain, continental rise, continental slope, and oceanic trench.
6. Describe alpine belts of the Earth’s surface.
7. Describe the process of subduction, and explain how it is related to island arc
development and mountain-building.
8. How and where does new crust develop?
9. What is an orogen, and what are the two types of orogens that are recognized?
10. Describe the process of continental rupture by which new ocean basins can form.
Which stages of this process are illustrated by the Rift Valley of East Africa and the
Red Sea?
11. What is the Wilson cycle of plate tectonics, and what is the net effect of a Wilson
cycle on the continental mass?
12. What mechanism is responsible for the movement of lithospheric plates, and what is
the power source for this mechanism?
CHAPTER QUIZ
Multiple Choice Questions
1. Which of the following statements about the asthenosphere is not correct?
a) It is in a liquid state.
b) It lies below the lithosphere.
2.
3.
4.
5.
c) It is a soft layer in the mantle.
d) It contains ultramafic rock.
Which of the following is an era of the geologic time scale?
a) Pliocene
b) Jurassic
c) Cenozoic
d) Carboniferous
The axial rift of the midoceanic ridge is:
a) a line along which seafloor spreading occurs
b) a zone of rising magma of mafic composition
c) both a and b
d) neither a nor b
Along transform plate boundaries, the plates move:
a) toward one another
b) horizontally past one another
c) away from one another
d) vertically past one another
Approximately how long ago is the supercontinent of Pangea thought to have begun
breaking apart?
a) one billion years ago
b) 500 million years ago
c) 250 million years ago
d) fifty million years ago
True/False Questions
1. The oldest rocks found on Earth are more than five billion years old. (T/F)
2. New lithosphere is formed along converging plate boundaries. (T/F)
3. The Himalayan Mountains near the northern border of India were formed during a
continental suture. (T/F)
4. Folding in the Earth’s crust is caused by extension. (T/F)
5. The “moho” is the boundary surface between the lithosphere and the aesthenosphere.
(T/F)
Short Answer Questions
1. What are the two major chains of mountain arcs or alpine belts found on the Earth’s
surface?
2. There are two different types of lithospheric plates. What are they, and how do they
differ?
3. What is the basis for radiometric dating?
Short Essay Questions (1 - 2 paragraphs)
1. Discuss the following statement:
The present pattern of plates and their motions explains the location of many
geologic surface phenomena such as major relief features, volcanoes,
earthquakes, and rock age.
2. Briefly describe the interior structure of the planet Earth.
Internet Resources
1. Map of the age of the ocean floor:
<http://gdcinfo.agg.nrcan.gc.ca/app/agemap_e.html>
2. Ocean floor topography: <http://topex.ucsd.edu/marine_topo/mar_topo.html>
3. Measuring plate tectonic motion from space:
<http://cddisa.gsfc.nasa.gov/926/slrtecto.html>
4. Interactive global relief: <http://www.ngdc.noaa.gov/mgg/image/relief_slides2.html>
5. Land surface topography: <http://www.ngdc.noaa.gov/mgg/image/2minrelief.html>
6. Images of Earth’s interior: <http://mahi.ucsd.edu/shearer/SEDI/main/imagelib.html>
7. Overview of plate tectonics theory:
<http://pubs.usgs.gov/publications/text/dynamic.html#anchor19978839>
8. Seafloor spreading and plate tectonics:
<http://www4.nas.edu/beyond/beyonddiscovery.nsf/web/seafloor?OpenDocument>