286 Seismology and Plate Tectonics
weaker asthenosphere below. The lithosphere and asthenosphere are mechanical units defined by their strength and the
way they deform. The lithosphere includes both the crust and
part of the upper mantle.
Figure 5.1-1 shows the three basic types of plate boundaries. Warm mantle material upwells at spreading centers,
also known as mid-ocean ridges, and then cools. Because the
strength of rock decreases with temperature (Section 5.7.3),
the cooling material forms strong plates of new oceanic lithosphere. The cooling oceanic lithosphere moves away from the
ridges, and eventually reaches subduction zones, or trenches, 2
where it descends in downgoing slabs back into the mantle, reheating as it goes. The direction of the relative motion between
two plates at a point on their common boundary determines
the nature of the boundary. At spreading centers both plates
move away from the boundary, whereas at subduction zones
the subducting plate moves toward the boundary. At the third
boundary type, transform faults, relative plate motion is parallel to the boundary.
As discussed in Section 3.8, seismology shows that the
structure of the mantle and the core varies with depth, due to
changes in temperature, pressure, mineralogy, and composition. Plate tectonics describes the behavior of the lithosphere,
the strong outer shell of the mantle, which is the cold outer
boundary layer of the thermal convection system involving the
mantle and the core that removes heat from the earth’s interior.
Although much remains to be learned about this convective
system, especially in the lower mantle and the core (Fig. 5.1-2),
there is general agreement that at shallow depths the warm,
5.1 Introduction
Two of the major advances in the earth sciences since the 1960s
have been the growth of global seismology and the development of our understanding of global plate tectonics. The two
are closely intertwined because seismological advances provided some of the crucial data that make plate tectonics the
conceptual framework used to think about large-scale processes in the solid earth.
The theory of plate tectonics grew out of the earlier theory
of continental drift, proposed in its modern form by Alfred
Wegener in 1915. The idea that continents drifted apart was an
old one, rooted in the remarkable fit of the coasts of South
America and Africa. Still, without compelling evidence for
motion between continents, the idea that such motions were
physically impossible prevented most geologists from accepting Wegener’s ideas. By the 1970s the story was very different.
Geologists accepted continental drift in large part because
paleomagnetic measurements, based on the geometry and history of the earth’s magnetic field, showed that continents had in
fact moved over millions of years. Combination of these observations with results from seismology and marine geology and
geophysics led to the realization that all parts of the earth’s
outer shell, not just the continents, were moving.
Plate tectonics is conceptually simple: it treats the earth’s
outer shell as made up of about 15 rigid plates, about 100 km
thick, which move relative to each other at speeds of a few cm
per year. 1 The plates are rigid in the sense that little (ideally
no) deformation occurs within them, so deformation occurs
at their boundaries, giving rise to earthquakes, mountain
building, volcanism, and other spectacular phenomena. These
strong plates form the earth’s lithosphere, and move over the
5 Seismology and Plate Tectonics
The acceptance of continental drift has transformed the earth sciences from a group of rather unimaginative studies based on pedestrian interpretations of natural phenomena into a unified science that holds the promise of great intellectual and practical advances.
J. Tuzo Wilson, Continents Adrift and Continental Aground, 1976
1 This is about the speed at which fingernails grow.
2 Boundaries are described either as mid-ocean ridges and trenches, emphasizing
their morphology, or as spreading centers and subduction zones, emphasizing
the plate motion there. The latter nomenclature is more precise, because there are
elevated features in the ocean basins that are not spreading ridges, and spreading
centers like the East African rift exist within continents.
weaker asthenosphere below. The lithosphere and asthenosphere are mechanical units defined by their strength and the
way they deform. The lithosphere includes both the crust and
part of the upper mantle.
Figure 5.1-1 shows the three basic types of plate boundaries. Warm mantle material upwells at spreading centers,
also known as mid-ocean ridges, and then cools. Because the
strength of rock decreases with temperature (Section 5.7.3),
the cooling material forms strong plates of new oceanic lithosphere. The cooling oceanic lithosphere moves away from the
ridges, and eventually reaches subduction zones, or trenches, 2
where it descends in downgoing slabs back into the mantle, reheating as it goes. The direction of the relative motion between
two plates at a point on their common boundary determines
the nature of the boundary. At spreading centers both plates
move away from the boundary, whereas at subduction zones
the subducting plate moves toward the boundary. At the third
boundary type, transform faults, relative plate motion is parallel to the boundary.
As discussed in Section 3.8, seismology shows that the
structure of the mantle and the core varies with depth, due to
changes in temperature, pressure, mineralogy, and composition. Plate tectonics describes the behavior of the lithosphere,
the strong outer shell of the mantle, which is the cold outer
boundary layer of the thermal convection system involving the
mantle and the core that removes heat from the earth’s interior.
Although much remains to be learned about this convective
system, especially in the lower mantle and the core (Fig. 5.1-2),
there is general agreement that at shallow depths the warm,
5.1 Introduction
Two of the major advances in the earth sciences since the 1960s
have been the growth of global seismology and the development of our understanding of global plate tectonics. The two
are closely intertwined because seismological advances provided some of the crucial data that make plate tectonics the
conceptual framework used to think about large-scale processes in the solid earth.
The theory of plate tectonics grew out of the earlier theory
of continental drift, proposed in its modern form by Alfred
Wegener in 1915. The idea that continents drifted apart was an
old one, rooted in the remarkable fit of the coasts of South
America and Africa. Still, without compelling evidence for
motion between continents, the idea that such motions were
physically impossible prevented most geologists from accepting Wegener’s ideas. By the 1970s the story was very different.
Geologists accepted continental drift in large part because
paleomagnetic measurements, based on the geometry and history of the earth’s magnetic field, showed that continents had in
fact moved over millions of years. Combination of these observations with results from seismology and marine geology and
geophysics led to the realization that all parts of the earth’s
outer shell, not just the continents, were moving.
Plate tectonics is conceptually simple: it treats the earth’s
outer shell as made up of about 15 rigid plates, about 100 km
thick, which move relative to each other at speeds of a few cm
per year. 1 The plates are rigid in the sense that little (ideally
no) deformation occurs within them, so deformation occurs
at their boundaries, giving rise to earthquakes, mountain
building, volcanism, and other spectacular phenomena. These
strong plates form the earth’s lithosphere, and move over the
5 Seismology and Plate Tectonics
The acceptance of continental drift has transformed the earth sciences from a group of rather unimaginative studies based on pedestrian interpretations of natural phenomena into a unified science that holds the promise of great intellectual and practical advances.
J. Tuzo Wilson, Continents Adrift and Continental Aground, 1976
1 This is about the speed at which fingernails grow.
2 Boundaries are described either as mid-ocean ridges and trenches, emphasizing
their morphology, or as spreading centers and subduction zones, emphasizing
the plate motion there. The latter nomenclature is more precise, because there are
elevated features in the ocean basins that are not spreading ridges, and spreading
centers like the East African rift exist within continents.