CHAPTER 4: PLATE TECTONICS
SUMMARY
This chapter approaches the development of the theory of plate tectonics as an excellent
example of the scientific method in action. Data are presented in such a way as to lead the
student to “discover” the ideas that were central to this Earth science paradigm shift. This begins
with a discussion that early evidence such as fit of the continents and matching of fossils and
mountains appeared to indicate that continents were once joined and later drifted apart. You are
reminded that the lack of a suitable mechanism to cause the continents to drift prevented that
hypothesis from being widely accepted, as is expected in science.
Figure Above: Alfred Wegener's (German meteorologist) Pangaea (formed between
300-200 million years ago and began separating 200 million years ago). Evidence Wegener
observed and collected to support his continental drift hypothesis included the fit of continents
(like a jigsaw puzzle), matching rocks, mountain belts, fossils and evidence of glaciation on five
continents. The above figure shows the fit of the continents and fossils found on
various continents of the same species showing they could have easily migrated across land that
is now separated by vast oceans.
4.2: Continental Drift (page 77): Lines of evidence discussed above are found in this
section of the book. Checkpoint 4.2 asks the following question: Which of these lines of
evidence were used to support Wegener's continental drift hypothesis? (select all that apply)
a) The distribution of fossils
b) Fit of the continents
c) Match of mountain belts
d) Earthquake locations
e) Paleoclimate data
Wegener showed strong evidence in support of his hypothesis but it was not accepted at
this time (see page 79). What was the major stumbling block to his hypothesis? He was unable
to propose an acceptable mechanism to explain how the continents moved. Study of the seafloor
50 years later would revive his hypothesis (but he would not live to see it).
4.3 Evidence from the Seafloor (page 80): The impact of technology on science is
revisited when discussing that new instruments became available beginning in the 1950s that
were used to collect data that provided additional support to the early ideas of Wegener. Such
data collection led to recognition of global patterns to earthquakes and volcanoes, and the
characteristics of the age and topography of the ocean floor led to the concept of seafloor
spreading. The two ideas of continental drift and seafloor spreading subsequently contributed to
the theory of plate tectonics.
Figure Above (see page 87 figure 4.14): evidence of seafloor spreading includes the
Mid-Oceanic Ridge System (seafloor topography) and magnetic patterns measured in volcanic
rock (paleomagnetism) as magma rises to the Earth's surface, solidifies and slides down both
sides of the ridge like a conveyor belt. As the magma solidifies iron atoms align with the Earth's
magnetic field and are "frozen" like tiny compasses showing the magnetic field has "switched"
between normal (today) and reversed polarity throughout Earth's history (magnetic field
reversals). These "stripes" are oriented parallel to the adjacent ridges and reveal a symmetrical
pattern of magnetic polarity reversals on either side of the oceanic ridges (see pages 85-86 for
this discussion and supporting diagrams). The inclination (deviation from horizontal or vertical)
of these minerals at time of formation also give the approximate latitude of formation of the
rocks (see page 85 in your text for discussion and diagrams). Other evidence supporting
seafloor spreading include the age of the ocean floor (youngest at the ridges and getting older
away from the ridges), heat flow (greatest at ridges), volcanoes and earthquake locations and
the thickness of oceanic sediments (thickens away from ridges).
NOTE: the following question (in red) has been omitted this semester (Checkpoint 4.8) in
favor of Checkpoint 4.7 (page 83)...One of your homework questions for this chapter asks you to
explain how the following patterns can be interpreted to contradict the contracting Earth model:
1) topography of the ocean floor, 2) age of the ocean floor, 3) heat flow, 4) volcanic activity, and
5) earthquake activity in the ocean floor.
To answer this, first read about the contracting earth model on page 77. Each of the
patterns are explained in the book beginning on page 80. Do these patterns fit the contracting
Earth model?? For example, in the contracting Earth model all features on Earth would be
approximately the same age; but when you read on the age of the ocean floor you will see that
the age of oceanic crust varies, and that oceanic crust is much younger than continental crust
(also see Figure 4.7)...
4.4 Plate Tectonics (page 87): The theory of plate tectonics is used to explain most of
the major landforms we observe on Earth. The theory suggests that tectonic plates are created at
oceanic ridge systems and eventually descends into the mantle at subduction zones. The surface
of the Earth is comprised of lithospheric (crust plus the upper rigid mantle) tectonic plates. These
plates interact at their boundaries where earthquakes and volcanoes are common. These plates
move only a few cm/yr about the surface of Earth. A significant portion of the chapter is
dedicated to explanations of plate-boundary interactions.
Figure Above: Relative motions of plates are shown in figure above. Notice several features of
this diagram, including the symbols (upper right) showing the 3 types of plate boundaries; the
arrows and the key for the arrows in lower left of diagram showing relative speeds in cm/yr and
that the plates can be composed of mostly oceanic crust (e.g., Pacific Plate; Nazca Plate); mostly
continental crust (e.g., Eurasion Plate); but that most plates are composed of both oceanic and
continental crust (e.g., South American Plate; North American Plate; African Plate). You have
to look closely at this diagram to see the continental areas.
Recall the stumbling block in Wegener's continental drift hypothesis: he could not explain how
the continents moved. With the discovery of ridges and seafloor spreading, a mechanism for
movement of crust is more easily hypothesized. Please see Figure 4.21 on page 94 for a
description of what many scientists believe is the mechanism for plate motions. Hot rock rises
and melts at the oceanic ridges and creates new seafloor that moves like a conveyor belt
symmetrically away from the ridge and as the seafloor becomes older, colder and denser it
eventually sinks back into the Earth's interior at subduction zones where the oceanic crust is
recycled (this model of plate motion is called mantle convection; hot rock rises, cool rock
sinks...similar to the atmosphere and oceans where warm air or water rises and cold air or water
sinks...the rocks also move down the ridges by gravity and are pulled into the subduction zone by
the weight of the descending slab of lithosphere). Keep in mind that the plates are comprised of
the lithosphere: the crust plus the upper-rigid mantle. Beneath the lithosphere is the
asthenosphere, a zone in the mantle approximatley 200 km thick where rock is mostly solid but
flows (ductile properties); this allows the rigid lithosphere to move along the ductile
asthenosphere.
4.5 Plate Boundaries (page 95): There are three types of plate boundaries: convergent,
divergent and transform. There are variations of each boundary type depending on the type of
crust involved.
Figure's Above: from top to bottom, showing cross sectional views of the crust involved during
subduction (convergent boundaries) and the features formed by each type of convergent
boundary. Top diagram is covergence of oceanic-oceanic crust and subduction of the
older/cooler/denser oceanic crust creating volcanic islands (arcs) such as the Aluetian Islands
off Alaska. The middle diagram shows convergence of continental-oceanic crust and subduction
of the denser oceanic crust creating volcanic belts on land such as the Cascade Range (and
volcanoes such as Mount St. Helens) of Oregon and Washington (and a portion of N.
California). The bottom (left) diagram shows convergence of continental-continental crust
where subduction will stop (continental crust is not dense enough to subduct), creating massive
mountain ranges such as the Himalayas and Mt. Everest (but not much volcanism since
subduction stops). The bottom right diagram shows all three types of plate boundaries
(divergent at the ridge; transform offets the ridges and convergent forming a subduction zone
between oceanic-continental crust). Also, note the passive continental margin (where
subduction is not occurring), and the active continental margin (where subduction is occurring).
Figure Above: note symbols depicting the various plate boundaries (convergent and subduction
of the Jaun de Fuca plate forming the Cascade volcanoes-the black triangles are on the
overriding or non-subducting North American plate); divergent at the ridges in red offset by
transform plate boundaries; and transform along the San Andreas Fault). Notice the San
Andreas is a transform plate boundary (and is also called a strike-slip fault) connecting the
ridge off of northern Ca to a ridge in the Gulf of California. This is the plate tectonic setting
where we live!! We are moving to the northwest and in millions of years should be a suburb of
San Francisco. As you can see, we are not going to split off and fall into the ocean.
The United States Geological Survey's (USGS) website is a great site to search and
investigate concepts related to this class. The following link will take you to the online
edition of This Dynamic Earth, the story of plate tectonics, published by USGS...it might be
helpful to review this story to enhance your understanding of plate tectonics, a very
important theory that can explain many of Earth's natural features and processes:
pubs.usgs.gov/gip/dynamic/dynamic.html
LEARNING OBJECTIVES
1.
2.
3.
4.
Students will explain concepts related to plate tectonics.
Students will explain principal observations behind Wegener’s continental drift hypothesis.
Students will describe the relationship between the age and topography of the ocean floor.
Students will explain changes in the magnetic record found in rocks to determine changes
over time.
5. Students will describe plate motions and relationships between tectonic plates.
6. Students will compare and contrast types of plate boundaries.