Big Idea #1- Earthquakes and volcanoes occur primarily along
plate boundaries; the frequency and type of events vary with
the type of boundary.
By 1962, the idea that pieces of the earth's surface moved around no longer seemed radical. The concepts of
continental drift and seafloor spreading had revolutionized geology(see our Plate Tectonics I module), and
researchers excitedly began to revise their interpretations of existing data into a comprehensive theory of plate
tectonics. For example, geologists had long recognized that earthquakes are not randomly distributed on the
earth (see Figure 1).
©Anne E. Egger
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Figure 1. Map showing earthquakes from 2003-2011 with magnitude greater than
3. Colors indicate depth of hypocenter, or origin of the earthquake: red is 0-33 km,
yellow is 33-100 km, green is 100-400 km, and blue is >400 km depth. Data are
from the Advanced National Seismic System.
In fact, earthquakes are concentrated along the plate boundaries drawn by Harry Hess. Not all earthquakes occur
at the same depth, however. Where Hess had postulated that the rocks of the ocean floor were diving down into
subduction zones, earthquakes occur at shallow depths of 0 to 33 km below the surface near the trenches, and
depths of almost 700 km below the surface further inland, illustrated in Figure 1 by different colored circles. On
the other hand, only shallow earthquakes (depths of 0 to 33 km, shown in red in Figure 1) are recorded at the
spreading ridges. These data helped geologists draw more detailed cross-sections showing that plates are thin at
spreading ridges, and that subduction extends long distances, taking plates deep beneath the continents.
Similar to earthquakes, volcanoes are located preferentially on or near plate boundaries (see Figure 2).
©Anne E. Egger
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Figure 2. Map showing volcanoes that have been active in the last 10,000 years.
Colored triangles indicate different volcano types: red triangles are primarily
calderas, green triangles are stratovolcanoes, blue triangles are shield volcanoes
and fissure vents. Data are from the Smithsonian Institution, Global Volcanism
Program.
Also similar to earthquakes, different kinds of volcanoes occur along different types of plate boundaries. Most
of the volcanic eruptions that make the news, such as the 1980 Mount St. Helens eruption, take place near
subduction zones. This type of volcano is represented by green triangles in Figure 2. These devastating,
explosive eruptions reflect the composition of the magma - it is extremely viscous and thus cannot flow easily.
In contrast, the volcanic eruptions that occur along spreading ridges are much gentler, in part because most of
these eruptions occur under 2 to 3 kilometers of water, but also because the magma is far less viscous. This type
of volcano is represented by blue triangles in Figure 2.
Big Idea #2 - Plates interact with one another at boundaries in
one of three ways: they diverge, converge, or slide past one
another.
Plate boundaries
These observations about the distribution of earthquakes and volcanoes helped geologists define the processes
that occur at spreading ridges and subduction zones. In addition, they helped scientists recognize that there are
other types of plate boundaries. In general, plate boundaries are the scene of much geologic action earthquakes, volcanoes, and dramatic topography such as mountain ranges like the Himalayas are all
concentrated where two or more plates meet along a boundary. There are three major ways that plates interact
along boundaries: they can move away from each other (diverge), they can move toward each other (converge),
or they can slide by each other (transform). Each of these interactions produces a different and characteristic
pattern of earthquakes, volcanism, and topography.
Divergent boundaries
Divergent boundaries are the mid-ocean ridges that launched the plate tectonics revolution - the Mid-Atlantic
Ridge is a classic example (see Figure 3). Shallow earthquakes and minor lava flows characterize mid-ocean
ridges. The seafloor at the ridges is higher than the surrounding abyssal plain because the rocks are hot and thus
less dense and more buoyant. As they move away from the spreading center, they cool and become denser and
less buoyant. Spreading has been occurring along the Mid-Atlantic Ridge for 180 million years, resulting in a
large ocean basin - the Atlantic Ocean.
©Anne E. Egger
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Figure 3. Cross-section of the Mid-Atlantic Ridge near latitude 14° S. Blue
triangle represents the location of fissure volcanoes. Colored circles represent
earthquakes, color-coded by depth (see Figure 1 for key).
Convergent boundaries
Convergent boundaries are the most geologically active, with different features depending on the type of crust
involved. There are two types of crust: oceanic and continental. Continental crust is thick and buoyant; oceanic
crust is thin, dense, and forms at mid-ocean ridges. Plates are made up of the crust and the upper mantle; this
combination of layers is called the lithosphere (see Earth Structure for more information). The activity that
takes place at convergent boundaries depends on the type of lithosphere involved, as explained next.
Big Idea #3 - Plates are made up of two types of crust -- oceanic
and continental; oceanic crust is thinner and denser that
continental crust. A single plate can have both continental and
oceanic crust.
Oceanic meets continental
These are the subduction zones first imagined by Hess, where dense oceanic lithosphere is diving beneath more
buoyant continental lithosphere. These boundaries are characterized by: (a) a very deep ocean trench next to a
high continental mountain range, (b) large numbers of earthquakes that progress from shallow to deep, and (c)
large numbers of intermediate composition volcanoes (see Figure 4). The Andes owe their existence to a
subduction zone on the western edge of the South American plate; in fact, this type of boundary is often called
an Andean margin.
©Anne E. Egger
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Figure 4. Cross-section of the South American subduction zone near latitude 22°
S. Green triangles represent the locations of stratovolcanoes. Colored circles
represent earthquakes, color-coded by depth (see Figure 1 for key).
Oceanic meets more oceanic
Where two oceanic plates converge, a subduction zone also occurs, but the result is slightly different than an
Andean margin. Since the densities of the two plates are similar, it is usually the older oceanic lithosphere that
is subducted because it is colder and slightly denser. Earthquakes progress from shallow to deep like in the
oceanic-continental convergence, and volcanoes form an island arc, like Mt. Fuji in Japan and Pinatubo in the
Philippines (see Figure 5). These volcanoes are slightly different from those that form the Andes because the
magma is produced from the melting of oceanic lithosphere rather than the melting of continental lithosphere.
©Anne E. Egger
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Figure 5. Cross-section of the Tonga trench near latitude 21° S. Colored triangles
represent the location of volcanoes, color-coded by type of volcano (see Figure 2
for key). Colored circles represent earthquakes, color-coded by depth (see Figure 1
for key).
Continental meets more continental
When two pieces of continental lithosphere converge, the result is a great pileup of continental material. Both
pieces of crust are buoyant and are not easily subducted. Continental convergence is exemplified by the
Himalayan mountain range, where the Indian plate runs into the Asian plate (see Figure 6). Numerous shallow
earthquakes occur, but there is very little volcanism.
Geologic activity away from plate boundaries
The boundaries described above account for the vast majority of seismic and volcanic activity on the earth. The
more data that began to fit into the plate tectonics scheme, however, the more the exceptions stood out. What
could account for Hawaii, for example, a scene of long-lived volcanic activity in the middle of the Pacific plate
where there is no subduction or spreading to generate magma?
There had to be something else. In 1963, J. Tuzo Wilson, a Canadian geophysicist, theorized that the mantle
contained immobile hotspots, thin plumes of hot magma that acted like Bunsen burners as plates moved over
them (Wilson, 1963). The Hawaiian Islands form a long, linear chain, with ongoing volcanic eruptions on the
island of Hawaii and extinct, highly eroded volcanic islands to the northwest. According to Wilson's hotspot
theory, the chain of islands represents the northwestward motion of the Pacific plate over a mantle plume.
Figure 9: J. Tuzo Wilson's original sketch of the
Hawaiian hot spot.(Used with the permission of the
Canadian Journal of Physics).
One important implication of Wilson's theory was that because hotspots were stationary, hotspot tracks could be
used to trace plate motion history. For example, the track of the Hawaiian chain continues to the northwest as an
underwater chain of progressively older, no longer active volcanoes. Once the volcanic eruptions stop, ocean
waves begin to take their toll, eroding the islands down to just below sea level, at which point they are called
seamounts. The islands and seamounts associated with the Hawaiian hotspot provide a history of motion for the
Pacific plate, which appears to have taken an eastward turn around 28 million years ago. Other hotspot tracks
around the world can be used in a similar manner to reconstruct a global plate tectonic history.
Big Idea #4 - Gravity and mantle convection are two driving
forces for the movement of plates.
What are the driving forces?
Hotspots added further proof to confirm that plates move constantly and steadily. Ironically, however, the
question that incited ridicule for Wegener continues to launch heated debate today: what ultimately drives plate
motion? Plates are constantly shifting and rearranging themselves in response to each other. Eventually, a new
Pangaea (or single supercontinent) will form, break apart, and form again on the earth. What keeps these plates
moving?
Hess assumed that mantle convection was the main driving force - hot, less dense material rises along midocean ridges, cools, and subsides at subduction zones, and the plates "ride" these convection cells (see our
Density module for more information). Though there is little doubt that convection does occur in the mantle,
current modeling suggests that it is not so simple. Many geologists argue that the force of convection is not
enough to push enormous lithospheric plates like the North American plate. They suggest instead that gravity is
the main driving force: cold, dense oceanic crust sinks at subduction zones, pulling the rest of the plate with it.
According to this theory, magmatic intrusions at spreading ridges are passive - the magma merely fills a hole
created by pulling two plates apart.
Figure 10: "Ridge push" and "slab pull" are both
ways that gravity can act to keep a plate in
motion. Note that arrows on convection cells and
overlying plate are going in the same direction.
Figure modified from This Dynamic Earth, a publication from the U.S.
Geological Survey.
Undoubtedly, gravity and convection both supply energy to keep plates moving. Their relative contributions,
however, are a matter of debate and ongoing research.
The strength of plate tectonic theory lies in its ability to explain everything about the processes we see both in
the geologic record and in the present. Our understanding of the subtleties continues to evolve as we learn more
about our planet, but plate tectonics is truly the foundation upon which the science of geology is built.
©Anne E. Egger
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Figure 6. Cross-section of the Himalayas along 88° E longitude. Colored circles
represent earthquakes, color-coded by depth (see Figure 1 for key).
Transform boundaries
Most boundaries are either convergent or divergent, but transform boundaries occur in a few places to
accommodate lateral motion, in which plates slide past one another. This type of boundary is very rare on
continents, but they are dramatic where they do occur. For example, the San Andreas Fault in California is a
continental transform boundary - frequent, shallow earthquakes occur (like the famous 1906 and 1989 San
Francisco earthquakes), but there is little associated volcanism or topographic relief (see Figure 7). The Alpine
Fault in New Zealand is very similar. Most transform boundaries occur not on land, however, but in short
segments along mid-ocean ridges.
©Anne E. Egger
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Figure 7. Cross-section of the San Andreas Fault in California near latitude 36°
N. Colored circles represent earthquakes, color-coded by depth (see Figure 1 for
key).
A few boundaries defy simple classification and are referred to as "plate boundary zones." For example, a
complicated earthquake pattern is produced by a wide, poorly understood plate boundary zone between the
Eurasian and African plates in the Mediterranean region.
Name___________________________________________________________ Period __________ Date ______
Tectonic Plate Jig-Saw
Directions- Read about your assigned section (1-4) and fill in details about each part of the plate tectonic
theory. Be ready to share your information with the rest of your table to fill out the entire chart below.
Big Idea
Big Idea #1
Big Idea #2
Big Idea #3
Big Idea #4
Details
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