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Introduction to Plate Tectonics
Courtesy: UCLA, ESS
The concept of plate tectonics provides a holistic view of how the Earth works.
According to the theory, the outermost division of the Earth consists of cool, rigid
lithospheric plates that are in constant motion, being driven by the internal heat of the
Earth. These plates are about 100 km thick and float on a hotter, more plastic region of
the upper mantle called the asthenosphere (you learned about this in the last lab).
New lithospheric material is created at the Oceanic Ridge and Rise System.
These are mountainous features on the seafloor where plates move apart (diverge). As
the sea floor spreads apart, basaltic magma derived from partial melting of the
asthenosphere rises to the surface, solidifies, and becomes new crust at the edge of the
lithospheric plates. As a result, the oceanic ridge system typically has high heat flow,
volcanic activity and shallow earthquakes (shallow because the rising asthenosphere and
magma bring heat close to the surface, allowing rocks below the crust to flow instead of
breaking).
New lithospheric material created at ridges is eventually destroyed elsewhere by
the process of subduction. In regions where plates converge, one lithospheric plate is
forced down into the mantle beneath the other plate. These zones of subduction are
visible on the seafloor as deep ocean trenches. They are seismically active areas
characterized by shallow-, intermediate-, and deep-focus earthquakes (deep earthquakes
are possible here because of the cold, brittle lithosphere being pushed deep into the
mantle). One consequence of subduction is the generation of magmatic arcs, which are
chains of volcanoes that lie parallel to trenches and above subducted slabs of lithosphere.
Partial melting of the mantle over subducting lithosphere produces magmas that rise to
the surface through fractures in the overriding plate. Compositionally, magmas produced
at subduction zones include andesites (diorites) and rhyolites (granites), which are more
siliceous in composition (i.e., higher in SiO2) than the basalts produced at mid-oceanic
ridges (For an overview see Figure 1). Heat flow is low in trenches, but high at the
adjacent magmatic arcs.
Fig. 1
Thus far we have described two different types of plate interactions: divergence
at mid-ocean ridges, and convergence at ocean trenches. There is a third type of
boundary, the transform fault, which occurs where two plates are moving in parallel but
opposing directions; in other words, the plates slide past one another. In some cases,
transform-fault boundaries are found on continents (such as the San Andreas fault in
California, and the Alpine fault system in New Zealand). In such cases, two blocks of
continental crust are sliding past one another. These areas are characterized by
intermittent shallow seismic activity (which we have all experienced living in Los
Angeles!). More commonly, however, transform faults (and associated "fracture zones")
are observed on the seafloor as offsets in the axis of ocean ridges (Figure 2). Heat flow is
low at transform margins.
Outline for Types of Lithospheric Plate Margins
I. Divergent -- Plate boundary where two plates are moving away from each other;
magmas derived from the asthenosphere rise upward to fill the gap between the
diverging lithospheric plates.
A. Oceanic Ridge and Rise System (aka Mid-ocean Ridges) -- Linear chains of
submerged volcanic mountains and associated central "rift valleys" where
basaltic magmas rise to the surface and are extruded as lavas onto the ocean
floor. This example represents an advanced stage of rifting and is characterized
by relatively slow rates of spreading (1 to 8 cm/yr, half-rate). Examples include
the Mid-Atlantic Ridge and East Pacific Rise.
B. Young or 'incipient' ocean basins -- Similar in appearance to mid-ocean ridges
but much more limited in extent. These areas represent intermediate stages of
continental rifting and ocean-basin formation. Examples include the Gulf of
California and Red Sea (See Figure 3).
C. On-land rifting -- Continental areas characterized by abnormally high heat flow,
extensional (pull apart) faulting and volcanic activity. These areas are thought
to represent the initial stages of rifting whereby continents 'drift' apart. A new
ocean basin is created between adjacent land masses as they separate. Examples
include the East African Rift Valley, the Rhine Valley Graben and the Rio
Grande Rift (New Mexico, Colorado).
Figure 3. Evolution of an ocean basin from initial rifting to maturity. A and B are similar
to the current East African Rift. C is like the Red Sea. D is like the Atlantic Ocean.
II. Convergent -- Plate boundary where two plates are moving toward each other, usually
one plate is subducted beneath the other. Convergence can be head-on, or oblique.
A. Oceanic plate-oceanic plate convergence - Convergence characterized by the
subduction of one oceanic plate beneath another oceanic plate, forming deep ocean
trenches and their associated volcanic island arcs. Examples include the trench-
island arc systems of the Aleutians, Puerto Rico-lesser Antilles, and Tonga. In
oceanic convergence, the more dense (older, and therefore colder) plate is
subducted, with the less dense (younger, warmer) crust forming the overriding
plate.
B. Oceanic plate - continental plate convergence - Characterized by subduction of
high-density oceanic plate beneath a lower-density continental plate, leading to the
formation of an ocean trench adjacent to the continent with a chain of volcanic
mountains on the continent. Examples include the Andes (South America), the
Cascade Range of Oregon and Washington, and Peninsular Ranges of Central
America.
C. Continental plate-continental plate convergence - In this case, neither of the low
density continental plates can be subducted into the relatively dense mantle. The
intense compression of pre-existing continental rocks forms linear belts of folded
mountains along the boundary (suture zone) where convergence occurs. In some
areas, pieces of ocean floor are caught up between colliding continents. Examples
of mountains formed by continental collisions include the Zagros (active; Iran), the
Himalayas (active; Nepal), the Alps (older; Italy/Switzerland), the Urals (ancient;
Russia, Kazakistan), and the Appalachians (ancient).
Figure 4. Three types of convergent plate boundaries.
III. Transform - Margins where two plates neither converge or diverge, but slide past
each other in parallel, but opposing directions.
When scientists first mapped the mid-ocean ridge in the Atlantic Ocean, it seemed
like a fairly regular mountain range except for being submerged below sea level. Then in
1956, Marie Tharp, a geologist at the Lamont Geological Observatory, discovered that
the Mid-Atlantic Ridge was split down the middle by a central rift valley (Fig. 2A).
Further mapping showed an even more curious thing - the ridge isn't a continuous line of
mountains. At many places, the ridge line is broken by offsets, along linear zones of
fracturing that extend for long distances away from the ridge axis at right angles to the
central rift valley. In 1965, J. Tuzo Wilson, a Canadian geophysicist, combined
information on earthquakes and seafloor spreading and showed that active faulting along
ridges was likely to occur in only a portion of the fracture zone: the portion lying
between offset ridge segments. This part of a fracture zone is called a ridge transform
fault (Fig. 2B).
A. Ridge transforms - faults oriented perpendicular to the axes of mid-ocean ridges,
offsetting the ridge axes. The transform fault between offset ridges is seismicallyactive. Seismically inactive fracture zones extend away from the ridge segments.
Ridge transforms are associated with shallow-focus seismic activity (Fig. 2B).
B. On-land transforms - Transform faults that develop where two continental blocks
slide past each other. An example of this is the San Andreas fault in California
(Figure 4).
IV. Intraplate Regions and Hotspots
It should be apparent from the preceding discussion that a very strong correlation
exists between the locations of plate boundaries and tectonic activity such as volcanism.
Volcanic and earthquake activity are not typical within the interiors of plates (intraplate
regions). However an important exception to this where large pulses, or plumes, of
basaltic magmas rise up from deep mantle sources. These mantle plumes, or hotspots, can
produce a stationary source of volcanism for millions of years. Such a hotspot is
responsible for the Hawaiian Island chain of volcanoes and the Emperor Seamount chain
in the North Pacific. These chains of volcanic islands and seamounts form as the
lithosphere passes over the stationary volcanic source at the hotspot (Figure 5). Several
island-seamount chains can be found in all ocean basins.
The large (4x5 foot) tectonics maps in the lab have nice illustrations of the
Hawaiian hot spot, located on the back of the map. A small bathymetry map of the
Hawaiian Island Chain is shown in Figure 6, below.
Figure 6
Bathymetry of the
Hawaiian Islands and
Emperor Seamounts
in the central Pacific
Ocean. Light shades
of gray indicate
shallow seafloor.
(USGS base map)