Lab 4: Plate Tectonics – Locating Geologic Hazards

Introduction
Lab 4: Plate Tectonics – Locating Geologic Hazards
The likelihood of major geologic hazards associated with the lithosphere, such as
earthquakes and volcanoes, is not uniform around the Earth's surface. Rather, there are
well-defined linear belts of earthquake and seismic activity that can be readily identified.
In the 1960s and 1970s, these belts were recognized as the boundaries of large plates that
moved around the Earth's surface. Thus, the theory of plate tectonics was developed.
This theory provides a ready explanation for the distribution of these types of geologic
hazards. It is useful for preparedness plan because it tells us what regions are subject to
what types of geologic hazards, the potential magnitude of these events and a crude
estimate of their recurrence times. Thus, a basic understanding of the theory of plate
tectonics is useful for understanding the major geologic hazards. (It is also helpful in the
exploration for Earth resources).
Fig.1: An understanding of the features of the ocean floor was critical to the development of the theory of
plate tectonics.
Subduction zones
Subduction zones are plate boundaries where two lithospheric plates converge or
move toward each other. During convergence, one plate is thrust (subducted) under the
other producing linear belts of volcanoes and earthquakes.
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Schematic cross-section through a subduction zone.
Subduction zones are regions of significant geologic hazards as well as the host of
certain types of Earth resources.
Types
Subduction zones are classified based on the types of plates that are colliding. Based
on this, there are three types of subduction zones.
• ocean-ocean: two oceanic plates are colliding therefore cannot predict which will
be subducted; associated volcanoes produce an oceanic island arc on the
overriding plate
• ocean-continent: an oceanic plate (the denser) is subducted beneath the
continental plate; volcanoes form a continental arc on the edge of the overriding
continental plate
• continent-continent: two continental plates collide, because they are both of low
density, neither is subducted, no volcanoes, forms high mountain range with large
earthquakes
Volcanoes
Some of the most explosive volcanoes are associated with subduction zones. They
are the classic composite volcano with its characteristic symmetrical profile. These
volcanoes erupt lavas with high gas content and viscosity which often leads to very high
explosivity.
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Some of the most powerful historic eruptions have been of subduction zone
volcanoes. These include:
• Tambora, Indonesia in 1815
• Krakatoa, Indonesia in 1883
• Mount Pele, Martinique in 1902
• Katami in 1910
• Mount Pinatubo, Philippines in 1986
Earthquakes
The largest earthquakes that occur on Earth are associated with subduction zones.
These quakes vary in depth from a few kilometers to more than 100 km. They also affect
very large areas and can have long durations, e.g. several minutes. Many of the most
destructive earthquakes have occurred along subduction zones. Unfortunately, a large
percentage of the world's population lives near subduction zones putting them at great
risk from this geologic hazard. Important subduction zone earthquakes include:
• Good Friday earthquake in Alaska, 1964
• Chile earthquake, 1960
These are the three strongest earthquakes ever recorded.
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Damage in Anchorage, Alaska produced by the 1964 earthquake.
Wadati-Benioff zone
A dipping planar (flat) zone of earthquakes that is produced by the interaction of a
downgoing oceanic crustal plate with a continental plate. These earthquakes can be
produced by slip along the subduction thrust fault or by slip on faults within the
downgoing plate as a result of bending and extension as the plate is pulled into the
mantle. (from http://earthquake.usgs.gov)
Schematic cross-section through a subduction zone.
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Divergent boundaries
1. rifting occurs when currents in the asthenosphere pulls-apart and tears a preexisting
plate
a. Great Rift Valley of East Africa is a prime example of rifting
b. Red Sea and the Gulf of Aden occur in basins produced by rifting that
separated the Arabian plate from the African plate
2. as the smaller plates continue separating from one another they diverge
a. typical rates are about 1 to 9 centimeters (0.5-4 inches) per year
3. molten rock rises in the center of the rift
a. cools and solidifies, becoming attached to the edges of the rifted plates
4. if divergence continues, further separating the older rifted segments, eventually an
ocean basin may be formed
5. creating new oceanic lithosphere that builds up to form the mid-ocean ridges
a. a continuous chain of submarine mountains that bisects the world’s oceans
6. process of plate growth at mid-ocean ridges is known as sea-floor spreading
(form http://stloe.most.go.th)
Transform margins
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Transforms, also referred to as transform faults, are conservative plate
boundaries, i.e. plates are neither created nor destroyed there rather they are conserved.
Along a transform, one plate slides past the other so there is neither subduction nor
magma ascent.
Transforms form the smallest percentage of the plate margins and represent a
fundamental development in the theory of plate tectonics. They were originally described
by the Canadian geologist J. Tuzo Wilson.
Connector
Transforms are the geometrical element that joins different types of plate boundaries.
Without them, the physics of rigid plates on a spherical surface would prevent plate
tectonics from occurring. There are three possible of plate boundaries that can be joined
by transforms:
• divergent-divergent:
• divergent-convergent: San Andreas Fault, Queen Charlotte Fault (off BC);
Mediterranean area, South Atlantic
• convergent-convergent: Caribbean, South Atlantic, Asia
Features
The most common type of transform connection is divergent-divergent (or ridgeridge).
A careful observation of the the mid-ocean ridge system reveals that is is not a
continuous linear feature, but that it is broken up into short linear segments 200-800 km
long. This produces the characteristic stair-step pattern of the mid-ocean ridge system.
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The linear features that offset the ridge crest are transforms. They extend for
thousands of kilometers across the ocean floor and form impressive cliffs that may tower
1-2 km above the surrounding ocean floor. Topographic features and magnetic anomalies
that cross these transforms may be displaced as much as 1000 km from each other. There
is also a significant difference in the age of oceanic crust on opposite sides of the
transform.
Hazards
Along transform boundaries, two lithospheric plates slide past each other.
However, in most cases motion is not uniform but occurs in a series of events or ruptures.
These occur when the stress accumulated along the fault exceeds the strength of the lock
that has been preventing the plates from moving. When this happens, an earthquake
occurs. Because plates are neither created nor destroyed at transforms, they are not
marked by volcanic activity. Thus, the primary lithospheric-driven geologic hazard
accompanying transform boundaries is earthquakes.
Transform fault earthquakes:
• occur at shallow depths (

outside of the ridge crests. These linear, aseismic extensions of the transforms are
fracture zones.
• between crest two plates moving in opposite directions - friction produces quakes
• outside crests plates moving in same direction and at same rate - no friction to
produce earthquakes
• the offset of the magnetic stripes was originally produced at the ridges and don't
get bigger with time
This distribution is a result of the plate motions along the ridge system.
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