Geoscience Reference
In-Depth Information
Galapago
s
Sala-y-G
omez
Seamounts or atolls
Plate boundaries
Active or extinct volcanoes
Location of other hot spots and associated island strings on the Pacific Plate (based on Holmes, 1965).
Fig. 9.7
that lines of volcanic activity represent tearing of the
Earth's crust perpendicular to mid-ocean ridges.
The Hawaiian and Tuamotu Island chains certainly
fit the locations where stress fractures should form
via this mechanism. It is now viewed that intra-plate
volcanism is more complex than originally theorized in
the 1970s using the concept of hot spots.
strain is excessive, ruptures or faults occur to relieve
the pressures that are being built up. Fracturing may
occur gradually, or as a sudden series of shocks that
radiate outwards from the strain zone in an uneven
fashion, dependent upon the spatial variation in rock
strength.
The type of faulting or rupturing that occurs
depends upon the characteristics of the fault. Figure
9.8 shows the typical range of faults that occur with
earthquakes. The zone of earthquake influence is
narrowest for the strike-slip fault, whereas normal or
thrust faulting produces a wider zone of influence.
However, the difference is really dependent upon the
angle of the fault. For example, as the dip of the San
Andreas Fault is virtually vertical, earthquake damage
rapidly diminishes within 20 km of the fault line. This
is why moderate earthquakes this century in southern
California have not caused widespread damage. On the
other hand, the high magnitude earthquakes in Chile
in 1960, and Alaska in 1964, occurred on low-angled
reverse faults (20ยบ or less in the case of Alaska) associ-
ated with subduction zones, a feature that extended
Oth
er faulting and dilatancy
The above factors account for the location of most
volcanic activity and a significant number of earth-
quakes. Most damaging earthquakes simply represent
the rapid release of strain energy stored within elastic
rocks. Such earthquakes are termed tectonic earth-
quakes. The Earth's crust is being continually stretched
and pulled in different directions as plates move
relative to each other, and as forces act within the
plates themselves. Even minor crustal movement sets
up elastic strain within the surface of the Earth, which
may be greater than the internal strength of the layers
of bedrock. This strain builds up a reservoir of energy
much like a coiled spring in a child's toy. Where this
