Plate Tectonics: A Unifying Theory - The Changing Earth: Exploring Geology and Evolution - page 47

in the Gulf of California with the Juan de Fuca and Pacifi c
plates off the coast of northern California (

Figure 2.21).
Many of the earthquakes that affect California are the result
of movement along this fault (see Chapter 8).
Unfortunately, transform faults generally do not leave
any characteristic or diagnostic features except for the ob-
vious displacement of the rocks with which they are asso-
ciated. This displacement is usually large, on the order of
tens to hundreds of kilometers. Such large displacements
in ancient rocks can sometimes be related to transform
fault systems.

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Deep-sea
sediments

Pillow
lavas

Sheeted
dikes

Oceanic
crust

Massive
gabbro

HOT SPOTS AND MANTLE PLUMES
Before leaving the topic of plate boundaries, we should men-
tion an intraplate feature found beneath both oceanic and
continental plates. A hot spot (Figure 2.15) is the location on
Earth's surface where a stationary column of magma, origi-
nating deep within the mantle ( mantle plume ), has slowly
risen to the surface and formed a volcano. Because the man-
tle plumes apparently remain stationary (although some
evidence suggests that they might not) within the mantle
while the plates move over them, the resulting hot spots
leave a trail of extinct and progressively older volcanoes
called aseismic ridges that record the movement of the
plate.
One of the best examples of aseismic ridges and hot
spots is the Emperor Seamount-Hawaiian Island chain
(

Layered
gabbro

Upper
mantle

Peridotite

◗ Figure 2.19 Ophiolites Ophiolites are sequences of rock on land
consisting of deep-sea sediment, oceanic crust, and upper mantle.
Ophiolites are one feature used to recognize ancient convergent
plate boundaries.

Elongated belts of folded and faulted marine sedimen-
tary rocks, andesites, and ophiolites are found in the Ap-
palachians, Alps, Himalayas, and Andes mountains. The
combination of such features is significant evidence that
these mountain ranges resulted from deformation along
convergent plate boundaries.

Figure 2.22). This chain of islands and seamounts (struc-
tures of volcanic origin rising higher than 1 km above the
seafl oor) extends from the island of Hawaii to the Aleutian
Trench off Alaska, a distance of some 6000 km, and consists
of more than 80 volcanic structures.
Currently, the only active volcanoes in this island chain
are on the island of Hawaii and the Loihi Seamount. The rest
of the islands are extinct volcanic structures that become pro-
gressively older toward the north and northwest. This means
that the Emperor Seamount-Hawaiian Island chain records
the direction that the Pacifi c plate traveled as it moved over
an apparently stationary mantle plume. In this case, the Pa-
cifi c plate fi rst moved in a north-northwesterly direction and
then, as indicated by the sharp bend in the chain, changed
to a west-northwesterly direction approximately 43 million
years ago. The reason that the Pacifi c plate changed direc-
tions is not known, but the shift might be related to the col-
lision of India with the Asian continent at about the same
time (see Figure 10.22).
Mantle plumes and hot spots help geologists explain
some of the geologic activity occurring within plates as
opposed to activity occurring at or near plate boundar-
ies. In addition, if mantle plumes are essentially fixed
with respect to Earth's rotational axis, they can be used
to determine not only the direction of plate movement,
but also the rate of movement. They can also provide

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Transform Boundaries
The third type of plate boundary is a transform plate
boundary . These mostly occur along fractures in the sea-
floor, known as transform faults, where plates slide later-
ally past one another roughly parallel to the direction of
plate movement. Although lithosphere is neither created
nor destroyed along a transform boundary, the movement
between plates results in a zone of intensely shattered rock
and numerous shallow-depth earthquakes.
Transform faults “transform” or change one type of
motion between plates into another type of motion. Most
commonly, transform faults connect two oceanic ridge seg-
ments; however, they can also connect ridges to trenches
and trenches to trenches (

Figure 2.20). Although the
majority of transform faults are in oceanic crust and are
marked by distinct fracture zones, they may also extend
into continents.
One of the best-known transform faults is the San An-
dreas fault in California. It separates the Pacifi c plate from
the North American plate and connects spreading ridges

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