Download Plate Tectonics Introduction Boundaries between crustal plates

Dr. Eric Heinen De Carlo, University of Hawaii at Manoa
Plate Tectonics
KEY WORDS: continental drift, seafloor spreading, plate tectonics, mid ocean ridge (MOR)
system, spreading center, rise, divergent plate boundary, subduction zone, convergent plate
boundary, suture zone, deep ocean trench, conservative boundary, transform fault, strike slip
fault, fracture zone, earthquakes, oceanic crust, continental crust, upwelling, convection.
Introduction
Geologists have hypothesized that the outer 60-100 km of the Earth are composed of
approximately seven principal slabs of rigid lithospheric (rock) material. These slabs, known as
plates, account for most of the dynamic features of the crust of the Earth. This includes
earthquakes, mountain chain building, and volcanic eruptions. The hypotheses developed to
explain the presence of and interactions between these plates have been widely tested and are
generally accepted today. They were, however, quite controversial when they were first proposed
some thirty- five years ago. The concepts behind these hypotheses are now collectively known as
plate tectonics and have been advanced from hypotheses to the level of a theory.
The theory of plate tectonics evolved from the hypothesis of continental drift, proposed
by Alfred Wegener in the early part of the 20th century. Wegener hypothesized that continental
masses plow through oceanic rock. Because oceanic rock is more rigid than continental rock, and
because he failed to provide an acceptable mechanism to account for how continents could have
drifted great distances (not to mention the fact that Wegener was a meteorologist) his ideas were
widely rejected. In the subsequently developed hypotheses of seafloor spreading and plate
tectonics, however, there was no problem of continents plowing through oceanic rock. Instead,
rigid plates, including those with continents, simply move about relative to one another. Much
evidence acquired in the early 1960’s and subsequent ly provided strong support for the new
hypothesis, and plate tectonics enjoyed wide-ranging support from earth scientists. It is now
regarded as a theory.
The boundaries between lithospheric plates are zones of considerable deformation caused
by the motions of the plates. These zones are characterized by abundant earthquakes.
Boundaries between crustal plates
There are three basic types of plate boundaries:
1. DIVERGENT. The first type of plate boundary is found on the mid ocean ridge system
(e.g., the Mid Atlantic Ridge, the East Pacific Rise), where lava of basaltic composition
upwells from mantle and flows out onto the ocean floor where it cools and solidifies to
create new oceanic crust. The young crust is then pulled apart as additional lava comes up
and newer crust is formed along the center of the ridge. As a result of this process,
oceanic crust moves outward (horizontally) on both sides of an oceanic ridge, and new
crust is continually added to older oceanic crust. This type of plate boundary is known as
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a divergent plate boundary, ridge, or rise where the dominant force is tension (i.e.,
PULLING apart).
2. CONVERGENT. Creation of crust along ridges must be balanced by consumption of
crust elsewhere. Many plates are bounded by zones where rigid crust descends deep into
the mantle and the crust is ultimately reabsorbed. These plate boundaries are known as
subduction zones. The dominant force is compression (i.e., PUSHING together). There
are three sub-types of convergent boundaries:
a) where dense oceanic crust converges with less dense continental crust. The
greater density oceanic crust descends below the continental crust.
b) where oceanic crust collides with oceanic crust from another plate. The older
(cooler and denser) plate descends and the younger (warmer and less dense) plate
overrides.
c) where two continental slabs encounter each other, a more complex convergent
plate boundary occurs. The interaction between these slabs causes intense
deformation and mountain building, leading to the formation of a suture zone .
3. CONSERVATIVE. The third type of plate boundary occurs where compensating
motion takes place without creating or consuming crust and plates slide past each other.
These conservative plate boundaries are characterized by long faults between the
adjoining plates that are known as transform faults (i.e., one feature is “transformed”
into another). They occur where new crust zones (i.e., ridges or rises) abruptly end. These
plate boundaries are called transform plate boundaries. The physiographic feature
formed by the transform fault and their extensions beyond the region connecting two
sections of ridge are called fracture zones. Note that the extensions of the fracture zones
beyond the active ridge segments DO NOT separate different plates but are embedded
within a single plate.
Major lithospheric plates of the Earth
There are seven major lithospheric plates on the rigid outer surface of the Earth (Fig. 1).
Several smaller plates, as well as regions of extremely complex tectonic morphology exist and
complete the puzzle of the Earth’s plate tectonic structure. The latter will not be considered in
this laboratory exercise. In the attached diagram of the Earth’s plates and that showing the
location of major earthquake epicenters (Fig. 2), note that the zones of earthquakes coincide with
plate boundaries. There is, however, a major distinction between the various zones in terms of
the depth of occurrence of the earthquakes. Shallow earthquakes occur at the MOR and along
transform faults, whereas deeper earthquakes occur at subduction zones, with the deepest ones
located at substantial distances beyond where the descending slab begins to be subducted. For
example, the deepest earthquakes associated with the Peru-Chile trench occur well eastward of
the Andes mountains of South America (see Fig. 2).
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Exercises
During this exercise you will use a map of the ocean floor to carry out a series of observations
and calculations about plate motions. The observations will help you recognize and familiarize
yourself with the primary physiographic features associated with plate tectonics (i.e., plate
boundaries). The calculations will illustrate some of the concepts of seafloor spreading and plate
tectonics and will demonstrate how application of basic principles using very simple
approximations allows you to draw relatively accurate conclusions. Certain island chains, those
created by hot spots, represent an aid to understanding plate motions as you will discover in the
exercises belo w.
1.
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Refer to Figures 1 and 2 and carry out the following:
Draw a dark (or red) line along all plate boundaries that show evidence of subduction
(convergence). These can be identified by the existence of deep earthquakes.
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The remaining plate boundaries in Figure 1 are either divergent plate boundaries or
conservative (transform) plate boundaries. Keep in mind that plate motion is towards
subduction zones, and away from (perpendicular to) ridges. Noting where you just
marked the location of subduction zones, use a different color pen to mark small double
arrows along the MOR system to show the relative spreading direction.
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Transform faults are zones where there is neither tension nor compression. These are
strike-slip faults and must trend parallel to the directions of plate motion. Using this as a
guide, draw blue lines along transform faults.
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Now that you have outlined the three types of boundaries along the major lithospheric
plates, you are prepared to consider the actual motions of these plates. Because it is
possible that all plates are moving relative to a stationary point on Earth, there is a
potential for great complexity in the absolute plate motions. It is, however, much simpler
to consider that one plate is fixed and that all the others move relative to it. For the sake
of this exercise let us assume that the American plate is stationary. Now… draw a single
large arrow on each plate to show its motion RELATIVE to the (stationary) American
Plate.
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Fill out Table 1.
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Figure 1: Major Lithospheric Plates of the World.
Figure 2: Locations of epicenters of eathquakes throughout the world
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Table 1. Classification of Geologic Processes
Geologic Process
Type of plate boundary
Divergent (ridges
Convergent
Conservative
or rises; zones of
(subduction zones;
(transform faults)
seafloor spreading)
colliding plates)
Creation of oceanic crust
Volcanism
Creation of fold- fault
mountain ranges
Compression
Tension
Shear
2. In this exercise you will calculate the age of various portions of the seafloor (and features
thereupon) based on their current location relative to the Mid Ocean Ridge (MOR) system
(i.e., their distance from the MOR) and the appropriate present-day MOR spreading rate. The
following spreading rates are given. Note that these are half spreading rates, i.e., the rate at
which one side of the MOR spreads. The full spreading rate is, of course, twice these values
because, as new seafloor is formed, it spreads on both sides of the MOR.
East Pacific Rise
Mid Atlantic Ridge
Indian Ocean Ridge
8 cm/yr
1 cm/yr
2.5 cm/yr
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Assuming that the Hawaiian Islands have been carried along with the motion of the
Pacific Plate, calculate the age difference between the (Big) island of Hawaii and the
bend in the Hawaiian Archipelago-Emperor Seamount chain.
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Estimate the age of the seafloor at Cape Hatteras, North Carolina.
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Knowing that the Himalaya represent a suture zone formed by the collision of India with
Asia, estimate when India first began colliding with Asia using the spreading rate of the
Indian Ocean Ridge.
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Assuming you have lived in the same house all your life and you have not changed the
configuration of your bedroom since birth (unlikely, but assume this is the case)…
calculate how far your bed has moved since you were born.
3. Compare the ages obtained above with radiometric age given below and discuss the
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agreement or disagreement with your calculations. If there is a substantial disagreement
between your calculation and the data provided, speculate what could have caused the
discrepancy in age.
Ages of various features in the World Ocean:
Hawaiian Archipelago- Emperor Seamount chain bend: 42 Ma
Initial collision of India with Asia: 53 Ma
Opening of North Atlantic Ocean: 165 Ma
4. You have noted in exercise two above that the Hawaiian-Emperor trend has a break in trend
slightly to the west of Midway Island. What could have caused this change in orientation
of the islands and seamounts?
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