Download Lab 06 - Las Positas College

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LAS POSITAS COLLEGE
OCEANOGRAPHY LAB
LAB 6
PLATE TECTONICS
(Modified from Exercise 4, Sea Floor Spreading and Plate Tectonics, Laboratory
Exercises in Oceanography, Bernard W. Pipkin et al, 2ed, W. H. Freeman and Co.
1987)
plate motions is heat produced by the
decay of radioactive atoms in the earth’s
1.
It has long been recognized that
interior. Although these atoms exist in
such features as mountains,
very small concentrations, the combined
earthquakes, and volcanoes are not
total heat production is large enough to
randomly distributed upon the surface of
keep the earth’s interior quite hot and
the earth. In 1912 the German
provide a constant flow of heat to the
meteorologist Alfred Wegener at
earth’s surface.
tempted to explain this distribution by
arguing that if continents could move
vertically, then horizontal motion, or
4.
Because of this heat production,
continental drift, was also possible. The
thermal gradients develop in the earth’s
theory of sea-floor spreading—a
interior, causing hot, low density
process whereby new sea floor is
material to rise toward the surface while
created as adjacent crust is moved
cold, higher-density material sinks. The
apart to make room —was first
convective motion thus generated in the
proposed in 1960 as a viable alternative
earth’s interior moves the lithospheric
to Wegener’s theory.
plates around on the surface of the
earth. In a few areas, strong plumes
may rise through the earth’s mantle and
2.
In essence, it has been proposed
penetrate the lithosphere to produce a
that the earth’s outer shell (called the
hot spot with magmatic activity, which
lithosphere, a zone about 100
may or may not lie on a plate boundary.
kilometers thick) is composed of a
Examples of hot spots include Hawaii,
number of rigid plates. Each of these
Iceland, Easter Island, the Galapagos
crustal plates moves in a different
Islands, and Yellowstone (figure 2).
direction, and thus plate boundaries are
sites of tectonic activity where
5.
There are three types of plate
earthquakes, volcanism, and mountain
building occur. Six large plates and a
boundaries: DIVERGENT,
number of smaller ones have been
CONVERGENT, AND TRANSFORM
identified (figure 1).
(figure 3). Where plates are separating,
magma rises and cools to form new
ocean crust. This boundary also called a
3.
The source of energy for driving
spreading center and is expressed
topographically as a mid-ocean ridge.
Examples of ridges include the MidAtlantic Ridge and East Pacific Rise.
Of course, if new sea floor is
continuously created at spreading
centers, old sea floor must be
destroyed. This occurs at destructive
boundaries, also known as subduction
zones, which are expressed
topographically by the presence of
trenches. As old oceanic crust is
subducted, it will be heated, causing
partial melting. Magma produced in this
fashion will rise, causing volcanism on
the overriding plate near the trench.
Examples of subduction zones include
the Aleutian Trench and Aleutian
Islands, and the Peru-Chile Trench and
Andes Mountains. The third type of
boundary lies along numerous offsets
on mid-ocean ridges. These form
conservative plate boundaries, where
plates slide past one another with no
crustal creation or destruction. These
are also known as transform faults and
may be expressed topographically as
fracture zones. Examples of these are
the Romanche, Oceanographer, and
Mendocino fracture zones.
Figure 1. A mosaic of plates forms the earth’s lithosphere, or outer shell.
According to the theory of plate tectonics, the plates are not only rigid but are in constant motion .
6.
The boundaries between plates
are seldom smooth, and thus plates
“catch” on each other, deforming
elastically on the edges until rock failure
occurs to produce the abrupt motion we
know as earthquakes. The largest and
most devastating of these events often
occur in subduction zones, but
transform faults such as the San
Andreas (in California) may also
produce very large earthquakes.
Occasionally two continental plates will
collide in a subduction zone, but
because continental crust has too low a
density to sink into the earth’s interior,
the result is the pushing up of large
mountain ranges. Collisions such as
these created the Himalayas and the
Alps.
7.
The critical evidence for sea-floor
spreading is based on earth magnetism
(the force exerted by the earth’s
magnetic field). In the past
(approximately every half
million years), the earth’s magnetic
field has reversed its polarity, so that
the north magnetic pole becomes the
south magnetic pole (or the presentday normal polarity becomes reverse
polarity). Each major reversal is termed
a polarity epoch. The sequence of
reversals occurring in the course of the
past several million years has been
dated with the use of radiometric
techniques (figure 4). Shorter reversals,
termed events, have also been recorded
within the longer epochs.
8.
When basalt is intruded and
cools in cracks at mid- ocean ridges, the
polarity of the earth’s magnetic field at
the time of cooling is preserved. As the
crust moves away from the spreading
center, each successive unit of the
cooled magma gradually moves
outward, revealing zones of fossil
magnetism (figure 5), in which the
orientation of the earth’s magnetic field
at the time of formation is preserved.
This fortunate preservation allowing us
to detect fossil magnetism gives us a
method for measuring the rate at which
new sea floor is formed.
9.
Where the rocks have the same
magnetic polarity as the present-day
normal magnetic field we find stronger
than average magnetisms and we have
a positive anomaly; where the rocks
preserve reverse polarity we measure
weaker than average magnetic field and
we have a negative anomaly. We can
determine the rate at which new sea
floor is formed by measuring the
distance from the ridge crest to a
magnetic anomaly of known age. To
calculate the spreading rate, simply
divide the distance traveled by the
age of the oldest reliably dated
anomaly. With the aid of this
technique, plate velocities of from 1
to 10 centimeters per year have been
calculated (figure 6).
10.
Although mid-ocean ridges are
areas in which relatively large
temperature increases should be
observed in sediments beneath the sea
floor, measurement of these gradients
has shown that they are only slightly
larger than those observed at locations
far away from ridge crests. At first, these
observations were difficult to reconcile
with the predictions of the plate tectonic
model. Then, suggestions were made
that perhaps seawater enters fractures
Figure 2. Location of hot spots. They are found on all major plates: on continental land masses as well
as oceanic plates. Their distribution is random.
Figure 3. New sea floor is created at the spreading center or oceanic ridge as magma rises from below.
The magma solidifies as lava and preserves the prevailing magnetic polarity. Transform faults are
perpendicular to the ridges. As they slide past each other, they produce earthquakes. Shallow
earthquakes occur at the ridges, while deep earthquakes occur at subduction zones.
Figure 5. Evidence for sea floor spreading has been
obtained by determining the polarity of fossil magnetism in
rocks lying on both sides of oceanic ridges. In the illustration
rocks of normal polarity are represented by the lighter gray
stripes. Two blocks represents a fracture zone. The symmetry
suggests that the rocks welled up on a molten or semimolten
state and gradually moved outward.
Figure 4. Illustration of how magnetic polarities of lave flows are used to construct the time scales of
magnetic reversals over the pas 5 million years. In no one place is the entire sequence found; the
sequence is worked out by patching together the ages and polarities from lava beds all over the world.
Each event is named after a geologist.
Figure 6. These are magnetic reversals recorded from the ocean floor. There are two values to note.
Distance, in terms of kilometers below the magnetic anomaly and time, in terms of millions of years,
above the magnetic anomalies. Do note that spreading rates are calculated in centimeters per year.
in the oceanic crust and flows toward
hot, recently formed rock. Where it
encounters hot rock, water would be
heated, causing it to rise and exit near
ridge crests. This suggestion seemed
reasonable because hot springs and
geysers are frequently observed on
continents near magma sources. This
hydrothermal flow would provide a
mechanism for cooling the newly formed
crust and thus reducing the temperature
gradients.
carry sulfide in solution, which bacteria
can combine with oxygen from seawater
in order to derive energy for growth. The
bacteria can then be consumed by
higher organisms, including large clams
and tube worms, allowing a unique
ecosystem to develop, fueled by
chemical energy. Similar vents have
since been found at a number of other
locations on ridge crests, some of which
may vent water nearly 300°C in
temperature.
11.
In 1977, an expedition went to
the Galapagos spreading center to
search for vents of hot hydrothermal
waters. Using the submersible Alvin,
they explored the sea floor for evidence
of hot springs. Their search was
successful, and resulted in some remark
able additional discoveries. Plumes of
hot water were found exiting from
fissures in the crust. These plumes
12.
Although each vent may have
only a modest flow and be active for
only a few thousands of years, there
may be large numbers of them. It has
been estimated that the entire volume of
the ocean is circulated through ridge
crests in 10-20 million years.
Examination of the composition of these
hot waters has answered some longstanding questions regarding ocean
chemistry, because reactions between
seawater and hot rocks help regulate
the abundance of some elements in
seawater. Metals are leached from hot
rocks and carried upward by
hydrothermal flow. As the water rises
and cools, some of these metals may
precipitate in the fractures or fissures
that permit flow, forming ore deposits.
Other metals are carried into bottom
water and provide a significant fraction
of the dissolved metal flux to the sea.
EXERCISES
1a. Examine figure 3 and state where you would expect to find the highest
temperatures on the seafloor.
b. Where would deep-focus earthquakes occur? (The focus is the location inside the
earth where an earthquake starts).
c. Why would you not expect to find deep-focus earthquakes under spreading centers?
2. What types of plate boundaries occur between the following? Refer to the box in
figure 1.
a. The North American and Pacific plates
b. The Nazca and South American plates
c. The South American and African plates
3. What stresses (tension = pulling, compression = pushing, or shearing = tearing
apart) characterize the following types of junctions?
a. Spreading centers
b. Subduction zones
c. Transform faults
4a. A transform fault (single line) is shown offsetting mid-ocean ridges (double lines) in
the sketch below. Use an X to show the possible locations of earthquake epicenters
(the point on the earth’s surface directly above an earthquake’s focus) along its strike.
Use arrows to show the directions of plate motions. NOTE: no more than 6 x’s are
allowed.
b. Explain the distribution of earthquake epicenters on the sea floor.
5. Refer to figure 6 and calculate the spreading rates in centimeters per year for the
three areas shown in the figure.
a. South Atlantic
b. North Pacific
c. Pacific Antarctic
d. Which of the ocean basins exhibits the slowest spreading rate?
e. Which one is spreading the fastest?
f. About how long did it take for the Atlantic Ocean to open to its present width at the
equator?
g. During what geologic time period did these events take place? Use geologic time
scale!
6. The Hawaiian Islands have been created by the passage of the Pacific Plate over a
hot spot. On which Hawaiian Island would you expect to find active volcanism and the
youngest rocks? (Indicate east or west, or identify the islands on a good map of the sea
floor.)
7. On a good map of the Pacific sea floor, find the following hot-spot ridges in the
Pacific: the Emperor Seamount Chain hot-spot ridge, the Tuamotu Archipelago—Line
Island hot-spot ridge, and the Austral—Gilbert—Marshall Island hot- spot ridge. If these
oceanic island chains are produced by hot spots, explain the change in the lineation of
each of them (mainly westerly for the Hawaiian, Tuamotu, and Austral parts; and mainly
northerly for the Emperor, Line, and Gilbert-Marshall parts).
8. If the Pacific Plate is moving at 10 centimeters per year, and this speed has not
changed, estimate the time (years before present) when the change described in
Question 7 took place.
9. On a good map showing detailed bottom topography, locate the Walvis Ridge and
Rio Grande Rise in the South Atlantic. It is believed that these features were produced
as volcanic ridges from a hot spot currently at about 40° S on the Mid-Atlantic Ridge. If
this is true, why do the lineations of the hot-spot ridges and the lineation of the fracture
zones (both produced by plate tectonics) show a different sense of movement
(lineations are in different directions)?
G12L Form 6 Rev: 13 Aug 2013