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Transcript
Continental Drift
Take a look at a globe sometime and observe the remarkable fit between South America and Africa.
Could they have, in fact, been connected?
During the 19th and early 20th centuries, several
geologists explored the idea of moving continents by observing the possible “fit” between Africa and
South America. In 1912 Alfred Wegener, a lecturer in astronomy and meteorology, hypothesized that
the earth’s continents were all together at one time, forming the supercontinent Pangaea, and then they
broke apart, drifting through the ocean floor to produce the present-day continental configurations. This
is known as the Continental Drift Hypothesis. Wegener supported his Continental Drift Hypothesis
with fossil evidence and lithological correlation. For example, Wegener correlated several land type
fossils with other land type fossils on different continents. Additionally, Wegener pointed out
remarkable lithologic characteristics that matched with those of other continents.
Continental drift was not immediately accepted by Wegener’s peers when he published his findings in
1915. Critics of continental drift indicated that his evidence was not substantial, and Wegener could not
explain how continents move. In fact, critics bitterly attacked Wegener, destroying his teaching career
and credibility as a scientist. During the early 1900’s, two primary viewpoints or doctrines were
currently accepted, thus dismissing continental drift. Permanentists believed that continents and
basins are virtually unchanged and continents remained stationary. Contractionists explained earth
features using the Shrinking Earth Hypothesis, which suggested that gradual contraction of the solid
earth allowed the ocean floor to become dry land and dry land to become the ocean floor.
The Birth of Plate Tectonics
Alfred Wegener died in 1935, found frozen in the Artic. However, some scientists stayed on board
supporting Wegener’s Continental Drift Hypothesis. In 1947, technology allowed the mapping of the
Atlantic Ocean floor and led to the discovery of a linear mountainous terrain with abundant volcanic and
earthquake activity. It was learned that this mountain range was produced by volcanism in which lava
pushes up between the mountains, spreading apart the ocean floor. This is known as sea floor
spreading or referred to as a spreading ridge. This particular ridge is called the mid-Atlantic ridge.
Magma rises and moves
in between the ocean plates,
producing volcanism and
earthquakes. Lava extrudes on
both sides of the ridge, spreading
the plates apart. This is known
as a spreading center.
Paleomagnetism
By the 1960’s, scientists discovered a series of linear magnetic anomalies or magnetic stripes located
on each side of the mid-Atlantic ridge. In other words, the magnetic stripes on one side of the ridge
matched magnetic stripes on the other side of the ridge. These stripes possessed alternating magnetic
orientations which reflected the earth’s magnetic polarity at the time of ocean floor formation. These
are known as magnetic reversals. The data and observation of these magnetic reversals show that the
sea floor is spreading to produce stripes on both sides of the ridge, giving a mirror pattern. The
discovery of these geometric magnetic patterns is referred to as paleomagnetism. Paleomagnetism
demonstrates that continents are on the move and provides the pivotal evidence necessary to explain
the theory of plate tectonics. Wouldn’t Alfred Wegener have been proud?
A
B
C
In each diagram A,B, and C, the earth’s
crust is slowly spreading. This is reflected
by the magnetic reversal stripes
producing the same pattern on either
side of the spreading center. This is
known as paleomagnetism. Which magnetic
stripe is the oldest? Why?
Plate Tectonics
By the late 1960’s, scientists had joined together to create the plate tectonic model. The plate tectonic
model is used to describe various geologic features, geological rock environments, and the pattern of
volcanism as well as earthquake activity. According to the plate tectonic model, the surface of the
Earth consists of a series of relatively thin but rigid plates which are in constant motion. The surface
layer of each plate is composed of oceanic crust, continental crust, or a combination of both. The lower
part consists of the rigid upper layer of the Earth's mantle. The rigid plates pass gradually downward
into the plastic (soft) layer of the mantle, the astenosphere. The plates may be up to 70 km thick if
composed of oceanic crust or 150 km thick if incorporating continental crust. Plates move at different
velocities; the African plate moves about 25 mm per year whereas the Australian plate moves about 60
mm per year. Volcanic and earthquake activities primarily occur along plate boundaries. Here, plates
may move away from each other, slam into each other, or grind past one another. There are three
types of plate boundaries: divergent, convergent, and transform plate interactions.
Divergent plate boundary
The divergent boundary represents two plates moving away or separating from each other, hence the
term divergent. At this type of boundary, new oceanic crust is formed in the gap between two diverging
plates as magma rises and fills the gap. The divergent boundary is characterized by high-rising ridges
at the boundary while plate material thins away from the divergent boundary. Presently, most divergent
boundaries occur along the central zone of the world's major ocean basins. The process by which the
plates move apart is referred to as sea floor spreading. The Mid-Atlantic Ridge and East Pacific Rise
provide good examples of this type of plate margin.
The divergent boundary is represented
by two plates moving apart or away
from each other. Magma rises, filling the
gap and pushing the plates apart. Earthquakes
and volcanic activity are common along the
divergent boundary.
Convergent plate boundaries
The convergent boundary is represented by two plates moving toward each other, hence the term
convergence. As the two plates converge, one plate typically slides beneath the other. The sliding
plate descends below the overriding plate and is assimilated into the Earth’s upper mantle. The
descending plate moves along the subduction zone before entering the mantle. In other words, it can
be stated that one plate subducts beneath another. The point of subduction is typically marked by a
deep ocean trench on the surface of the earth. A major consequence of plate subductiion is marked by
deeply focused earthquakes along the subduction zone. Once the subducting plate reaches depths
ranging between 90-150 km, it begins to melt forming magma. The less dense magma begins to rise
(like a balloon in a deep swimming pool) and reaches the Earth’s surface, forming a volcanic arc or
volcanic chain above the earth’s surface on the non-descending plate. There are three types of
convergent plate interactions: ocean to continent, ocean to ocean, and continent to continent
convergence.
A
B
C
Convergent boundaries showing
plate subduction. There are three
primary convergent boundaries:
(A) ocean to continent, (B) ocean
to ocean, and (C) continent to continent.
Each convergent boundary produces
various types of geologic features:
(A): volcanic chain on the continent
(B): volcanic arc rising above the ocean
(C): high-rising mountains on the continent
Transform Plate Boundaries
The transform boundary is represented by areas where two plates are grinding or sliding past one
another. In the area of the grinding or fracture zone, faults typically occur and are known as transform
faults. Most transform faults are located on the ocean floor where they primarily offset spreading ridges
(mid-ocean ridges). However, some transform boundaries are located on continental crust, creating
transform faults that traverse long distances and result in “sliding” between sections of continental
crust. The most famous example is the San Andreas Fault Zone (SAF). The SAF begins in the Gulf
of California, traverses through southern and central California (approximately 70 miles west of
Bakersfield), and exits north of San Francisco where it changes into offset spreading ridges on the
ocean floor. Another example of a transform boundary on land is the Alpine Fault of New Zealand.
Typically, one can expect recurring earthquake activity along the transform fault zone. Earthquakes are
usually shallow due to the lack of subduction processes. Volcanic activity is not present because of the
nature of plate motion. Below is a diagram illustrating the occurrence of a transform boundary and the
location of the SAF:
Transform
Faults
A
Diagram (A) shows typical transform faults located on the
ocean floor. Note that oceanic ridges are offset due to
the “sliding” motion of transform boundaries.
B
Diagram (B) illustrates the geographical location of the
San Andreas Fault zone. Note the arrows indicating
the transform motion along the plate boundary. Will
the Los Angeles Dodgers be city rivals to the San Francisco
Giants someday?
How do plates move?
How does convection work? Convection describes the process in which either hot liquids or
gasses rise from a heat source and cool when away from the heat source. The cooling material
sinks and is again heated by the heat source, rising and then cooling. The repeated process
creates circular motions of energy or convective cells. Below, a diagram illustrates convection in
boiling water.
Boiling Water
(A) Hot water molecules rise to the surface.
(B) Water molecules cool and begin to sink.
(C) Convection cells are created.
B
C
A
Scientists hypothesize that convection processes deep within the earth drive the motion behind
moving continents. The heat source is believed to be generated from leftover heat during Earth’s
formation, radioactive decay of elements, and the effects of the geothermal gradient. Applying
convective motions in the Earth’s interior, heat at the Earth’s lower mantle creates rising plumes of
magma which move toward the surface. Once the plumes reach the upper mantle, they cool,
causing a sinking effect. As the plume sinks, it “drags” the plate, which causes the plate to move.
This is known as mantle convection. Is mantle convection uniform in the mantle? Most scientists
consider convection within the mantle analogous to the characteristics of a lava lamp. Below are
illustrations of mantle convection in an ideal earth and a lava lamp version.
A
B
Diagram (A) shows mantle convection in which magma rises from the core (heat source) and
sinks as it nears the upper mantle. Convection here illustrates a series of homogenous-type
convective cells. Diagram (B) shows a more realistic view of mantle convection. Here
magma plumes rise individually (as magma blobs) at different rates but still maintain the
process of convection. This type of convection is observed in a typical lava lamp.
NOTES:
PART A – Plate Tectonic Definitions
Below are various plate tectonic terms that must be mastered in order to perform successfully on
lab tests as well as lecture tests:
Continental Drift Hypothesis
Permanentists
Contractionists
paleomagnetism
divergent boundary
convergent boundary
subduction zone
ocean trench
transform boundary
San Andreas Fault Zone
convection
mantle convection
lava lamp
PART B- Identification of plate boundaries
In each space provided, draw a diagram that illustrates each plate boundary. In your diagram, label all
pertinent characteristics, color the various characteristics, and identify at least two geographical regions
that represent your plate boundary. Use text information from this lab and other text book sources.
Divergent Boundary
Convergent Boundary
(O-O), (O-C), (C-C)
Transform Boundary
PART C- Plotting Earthquake Locations and Identifying Plate Boundaries
Using longitude and latitude data (next page), plot the various locations of earthquakes. Note the
location and depth of each earthquake. After the earthquakes have been plotted, determine what type
of plate tectonic boundary is present.
PART D – Critical Plate Tectonic Thinking Questions
1. What evidence do scientists observe or look for when proposing the convection hypothesis for the
movement of continents?
2. Investigate what the Ring of Fire represents (not from the Johnny Cash song or a particularly
distressing episode of diarrhea). What tectonic boundaries are involved?
Where is the Ring of Fire located?
3. How is plate tectonic theory used to support the believed age of the earth?
4. Using the steps in the scientific method, how did scientists formulate the theory of plate tectonics? In
other words, what observations led to hypotheses, experiments, and, finally, a theory?
5. What evidence do you think scientists use to determine the motion of the North American and Pacific
plates along the San Andreas Fault?