Download CLASS SET - Plate tectonics reading packet

CLASS SET!!!
Continental Drift
Define/answer questions on a page in
your NOTES section.
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 (define) 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 (define) believed that continents and basins are
virtually unchanged and continents remained stationary. Contractionists (define) 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 Arctic, largely as a discredited crack-pot scientist.
However, some scientists supported 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 (define). 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?
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.
A
B
Which magnetic stripe is the
oldest? How do you know?
C
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 (define) 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 (define) 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 (define) 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
(define) 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 90150 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 (draw, give examples).
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
B
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.
Diagram (B) illustrates the geographical location of the
San Andreas Fault zone. Note the arrows indicating
the transform motion along the plate boundary.
Question: Will the Los Angeles Dodgers be
cross-town 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
Question: The same process works for magma
in which two layers of the earth?
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.