Download plate tectonics

(10) Solid Earth. The student knows that plate
tectonics is the global mechanism for major
geologic processes and that heat transfer,
governed by the principles of
thermodynamics, is the driving force. The
student is expected to:
(a) investigate how new conceptual interpretations of
data and innovative geophysical technologies led to
the current theory of plate tectonics;
(b)
describe how heat and rock composition affect
density within Earth's interior and how density
influences the development and motion of Earth's
tectonic plates;
There are many forms of energy in our Universe, only some of
which we have learned about so far. The principles of
thermodynamics govern how and why these energy forms are
transferred.
Law 1: Conservation of Energy.
 The total amount of energy in the universe is constant. This
means that all of the energy has to end up somewhere, either
in the original form or in a different form. Tell one example that
illustrates the first law that we’ve learned about so far.
Law 2: Entropy
 Disorder in the universe always increases. As the disorder
in the universe increases, the energy is transformed into less
usable forms. Thus, the efficiency of any process will always
be less than 100%. Tell one example that illustrates the second
law that we’ve learned about so far.
Law 3: Law of Absolute Zero
All molecular movement stops at a temperature we call absolute
zero, or 0 Kelvin (-273oC). Since temperature is a measure of
molecular movement, there can be no temperature lower than
absolute zero. Since we don’t really focus on this 3rd Law, you really
won’t find an example illustrating this that we’ve learned…however,
you should still be familiar with it!
Zeroth Law:
This law states that if object A is in
thermal equilibrium with object B,
and object B is in thermal
equilibrium with object C, then
object C is also in thermal
equilibrium with object A.
This law allows us to build
thermometers. For example, the
length of a mercury column (object
B) may be used as a measure to
compare the temperatures of the
two other objects.
1. Describe the first Law of Thermodynamics and
tell how it applies to something we have
already learned in Earth Science.
2. Describe the second Law of Thermodynamics
and tell how it applies to something we have
already learned in Earth Science.
What we have learned
before about convection
currents within the mantle,
should help you to make a
connection with the science
of how the Earth’s tectonic
plates move about the
surface.
You can see the hot plumes
here, formed at the mantlecore boundary.
These plumes not only form the Earth’s “hot spots”, but they are
responsible for helping to form the convection currents within the
asthenosphere which drive the plates.
• The current scientific theory that explains the large-scale
movements of the lithospheric plates.
 developed during the early 1900s, and
 later accepted by the majority of the scientific
community when the concepts of seafloor spreading
were developed in the late 1950s and early 1960s
Let’s start at the beginning…with Alfred Wegener
It’s not easy being a scientific genius, as Wegener
found out early on! He was met with:
“Utter, damned rot!”
 “If we are to believe the continental drift
hypothesis, we must forget everything we have
learned in the last 70 years and start all over
again,”
 Anyone who “values his reputation for
scientific sanity” would never dare support
such a theory”
Wegener began as a meteorologist, and
studied climate patterns on Earth, largely
in Greenland and polar regions. It is here
he observed the movement of icebergs
and glaciers.
As a Meteorologist,
• he pioneered the use of balloons to track air
circulation
• became the first to use kites and tethered balloons to
study the polar atmosphere as well.
• Early 20th century geologists viewed continents as fixed features that could rise
and fall, but not move around.
• Slow shrinking of the Earth was considered to be the cause of mountain building.
• Connections of former land bridges and seaways could explain all stratigraphy
and distributions of fossils. This was essentially the Atlantis myth of appearing and
disappearing continents.
Wegener noted several inconsistencies.
3. How do thermodynamics influence plate tectonics?
4. What is plate tectonics theory, and who “piloted” the
notion?
5. What did we think was happening before we knew
about plate tectonics:
• How did continents move or change?
• Mountain building?
• Fossil and other geologic consistencies from one
continent to another?
• Distribution of ancient climatic
indicators does not make sense.
• Close geographic fit of South
America and Africa--like a torn
newspaper.
• Narrow mountain belts restricted
to continental margins.
• Isostasy of crust at two levels-oceanic and continental.
• Distribution of certain fossils, such
as Mesosaurus.
• In 1912 the meteorologist Alfred Wegener
described what he called continental drift, which
spurred a debate that would end up fifty years
later in the theory of plate tectonics.
• Wegener thought that the present continents
once formed a single land mass (which we now call
Pangea) that drifted apart, thus releasing the
continents from the Earth's mantle and likening
them to "icebergs" of low density granite floating
on a sea of denser basalt.
The biggest problem for Wegener was that he had no
convincing mechanism for how the continents might
move. Wegener thought that the continents were
moving through the Earth's crust, like icebreakers
plowing through ice sheets, and that centrifugal
(caused by Earth’s rotation) and tidal forces were
responsible for moving the continents.
6. Describe five inconsistencies that Wegener found
with current theory:
• 1. Climate:
•
2. Fit:
•
3. Mountains:
•
4. Density and isostasy:
•
5. Distribution:
7. Wegener couldn’t come up with a “mechanism” for
his continental drift hypothesis. How did he think the
continents moved around?
During the 1940s, scientists began proposing that convection currents
might have driven the plate movements, and that spreading of the
continents may have occurred below the sea within the oceanic crust.
• One of the first pieces of geophysical evidence that was used to
support the movement of lithospheric plates came from
paleomagnetism.
 This is based on the fact that rocks of different ages show a
variable magnetic field direction. The magnetic north and south
poles reverse through time, and, especially important in
paleotectonic studies, the relative position of the magnetic north
pole varies through time.
 Initially, this phenomenon was explained by what was called
"polar wander" . It was assumed that the north pole location had
been shifting through time, and that the plates and continents
were “fixed”. An alternative explanation, though, was that the
continents had moved relative to the north pole. This, of course,
turned out to be true, and the “Theory of Plate Tectonics” was
born.
The second line of evidence for plate tectonics was our
knowledge of Earth’s interior, and how it could influence the
Pillow Basalt
surface. We already know that the Earth’s interior is made up of
progressively more and more dense material, as you travel
towards the core. It also gets hotter, and hotter. Both of these
facts influence the motion of Earth’s tectonic plates.
You are also aware that the
continental crust (largely made
up of granite) is less dense
than the oceanic crust (largely
When the new crust forms at mid-ocean ridges, it is higher than
made up of basalt).
the surrounding oceanic plate. It forms a mountain. This “height”
In
places
crusttomeet
(subduction
above
thewhere
rest ofoceanic
the seaand
floorcontinental
allows gravity
“push”
on the
zones),
the that
greater
of oldthe
lithosphere
lithosphere
liesdensity
farther from
ridge. relative to the
underlying asthenosphere allows it to sink into the deep mantle at
subduction zones, providing most of the driving force for plate
motions. (Ridge push, Slab pull)
8. Explain one piece of evidence that helped prove Plate
Tectonics Theory.
9. Explain another piece of evidence that helped prove Plate
Tectonics Theory.
10. How does gravity influence plate movements?
• At the oceanic ridges?
• At the subduction zones?
11. Which is the greater influence on tectonic movement, slab
push, or slab pull?
Although subduction is believed to be the strongest force driving
plate motions, it cannot be the only force since there are plates
such as the North American Plate which are moving, yet are
nowhere being subducted.
Three Thoughts:
 Mantle Dynamics
Large scale convection currents in the upper mantle which are transmitted
through the asthenosphere as the main driving force of the tectonic plates
This theory was launched by Arthur Holmes
 Gravity Dynamics
Ridge push, Slab pull. Remember, gravity dictates these two
geological forces. At the mid-ocean ridge, gravity helps to “push”
the plates apart. At convergent zones, gravity “pulls” one plate
beneath the other. Slab pull is stronger than ridge push, by far.
 Rotation Dynamics
Was Wegener right? Does the Earth’s rotation influence tectonic plate
motion at all? There is new scientific evidence that centrifugal force,
which causes Coriolis Effect, influences plate direction.
12. How do mantle dynamics influence plate movement?
13. How do gravity dynamics influence plate movement?
14. How do rotation dynamics influence plate
movement?
In 1971, geophysicist W. Morgan proposed the hypothesis of
mantle plumes.
In this hypothesis, convection in the mantle transports heat from
the core to the Earth's surface in thermal columns, known as
“plumes”.
Mantle plumes carry heat upward in narrow, rising columns, driven
by heat exchange (thermodynamics) across the core-mantle
boundary. (known as the Gutenberg Discontinuity)
Plume
Lithosphere
Mantle
Core
Heat
These “hot spots” occur along
plate boundaries, as well as midplate, such as with the Hawaiian
Islands.
This cycle is nothing more than an “updated” version of the rock
cycle, that includes plate tectonic theory. It was developed by J.
Tuzo Wilson in the late 1950s and 60s.
The problem with the traditional rock
cycle is that it implies that rocks just
cycle endlessly from one to the other,
as you can see from this simplistic
diagram. James Hutton in his Law of
Uniformitarianism envisioned earth
processes cyclically but never getting
anywhere, never evolving.
The Wilson cycle, however, contains
an evolutionary component: that is, it
is not just cyclical, but it is cyclical
with direction.
In the Wilson Cycle, there are 9 stages (or A-I). If you were to
compare the Wilson Cycle Stage A continent with the Wilson
Cycle Stage I continent, you would find stage I is a far more
complex continent containing more felsic igneous rock because
of all the rock evolution taking place in the Wilson cycle.
Stage A
The Wilson Cycle simply depicts the rocks on planet Earth as
having gone through a series of changes, and evolutions, and not
a simple cycle, and describes the movement and formation of
these rocks in terms of plate tectonics.
Stage I
15. What are hot plumes, and how do they form?
16. What name is given to the boundary where hot plumes form?
17. Why is Hawaii (the big Island) the only Island within the
Hawaiian chain to maintain an active volcano?
18. How is the rock cycle different from the Wilson Cycle?
19. How might a stage A continent differ from a stage I continent in
the Wilson Cycle?
20. What is a “felsic” rock?