Transcript Science 3360 - Kennesaw State University College of Science and

Science 3360
Lecture 16: Plate Tectonics
A look at the historical development
of the theory of plate tectonics
Questions to Answer
Two lectures ago we ended our discussion of the Rock Cycle with some
nagging questions that need to be answered…
•How does sedimentary material get transported to the deep interior
where it can be metamorphized?
•What causes uplift to move the igneous and metamorphic rocks to the
surface to be weathered?
•Where does the energy come from to drive this cycle?
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Aside: Scientific Laws
Theory and hypothesis are both often used colloquially to
mean an untested idea or opinion.
According to the American College Dictionary:
•Hypothesis: “a proposition or conjecture proposed as an
explanation for the occurrence of some specified group of
phenomenon.”
•Often serves as the basis of argument or
experimentation by which to reach the truth
•Theory: “a coherent group of general propositions used
as principles of explanation for a class of phenomenon
•Law: “well established propositions that are regarded as
reporting matters of fact
Plate Tectonics
In the past two lectures we reviewed the basic properties
of the solid earth and the minerals and rocks that we find
on its surface. The rocks hint at the existence of a very
dynamic earth. In this lecture we begin our discussion of
Plate Tectonics
Plate Tectonics represents a unified theory of the solid earth
that explains many of the puzzles that we identified in
previous lectures.
But first we need to define two new regions…
The Lithosphere and Asthenosphere
The crust and upper mantle form 2 distinct regions:
• The Lithosphere (comprising the first 70 - 125 km of the solid earth)
consists of the oceanic and continental crustal material plus the
uppermost portion of the mantle. It is rigid and acts as a single unit.
• the Asthenosphere ( ~ 100’s km in depth) is characterized by low Pwave velocities. Because of these low velocities, geologists have
deduced that this layer is plastic (i.e. not a liquid, but not rigid either)
due to it probably being close to its melting point. As a result it:
– provides source of material
ejected in volcanoes
– accounts for “rebound”
of crust following ice ages
– provides malleable
layer upon which
plates can flow
Plate Tectonics: The Origin
We start our discussion with the life of Alfred Wegener.
Alfred Wegener (1880 - 1930)
1880 – Born
1904 - Earned PhD in astronomy from University of Berlin
 1906 - Joined expedition to Greenland to study polar air circulation; took
position at University of Marburg
1911 - Began to formulate his theory of ”continental drift”
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1912-13 - joined another expedition to Greenland
1914 - Drafted into the German army, but was released from combat duty
after being wounded, and served out the war in the Army weather forecasting
service.
1915 - 1st edition of The Origin of Continents and Oceans published
1924 - Took professorship in meteorology and geophysics at the University
of Graz, in Austria.
1930 - While returning from a rescue expedition that brought food to a party
of his colleagues camped in the middle of the Greenland icecap, Wegener
died, a day or two after his fiftieth birthday.
Acceptance of Wegener’s theory by the scientific community would
not come for decades.
The Continental Drift Theory
In 1908, Wegener read a paper on the similarities of fossils in North
America and Europe. From this paper, Wegener pieced together
evidence from a broad range of the Earth sciences to hypothesize
that:
All of the present-day continents were once part of a single
supercontinent called Pangaea
About 200 - 300 million years ago Pangaea had rifted, or split, and
its pieces had been moving away from each other ever since
The Continental Drift Theory
Evidence upon which Wegener built his hypothesis
1. The close fit between the coastlines of Africa and South America –
actually fit is even better when using the continental shelf edges (i.e.,
the shoreline has been deformed by erosion).
2. Glacial features from essentially the same time period appear in
South America, Africa, India, and Australia. This is perhaps is best
explained if these continents were once connected.
The Continental Drift Theory
More of the evidence upon which Wegener built his hypothesis.
3. Fossils of identical plants and animals found on all continents. This had
been explained at the time by postulating that land bridges, now sunken,
had once connected the continents.
Glossopteris: grew in
temperate, humid areas
during Permian Period
(245 – 285 million ybp.
But fossil remains are
found in rocks in regions
with current climates
ranging from tropical to
polar.
The Continental Drift Theory
More of the evidence upon which Wegener built his
hypothesis.
4. Large-scale geological features on different
continents matched when the continents were
brought together (e.g., the Appalachian mountains
in North America and those of the Scottish
Highlands; the distinctive rock strata of the Karroo
system in South Africa and those of the Santa
Catarina in Brazil).
5. Fossils found in some places do not match the
current climates:
Glossopteri (see previous page)
Australian dinosaur fossils in Dinosaur
Cove;
Fossils of tropical plants, such as ferns
and cycads found on the Arctic island of
Spitsbergen.
The Continental Drift Theory
Wegener constructed Pangaea by using the location of specific rock types
to determine the distribution of climate zones in the geologic past.
•Glacials from tills and striations were linked to polar climates
•Sand Dunes were linked to deserts
•Coral Reefs were linked to the tropics
The resulting distribution is quite different from today. There are two
explanations of this:
1. The poles have wandered or
2. The continents have drifted
The Continental Drift Theory
The motion of the continents. The white numbers
represent millions of years from present.
From the paleomap project
The Continental Drift Theory
Wegener’s theory was not without problems or criticism.
The main weakness was the lack of a viable mechanism for the “drifting
continents”.
Continents would have
become deformed by
the motion plowing
through the ocean
crust. We would expect
to see large mountains
along each coast which
is not the case.
The Continental Drift Theory
In 1928, Arthur Holmes proposed that convection currents – which
bring hot magma (actually rock) up from the deep mantle and cooler
rock downward - could drive continental motion.
In the 1940's, sophisticated
mapping of the seafloor during
the WWII revealed a
complicated chain of subsea
volcanic mountains along the
center of ocean basins forming a
system of Mid-ocean ridges that
appear as dark seams on the
figure
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The Continental Drift Theory
In the 1950’s, seismologists showed
that the mid-ocean ridges were also an
active seismic belt (or zone of
earthquakes). It was proposed that the
seismic belt corresponded to a trough,
or rift, system running down the center
of the ridges. The rifts are about 20
miles (30 km) wide and 6,500 feet
(2,000 m) deep. In all, the oceanic
ridges and their rifts extend for more
than 37,500 miles (60,000 km) in all
the world’s oceans. The mid-ocean
ridge system represents the largest
single volcanic feature on the Earth.
The Continental Drift Theory
The Deep Sea Drilling Project (DSDP) was conceived in the mid-1960s
but active drilling did not begin until 1968. The goal of the program was
to investigate the evolution of the ocean basins by core drilling the ocean
sediments and the underlying oceanic crust.
Drilling operations were carried out aboard the Glomar Challenger, a
120m long ship built for the DSDP program. The Glomar Challenger
was capable of drilling 2,500ft of sediment in 20,000 feet of water.
Tracks of the Glomar Challenger
from 1968-1975.
The Continental Drift Theory
The DSDP revealed two very important pieces of information that
would prove vital to the development of the theory of plate tectonics:
1. The ocean floor is geologically young (no older than ~180 million
year)
2. The youngest rocks are near the central ridge and get older as you
move away from the mid-ocean ridges.
Sea Floor Spreading
In 1962 Harry Hess and Robert Dietz independently developed the
theory of sea-floor spreading.
Their theories were based on the age variation seen on the previous
slide and the fact that the ocean floor is basaltic which differs from
continental rocks which are granitic What is the main difference
between basaltic rocks and granitic rocks?
The theory of sea floor spreading says:
•Magma comes to the ocean floor at mid-ocean ridges
•The magma erupts at sub-sea volcanoes and produces new basaltic
crust
•The basaltic crust slowly spreads causing the ocean floor to move
away from the central ridges
•When the spreading ocean floor reaches a continent, it plunges
underneath the continent (subduction) Why? Granitic material is
less dense than basaltic material (more on this with isostasy)
•New mountains are formed along the boundaries. More on this
later.
Isostasy
Ocean crust is composed primarily of basalt and
has a density of 2.9 g/cm3.
It’s relatively thin, about 5-7km.
Continental crust is composed primarily of granite
and has a density of 2.7 g/cm3. It’s relatively thick,
about 25-50km.
Granite is less dense than basalt so continental
crust tends to “float” above oceanic crust.
Continental crust is thickest beneath high mountain
ranges and thinnest beneath lowlands.
The lithosphere is supported by the asthenosphere.
The amount of athenosphere displaced is equal to
the mass of the overlying object
Isostasy
We know that the amount of the
athenosphere that is displaced is equal to
the mass of the overlying object. Think of
a placing a heavy ball onto a large
balloon. What happens?
Thick continental crust must have deep “root” below to support its weight.
Some areas of continental lithosphere are not in isostatic equilibrium
because the mass in that area has recently and abruptly changed.
Isostatic equilibrium (or isostasy) refers to the balance between the
gravitation downward pull and the upward buoyant force provided by the
athenosphere.
As continents erode, the base of granitic crust rises. Why? To maintain
isostatic equilibrium. Does this make sense?
Isostasy
As we saw earlier in the semester, we can calculate the buoyant force by
using Archimedes’ Principle. A.P. states that the buoyant force on an object
is equal to the weight of the fluid it displaces. It can be shown that the same
percentage of an object will always be below the surface of the fluid.
So a larger percentage
of the heavier blocks
remain under the
surface.
Images from geology.ohio-state.edu
Same percentage of the
blocks remain below the
water because all the
same density.
If we have a large
amount above, we
have to have a large
amount below
Isostasy
What happens as a mountain range is eroded?
The land will rise to compensate for the removal of the eroded material.
This is to maintain isostatic equilibrium. This means that some of the
rock that is visible may have spent much of its history under the ground.
Using this information, how was Stone Mountain formed?
Stone Mountain is granite, an intrusive metamorphic rock. It formed
under the ground about 350million year ago. There are a few theories as
to how it became exposed but the most accepted one appears to be as
follows. Surface erosion caused the underlying material (which SM was
a part of, along with metamorphic rock) to be lifted. Erosion caused the
metamorphic rock to weather away, leaving the tougher, more erosion
resistant granite of Stone Mountain visible.
Sea Floor Spreading
Support for sea-floor spreading theory came from a study of the magnetic
polarity of the basaltic material of the seafloor.
Background: the earth’s magnetic polarity (i.e., which pole is north and which is
south) has flipped many times in earth’s history. As lava cools and forms rocks, the
material becomes magnetized in the direction of the earth’s magnetic pole at that
time and remains that way. So the magnetic orientation of igneous rock tells
us the polarity of earth’s magnetic field at the time the rock was formed.
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Sea Floor Spreading
In 1963, Vine and Mathews explained the pattern of magnetic anomalies
in the oceanic crust. They proposed that the lava erupted at different
times along the mid-ocean ridge and preserved the record of magnetic
reversals. The creating of these magnetic anomalies requires sea floor
spreading.
Almost There…
Before we can have a fully functional theory of the dynamic Earth,
we still have to answer a few questions.
Why don’t the continents just break apart?
and
Where does the old seafloor go when it meets the continents?
The Theory of Plates introduced by Dietz, McKenzie, and Parker in
1968 will wrap help. The theory of plates combines sea-floor
spreading with the theory of continental drift.
1. Earth’s outer shell (the lithosphere) consists of rigid plates
2. The plates slide on the softer athenosphere
3. As the plates move, they carry both the continents and the
oceans
4. When continents and oceans meet, a deep valley is formed and
the ocean floor “subducts” beneath the continent.
Plate Tectonics
• Continents do not plow through oceanic crust. Instead
continents and oceanic crust are considered to be part of the
plates that move on the plastic asthenosphere.
• A driving force, convective currents, moves the plates.
• Seafloor is constantly being regenerated so it is younger than
continental crust
• Seafloor moves toward continental boundaries where it
subducts beneath the continents and melts, forming magma.
Thus the oceans don’t fill up with sediment.
• Sediments melted and then turned into igneous rock beneath
the continents. Rocks are slowly pushed upward, where they
can become exposed and weather. Thus the system does
not run out of nutrients.