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Plate Tectonics
The theory of plate tectonics indicates that the Earth’s surface sits on a large floating
island of rock. Although it feels solid and hard beneath our feet, the outer surface of the
Earth is a thin crust of fragile rock, fracture like the cracked shell of an egg. The pieces
of the shell are Earth’s tectonic plates – there are 12 major ones – and they float across a
layer of soft rock like rafts in a stream, their motions driven by forces generated deep in
the Earth. At their boundaries, the plates spread apart, converge, and slide past one
another. Use the following link to read and observe the most common plate boundaries,
www.pbs.org/wnet/savageearth/animations/hellscrust/index.html. Then access this link
to experiment with what happens at each boundary.
www.pbs.org/wgbh/aso/tryit/tectonics/shockwave.html.
Question 1 (As you proceed, answer all questions on a separate sheet of paper.)
What is tectonic plate theory? Identify and describe the major types of plate boundaries
indicating causes and effects.
Most earthquakes happen near the boundaries of tectonic plates, where the plates spread
apart and where they crunch together (although temblors also strike from time to time in
the normally stable interior of continents). Along plate boundaries, the brittle outer part
of the Earth fractures along faults. The infamous San Andreas fault is not a single crack
where the North American and Pacific plates slide past each other. It is the largest of a
thicket of faults that collectively absorb the motion of the plates.
A typical earthquake sends out at least three different types of waves. These waves travel
differently in the Earth and make different patterns on the seismogram, a read-out from a
sensitive detecting instrument called a seismometer measuring tectonic activity. These
are quite useful in detecting the epicenter or the location on the surface of the Earth
directly above the focus of an earthquake.
Following an earthquake, the body waves strike first. The fastest kind is the primary
waves, or P-waves. People often report a sound like a train just before they feel a quake,
which is the P-wave moving as an acoustic wave in the air. Then the secondary, or Swaves, arrive. A person in a building perceives the arrival of S-waves as a sudden
powerful jolt, as if a giant has pounded his fist down on the roof. Finally, the surface
waves strike. In very strong earthquakes, the up-and-down and back-and-forth motions
caused by surface waves can make the ground appear to roll like the surface of the ocean,
and can literally topple buildings over. The L-waves or surface waves usually cause the
most damage. Use the following web sites to observe these waves in a simulation:
www.pbs.org/wnet/savageearth/animations/wave-primary.html
www.pbs.org/wnet/savageearth/animations/wave-secondary.html
www.pbs.org/wnet/savageearth/animations/wave-surface.html
Look carefully over the eight seismograms from the seismic stations around the world at
the following link: http://cse.ssl.berkeley.edu/img/earthquakes/seismogram000.jpeg
Question #2
a) Which location experienced the greatest shock? How can you tell?
b) Which location is probably closest to the epicenter?
c) Which location is the farthest from the epicenter?
Question #3
a) Name the four locations on the seismogram that received all three types of waves (P,
S, and L) and identify the times each of the three waves arrived.
b) Which type of wave is the fastest?
c) Which type of wave is the slowest?
Question #4
At which location did the P and S waves arrive closer together – Davis or Hawaii? What
does this tell you about the location of the epicenter?
Question #5: At some locations not all of the three waves arrived.
a) Which locations appear to have received only P and L waves, but no S waves?
b) Which locations appear to have received only L waves?
Question #6: Some locations that are close to each other did not receive the same waves.
a) Did Los Angeles receive a P-wave? Did Davis?
Question #7: Look at the L-waves for Kingston, Jamaica and Miami, Florida.
a) Which one – Kingston or Miami – is probably closer to the epicenter? How can
you tell from the seismograms?
b) Did Kingston receive a P-wave? Did Miami receive a P-wave?
c) These two cities are not very far from each other on the Earth. Why did the P-wave
arrive in one of these cities but not in the other? Make a good hypothesis as to
why.
Volcanic Eruptions and Weather/Climate Change
Though environmental scientists argue that global warming is a real phenomena causing
significant reaching effects on a variety of global systems, outspoken skeptics argue that
climate change is a fabrication and fallacy. Media personality Rush Limbaugh has
repeatedly asserted his opinion that “natural phenomena such as volcanic eruptions emit
pollutants that dwarf those produced by human activity, thus proving that fears of global
climate change are fabricated.”
More than 80% of the Earth’s surface – above and below sea level – is of volcanic origin.
Gas emissions from volcanic vents over hundreds of millions of years formed the Earth’s
earliest oceans and atmosphere, which supplied the ingredients vital to evolve and sustain
life. Over geologic eons, countless volcanic eruptions have produced mountains,
plateaus, and plains, which subsequent erosion and weathering have sculpted into
majestic landscapes and formed fertile soils. But, although there are hundreds of active
volcanoes worldwide, most erupt intermittently. A few erupt only once every few
decades to once in several centuries. A few massive eruptions have occurred during the
past several thousand years, e.g., Mt. Mazama in Oregon, whose remains make up Crater
Lake National Park. Once every few hundred thousand years, a truly gigantic eruption
occurs, producing deposits such as California’s Bishop Tuff and have profound local and
regional effects on climate and life.
Within the past few centuries, several volcanic eruptions have had well-documented
temporary effects on terrestrial climate. One such event occurred in the late 1880’s, the
eruption of Krakatau in what is presently Indonesia. Such eruptions are so powerful that
they blast vast quantities of ash and gas into the stratosphere (upper atmosphere) above
the zone of atmospheric turbulence. This ash and gas layer stays in the atmosphere for up
to several years and is distributed globally by prevailing high-altitude winds. The ash
and gas reflect the solar radiant energy back to space before it can reach the lower
atmosphere or the planet’s surface, thus resulting in cooling the Earth called the albedo
effect. This cooling may last, as in the case of Mt. Pinatubo in 1991, for several years.
In 1815, the volcano Tambura erupted in the East Indies, producing the so-called “year
without a summer” in parts of the Northern Hemisphere such as New England, where
snow fell in July, and much of Western Europe, where crops failed.
Do volcanic eruptions have long-term effects? Pinatubo’s eruptive volume was estimated
at 9 billion cubic yards (yd3). We will study the effects of the eruption in 1991 to
determine if it played a role in changing the weather patterns or inspiring global climate
change.
Some Useful Conversions
1 yd3 = 0.76 m3
1 ton = 2000 lb
1 metric ton (tonne) = 1000 kg = 2200 lb
1 kg = 1000 g = 2.2 lb
454 g = 1 lb
Question #8
A good average for Mt. Pinatubo’s eruptive density is 2000 kg/m3, consisting of ash,
pumice, and some basalts. How much did the entire eruptive mass weigh?
Question #9
If 1% of this mass was gas, what was the mass of the gas within the eruptive material?
The U.S. Geological Survey estimates CO2 content at 15% of the total gas mass, 84 %
was water vapor, and 1% was trace gases such as heavy metals. What is the approximate
weight of CO2 from the Mt. Pinatubo eruption?
Question #10
The total anthropogenic (human-produced) CO2 is approximately 7700 million tons per
year. What % of this number is the calculated CO2 content of the Mt. Pinatubo eruption?
How many Mt. Pinatubo’s would have to erupt to produce 10% of the total annual
anthropogenic CO2? On the basis of these calculations, do you conclude that volcanic
eruptions dwarf human sources of CO2? Likewise, do they create long-term climatic
changes?
Continental Drift and Plate Tectonics
In 1912, Alfred Wegener proposed that continental drift has provided a mechanism for
the diversification of life on the planet and provides an explanation for the similarities
between species on continents completely separated by oceans. According to the theory
of plate tectonics, large plates under the Earth’s surface drifted away from one another at
a rate of 2 cm/yr and are still moving away from one another. The original supercontinent of Pangaea, gradually broke apart into pieces as the plates drifted, caused by
degassing of the Earth’s hot magma interior. The movement of plates has led to the
further understanding of speciation and biogeography, the geographical distribution of
flora and fauna. Support for Wegener’s hypothesis is found in the location of marsupials
(ex/ kangaroos) as far away from Australia as the United States (ex/ opossum). As well,
lungfish are found in three continents suggested that they rose from a common ancestor
on Pangaea.
Continental Drift Timeline
Era
Time
(mya)
Action
Result
Paleozo
ic
543
Pangaea is a
single
Continent.
Migration and movement of
Species between temperate
& tropical climates.
Mesozo
ic
180
Pangaea begins
to
separate
Gondwanaland (south) and
Laurasia (north)
Cenozo
ic
65
Modern
continents
Formation of 5 continents:
Australia continues to move south.
Question #12
According to the data provided and estimates for the rate of separation, how far had
Gondwanaland and Laurasia separated before they began to break apart? How wide
should the Atlantic Ocean be at its widest point? If the Atlantic Ocean is estimated to be
2,575 km wide at its shortest point according to modern cartographic data, how has the
rate of continental drift changed?