Download A Continental Drift Theory

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Lesson 1.1A
A Continental Drift Theory
Overview
Since the European discovery of the Americas over 600 years ago,
explorers and scientists have observed many curious land formations.
From newly drawn maps, they noticed similar coastline shapes of
certain continental landmasses. Those travelling to the new world saw
similar mountain formations to those found in northern Europe. They
saw features in warm equatorial locations that looked very similar to
features seen in ice-covered regions such as Greenland and Antarctica.
These and other discoveries puzzled scientists at the time. Why were
features on such distant lands so similar? And why were some features
found in places where they didn't seem to belong?
The first person who attempted to make sense of it all was a young
German scientist called Alfred Wegener, and he did so in the early part
of the 20th century. In this lesson, you will learn about his new idea
about continental movement. You will also read about the evidence he
used to support his radical theory. Finally, you will discover the reasons
why most scientists at the time disagreed with his ideas.
Resource List
• Science 10 Website
http://www.openschool.bc.ca/courses/science /sc10/mod1.html
• Science 10 Media CD
• Science 10 Data Booklet
http://www.openschool.bc.ca/courses/science /sc10/index.html
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The Birth of a Theory
Thanks to exploration, the first world maps were created over 350 years
ago. Explorers and scientists who read these maps noticed something
odd. They noticed that the edges of some continents and landmasses
seem as though they could 'fit' together. A good example is the
coastline of South America, which looks as though it could neatly fit
into the West African coastline.
While similar coastlines do seem to fit together like a jigsaw puzzle, no
one seriously believed that large landmasses could move around on the
Earth's surface—no one that is, until scientist Alfred Wegener, in the
early 20th century.
Evidence Supporting the Theory of Continental Drift
Scientists are curious people. They always try to explain what they see
happening around them. If they can provide lots of proof for their
explanation, then their explanation becomes a theory. On the other
hand, if there is enough evidence to show the explanation is wrong,
the theory is thrown out.
A theory, then, is an explanation that has been tested, and has lasted
for a long time. A theory requires supporting evidence; Wegener found
lots of evidence to support continental drift.
The Fit of Continental Coastlines — Wegener used the
jigsaw puzzle fit between the South American and
African coastlines as his first piece of evidence to
support continental drift. He didn't believe that these
“pieces” would be so well matched-that is, not unless
they had actually once been connected.
Similar Mountain Ranges and Rock Sequences —
Explorers quickly discovered that distant continents
contained rock of similar ages and features. These
findings seemed to show that the continents may not
have always been separated as they are now. For
example, this is what they found when they looked at
the Appalachian Mountain Range of North America:
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• This range stretches northward from the
eastern United States up into the Atlantic
provinces of eastern Canada. There, it seems
to suddenly stop at the island of
Newfoundland.
• Very similar mountain ranges of the same age
and rock-type also appear in eastern
Greenland, Ireland, Great Britain, and
Norway.When these landmasses are placed
together, the mountains form a single long
range, as shown in the following image.
Fossil Evidence — A fossil is any evidence of ancient life.
In the beginning of the 20h Century, fossil evidence
was also found to support continental drift. Identical
fossilized plant and animal species have been found
in many different places, on different continents. It
seems hard to believe that such similar organisms
would exist so far away from each other, or that they
could have swam from one continent to another. It is
more likely that these life forms once lived all
together on a single continent, as shown in the
following image.
Credit: Fossil Evidence for Continental Drift, USGS, United
States Geological Survey
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Lesson 1.2D
Tying It All Together
Overview
You will recall from previous lessons that there are three types of plate
boundaries: convergent, divergent and transform. Certain types of
volcanoes form at convergent boundaries. Other types form at
divergent boundaries. You will also recall that the edges of the Pacific
plate are termed the Ring of Fire for a very good reason - this is where
the vast majority of volcanoes are located.
However, as you will see, volcanoes can form in places other than plate
boundaries. Does this knowledge disprove plate tectonic theory? Not at
all, as you will soon see. In fact, such features add to our knowledge of
earthquake, mountain range, and volcano distribution patterns, and
help to prove and explain tectonic processes.
As you read through this last lesson, think about all the information
you have learned that provides compelling evidence for Plate Tectonic
Theory.
Resource List
• Science 10 Website
http://www.openschool.bc.ca/courses/science/sc10/mod1.html
• Science 10 Media CD
• Google Earth
Http://www.google.com
Volcanoes and Subduction Zones
As you probably know by now, the location of volcanoes around the
world tells a lot about how plates interact. The position of volcanic
island arc chains beside deep-sea trenches tells us the following:
• Trenches mark the location of oceanic-oceanic convergent
plate boundaries.
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• Subduction and melting occur at these trenches.
• The plate containing the island arcs must be overriding the
other plate, which in turn is being subducted and destroyed.
At the same time, volcanic mountain ranges that run along the edges
of continents, with deep-sea trenches following along just offshore, tell
us another part of the story:
• Trenches also mark the location of oceanic-continental
convergent plate boundaries.
• Subduction and melting occur at these trenches, too.
• The oceanic plate must be subducting and melting beneath
the continental plate in order to create the volcanoes seen on
these continents.
As you can see, the processes that create these volcanic features are very
similar. It should not surprise you to learn that all volcanoes formed at
subduction zones erupt in much the same way. The magma that forms
these volcanoes contains a large number of gases. These gases make the
molten rock very explosive. When these volcanoes erupt, they can
create relatively “quiet” flows of lava. Or, they can produce explosive
pyroclastic eruptions of hot ash and gases. Such volcanoes are referred
to as composite volcanoes because of the different ways in which they
erupt. Composite volcanoes are also sometimes called strato-volcanoes.
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Here is a picture of
Mt. St. Helens, a
composite volcano in the Cascade Mountains of
western North
America, before
1980.
Credit: USGS; United States Geological Survey, Photo
by D.R. Mullineaux
Here is the same
volcano during a
massive pyroclastic eruption on
May 18, 1980…
Credit: USGS; United States Geological Survey, Photo
by Donald A. Swanson
…and this is how
the volcano now
appears, after the
eruption. Ash
from this eruption
was discovered as
far away as
Europe!
Credit: USGS; United States Geological Survey, Photo by
Gene Iwatsubo
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If you are interested and are able to do so, go to Google Earth (Http://
www.google.com) and type in 'Mt. St. Helens'. By tilting the view, you
can take a fantastic 3-D tour of the volcano and look inside the blast
zone.
Volcanoes and Spreading Ridges
Volcanism is not limited to subduction zones. Recall that spreading
ridges form due to hot, rising magma. As the magma cools and
hardens, it pushes old plate material aside to create new plate material
at a divergent boundary. The spreading process causes rift valleys to
form in the middle of these ridges.
Sometimes at these boundaries, large amounts of magma flow out of
these rift valleys. Over time, many eruptions build up the magma. As
this process continues, an island rises above sea level. Volcanoes will
appear on this island. However, since the magma is actually flowing
from a rift rather than a vent, sometimes rift eruptions also occur on
such an island. An example of a rift eruption is shown here.
Credit: USGS; US Geological Survey, Photo by D.A. Clague
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Lesson 2.5A
Atoms and Isotopes
Overview
You know that atoms contain protons, neutrons, and electrons. We’ve
learned a lot about the arrangement of electrons in atoms and
molecules in order to understand chemical reactions. In chemical
reactions, like the ones we’ve studied so far, the nuclei of the atoms
involved don’t change at all. Are there any reactions that involve a
change in the nucleus?
In this lesson we’ll introduce the topic of radioactive decay. Let’s start
with a brief review of atoms and isotopes.
Resource List
• Science 10 Media CD (Radioactive Decay)
Atoms and Isotopes
The nucleus of an atom contains protons and neutrons. Each atom has
a specific number of protons, which is denoted by its atomic number.
Many nuclei also contain neutrons, which have about the same mass
as protons, but without a charge. Two atoms that have the same
number of protons and electrons, but a different number of neutrons,
are called isotopes.
The mass number is the number of protons plus the number of
neutrons in the nucleus. Thus, two atoms with the same atomic
number and different mass numbers are called isotopes. The example
on the next page shows two isotopes of hydrogen: hydrogen-1 and
hydrogen-2 (deuterium).
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Nuclear Notations
Number of Protons
1
1
Number of Electrons
1
1
Number of Neutrons
0
1
Atomic Number
1
1
Mass Number
1
2
For a demonstration of these concepts,
:
Science 10 Media CD > Module 2 > Radioactive Decay > What is
Radioactivity?
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Guided Practice 2.5A 1
Reviewing Atoms and Isotopes
Before moving to the next topic, take some time to practice finding the
numbers of protons, electrons, and neutrons in some different
isotopes. Complete the following chart:
Isotope
Symbol
Uranium-235
Number of
Number of
Number of
Protons
Electrons
Neutrons
92
92
143
Carbon-14
Carbon-12
Plutonium-239
Aluminum-27
Alpha, Beta, and Gamma Radiation
Much of chemistry involves stable atoms, but some atoms in nature are
unstable. The nuclei of such atoms decay, spitting out particles and/or
energy. We call these unstable nuclei radioactive. Radioactive nuclei,
when they decay, emit a form of energy called radiation. When a
radioactive atom emits radiation from its nucleus, it is doing so in order
to achieve its stable form. This process is called radioactive decay.
All atoms having an atomic number greater than 83 are radioactive.
Some atoms with a lower atomic number can also be radioactive. One
example is carbon-14. (We’ll look at carbon-14 again in a later lesson,
when we talk about radioactive dating.)
There are three ways in which radioactive decay can occur. Radioactive
nuclei may emit alpha particles, beta particles, or gamma rays.
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Alpha Particles (α)
Alpha particles, denoted by the Greek letter a, are the same as helium
nuclei. They consist of two protons and two neutrons, and they carry
a charge of +2. You may also see the symbol written as
or
.
Of the three types of radiation, alpha particles have the lowest
penetrating power. In other words, alpha particles cannot travel very
far, and they are easily stopped with a single piece of paper. In fact,
alpha particles rarely penetrate even the dead cells covering your skin!
Ionization smoke detectors use radiation to detect smoke in the air. The
radiation source used is americium-241, an alpha emitter. The alpha
particles, which carry an electric charge, complete an electric circuit in
the smoke detector. When smoke is present in the air, the electric
current is interrupted and the alarm is activated.
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Beta Particles (β)
Sometimes, a neutron in the nucleus of an atom may split into a proton
and a high-energy electron. The high-speed electron that is emitted
from the nucleus is called a beta particle, denoted by the Greek
letter b. You’ll recall that electrons have very small mass and carry a
charge of -1. Beta particles are no different from electrons; the symbol,
, is used to represent a beta particle.
Beta particles are much smaller and faster than alpha particles, giving
them greater penetrating power than alpha particles. They can travel
up to a meter in the air, and can penetrate body tissue. For this reason,
beta emitters can be harmful, especially if they enter the body. Beta
particles can be easily stopped by a sheet of plastic or metal.
Gamma Rays (γ)
Unlike alpha and beta radiation, gamma rays are waves, not particles.
They have no mass and carry no charge. Gamma rays are a type of
electromagnetic radiation (energy that travels in waves). Some types
of electromagnetic radiation are shown in the diagram below.
Long wavelength
Low frequency
Low energy
Radio
Microwave
Short wavelength
High frequency
High energy
Infrared
Red light
Visible
Ultraviolet
X-ray
Violet light
Gamma Ray
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From the diagram, you should notice that, compared to other types of
electromagnetic radiation, gamma rays have very high energy, high
frequency, and a short wavelength. We use the Greek letter g to
represent gamma rays.
Gamma rays have a very high penetrating power and require a dense
substance, such as lead, or thick concrete, to stop them. In fact, lead
can be used as a radiation shield. You may have worn a lead apron if
you have ever had a dental X-ray. Gamma rays are even more powerful
than X-rays and they are very harmful to living things.
The image below summarizes the penetrating power of the three types
of radiation.
How to Spot Radiation
Since we cannot see alpha, beta particles, or gamma rays, how do we
know if a substance is radioactive? The most commonly used radiation
detector is called a Geiger counter (shown on the next page).
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Lesson 3.3B
Human Effects on Climate
Overview
We've seen in the previous lesson that natural causes can have
significant effects on weather and climate. However, there has been
much talk in the news lately about what humans are doing to the
environment and how our actions are affecting global climate and
weather.
In this lesson we will take a critical look at some of the evidence
indicating that human activity is causing our climate to change faster
than ever before
The Greenhouse Effect
The atmosphere has trace amounts of carbon dioxide gas in it.
Although there are only very small amounts, it plays a very important
role in maintaining the Earth's surface temperature. To help us
understand how, let's look at the example of a greenhouse.
The Greenhouse Effect
Visible energy from the sun
passes through the glass
and heats the ground.
Radiating thermal energy
from the ground is partly
reflected by the glass and
some is trapped inside
the greenhouse.
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You can see that the glass of the greenhouse traps some of the thermal
energy radiating from the ground, causing the air in the greenhouse to
be warmer than the air outside. A similar effect is found in the
atmosphere:
Greenhouse Effect in the Atmosphere
Source: UNEP/GRID-Arendal. Cartographer: Phllippe Rekacewicz. Reproduced
with permission of UNEP/GRID-Arendal.
Carbon dioxide in the atmosphere acts much like the glass of a
greenhouse, trapping thermal energy, and keeping the Earth warmer
than it otherwise would be. The greenhouse effect is necessary for our
survival on Earth. Without it, temperatures on Earth would be too cold
for most life forms to survive.
The greenhouse effect only becomes a problem when too much thermal
energy is trapped in the atmosphere. So, what does this have to do with
human activity?
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Burning Fossil Fuels
For the last 200 years or so, humans have been burning fossil fuels
(coal, oil, natural gas) to provide energy to heat our homes, drive our
cars, and make electricity. We have become very dependent on this
energy (try to imagine life without cars or buses for example). The
problem lies in what happens to the fossil fuel after we burn it.
One of the products of burning a fossil fuel is carbon dioxide gas. Every
time you run your car, you add carbon dioxide to the atmosphere. In
small amounts this might not be a big deal. Over time, and with the
increasing use of fossil fuels, the layer of carbon dioxide in the
atmosphere builds up. As carbon dioxide builds up in the atmosphere,
more and more energy is reflected back to Earth. When this happens,
the Earth's temperature starts to increase.
An increase in the Earth's average surface temperature is what scientists
refer to as global warming. Global warming is one example of
climate change.
Deforestation
Deforestation is another human activity that affects climate.
Trees and other plants absorb carbon dioxide from the atmosphere.
Through the process of photosynthesis, plants turn carbon dioxide
into food (glucose).
Around the world, humans cut down a lot of trees for a variety of
reasons. On a small scale, one tree is probably not going to make that
big a difference to the amount of carbon dioxide in the atmosphere.
However, when a whole forest is cut down it starts to add up.
When you look at deforestation on a global scale, the effects on climate
become very apparent. For example it is estimated that one tree will
absorb 730 kg of carbon dioxide over its lifetime. Over the last 10 years
the average timber harvest in BC has been 77,000,000 m3. That's a lot
of trees! And that's a lot of carbon dioxide that will not be absorbed.
Forests also release water vapour into the atmosphere increasing cloud
formation. This helps keep the Earth cool by increasing the amount of
solar radiation that is reflected back into space.
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You can begin to see how humans are making an impact on climate.
Not only are we adding carbon dioxide to the atmosphere by burning
fossil fuels, but we are also getting rid of things that can help remove
that excess carbon dioxide.
A Fine Balance
Anything that causes an increase in the concentration of greenhouse
gases in the atmosphere will disturb the Earth's energy balance to some
degree. Changes in the amount of thermal energy that is trapped in the
atmosphere influence global climate. Some events have more of an
effect than others.
The Earth's energy balance is a complex system. Natural phenomena
like volcanoes and forest fires increase the amount of carbon dioxide in
the air. However, these same phenomena also increase the Earth's
albedo temporarily, increasing the amount of energy that is reflected
back into space. It can be hard to consider all of the possible effects of
any one event.
Human activities are no less complex. Everything we do affects the
Earth in some way. We have examined a few human activities that
influence climate. Remember, this only touches the surface of the
many influences humans have on the environment. Human actions
change the Earth's thermal energy balance in many ways. It is
important to look at various impacts of our actions to get a big picture
of how we are affecting the global climate.
Note:
Complete the following guided practice activity, then do the
Section 3.3 Assignment Part A: Global Warming Matching Quiz.