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MODULE 1 ■ SCIENCE 10 7 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 8 MODULE 1 ■ SCIENCE 10 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: MODULE 1 ■ SCIENCE 10 • 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 9 96 MODULE 1 ■ SCIENCE 10 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. MODULE 1 ■ SCIENCE 10 97 • 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. 98 MODULE 1 ■ SCIENCE 10 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 MODULE 1 ■ SCIENCE 10 99 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 MODULE 2 J SCIENCE 10 187 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). 188 J MODULE 2 SCIENCE 10 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? MODULE 2 J SCIENCE 10 189 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. 190 MODULE 2 J SCIENCE 10 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. MODULE 2 J SCIENCE 10 191 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 192 MODULE 2 J SCIENCE 10 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). MODULE 3 ■ SCIENCE 10 81 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. 82 MODULE 3 ■ SCIENCE 10 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? MODULE 3 ■ SCIENCE 10 83 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. 84 MODULE 3 ■ SCIENCE 10 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.