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Transcript
Anna Cajiga SME 301 Geology Summary Corrections April 12, 2005 Title Make Your Own Fossils! Benchmark Area: Earth Science Category: Geosphere Benchmark: Explain how rocks are formed (Middle School). Misconception Fossils are actual preserved animal or plant parts. Background There is a common misconception that fossils are all preserved animal or plant parts. However, this activity illustrates that many fossils are in fact molds or casts of the original plant or animal. Although some fossils are preserved animal or plant parts, such as petrified wood, there are also other types of fossils. When a living organism falls into mud and leaves an impression, the sediments in the mud can become compacted and cemented, forming a sedimentary rock that still has the impression of the organism in it. This is known as a mold fossil. Sometimes, this impression can be filled into with sediments that eventually become compacted and cemented to form a sedimentary rock that has the same shape as the living organism. This is called a cast fossil. In both cases, the fossil that remains is not the preserved plant or animal, but instead a cast or mold of it. Materials An example of a real mold fossil An example of a real cast fossil Clay Shell Two paper cups Plaster of Paris Scissors Water Directions 1. Flatten a piece of clay until it looks like a thick pancake. 2. Press the shell into the clay and then remove it. 3. Observe the clay and compare it to the example of a real mold fossil. 4. Cut the bottom off of a small paper cup to make a collar. 5. Put the collar around the mold fossil you just created. Pour about half an inch of water into another paper cup. Slowly add plaster of Paris and stir until it has the consistency of thick soup. 6. Pour the plaster of Paris into the collar on the mold fossil. Allow it to dry for 24 hours, then remove the dry plaster of Paris cast from the mold. 7. Observe the cast and compare it to the example of a real cast fossil. Results At the conclusion of this experiment, students will have one mold “fossil” where they can clearly observe the imprint of the shell in the clay. They will also have one cast “fossil” in which they can see that the plaster of Paris has formed a shape that is identical to the original shell. Discussion There are three types of rocks in existence – sedimentary, igneous and metamorphic. Sedimentary rocks form when weathering of rocks and landforms creates sediments on the surface of the earth. When sediments pile up on top of each other, the pressure from the sediments on top presses the sediments on the bottom together. Minerals, usually calcium carbonate, dissolve in the water and become like cement that “glues” the sediments together. This is called lithification. When sand sediments are compacted and cemented, sandstone is formed. Silt and clay sediments can go through lithification and become shale. When shells, or calcium carbonate, are cemented and compacted, limestone is formed. You can often see layers in sedimentary rocks since they are formed from different sediments building up over time. Fossils are also found in sedimentary rocks because sedimentary rocks form at the earth’s surface, which is where living organisms are found. Mold fossils and cast fossils are two types of fossils. Mold fossils are formed when a living organism falls into mud and leaves an impression in it. In this activity, mold fossils were illustrated by pressing a shell, which represents a living organism, into the clay, which represents mud. In the real world, after an organism falls into mud, the mud can harden into rock over time so the impression of the animal is left in the solid rock. A cast fossil is formed when a mold fossil is filled in with sediments over time. These sediments can then be compacted and cemented to form a sedimentary rock that has the exact same shape as the original living organism. In this activity, the impression of the shell in the clay represents the mold fossil, and the plaster of Paris represents the cast fossil that was created as sediments built up over time and became compacted into a sedimentary rock. Sedimentary rocks are not the only types of rocks. After a sedimentary rock has formed, it can either be weathered back into sediments, or it can sink below ground through cracks in the earth’s surface. Once underground, one of two things can happen to the rock. Heat and pressure can be applied to the rock until a chemical change takes place and a new material is formed. This new material would be a metamorphic rock, which are formed when pre-existing rocks undergo pressure and heat. This process is called metamorphism. When sandstone becomes a metamorphic rock, it is called quartzite. Shale becomes slate when it is put under intense pressure and heat, and limestone becomes marble. After their creation, metamorphic rocks can undergo weathering and turn into sediments. The other option that both metamorphic rocks and sedimentary rocks can undergo is sinking underground through cracks in the earth’s crust and eventually melting into magma in the hot mantle of the earth. This will result in the creation of igneous rocks. Like the formation of metamorphic rocks, the creation of igneous rocks occurs underground. If rocks melt into magma, they will eventually cool again, forming igneous rocks. Igneous rocks can either cool slowly underground, called intrusive igneous rocks, or quickly above ground, called extrusive igneous rocks. When the rocks cool quickly, the crystals in them are too small to be seen with the naked eye. When they cool slowly, they have time to form large crystals that can be seen with the naked eye. Granite, basalt, pumice and obsidian are all examples of igneous rocks. Granite has large crystals and basalt has small crystals. When pumice forms, gasses get trapped in it and form holes. Igneous rocks can then be melted again and undergo metamorphism to change into a metamorphic rock. Igneous rocks can also be weathered into sediments and then undergo lithification to be formed into a sedimentary rock. This whole process of one type of rock having the ability to become any other type of rock is known as the rock cycle. It becomes clear that, as the Law of Conservation of Matter states, the matter on earth is never created or destroyed, just changed. One type of rock can eventually become another type of rock. All sediments on the earth will also eventually be turned into new rocks. There are many ways for the sediments that become rocks to come about in the first place. To understand this, we must first understand landforms and the various materials that cover the earth’s surface. The earth’s surface does not look the same everywhere. One reason for this is because of landforms. Landforms are the natural physical features of the earth’s surface. For example, some areas are called plains. Plains are large areas of level land that is low in elevation, meaning no higher than 1000 feet above sea level. Other areas are plateaus, or large areas of level land that are high in elevation, meaning higher than 1000 feet above sea level. Plateaus are surrounded in steep cliffs. Mountains are land that rises steeply and comes to very narrow tops. Highlands are similar to mountains in that they are elevated land, but highlands are not as high or steep as mountains. Basin and range is another landform. Basin and range is lowlying lands surrounded by higher ground on all sides. Landforms are not the only way that the surface of the earth varies. The earth’s surface is also covered in many different kinds of materials. One of these materials is soil, which is made of rock sediments, living and dead organisms, water, and air. Some places on earth are just covered in sand, or small pieces of rock sediments. Other kinds of rock sediments that can be found on the earth are, from smallest to largest, clay, silt, pebbles, cobbles and boulders. All of these landforms and different materials on the earth’s surface are connected to each other. The landforms can be broken down into smaller pieces, and these smaller pieces are in turn broken down into even smaller pieces. This process is called weathering, which is the breakdown of rocks and landforms into smaller pieces called sediments. There are many ways that weathering can take place, through both physical means, when just the appearance of the rock changes and the rock is made of the same materials before, and chemical means, when chemical changes take place and the rock is no longer the same material. Physical changes are caused by many factors. The force of moving water can break down rocks into smaller pieces. This often happens in mountains where water can rush down the sides of the landform. Eventually, this weathering can result in mountains becoming highlands. Weathering from water also occurs in the floodplains of bodies of water, where rising waters can wear away at rocks and landforms, leveling the land. Water can also fill tiny cracks in rocks and then expand when it freezes, thus creating pressure that breaks the rocks into smaller pieces. Large blocks of moving ice called glaciers also create a lot of force that has the potential to break rocks apart. Glaciers can contribute to the creation of basin and range landforms. The force created from rocks or sediments falling down a hill because of gravity can also weather rocks. This frequently occurs on mountains and highlands because of the high elevations and sloped sides. Mudslides are a great example of gravity causing weathering. Wind can remove small sediments from rocks, and it can also carry sediments that hit other rocks and contribute to them breaking apart. This factor plays a big role in weathering on plains and plateaus, where the wind sweeps across the flat land, breaking apart rocks as it passes. In all landforms, animals burying in the earth’s surface and plants growing in the soil both also contribute to weathering. Another significant reason for weathering in all landforms is the actions of humans. Among many other things, we build buildings and roads, dig mines, and plant food to eat, all of which break apart and change the earth’s surface. Chemical changes can be caused when minerals in rocks react with oxygen, called oxidation, or through minerals in rocks reacting with acid, which often occurs because of acid rain. When earth materials are weathered and broken into smaller pieces, these smaller pieces can then be transported to other locations, usually by water or wind. This is called erosion. Together, weathering and erosion cause a leveling of the earth’s surface as hills are worn down by weathering and valleys are filled in with sediments that have eroded. While weathering and erosion work to level the earth’s surface, there are forces inside the earth that work to build up the earth’s surface. Inside the earth, there are three layers. On the outside, there is the outer crust, which is a brittle solid that is very thin, only about 10 to 25 miles thick. Next is the mantle, which is about 2000 miles thick and is a moldable solid, similar to the consistency of cornstarch and water. Lastly, in the center is the core, which is also about 2000 miles thick. There are two parts to the core, the outer core, which is a liquid layer, and the inner core, which is solid. The outer crust is broken into 12 very large pieces that float on top of the mantle, called tectonic plates. The continents and oceans sit on top of these plates. Plates holding continents are called “continental plates” and plates holding oceans are called “oceanic plates” Continental plates are made of granite and oceanic plates are made of basalt, which is denser than granite. These plates do not sit still, they are actually always moving very slowly, a movement that is called plate tectonics. Millions of years ago, all of the continents were one land mass called Pangaea, but because of plate tectonics, they have slowly moved into their present positions. The reason plate tectonics occur is that the mantle and the core are very hot because radioactive decay of rocks heats the surrounding areas. Temperatures raise higher and higher closer to the center of the earth. This allows convection currents to occur. As the mantle closer to the center of the earth heats, some of the mantle melts and magma is created. The magma, or molten rock, is very hot, so the molecules spread further apart and it becomes less dense than the surrounding mantle and it begins to rise. As it rises towards the outer crust, temperatures being to cool, so the molecules move closer together, it becomes denser, and begins to sink once again. This cycle causes the upper mantle to move, and in turn, the plates riding on the mantle also move. Different things happen when the plates move towards each other or away from each other. When tectonic plates pull apart from each other, it is called divergence. When the plates pull apart from each other, an opening appears, called a trench. Magma from the mantle then comes up through this crack, and then cools, creating new, mountainous crust. This is happening in the middle of the Atlantic Ocean. The chain of mountains and volcanoes in the Atlantic Ocean is called the Mid-Atlantic Ridge. Tectonic plates can also move towards each other, called convergence. Two things can happen when plates converge. If the plates have the same density, they simply crumble and fold as they push together, creating mountains. If the plates are of different densities, the plate with the higher density sinks below the plate with the lesser density. In other words, oceanic plates will sink below continental plates because oceanic plates have a higher density. This is called subduction. The plate that sinks melts in the mantle and becomes magma because of the high temperatures here. This magma then comes back to the surface through cracks in the crust and creates volcanoes. Lastly, tectonic plates can slide past each other, which is called a transform fault. When this happens, the top part of the plates sticks together but the lower part slides past each other. This causes strain to build up in the rock, which can eventually be released and an earthquake results. Plate tectonics cause the surface of the earth to constantly change, but it is important to remember that since the plates in the earth’s crust are moving so incredibly slow, these changes take a very long time to take place. Gaining more knowledge about geology helps us to explain many facts we may not have understood before. For example, learning about plate tectonics explains why earthquakes and volcanoes always tend to occur in certain places around the globe. This is because earthquakes and volcanoes are caused by the movement of tectonic plates – earthquakes by plates that slide past each other, and volcanoes by one plate that sinks below another plate. Since this is what causes volcanoes and earthquakes, it makes sense that volcanoes and earthquakes would occur at locations around the globe where two tectonic plates meet. Understanding plate tectonics also explains why the continents appear as if they could fit together like puzzle pieces. Since tectonic plates are always slowly moving as a result of the convection currents caused by the heating and cooling of the earth’s mantle, it is possible for them to drastically change locations over time. Because of this, the continental plates were, at one time, one large land mass, that has since then broken apart into the separate continents. Weathering helps us understand how potholes in the road form. When water melts, it can fill the cracks and holes in the roads. If the temperature then drops, the water will freeze and form ice, which takes up more space than water does. This applies pressure to the road and can contribute to it cracking more, and eventually crumbling. When this happens repeatedly, large potholes will form. Lastly, learning about the rock cycle allows us to see how various rocks are formed. This can help us explain why we can see crystals in some rocks and we cannot see crystals in other rocks. When igneous rocks are formed from the cooling of magma, the magma can either cool quickly above the surface of the earth, or slowly below the surface of the earth. When magma cools slowly, it has time to form crystals that we can see with the naked eye, but when it cools rapidly, it does not have time to form crystals large enough to be seen with the naked eye. Classroom Resources 1. Plate Tectonics with an Orange (Found at <http://www.coaleducation.org/lessons/wim/10.htm>.) In this activity, students are asked to peel an orange in as few pieces as possible and then use toothpicks to re-secure the pieces of peel to the orange in order to gain a clear visual representation of the earth’s crust, plate tectonics and fault lines. 2. Everybody Needs a Rock by Byrd Baylor This picture book teaches children to explore the characteristics and qualities of rocks using their senses, and even encourages them to start their own rock collection so they can further explore aspects of geology. 3. “The Earth Field Trip” NOVA Video Set This set of videos includes a detailed teacher’s guide and videos about volcanoes and earthquakes with real life footage, allowing students to understand and visualize the effects of plate tectonics and the forces within the earth. Credits I found this activity in the course pack for SME 301. I reworded the directions a bit in some places, and I added steps for students to compare the “fossils” they created to real cast and mold fossils, because I felt that this would help strengthen their understanding of the different kinds of fossils and how they are created.