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Introducing the Theory of Plate Tectonics

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1 Introducing the Theory of Plate Tectonics
Prepared by: Mrs. C. Hildreth March 5, 2004 Shenendehowa High School Clifton Park, NY Diagrams and information from: Tarbuck, E. & Lutgens, F. Earth Science. Prentice-Hall, Inc. 2000 Earth Science. McDougal Littell Inc. 2003 Spaulding, N & Namowitz, S. Heath Earth Science. DC Heath & Co NEXT 

2 New York State Physical Setting / Earth Science Core Curriculum
Standard 2, Key Idea 1-- Information technology is used to retrieve, process, and communicate information as a tool to enhance learning. Standard 4, Key Idea 2 -- Many of the phenomenon that we observe on Earth involve interactions among components of air, water, and land. Earth’s internal heat engine is powered by heat from the decay of radioactive materials and residual heat from Earth’s formation. Differences in density resulting from heat flow within Earth’s interior caused the changes explained by the theory of plate tectonics: movement of the lithospheric plates; earthquakes; volcanoes; and the deformation and metamorphism of rocks during the formation of young mountains. Major Understandings: 2.1 a, b, k, l, m, n The outward transfer of Earth’s internal heat drives convective circulation in the mantle that moves the lithospheric plates comprising Earth’s surface. The lithosphere consists of separate plates that ride on the more fluid asthenosphere and move slowly in relationship to one another, creating convergent, divergent, and transform plate boundaries. Many processes of the rock cycle are consequences of plate dynamics. Many of Earth’s surface features are a consequence of forces associated with plate motion and interaction. Standard 6, Key Idea 1, 2, 5 -- Process skills: systems thinking; models; patterns of change. NEXT 

3 EARTH’S INTERNAL HEAT ENGINE AND CONVECTION The diagram below shows a cross section of Earth to a depth of approximately 700 km (not to scale). Arrows show motion of Earth’s interior and surface. In this lesson you will learn about the dynamics of heat generated motion. You will also explore evidence of the Theory of Plate Tectonics. TO VIEW THE LESSON: Click on each number in order from # 1 to # Answer the questions on the accompanying notesheet. 6. Convergence 4. Divergence 5. Convergence 2. Asthenosphere 3. Convection 1. Earth’s Interior

4 CROSS-SECTION VIEW OF EARTH’S INTERNAL STRUCTURE
Since its formation, Earth has been losing heat (cooling) through two processes: 1. Decay of radioactive materials 2. Residual heat from Earth’s formation See your Earth Science Reference Tables, page 10. Find the following: Inner core Outer core Stiffer mantle Asthenosphere Observe the temperature ranges within each of these interior layers. See Earth Science Reference Tables, page 10. What is the temperature at Earth’s inner core?________ Earth’s outer core is [liquid, solid] because the actual temperature is [less than, greater than] the melting point of the rock material. BACK

5 ASTHENOSPHERE AND LITHOSPHERE
The surface of Earth is broken into huge lithospheric or crustal plates. The lithosphere consists of the upper rigid mantle and the overlying oceanic or continental crust. The asthenosphere is capable of flow. This is because the temperature in the asthenosphere is slightly greater than the melting point of the ultramafic rock material which makes up the asthenosphere. This causes a partial melting of the mantle. See your Earth Science Reference Tables, page 10. Find the following: lithosphere rigid mantle oceanic crust continental crust asthenosphere. The temperature in the asthenosphere is slightly [less than, greater than] the melting point of the rock material which makes up the asthenosphere. This causes [partial, total] melting of the mantle. Continental crust is [thinner, thicker] and [less, more] dense than oceanic crust. BACK

6 CONVECTION Heat from Earth’s interior causes density differences and results in convection in the asthenosphere. This region of high heat flow pushes magma upward into the solid lithospheric plates (upper rigid mantle and crust), forcing the plates aside and forming new rock on the sea floor. See the Earth Science Reference Tables, page Observe the convection arrows in the asthenosphere. Where the arrows are shown rising, they diverge, or split apart. Where arrows are shown sinking, they converge, or come together. Convections currents are located in the [asthenosphere, lithosphere]. Heat causes matter to [expand, contact], making it [less, more] dense. In this way, heat is transferred from Earth interior to the surface. Rising arrows [diverge, converge] in the upper part of the asthenosphere. BACK

7 DIVERGENCE Heat flow causes the lithosphere to break apart (divergence) and form a rift valley. These rift valleys form the boundaries between plates and new rock is formed as the magma cools. This is referred to as sea-floor spreading. Older rock is pushed aside where new rock is forming. As distance from mid-ocean ridges increases, age of rock also increases. Most divergent boundaries have mid-ocean ridges with deep rift valleys along their entire length. Find the rift valley and mid-ocean ridge in the diagram below. See your Earth Science Reference Tables, page 10. Find the Mid-Atlantic Ridge. See your Earth Science Reference Tables, page 5. Again find the Mid-Atlantic Ridge. Below the page 5 map is a diagram of a divergent plate boundary. Locate other divergent plate boundaries (mid-ocean ridges) on the map. OLDER ROCK OLDER ROCK Write the names of the major mid-ocean ridges found at divergent boundaries. 1. 2. 3. 4. 5. Arrows at these plate boundaries show that the plates are [converging, diverging]. HEAT FLOW NEXT

8 DIVERGENCE WITH TRANSFORM FAULTS Rift valleys are broken into segments by transform faults. Movements along these fractures are a source of earthquakes that occur along the ridge. Observe the many transform faults along the mid-ocean ridges on page 5 of the Earth Science Reference Tables. Rift valleys [do, do not] have equal and constant activity all along the rift. Rift valleys, and hence, divergent boundaries are broken by _________________ faults. NEXT

9 TRANSFORM BOUNDARY At the boundaries of some areas, the lithospheric plates are sliding past each other. Frequent earthquakes occur along these transform boundaries as huge masses of rock move. Find a transform boundary on the US west coast on page 5 of the Earth Science Reference Tables. Transform plate boundaries occur where tectonic plates [are pulled apart, slide past each other]. NEXT

10 TRANSFORM BOUNDARY The San Andreas Fault, located in western California, is the source of frequent earthquakes. Here, the Pacific Plate and the North American Plates are sliding past each other. The average rate of movement along the San Andreas Fault is about 5 cm per year. Some areas have not moved for centuries. Pressure has been building and is thought to be the most likely places for future earthquakes. Find the San Andreas Fault on page 5 of the Earth Science Reference Tables. San Andreas Fault The lithospheric plate on the western side of the San Andreas Fault is moving [SE, NW], while the lithospheric plate on the eastern side of the San Andreas Fault is moving [SE, NW]. The San Andreas Fault forms a [divergent, transform] plate boundary. BACK

11 CONVERGENCE: ocean-continental plate boundary Continental crust is made of granite, which is less dense than ocean crust, which is made of basalt. (See Earth Science Reference Tables, pg. 6 and 10) The more dense ocean crust subducts (goes under) continental crust, forming a deep trench on the ocean floor. High temperatures and pressures in the asthenosphere cause the subducting plate to melt. Magma then rises to the surface and escapes in volcanic eruptions, forming major mountain ranges on the continent. Eruptions can be very violent and earthquakes at subduction boundaries occur much deeper in the lithosphere than at other plate boundaries. It is important to recognize that plates formed at mid-ocean ridges are destroyed at subduction zones. The Convergent Plate Boundary diagram, at the bottom of the Earth Science Reference Tables page 5, indicates which plate is subducting and which plate is overriding. Find the Peru-Chile Trench to the west of South America. The Andes Mountains in western South America have formed from volcanic eruptions along this [convergent, divergent] boundary. The Cascade Mountains, including Mt. St. Helens which erupted in 1980, are found in the northwest region of the United States where the Juan de Fuca Plate is [subducting under, overriding] the North American Plate. BACK

12 CONVERGENCE: oceanic-oceanic plate boundary Where two ocean plates converge, the older, more dense ocean crustal plate subducts under the younger, less dense. Remember, the older crust is farther from the mid-ocean ridge where it was originally formed as new seafloor. Island arcs are formed near ocean trenches as subducting plates are melted and rise to the surface as volcanic eruptions. Island arcs are found along the Aleutian Trench and Tonga Trench in the Pacific Ocean. Find the Aleutian Trench and Tonga Trench on page 5.. At the Aleutian Trench, the Pacific Plate is the [overriding, subducting] plate. At the Tonga Trench, the Pacific Plate is the [overriding, subducting] plate. So older, more dense rock makes up the edge of the [Indian-Australian, Pacific] Plate. NEXT

13 CONVERGENCE: continental-continental plate boundary If converging plates are both carrying continents, the continents may be welded into a single larger continent. This converging boundary is known as a collision boundary. The collision causes the lithosphere to be pushed up into a mountain range. Describe a collision boundary.__________________________________________________________________ NEXT

14 CONVERGENCE: India colliding with Asia The Himalayan Mountains are an example of a collision boundary that is still forming today. Here India is pushing northward into China at a rate of about 5 cm each year. India is now welded to the Eurasian continent with the world’s highest mountains and frequent earthquakes. The Appalachian Mountains formed by the same process during the collision of North America and northern Africa. At a much later time, these continents were rifted apart, forming the Atlantic Ocean. Locate India and the Eurasian Plate. The Indian-Australian Plate is the [subducting, overriding] plate. NEXT

15 CONVERGENCE: This map shows the Australian-Indian plate colliding with the Eurasian plate. Find India and the Himalayan Mountains. HIMALAYAN MOUNTAINS INDIA The Himalaya Mtns. are located at the boundary between the _____________________ and ____________________________ Plates. NEXT

16 WORLD MAP – TECTONIC PLATE BOUNDARIES Study the three types of tectonic plate boundaries shown below. Locate these boundaries on the map on pg. 5 in your Earth Science Reference Tables. Fill in the chart below with the three types of tectonic plate boundaries shown in the map, their locations on this map, and other locations from pg. 5 in your Earth Science Reference Tables. NEXT

17 EVIDENCE: This world map shows the distribution of shallow-, intermediate-, and deep-focus earthquakes. Deeper focus earthquakes occur in subduction zones along convergent plate boundaries. Shallow focus earthquakes occur along mid-ocean ridges and transform boundaries. Deeper earthquakes are evidence for the existence of a [convergent, divergent] plate boundary. On South America’s west coast, earthquakes that occur farther inland are [deeper, shallower] than earthquakes that occur in the ocean. This [is, is not] consistent with a subduction zone. NEXT

18 EVIDENCE: EARTHQUAKE FOCI DEPTH [Japan trench region] Japan is an island arc created from the subduction of the Pacific Plate. Earthquakes in western Japan are deeper than earthquakes in eastern Japan. Earthquakes that occur in Japan have deeper foci the farther [east, west] that they occur, which is typical for a subducting plate. NEXT

19 EVIDENCE: Many earthquakes and volcanic eruptions surround the Pacific Ocean. This is known as the “Ring of Fire”. The locations of earthquakes and volcanoes surround the entire [Atlantic, Pacific] ocean. This is known as the “Ring of Fire”. Locations of earthquakes and volcanoes [are, are not] consistent with tectonic plate boundaries. NEXT

20 Figure #1 Mount St. Helen’s
EVIDENCE: Volcanic activity occurring at subduction boundaries usually results in more violent eruptions. Figure #1 Mount St. Helen’s Figure #2 Mt. Uluigalau Volcano, Fiji Figure #1 shows that volcanoes form [on land, in the ocean] when an ocean plate [subducts, overrides]. Figure #2 shows the formation of a [mid-ocean ridge, deep ocean trench] and island arc. The island arc is formed from [volcanoes, earthquakes, both of these]. NEXT

21 EVIDENCE: map of the ocean floor Red areas indicate shallower regions along continental shelves. Blue areas show the deepest regions in the oceans. Green areas show the locations of undersea mountain ranges. Undersea mountain ranges [are, are not] consistent with the locations of mid-ocean ridges formed at [convergent, divergent] plate boundaries. NEXT

22 EVIDENCE: physical world map…locations of mountain ranges Mountain ranges are formed at active plate boundaries where tectonic plates are converging. Find the Cascade Mts., the Andes Mts. and the Himalaya Mts. in the map below. Cascades Himalayas Andes Mountains on land are usually formed at [convergent, divergent] plate boundaries. NEXT

23 Rift Valley Mid-Ocean Ridge OLDER ROCK OLDER ROCK 4 4 NEXT
EVIDENCE: magnetic polarity patterns in seafloor rocks There have been four major polarity reversals in the past four million years. Geologists use the knowledge of magnetic polarity to compare ages of rocks near spreading centers on the ocean floor. The diagram below shows divergent plates at a mid-ocean ridge. Solid arrows show direction of normal polarity. Currently forming rocks at the spreading center show normal polarity. North Reverse Polarity Sea Level Rift Valley Mid-Ocean Ridge OLDER ROCK OLDER ROCK Rocks that form at spreading centers have [normal, reverse] polarity. Bands of rock at equal distances on opposite sides of a spreading have [different, the same] polarity. Spreading center 4 4 NEXT Millions of years

24 Direction of Plate Movement
EVIDENCE : TECTONIC HOT SPOTS Not all volcanism occurs at plate boundaries. Hot spots are usually located in the middle of lithospheric plates. The hot spot seems to remain in the same location while the plate moves over it. This results in a chain of extinct volcanic islands like the Hawaiin Islands. Direction of Plate Movement Name the four other hot spots shown on the map on pg. 5 of your Earth Science Reference Tables. North KAUAI OAHU MOLOKAI Active Volcano is over the Hot Spot MAUI NEXT HAWAII

25 PLATE TECTONICS AND THE ROCK CYCLE New rocks are continually being created due to tectonic forces. Igneous rocks form from the solidification of magma and volcanic lava. Metamorphic rocks are changed by the heat and pressure caused by converging of tectonic plates. Sedimentary rocks are created when other rocks are weathered and eroded, deposited as sediments, and then compacted and cemented. Find the location of each of these processes in the diagram below. Igneous Tectonic forces [do, do not] contribute to the formation of Earth’s rocks. State the two types of rock that are formed as a direct result of tectonic plate interactions. ________________ and ____________________ Sedimentary Metamorphic NEXT

26 PLATE BOUNDARIES MAP The map below shows the major plate boundaries currently identified by the Theory of Plate Tectonics. You should now be familiar with the evidence which supports this theory, as well as with the interactions and types of plate boundaries as shown on page 5 of the Earth Science Reference Tables. Pacific Plate The Pacific Plate is forming at the [eastern and southern edges, western and northern edges] and being destroyed in subduction zones at the [eastern and southern edges, western and northern edges]. We can infer that the Atlantic Ocean is becoming [narrower, wider] since at the Mid-Atlantic Ridge, new sea floor is being [destroyed, formed]. Most of the tectonic plates [are, are not] named for the large land masses which sit upon them. NEXT

27 QUIZ YOURSELF Use your Earth Science Reference Tables, pg
QUIZ YOURSELF Use your Earth Science Reference Tables, pg. 5 and 10 Then click NEXT for the KEY. Correct your answers. Then you may go back and review the slide presentation if you have time. 1. What type of plate boundary occurs at the Peru-Chile Trench? 2. The Mid-Atlantic Ridge forms what type of plate boundary? 3. At the Tonga Trench, lithospheric plates are [converging, diverging, sliding past]. 4. The African Plate is moving toward which direction? 5. Which hot spot is located on the North American Plate? 6. Where is the Nazca Plate subducting and being destroyed? 7. Where is the African Plate beginning to break apart? 8. Which direction is the Pacific Plate moving relative to the North American Plate? 9. What is the name of the fault that separates the Pacific and N. American Plates? 10. What is happening in the asthenosphere which results in volcanism in rift valleys along the Mid-Atlantic Ridge? NEXT

28 KEY NEXT CHECK YOUR ANSWERS: 1. CONVERGENT 6. PERU-CHILE TRENCH
1. What type of plate boundary occurs at the Peru-Chile Trench? 2. The Mid-Atlantic Ridge forms what type of plate boundary? 3. At the Tonga Trench, lithospheric plates are [converging, diverging, sliding past]. 4. The African Plate is moving toward which direction? 5. Which hot spot is located on the North American Plate? 6. Where is the Nazca Plate subducting and being destroyed? 7. Where is the African Plate beginning to break apart? 8. Which direction is the Pacific Plate moving relative to the North American Plate? 9. What is the name of the fault that separates the Pacific and N. American Plates? 10. What is happening in the asthenosphere which results in volcanism in rift valleys along the Mid-Atlantic Ridge? CHECK YOUR ANSWERS: 1. CONVERGENT 6. PERU-CHILE TRENCH 2. DIVERGENT 7. EAST AFRICAN RIFT 3. CONVERGING 8. NORTHWEST 4. EAST SAN ANDREAS FAULT 5. YELLOWSTONE 10. RISING HEAT FLOW or CONVECTION NEXT

29 RELATED WEBSITES Click on some of the following to learn more about the Theory of Plate Tectonics… Click to Review This Lesson QUIT


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