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The Biggest Plates on Earth

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The Biggest Plates on Earth Submarine Ring of Fire Expedition The Biggest Plates on Earth FOCUS ADDITIONAL INFORMATION FOR TEACHERS OF DEAF STUDENTS Plate tectonics In addition to the words listed as Key Words, the fol- lowing word should be part of the vocabulary list. GRADE LEVEL Crust 5-6 The words listed as Key Words are integral to the FOCUS QUESTION unit but will be very difficult to introduced prior to How do tectonic plates move, what are some con- the activity. They are really the material of the les- sequences of this motion, and how do magnetic son. There are no formal signs in American Sign anomalies document the motion at spreading cen- Language for any of these words and many are ters? difficult to lipread. Having the vocabulary list on the board as a reference during the lesson will be LEARNING OBJECTIVES extremely helpful. Students will be able to describe the motion of tec- tonic plates, and differentiate between three typical The second paragraph of the Background Information boundary types that occur between tectonic plates. has important information for use during the initial step of the Learning Procedure when you are explain- Students will be able to infer what type of bound- ing concepts of plate tectonics and drift. You should ary exists between two tectonic plates, given infor- demonstrate convection currents by setting up a heat- mation on earthquakes and volcanism in the vicin- ed container of water and adding food coloring to it. ity of the boundary. Also it would be helpful to gather several flat rocks and demonstrate the motion of tectonic plates. It is Students will understand how magnetic anoma- important to demonstrate how small the movement is lies provide a record of geologic history around annually (several centimeters). spreading centers. MATERIALS Students will be able to infer the direction of motion Copies of “Pacific Basin Tectonic Plates,” one between two tectonic plates given information on copy per student or student group magnetic anomalies surrounding the spreading Copies of “Magnetic Anomalies on the Juan ridge between the plates. de Fuca Plate,” one copy per student or student group Students will be able to describe plate boundaries and tectonic activity in the vicinity of the Juan de Fuca plate. 1 Submarine Ring of Fire Expedition – Grades 5-6 Submarine Ring of Fire Expedition – Grades 5-6 Focus: Plate tectonics oceanexplorer.noaa.gov oceanexplorer.noaa.gov Focus: Plate tectonics AUDIO/VISUAL MATERIALS charted the location of deep earthquakes in the (Optional) Overhead transparency of cross- sec- Pacific Ocean. He found that earthquake sites are tion illustrating the main types of plate boundar- distributed in an arc that spanned both sides of the ies (download from U.S. Geological Survey web ocean. Five years earlier, Japanese seismologist Kiyoo Wadati had noticed a similar pattern, and site: http://pubs.usgs.gov/publications/text/Vigil.html) suspected that it was connected in some way to the (Optional) Overhead transparency of map illus- idea of continental drift. He was right. trating the Ring of Fire (from U.S. Geological Survey web site: http://pubs.usgs.gov/publications/text/ Today we know that the outer shell of the Earth fire.html) (called the lithosphere) consists of about a dozen (Optional) Overhead projector large plates of rock (called tectonic plates) that move several centimeters per year relative to each TEACHING TIME other. These plates consist of a crust about 5 km Two or three 45-minute class periods thick, and the upper 60 - 75 km of the Earth’s mantle. The plates that make up the lithosphere SEATING ARRANGEMENT move on a hot flowing mantle layer called the Classroom or groups of two to four students asthenosphere, which is several hundred kilometers thick. Heat within the asthenosphere creates con- MAXIMUM NUMBER OF STUDENTS vection currents (similar to the currents that can be 30 seen if food coloring is added to a heated contain- er of water). These convection currents cause the KEY WORDS tectonic plates to move. Plates may slide horizon- Basalt tally past each other at transform plate boundaries. Ring of Fire The motion of the plates rubbing against each other Asthenosphere sets up huge stresses that can cause portions of Lithosphere the rock to break, resulting in earthquakes. Places Magma where these breaks occur are called faults. A well- Fault known example of a transform plate boundary is Transform boundary the San Andreas Fault in California. Convergent boundary Divergent boundary Another type of plate boundary is formed where Subduction tectonic plates are moving apart. At these diver- Magnetic anomaly gent plate boundaries, magma (molten rock) rises Tectonic plate from deep within the Earth and erupts to form Spreading center new crust on the lithosphere. Most divergent plates boundaries are underwater (Iceland is an excep- BACKGROUND tion), and form submarine mountain ranges called The Ring of Fire is an arc of active volcanoes and oceanic spreading ridges. While the process is earthquake sites that partially encircles the Pacific volcanic, volcanoes and earthquakes along oce- Ocean Basin. The location of the Ring of Fire coin- anic spreading ridges are not as violent as they cides with the location of oceanic trenches and are at convergent plate boundaries where tectonic volcanic island arcs, and was one of the early clues plates are being pushed together. Usually one of that led to the theories of continental drift and plate the converging plates moves beneath the other (a tectonics. In 1940, Hugo Benioff, an American process called subduction). Deep trenches are often seismologist (a scientist who studies earthquakes), formed where tectonic plates are being subducted, 2 3 Submarine Ring of Fire Expedition – Grades 5-6 Submarine Ring of Fire Expedition – Grades 5-6 Focus: Plate tectonics oceanexplorer.noaa.gov oceanexplorer.noaa.gov Focus: Plate tectonics and earthquakes are common. As the sinking plate ics. When magma erupts along oceanic spreading moves deeper into the mantle, fluids are released ridges, it contains high concentrations of various from the rock causing the overlying mantle to par- minerals, including magnetic minerals, such as iron tially melt. The new magma rises and may erupt to and magnetite that align with the Earth’s magnetic form volcanoes, often forming arcs of islands along field like a compass needle. When magma cools the convergent boundary. to form solid rock (basalt), particles of magnetite become immobilized, and provide a record of On the Pacific Ocean tectonic plate, the production the position of the Earth’s magnetic field when the and recycling of lithospheric crust has been likened magma erupted. to a huge conveyor belt system in which new crust is formed at the oceanic spreading ridges off the The Earth’s magnetic field periodically reverses (so western coasts of North and South America, and that compass needles point to the south instead older curst is returned to the lower mantle at the of the north) at irregular intervals that average convergent plate boundaries along the underwater around 400,000 years. The alignment of magnetic volcanoes and island arcs of the western Pacific. particles in crust that was formed during periods The 2002 Submarine Ring of Fire Expedition is of reversal is opposite to that of particles produced investigating the formation of new ocean crust at when the Earth’s magnetic field is oriented simi- the edge of the Juan de Fuca tectonic plate off the larly to the present. Scientists have found that the coast of western North America. This is a com- intensity of the Earth’s magnetic field varies around paratively small tectonic plate, and as a result has spreading centers; in some locations it is stronger a divergent boundary (with the Pacific plate) as than normal, in other locations it is weaker. These well as a convergent boundary (with the North variations are called magnetic anomalies, and are American plate) relatively close together. The erup- the result of magnetic minerals in the basalt rocks. tion of Mt. St. Helen in 1980 was a result of the If the minerals were “locked” into the rocks when subduction of the Juan de Fuca plate beneath the the Earth’s magnetic field was similar to its present North American plate. The divergent boundary is magnetic field, then the magnetism of the minerals an active spreading center that is organized along is added to the Earth’s magnetic field, so the total three ridges: Gorda Ridge, Juan de Fuca Ridge, magnetic intensity is greater than normal; this is and Explorer Ridge. While Gorda and Juan de called a “positive magnetic anomaly.” On the other Fuca Ridges have been intensively studied, Explorer hand, if the minerals were “locked” into the rocks Ridge is virtually unexplored and is the focus of the when the Earth’s magnetic field was opposite to its 2002 Submarine Ring of Fire Expedition. present magnetic field, then the magnetism of the minerals opposes the Earth’s magnetic field, so the One of the first tasks for the expedition is to pre- total magnetic intensity is less than normal; this is pare maps of the seafloor around Explorer Ridge called a “negative magnetic anomaly.” to aid in identifying the most promising sites for investigation. In addition to topographic maps When positive and negative magnetic anoma- prepared with a multibeam sonar system (see http: lies are measured on either side of an oceanic //oceanexplorer.noaa.gov/explorations/02alaska/background/edu/ spreading center, they form a zebra stripe-like media/mapping7_8.pdf, scientists will also make detailed pattern with the stripes running parallel with the measurements of magnetic fields around the Ridge ridge of the spreading center. Scientists have to reveal features that will not be detected by the reconstructed the history of magnetic reversals for sonar system.
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