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A Plate Tectonic Primer
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  1. A Plate Tectonic Primer Adapted from Dr Lynn Fichter James Madison University

  2. “No Rock (or Mineral) Is an Accident” • Dr Lynn Fichter has an extensive www site, some of which is very useful for our class: • Lecture material (this and others) • Ch 1 and 2 in the M&RCP • Fichter’s site was constructed to assist teachers in preparation of their earth science classes and should be useful now and in the future (for educators, especially).

  3. Synopsis of Plate Tectonic Theory • Plate Tectonics : • The outer rigid layer of the earth (the lithosphere) is divided into about a dozen "plates" • Plates move across the earth's surface relative to each other, like slabs of ice on a partially frozen lake. • Describes the ongoing dynamics of the earth • Explains most geologic phenomena both past and present, including the distribution and evolution of earth materials

  4. A model summarizing the evolution of tectonic plates and plate interactions through geological time Simplified as the opening and closing of ocean basins The Wilson Cycle

  5. Minerals and rocks are stable only under the conditions at which they form Temperature Pressure Chemical composition of the system Change the conditions and the rocks change too The Rock Cycle

  6. Evolution of the earth is inherent in the Wilson Cycle model Plate tectonic processes Fractionate (separate into discrete fractions) earth materials Create rocks with different compositions Increase the diversity of rocks with time, Early Earth ≠ Modern Earth! The Tectonic Rock Cycle

  7. With your neighbor(s): Draw, label, and describe the internal structure of the earth. Structure of the Earth ?

  8. Approximately 12,740 kilometers in diameter Stratified into layers Core: The central portion of the earth About 7000 km in diameter. Composed of an iron-nickel alloy. The outer core is molten, The inner core, even though just as hot, is a solid because of the increased pressure. Structure Of The Earth

  9. Approximately 12,740 kilometers in diameter Stratified into layers Mantle Below the crust and above the core About 2800 km thick Subdivided into many layers (lithosphere, asthenosphere, upper, and lower mantle) Composed mostly of ultramafic rocks such as peridotite and dunite and their metamorphic equivalents (e.g. ecologite). Structure Of The Earth

  10. Approximately 12,740 kilometers in diameter Stratified into layers Crust Rigid, rocky outer shell 0 to ~40 km thick <1% of the total volume of the earth Composed mostly of mafic to felsic composition igneous and metamorphic rock, plus sedimentary cover Provides all economic earth resources Structure Of The Earth

  11. The outer layers of the earth are where most plate tectonic activity occurs Two major divisions: Behavior: LITHOSPHERE (rigid crust and mantle) ASTHENOSPHERE (weak, plastic mantle) Composition: CRUST (rocks - oceanic and continental) MANTLE (ultramafic rocks – peridotite, eclogite) Structure Of The Earth's Outer Layers

  12. The Lithosphere Outer rigid shell of the earth Extends from the surface to about 70-100 km deep Consists of the crust and the uppermost mantle The Asthenosphere About 150-200 km thick Composed mostly of ultramafic rocks such as peridotite and dunite Weak and plastic, flows slowly under stress Structure of the Earth's Outer Layers

  13. The Moho Lower boundary of the crust Marks the transition from the granite and basalt of the crust to the ultramafic rocks of the mantle below Structure of the Earth's Outer Layers

  14. The Crust Ocean Basins black layer composed of mafic rocks like basalt and gabbro 7-10 km thick Continents brown layer composed of felsic rocks such as granite 10-30 (as much as 70) km thick Structure of the Earth's Outer Layers

  15. The Origin and Heat History of the Earth • Gravitational contraction of planetismals (meteorites and asteroids) • Extraterrestrial impacts • Radioactive decay • Resulted in molten proto-earth

  16. The Cooling History and Origin of Concentric Layering • This “ancient” heat (plus ongoing radioactive decay) drives planetary scale dynamics • Convective separation and density stratification through • Hot spots “piping” magma and heat from the deep interior • Convection cells in the mantle

  17. What is a plate? Composed of rocks of the Lithosphere, plus overlying soil, etc. Plates “ride” on the weak, plastic asthenosphere Plate Tectonics

  18. Divergent boundaries, Convergent boundaries Transform boundaries Plate Tectonics • Is driven by convection cells in the mantle • Results in continuous jostling of plates against one another at • Plate Boundaries:

  19. Convection cells in the mantle. Hot, low density material from the lower mantle, heated by the core, flows upward towards the surface. Heat escapes through volcanic activity. Cooled (now denser) material then sinks back toward the core to be heated again. This creates a cycle of movement that drives plate tectonics and chemically separates and initially homogeneous earth The Main Dynamic Process of Plate Tectonics

  20. Cratons (stable continents) Ocean Basins Plate boundaries Divergent Convergent Transform Hot Spots The Six Lithospheric Tectonic Regimes

  21. The Plates, Plate Boundaries, and Interplate Relationships • Components of the Plate Tectonic Theory

  22. The evolution of earth materials is driven by chemical differentiation (fractionation) in the context of plate tectonics Igneous rocks evolve compositionally at Convergent plate boundaries Divergent plate boundaries Hotspots Plate Tectonics and Evolution of the Earth’s Composition

  23. Origins of Magmas and Magmatic Differentiation • At these three sites magma is generated • Through processes of fractional melting (partial melting) and magmatic differentiation entirely new rocks are created Primitive magma Recycled magma

  24. Origins of Magmas and Magmatic Differentiation • Primitive magmas: • Derived through partial melting (anatexis) of ultramafic mantle rock • Source of much mafic (basaltic/gabbroic) magma • Generally low viscosity, shield volcanoes and oceanic crust • Hot spots • Hawaii islands, Iceland

  25. Origins of Magmas and Magmatic Differentiation • Primitive magmas: • Ocean spreading centers, • MORB (mid-ocean ridge basalt) • Ophiolite (oceanic crust profile)

  26. Origins of Magmas and Magmatic Differentiation • Primitive magmas: • Continental rifts (bi-modal {basalt/rhyolite} volcanism) • East African rift volcanism • Volcanism of NA great basin

  27. Recycled (subduction zone) magmas Derived through dehydration of subducted plate, causing partial melting of overlying ocean crust and mantle at subduction zones Complex interaction of pre-existing ocean crust/ transitional mantle at areas of high heat flow Ocean-continent convergence Chile/Peru trench, composite Andesitic volcanoes Convergent plate margin volcano-plutonic arcs Origins of Magmas and Magmatic Differentiation

  28. Recycled (subduction zone) magmas Derived through partial melting of ocean crust and mantle at subduction zones Ocean-ocean convergence Calc-alkaline basalt/Andesite volcanic Volcanic (plutonic) island arcs Origins of Magmas and Magmatic Differentiation

  29. Plate Tectonics, Magmatic Differentiation, and Earth Evolution • Through plate tectonic processes the earth has evolved: • First, to form volcanic island chains (volcanic arcs scattered across a single world wide ocean) • Then to enlarge these features to form the large continental masses we live on today.

  30. Collaborative Assignment Answer in complete sentences (from M&RCP Chap 1): • Is there a difference between a plate boundary and a continental margin? Yes – a plate boundary is where 2 plates meet, resulting in earthquakes, volcanoes, etc. A continental margin is where felsic crust of the continent meets mafic crust of the ocean basin. Continental margins can be plate boundaries, and plate boundaries can occur at continental margins. But some plate boundaries can be in the middle of continents or oceans.

  31. Collaborative Assignment Answer in complete sentences (from M&RCP Chap 1): 2. Why does it make sense to discriminate the layers of the earth on the basis of both composition and behavior? Why is it critical for an understanding of plate tectonics that the earth has a plastic layer relatively close to the surface? Composition and behavior of earth layers are both important for understanding plate tectonics. Some layers of the earth have the same composition but different behavior (inner and outer core – both Fe; lithospheric and asthenospheric mantle – both ultramafic rock). Some layers have similar behavior but different compositions (oceanic and continental crust are both “brittle”). The plastic layer (asthenosphere) is critical because it supports the plates and allows them to move. Without the asthenosphere we wouldn’t have plate tectonics.

  32. Collaborative Assignment Answer in complete sentences (from M&RCP Chap 1): 3. Why is oceanic crust so much younger than continental crust? Oceanic crust is more easily subducted because it is more dense than continental crust, so it is continually consumed. 4. Why have continents continued to grow through earth’s evolution, but oceans do not? Ocean crust is continually consumed by subduction (see above question). The process of subduction recycles rock to create felsic continental crust – continents are low density and not consumed by subduction. Once created, continental rock is pretty much permanent.

  33. Collaborative Assignment Answer in complete sentences (from M&RCP Chap 1): 5. Why does Dr. Fichter say that “Most of what is interesting about the earth happens at these boundaries [plate boundaries]”? What context is he referring to? Do you agree? Make an argument from a different perspective that this statement is false. He is referring to a geological context, and in particular the context of plate tectonics – for example, volcanoes, metamorphism, earthquakes, and mountain building all take place predominantly at plate boundaries due to plate interactions. Several examples show the statement is false - not all geologic activity happens at plate boundaries (hot spots, some earthquakes, weathering); also weather and climate are not entirely tied to plate boundaries; and meteorite impacts are definitely NOT tied to tectonics.

  34. Collaborative Assignment Answer in complete sentences (from M&RCP Chap 1): 6. Do you think there is geological data (ancient rocks) that Dr. Fichter might have used to support the statement that “the earth’s heat engine ran faster at the beginning than now”? Use this data to make an argument that would support this statement. If the earth was hotter in the past, it may have made rock that it cannot make now, because specific rock types form at certain Pressure-Temperature-Composition conditions. Komatiite (ultramafic lava flows) and anorthosite (plagioclase feldspar) rocks require hot conditions to fomr and are largely restricted to Archean age – suggesting that the modern, cooler earth can no longer make these rock types.