A Plate Tectonic Primer

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 tied to 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 and chemically separates and initially homogeneous earth BUT does convection drive plate motion, or does plate motion drive convection?? 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.