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Geologic Time

Geologic Time. Deposition of the Barkley Sandstone Deposition of the Grin Sandstone Deposition of the Lemin Ashtone Deposition of the Cave Limestone Deposition of the Lewin Mudstone Folding Possible unconformity dividing upper and lower sequences (only inferred)

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Geologic Time

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  1. Geologic Time

  2. Deposition of the Barkley Sandstone • Deposition of the Grin Sandstone • Deposition of the LeminAshtone • Deposition of the Cave Limestone • Deposition of the Lewin Mudstone • Folding • Possible unconformity dividing upper and lower sequences (only inferred) • Deposition of the Jep Shale • Deposition of the Peaty Shale • Deposition of the Coal Measure • Deposition of the Basin Conglomerate • Deposition of the Deekin Sandstone • Deposition of the Shallow Shale • Deposition of the Park Claystone • Faulting lower sequence into blocks • Thrust faulting of upper sequence over lower sequence • Intrusion of Basalt • Continuation of thrust faulting

  3. A little quantitative exercise: • A radioactive isotope A has a half life of T1/2 = 10,000 years and decays into a stable daughter isotope B. Att = 0, the rock cools just below its blocking temperature and begins to trap the daughter isotope B, which would otherwise be lost. • On the provided sheet of graph paper carefully plot the relative abundance of A and B from t = 0 to t = 5 T1/2. • The analysis of an igneous rock specimen yields 12,500 isotopes of A and 87,500 isotopes of B. How old is the rock?

  4. A brief history of oceans, continents and Connecticut • oceans : mostly basalt • continents : mostly granite If Earth started out as one molten blob – how do we separate crust into continents and oceans ?

  5. Magmatism of Subduction Zones Which magma types are produced ?

  6. (wet) partial melting in subduction zones can produce • andesitic • basaltic • rhyolithic magma

  7. Mt. Stromboli, Italy image: USGS volcano hazards program Mt. Spurr, AK image: ETH

  8. Clark AFB, Philippines, image: USGS volcano hazards program

  9. http://www.elams.org/Places/Yosemite%202001/Half%20Dome%20with%20Royal%20Arches.jpghttp://www.elams.org/Places/Yosemite%202001/Half%20Dome%20with%20Royal%20Arches.jpg

  10. Early Continents • small island arcs (e.g., Japan) • have core of granitic rocks (light, won’t be subducted) • are pushed across surface of earth by plate tectonics • combine with other small (micro)continents • form accreted terranes

  11. A Closer Look at Subduction Zones • most of the oceanic slab goes down • some sediment and oceanic rock will be caught in subduction zone to for accretionary prism • small microcontinents will not be subducted • accreted terranes

  12. Early Terranes of North America

  13. More Recent Additions

  14. From: Bell, 1985

  15. The Wilson Cycle • continents are hard to destroy → float on top of crust • they are bound to crash into each other • form super continent (Pangea) • what drives plate movement? • what makes continents break up?

  16. The Elusive Mechanism:What Drives Plate Tectonics ? • Ridge-Push forces

  17. from: NOAA's National Geophysical Data Center

  18. Why do Ridges Stand High? • temperature of rocks ? • density of rocks ? • what’s below the lithosphere ? • what are the physical properties of the asthenosphere?

  19. hot rocks are less dense • buoyant • float high on asthenosphere • become denser as they cool • oceanic lithosphere slides down the ridges

  20. Temperature Distribution in Subducted Slab

  21. Which Processes are Important? • how would you investigate importance of slab pull and ridge push forces? • where does ridge push occur? • where does slab pull occur • what is easier to move: a small or a large plate? • which plates should move the fastest?

  22. from: Cox and Hart, 1986

  23. What Makes Continents Break up? • Continents are thick, good thermal insulators • How will temperature change under thick continental crust? • How will buoyancy change? • How might this affect stability of continent?

  24. The Wilson Cycle • continents are pushed together by plate tectonics • can form super continent • thick layer of insulating rocks • base of continent heats up • softens rocks, makes them buoyant • continents can drift apart

  25. from: Bell, 1985

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