Our Hazardous Environment GEOG 1110 Dr. Thieme

1. Our Hazardous EnvironmentGEOG 1110Dr. Thieme Lecture 2: Earth's Structure, Plate Tectonics, and the Rock Cycle

2. Structure of Earth Average density of the solid Earth = 5.5 g/cm3 Rocks of continental crust average 2.5 to 3.0 g/cm3 Dense (Fe, Ni) core required by planetary motions

3. What Is Inside Earth? Upper layer is crust; two types:continental oceanic Thickest layer: mantle Lowest layer: iron-nickel core (molten outer core; solid inner core)

4. Relief and Tectonics • Ocean Basins are spreading away from mid-ocean ridges • Continental collisions build mountains and increase land surface elevation: • ocean-ocean • ocean-continent • continent-continent

5. Earth's crust is divided into 7 major and 20 smaller plates

6. Plate boundaries are defined by areas of seismic activity

7. Tectonic Cycle: Earth's crust is constantly being recycled as lithosphere is created at spreading ridges, rides on aesthenosphere, and is subducted into the mantle

8. Tectonic Cycle • New Ocean Crust is produced at spreading ridges • Different spreading rates along the ridge are accomodated by Transform Faults • Compressional Stress at Convergent Boundaries results in Folding and Faulting of Crustal Rocks • Earthquakes are responses to tectonic stresses

9. Divergence at Spreading Ridges - most important area for creation of new crust - sea floor spreading apart at gradual and constant rate - ocean floor subducted at trenches, remains geologically young

10. Ocean-Continent Convergence - deep sea trenches on the ocean floor - denser plate of oceanic crust is "subducted" beneath continental crust - ocean floor remains geologically young

11. Convergent Boundary - Oceanic-Oceanic • both plates are the same density • Aleutian, Mariana, and Tonga Islands in Pacific • Antilles in Caribbean

13. Convergent Boundary - Continent-Continent • both plates are the same density • edges of colliding plates crumple into mountains • Himalayan Mountains in Nepal and India

16. divergent plate boundary convergent plate boundary transform faults/ triple junctions

17. Strike-slip (Transform) fault Reverse (Thrust) fault Normal fault

18. Transform Boundary • most transforms offset spreading ridges of oceanic crust • the San Andreas fault zone in California is a transform plate boundary • the Pacific plate is sliding horizontally to the northwest past a segment of the North American plate • transform plate motions result in the strongest shaking by earthquakes at the Earth's surface

19. Continental Transform Fault

20. San Andreas Transform Fault running across the Carrizo Plain in California

21. Figure 1.18a San Andreas Fault impounds drainage used by palm trees

22. Figure 1.18b Marsh in Pool impounded by San Andreas Fault

23. Rock Cycle • all types of rocks can be changed into other types by: • time • heat • pressure • beginning the cycle with igneous rocks (high temperature) is arbitrary

25. Igneous Rocks -crystallized from magma which either intruded deep beneath the surface ("plutonic") or extruded to the surface ("volcanic") Sediment -particles and dissolved substances which settle out of a liquid (oceans, river floodplains, swamps), also windblown (dunes) Sedimentary Rocks -sediment compacted by weight of overlying layers, cemented by percolating water, or chemically precipitated. Metamorphic Rocks -Igneous rocks, sediment, or sedimentary rocks altered by being subjected to temperature or pressure conditions above those at the Earth's surface.