Review of the structure of the Earth (intro to geophysical lingo)

1. Review of the structure of the Earth(intro to geophysical lingo)

2. Basic facts you about the Earth HAVE TO KNOW

3. Kind of true: The Earth is a sphere. More true: The Earth is a spheroid. Best description: The Earth is an oblate spheroid (like a basketball someone’s sitting on). The Earth has a radius of 6371 km from pole to pole (6378 in the perpendicular direction). Basic Fact #1:

4. Aside …. Measuring the Earth …. • Latitude = equator-parallel • Longitude = meridians, pole to pole, equator-perpendicular • One degree of latitude = 111 km • (longitude varies depending on latitude) • 1 knot = 1 nautical mile per hour • (= 1.15 mph = 1.85 km/h) • 1 nautical mile = 1 minute (‘) of latitude Latitude Longitude

5. Basic Fact #2: The Earth is old. 4.55 billion years old.

6. Basic Fact #3: The Earth’s surface is mainly water (71%). • Submarine topography is • very rugged: mid-ocean • ridges, continental • margins (drowned edges • of the continents), deep- • ocean basins, deep-sea • trenches. • Deepest trench (Mariana • in western Pacific) is 11 • km deep; Hawaiian • Islands are taller than the • Himalayas when • measured from the • seafloor.

7. Basic Fact #4: The Earth’s surface is divided into tectonic plates. Plate boundaries can be (1)mid-ocean ridges (Mid-Atlantic Ridge) subduction zones (Aleutians, western Pacific, Chile) (3)transform faults where plates slide past one another (San Andreas Fault). Hotspots (Hawaii, Iceland)

8. Plate Boundaries: Ridges • Mid-ocean ridges • Divergent boundaries • Where oceanic crust is created • Spread 1-10 cm/yr • Slow-spreading ridge: Mid-Atlantic (1 cm/yr) • Intermediate-spreading ridge: Southeast Indian (3 cm/yr) • Fast-spreading ridge: East Pacific Rise (7.5 cm/yr)

9. Plate Tectonics A schematic cross-section of the Pacific Ocean

10. Plate Boundaries: Subduction • Trenches (subduction zones) • Convergent boundaries • One plate dives beneath another • Where oceanic crust is recycled • Converge 1-10 cm/yr • Subducted slabs can be traced seismically to at least 700 km • Large earthquakes

11. Plate Tectonics A schematic cross-section of the Pacific Ocean

12. Mid-Ocean Ridge Examples Coaxial segment, Juan de Fuca Ridge East Pacific Rise 9oN

13. Trench Example: Mariana • Created by the Pacific Plate diving beneath the Philippine Plate. • 10,912 m deep. “Pressure is over 8 tons per square inch, or the equivalent of an average-sized woman holding up 48 jumbo jets” (from Smithsonian Institution Ocean Planet website).

14. Hotspot Example: Hawaii The Hawaii-Emperor island chain was created by a long-lived hotspot or mantle plume that burns through the Pacific lithosphere.

15. Hotspot example: Iceland Seismically-derived image of the plume beneath Iceland. Thingvellir, Iceland -- the on-land expression of the Mid-Atlantic Ridge. N. American plate is to the right of the photo; Eurasian plate is to the left.

16. Basic Fact #5: The Earth is layered. There are two naming schemes for the layers, and which one you use depends on what you care about: (1) Lithosphere and asthenosphere (2) Crust, mantle, and core

17. Basic Fact #6: Things happen very differently at depth than on the observable surface of the Earth.

18. “like studying a car engine without being able to open the hood” --A. Alden Basic Fact #7: The movie “The Core” is terrible. Or, rephrasing, humans can’t explore the subsurface Earth. We have to rely on interpreting measurements that we make at the surface. What can be measured? -- How seismic waves travel through the Earth -- What the Earth’s gravity field looks like -- What the Earth’s magnetic field looks like -- How much heat the Earth produces That’s what we’ll cover the rest of the semester.