10.02.d 4 th Piece of Evidence: Magnetic Striping of Sea Floor Blackboard Exercise: Calculate Sea floor spreading rate…

Thinnest near mid-ocean ridges Thickest along passive continental margins Thick offshore of large rivers 5 th Piece of Evidence: Sediment Thickness Pattern

Age increases systematically out from ridge Mid-ocean ridges less deep because young Deepest seafloor is oldest Age patterns truncated at trenches Depth (dark is deep) Age (orange is young) Correlation of sea floor depth and age

6 th Piece of Evidence: Sea floor heat flow pattern

Earth’s Plates / Plate Tectonic Theory

Current Plate Tectonic Theory "Chocolate covered cherry" analogy Rigid outer shell Solid core Moveable liquid between the two Earth's Structure 6371 km mean diameter Internal structure characteristics Composition and density Behavior (solid:liquid; weak:strong) Unifying concept of geology Evolution to biology Relativity to physics

Current Plate Tectonic Theory Tectonics (Greek tecton = builder) Movement of Lithospheric Plates Large scale geologic processes (landforms, ocean basins, and mountains) Driven by forces deep within the Earth Lithosphere: 12 major plates (boiled egg-shell mode)12 major plates Plate tectonics: processes related to creation, movement, and destruction of plates Plates may include both continents and parts of ocean basins or ocean basins alone; may large (Pacific Plate) or small (Juan de Fuca Plate)

How do we know Internal Structure? Primarily based on seismology (earthquakes and seismic waves) – Primary waves (compressional) propagate the fastest (6.5 km/sec in the crust) and pass through liquids and solids. –Secondary (shear) waves propagate through solid materials, but not through liquid; about 4 km/sec in crust –Focus--the site where energy is first released –Focus depth--distance below the surface Link to seismic waves animation:

Internal Structure Inner core (1,300 km dia.) Mostly iron (90%); Some Ni, S, and O Outer core (2,000 km dia.) Liquid similar in composition to inner core Densities of inner and outer cores about same =10.7 g/cm 3 Earth's average density; ~5.5 g/cm 3 Mantle (3000 km dia.) Average density=4.5 g/cm 3 Iron & magnesium silicates The Mohorovicic discontinuity = Between the crust and lithosphere Lithosphere –Made up of the rigid mantle and crust –Cool, strong, outermost layer of Earth; averages about 100 km thick –Thin at mid-oceanic ridges; 120 km under oceans – km thick under continents Asthenosphere – Hot, slowly flowing layer of relatively weak rock – Low seismic velocity zone

Crust –Top of the lithosphere –Less dense than mantle –Oceanic crust »6-7 km thick »More dense than continental crust »Less than 200,000 My years old –Continental crust »May be billions of years old »Different geologic histories »Average thickness about 35 km (70 km max.) Internal Structure Continued

Processes Driving Plate Motion –Convection cells to cycle materials on long residence times (500 my) –Powered by heat from outer core and radioactivity.

Internal Structure –Epicenter-- surface projection from center through the focus –Seismic waves can be reflected and refracted (Snell's law: n 1 sin  1 =n 2 sin  2 ) –P-waves show low velocity zone at core-mantle boundary; some reflected or refracted –S-waves dissipated at the core- mantle boundary suggesting a liquid outer core

Plate Boundaries Divergent (spreading centers)Divergent – Mid-Oceanic ridges – Iceland – African Rift Valley Convergent (subduction)Convergent – Ocean-ocean (Japan and other Pacific trenches) – Ocean-continent (Andes Mts. in Latin America) – Continent-continent (Himalayan Mts. between India –and China) Transform (San Andreas fault)Transform Triple junctions (Mendocino triple junction, Red Sea, and others)Triple junctions Show animation (Atwater) of plate boundary movement/migration

Plate Boundaries

Fig.4.17b R. E. Wallace (228), U.S. Geological Survey Plate Boundaries in the field

Fig.4.21 W. W. Norton Application of Plate Tectonics – Hawaiian Island Chain and Plate Motion History

Fig.4.22a W. W. Norton Application of Plate Tectonics – Hawaiian Island Chain and Plate Motion History

Fig.4.22b W. W. Norton Origin of Hawaiian Island Chain – Hotspot/Mantle Plume

Plate Tectonics and Environmental Geology Effects Distribution of mineral resources Earthquakes and volcanoes Ocean currents and global climate

Rock Cycle

Hydrologic Cycle

Biogeochemical Cycle