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

10.02.d

4th 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

5th 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

6th 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)

• 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:http://www.classzone.com/books/earth_science/terc/content/visualizations/es1002/es1002page01.cfm?chapter_no=visualization

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/cm3

Earth's average density; ~5.5 g/cm3

Mantle (3000 km dia.)• Average density=4.5 g/cm3

• 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– 40-400 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 andradioactivity.

Internal Structure

– Epicenter-- surface projection from center through the focus

– Seismic waves can be reflected and refracted (Snell's law: n1sin1 =n2sin2)

– 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)– Mid-Oceanic ridges– Iceland– African Rift Valley

Convergent (subduction)– 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)

Triple junctions (Mendocino triple junction, Red Sea, and others)

Show animation (Atwater) of plate boundary movement/migration

Plate Boundaries

R. E. Wallace (228), U.S. Geological Survey

Plate Boundaries in the field

W. W. Norton

Application of Plate Tectonics – Hawaiian Island Chain and Plate Motion History

W. W. Norton

Application of Plate Tectonics – Hawaiian Island Chain and Plate Motion History

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

Rock Cycle

Hydrologic Cycle

Biogeochemical Cycle