4th Piece of Evidence:
Magnetic Striping of Sea Floor
10.02.d
Blackboard Exercise: Calculate Sea floor spreading rate…
5th Piece of Evidence:
Sediment Thickness Pattern
Thickest along passive
continental margins
Thinnest near
mid-ocean ridges
Thick offshore
of large rivers
Correlation of sea floor depth and age
Deepest seafloor is oldest
Mid-ocean ridges less
deep because young
Depth (dark is deep)
Age increases
systematically out from
ridge
Age patterns
truncated at trenches
Age (orange is young)
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/cm 3
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
Internal Structure Continued
• 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.)
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: n1sinq1
=n2sinq2)
– P-waves show low velocity zone
at core-mantle boundary; some
reflected or refracted
– S-waves dissipated at the coremantle 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
Plate Boundaries in the field
R. E. Wallace (228), U.S. Geological Survey
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
W. W. Norton
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