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Chapter 2: The Earth’s Mobile Crust
Plate Tectonics
The Earth’s interior
Plate Boundaries
Continental Margins
Tectonic Classification of Coasts
Tectonic effects on coastal sediment supply
Continental Drift-Plate Tectonics continents had once been together
Alfred Wegener 1920’s
Rejected (gravity Earth, Moon, Sun)
Continental Drift-Plate Tectonics New evidenceSea floor mappingMagnetic anomaliesEarthquake distributionHeat flowRadiometric datingSatellites
Theory of Mantle Convection (Hess, 1960s)
Shallow epicenters-Crustal movement
Mid-deep epicenterssubduction
Seismic refraction = change in speed of P & S waves
FactorsChemistryDensityPhysical State (liquid or solid)
Layered Structure of Earth
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Formation of the Universe, Solar System, Earth
– Big Bang, 13 bya– Formation of elementary particles– Gravitational formation of dense regions– 12 bya first stars– 4.55 bya Rotating cloud of gas from which sun
and planets formed, initiated by “supernova” = exploding star
– Accretion (Gaining material)– Differentiation (Separating based on density)– Evidence of rocks and water- 3.9 bya
Layered Structure of EarthInner core: r=1222km, primarily Fe & Ni T=4000-5500COuter core: 2258km thick, 3200C, liquid (partially melted)
Layered Structure of EarthMantle: 70% Earth’s volume, 2866m thick, 100-3200C,
Mg-Fe silicates, solid but can flowMohorovicic discontinuity: chemical boundary between
Mantle and Crust
Layered Structure of EarthContinental Crust:
Primarily granitic type rock (Na, K, Al, SiO2)40km thick on averageRelatively light
Oceanic CrustPrimarily basaltic (Fe, Mg, Ca, low SiO2)7km thickRelatively dense
Layered Structure of Earth (Physical Properties)
Lithosphere: 100km thickRigid slab (plate)
Asthenosphere350km thickPartially melted
(P,T,H2O)Mesosphere
Extends to mantle-core boundaryPressure=compact mineralogy, mechanically
strong
Tectonic or Lithospheric Plates
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Direction of Plate Movement
Rates: average 5cm/yearMid-Atlantic Ridge = 2.5 – 3.0 cm/yrEast-Pacific Rise = 8.0 – 13.0 cm/yr
Divergent
Convergent
Transform
Types of Plate Boundaries
Modern divergenceEast African Rift System
East African Rift System
Mid Atlantic Ridge Mid Atlantic Ridge
South Indian Ridge
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• Continent – Ocean
• Ocean – Ocean
• Continent – Continent
Convergent Plate Boundaries Convergent Plate Boundaries
Ocean-Ocean
Aleutian Islands, Alaska
Ocean – OceanCaribbean Islands
Continent – OceanWest Coast of South America
• Continent – Ocean
• Mount St. Helens
Fracture Zones-Transform faults
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Improved Mapping, WWII
Model of Mantle Convection (Hess, 1960’s)
radioactive decay, magma coming from the mantle or core-mantle boundary, gravitational sliding, slab pull
Continental Margins Continental Shelves:
broad shallow extension of the continents (~75km wide)
Regions of deposition (rivers, glaciers, scrapped marine deposits, calcium carbonate)
Large bedform features, reworked by tides, storms, waves
Continental Slopes:
continental crust thins into oceanic crust
steep (~20km, 1-25 degrees), 5deg Pacific, 3deg Atlantic
extend to depths between 1500-4000 m
Continental Rise:
Fan like deposit where the continental slope intersects the abyssal plains
Formed by turbidity currents
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Tectonic Evolution of Continental Margins
Atlantic Margin: passive margin, edge of the continent is in the middle of a lithospheric plate, little seismic activity, no volcanism
Pacific Margin: active margin, edge of the continent coincides with a plate boundary, rim of the Pacific Ocean, ring of fire, seismic activity and volcanism
Tectonic Setting (Pacific-vs-Atlantic type margin)
Tectonic Setting of opposite side of the continent
Geologic Age of the coast
Exposure to open ocean conditions
1. Collision Coasts
Continental collision coasts
Island arc collision coasts
2. Trailing Edge Coasts
Neo-trailing edge coasts
Afro-trailing edge coasts
Amero-trailing edgo coasts
3. Marginal Sea Coasts
Tectonic Coastline Classification (Inman & Nordstrom)
Continental collision coasts
continental – oceanic plate boundary
narrow margin, deep trench
seismic activity, coast backed by high mountains
Island Arc collision coasts
oceanic – oceanic plate boundary
narrow margin, deep trench
moderate mountain ranges
Ex Japan, Philippines, Aleutian Islands
Neo-Trailing Edge Coasts
Atlantic type margins
geologically young
Ex Red Sea, Gulf of Aden, Gulf of California
Afro-Trailing Edge Coasts
both sides of the continent are passive margins
lack organized drainage pattern
Ex Africa, Greenland
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Amero-Trailing Edge Coasts
opposite side of the continent is an active margin
organized drainage pattern
Ex Atlantic coast of N&S America and Europe, India
Marginal Sea Coasts
direct exposure to the ocean is reduced due to island arc system
Ex Gulf of Mexico, Bearing Sea, Sea of Japan, East and South China Seas
Microtidal: < 2 m
Mesotidal: 2 – 4 m
Macrotidal: > 2 m
Classification Based on Tidal Range
Walton and Adams, 1978
correlation of morphology based on energy regime
Hs2T2
Hs = significant wave height
T = significant wave period
Hs2T2 0 – 30 = mildly exposed coast
Hs2T2 30 – 300 = moderately exposed coast
Hs2T2 > 300 = highly exposed coast
Classification Based on Wave Energy
ShinnecockInlet
Tide Dominated
Wave Dominated
Mixed Energy
Hydrodynamic Coastline Classification
(Hayes, 1975)
Amero-Trailing Edge Coast
South Shore Significant Wave Conditions: H ~= 1.5 m, T = 8 sec
Micro-Tidal Environments
Long Island Classification
2 m
Tide Dominated
Tide Dominated &
Riverine
Wave Dominated
Mixed Energy
Gravel, sand
Gravel
Sand
Barrier Island
Cliff or Bluff Coast