Download Chapter 2: The Earth`s Mobile Crust Continental Drift

Continental Drift-Plate Tectonics
continents had once been
together
Chapter 2: The Earth’s Mobile Crust
Plate Tectonics
Alfred Wegener 1920’s
The Earth’s interior
Plate Boundaries
Rejected (gravity Earth,
Moon, Sun)
Continental Margins
Tectonic Classification of Coasts
Tectonic effects on coastal sediment supply
Continental Drift-Plate Tectonics
New evidence
Sea floor mapping
Magnetic anomalies
Earthquake distribution
Heat flow
Radiometric dating
Satellites
Theory of Mantle
Convection (Hess, 1960s)
Seismic refraction =
change in speed of P & S waves
Shallow epicentersCrustal movement
Mid-deep epicenters
subduction
Layered Structure of Earth
Factors
Chemistry
Density
Physical State (liquid or solid)
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Formation of the Universe, Solar System, Earth
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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 Earth
Mantle: 70% Earth’s volume, 2866m thick, 100-3200C,
Mg-Fe silicates, solid but can flow
Mohorovicic discontinuity: chemical boundary between
Mantle and Crust
Layered Structure of Earth
Inner core: r=1222km, primarily Fe & Ni T=4000-5500C
Outer core: 2258km thick, 3200C, liquid (partially melted)
Layered Structure of Earth
Continental Crust:
Primarily granitic type rock (Na, K, Al, SiO2)
40km thick on average
Relatively light
Oceanic Crust
Primarily basaltic (Fe, Mg, Ca, low SiO2)
7km thick
Relatively dense
Layered Structure of Earth (Physical Properties)
Tectonic or Lithospheric Plates
Lithosphere:
100km thick
Rigid slab (plate)
Asthenosphere
350km thick
Partially melted
(P,T,H2O)
Mesosphere
Extends to mantle-core boundary
Pressure=compact mineralogy, mechanically
strong
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Direction of Plate Movement
Divergent
Types of Plate Boundaries
Convergent
Rates: average 5cm/year
Mid-Atlantic Ridge = 2.5 – 3.0 cm/yr
East-Pacific Rise = 8.0 – 13.0 cm/yr
Modern divergence
East African Rift System
Mid Atlantic Ridge
Transform
East African Rift System
Mid Atlantic Ridge
South Indian Ridge
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Convergent Plate Boundaries
• Continent – Ocean
Convergent Plate Boundaries
Ocean-Ocean
Aleutian Islands, Alaska
• Ocean – Ocean
• Continent – Continent
Ocean – Ocean
Caribbean Islands
Continent – Ocean
West Coast of South America
• Continent – Ocean
Fracture Zones-Transform faults
• Mount St. Helens
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Model of Mantle Convection (Hess, 1960’s)
Improved Mapping, WWII
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 Rise:
continental crust thins into oceanic crust
Fan like deposit where the continental slope intersects
the abyssal plains
steep (~20km, 1-25 degrees), 5deg Pacific, 3deg Atlantic
extend to depths between 1500-4000 m
Formed by turbidity currents
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Tectonic Evolution of Continental Margins
Tectonic Coastline Classification (Inman & Nordstrom)
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
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
Continental collision coasts
2.
Trailing Edge Coasts
Neo-trailing edge coasts
Afro-trailing edge coasts
Amero-trailing edgo coasts
3.
Marginal Sea Coasts
Island Arc collision coasts
continental – oceanic plate boundary
oceanic – oceanic plate boundary
narrow margin, deep trench
narrow margin, deep trench
seismic activity, coast backed by high
mountains
Neo-Trailing Edge Coasts
Atlantic type margins
geologically young
Ex Red Sea, Gulf of Aden, Gulf of California
moderate mountain ranges
Ex Japan, Philippines, Aleutian Islands
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
Classification Based on Tidal Range
Classification Based on Wave Energy
correlation of morphology based on energy regime
Hs2T2
Microtidal:
<2m
Mesotidal:
2–4m
Macrotidal:
>2m
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
Walton and Adams, 1978
Long Island Classification
Hydrodynamic Coastline Classification
Amero-Trailing Edge Coast
South Shore Significant Wave Conditions: H ~= 1.5 m, T = 8 sec
Tide Dominated
Micro-Tidal Environments
Tide Dominated
Wave Dominated
Shinnecock
Inlet
Tide Dominated
&
Cliff or Bluff
Coast
Gravel, sand
Riverine
Mixed Energy
Gravel
2m
Wave Dominated
Sand
Barrier Island
Mixed Energy
(Hayes, 1975)
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