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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