Chapter 2 plate tectonics and physical hazards
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Chapter 2 - Plate Tectonics and Physical Hazards. The Big Picture. Pieces of Earth’s surface Move around Grind sideways Collide Sink into Earth’s hot interior Collisions create mountain ranges, cause tsunami Less directly, mountain ranges affect weather and climate.

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Chapter 2 plate tectonics and physical hazards

Chapter 2 - Plate Tectonics and Physical Hazards

The big picture
The Big Picture

  • Pieces of Earth’s surface

    • Move around

    • Grind sideways

    • Collide

    • Sink into Earth’s hot interior

  • Collisions create mountain ranges, cause tsunami

    • Less directly, mountain ranges affect weather and climate

Development of the theory
Development of the Theory

  • Continents drifted through oceanic crust  continental drift

  • Fit across Atlantic is best using edges of continental shelves

  • Mountain ranges, rocks, fossils and glacial erosion patterns match across Atlantic

  • Glaciers existed in areas now warm and tropical

  • Unsure of the mechanism causing the drift

Continents of Africa and South America fit together like puzzle pieces

Alfred Wegener (1912) proposed supercontinent Pangaea

The Origins of Continents and Oceans (1915)

Development of the theory2
Development of the Theory

Wegener’s continental drift hypothesis was rejected because its mechanism (continents plowing through ocean floors) was deemed not to be physically possible

More information about topography, appearance, age of ocean floor led to understanding of separation of continents

Late 1940s through the 1960s: discovery of mid-oceanic ridges and the science of paleomagnetism

Development of the theory3
Development of the Theory

  • Harry Hess (1960): ocean floors act as conveyor belts carrying continents in process of seafloor spreading

    • New oceanic crust wells up at mid-oceanic ridges

    • Oceanic crust spreads away from ridges

    • Oceanic crust sinks into deep oceanic trenches at continents’ edges

    • Spreading rate calculated to be 2.5 cm/year

    • Whole Atlantic Ocean floor created in about 180 million years

Development of the theory5
Development of the Theory

  • Confirmation of seafloor spreading in mid-1960s

    • Alternating polarity of Earth’s magnetic field is recorded in basaltic rocks of ocean floors

    • Vine and Matthews (early 1960s) found striped alternating magnetic pattern, parallel to mid-oceanic ridge, representing bands of rocks formed at ridge in past periods of Earth’s switching magnetic field polarity

      • Came to be known as the Vine-Matthews hypothesis

    • Rocks at ridges found to be young (< 1 million years old); rocks at trenches much older

Development of the theory8
Development of the Theory

  • Seafloor spreading and other evidence supports theory of plate tectonics (describes movement of Earth’s plates)

    • Landmasses formed Pangaea 225 million years ago

    • Pangaea breakup moved continents into current positions

Development of the theory10
Development of the Theory

  • Development of theory of plate tectonics as example of scientific method

    • Logical analysis of data to solve problems

    • Scientists make observations, develop tentative explanations (hypotheses)

    • Hypothesis must be testable

    • Tests and observations lead to acceptance, revision or rejection of hypothesis

    • If hypothesis continues to be supported by evidence over long time, it becomes a theory

    • Wegener’s continental drift hypothesis was modified to become theory of plate tectonics

Earth structure
Earth Structure

  • Layers of Earth based on rock composition: core, mantle, crust

  • Layers of Earth based on rock rigidity or strength:

    • Lithosphere is stiff, rigid outer rind of Earth, which makes up plates from 60 km (oceanic) to 200 km (continental) thick, including crust and denser underlying mantle

    • Asthenosphere is inner, hotter, more easily deformed layer

Earth structure2
Earth Structure

  • Continental crust

    • Silica-rich

    • Low density 2.7 g/cm3

    • 30-50 km thick

    • Stands higher than denser oceanic crust

  • Oceanic crust

    • Iron-, magnesium-rich, silica-poor

    • Higher density 3.0 g/cm3

    • 7 km thick

    • Floats lower than continental crust, on top of denser mantle

Earth structure do density lab here
Earth Structure (Do Density Lab here!)

  • Elevation difference between continental and oceanic crust explained by isostacy

    • Floating solid object displaces liquid of same mass

    • Crust (continental or oceanic) floats atop mantle (denser mantle slowly flows away to accommodate crust)

Earth structure4
Earth Structure

Boundary between crust and mantle identified as density difference: Mohorovicic Discontinuity (Moho)

Boundary between lithosphere and asthenosphere has been identified as zone of lower velocity for seismic (earthquake) waves traveling through mantle: low-velocity zone (LVZ)

Earth structure6
Earth Structure

  • Lithosphere is broken into about 12 large plates, most of which are combination of continental and oceanic areas, moving up to 11 cm/year

  • Plates move away from each other at divergent boundaries – usually mid-oceanic ridges

  • Plates move toward each other at convergent boundaries

    • Subduction zone: one plate dives under other, into mantle

    • Continent-continent collision: low density of continents prevents subduction, crumple up into each other instead

  • Plates slide past each other at transform boundaries

Earth structure7
Earth Structure

Lithosphere is broken into about 12 large plates, most of which are combination of continental and oceanic areas, moving up to 11 cm/year

Earth structure8
Earth Structure

Three types of boundaries, divergent, convergent and transform, found at western edge of North America

Hazards and plate boundaries
Hazards and Plate Boundaries

Most earthquake and volcanic activity occurs at plate boundaries

Subduction zones between oceanic and continental plates along Pacific coasts

Collisions between continents in mountain belts of southern Europe and Asia

Rapidly spreading divergent boundaries follow mid-oceanic ridges

Slowly spreading divergent boundaries pull apart continents (East African Rift zone)

Divergent boundaries
Divergent Boundaries

  • Plates pull apart (by sinking of heavy lithosphere at trenches)

  • System of connected oceanic ridges through ocean basins

    • Only above sea level at Iceland

    • Fissures open every 200-300 years in Iceland’s central valley, erupting large basalt flows

  • Spreading centers are source of basalt that covers all ocean floors, 2/3 Earth’s surface

  • Little volcanic or earthquake hazard

Convergent boundaries
Convergent Boundaries

  • Subduction zones – where two plates meet, denser (oceanic) plate slides beneath other and sinks into mantle

  • Contact zone sticks then slips, in huge earthquakes

    • Sudden movement of ocean floor can generate tsunami

  • Line of volcanoes forms inland from subduction trench

    • After being subducted, water-rich serpentinite heats up and breaks down, releases water

    • Released water rises into mantle, causes partial melting

    • Basalt magma rises to surface to form volcano

    • If basalt magma rises through continental crust, mixes to form rhyolite magma

Collision of continents
Collision of Continents

  • Neither plate sinks (too light to penetrate mantle)

  • High mountains are pushed up

    • Accompanied by large earthquakes

    • Forms Himalayas, Tibetan plateau, Caucasus

Transform boundaries
Transform Boundaries

  • Plates slide past each other without pulling apart or colliding

  • Oceanic transform faults offset mid-oceanic ridges

    • Direction of offset does not indicate relative movement on transform fault

  • Transform faults on continents generate large earthquakes, near large population centers

    • North Anatolian fault in Turkey

    • San Andreas fault in California

Hotspot volcanoes
Hotspot Volcanoes

  • Volcano is surface expression of hotspot:

    • Column of hot, partially molten rock rising through mantle, burning through overlying moving plate, to form track of volcanoes, far from plate boundary

    • Hotspot volcanoes erupt at active end of chain of extinct volcanoes, which get progressively older farther from active end

    • Hawaiian-Emperor chain in Pacific, Yellowstone line of resurgent calderas

    • Columns of hot rock (plume) appear fixed in mantle

    • Hot rock melts near surface as pressure drops