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Plate Tectonics
Chapter 17
Great Idea:The entire earth is still changing, due to the slow
convection of soft, hot rocks deep within the planet.
Chapter Outline
• The Dynamic Earth• Plate Tectonics: A Unifying View of
Earth• Another Look at Volcanoes and
Earthquakes
Science by the Numbers
• How long could a mountain last?• The case of the disappearing
mountains
The Case of the Disappearing Mountains
• Erosion– Few hundred million years– Mountains continually forming
• Earth’s surface is not static
Pike’s Peak in Colorado, US
Slopes and Peaks in Young and Older Mountains
The Dynamic Earth
The Dynamic Earth
• Small-scale changes– Construction site
• Erosion by rain
• Large-scale changes– Volcanoes – Earthquakes – Erosion
Volcanoes and Earthquakes-Evidence of Earth’s Inner Forces
• Volcano– Magma breaks through surface
• Earthquake– Rocks breaks along fault– Energy transmitted as wave– Richter scale
Mount St. Helens – Active Volcano
Cross Section of a Volcano
Earthquake Scarp
Tsunami
The Movement of theContinents
• Francis Bacon– Continents like a puzzle
• Alfred Wegener– Continental Drift
• Continents in motion
Continental Atlantic Coastlines
The Movement of theContinents – cont.
• Current Evidence– Ocean floors– Magnetic reversals– Rock ages
Ocean Floors
• Mapping– Ocean floor dynamic
• Canyons, mountains• Mid-Atlantic Ridge
– Earthquakes, volcanoes, lava flows
Topographic Map of the Ocean Floor
Magnetic Reversals
• Earth’s magnetic field– Changes periodically
• Magnetite– Crystals in lava align to magnetic field
• Paleomagnetism• Seafloor spreading
– New rock comes to surface
Parallel Magnetic Strips
Magnetic Strips that Parallel Ocean Ridges
Rock Ages
• Radioactive isotopes– Rocks near Mid-Atlantic Ridge younger– Rocks farther away older
New Support for the Theory
• Measuring motion of continents• Radio astronomy
– Measured arrival of radio waves– Repeated over several years
• North America and Europe– Separating at 5 cm per year
Science by the Numbers
• The age of the Atlantic Ocean
• Pangaea
Plate Tectonics: A Unifying View of Earth
Plate Tectonics
• Plate tectonics– Large-scale surface features– Related phenomena
Plate Tectonics – cont.
• Tectonic plates– Rigid, moving sheet of rock– Crust and upper mantle– Continental
• 100 km thick• Lower density (granite)
– Oceanic• 8-10 km thick• Dense rock (basalt)
Plate Tectonics – cont.
• Earth’s surface– ¼ continent, ¾ water
Earth’s Major Plates and Direction of Motion
The Convecting Mantle
• Mantle convection– Motion driven by Earth’s interior heat
energy• Sources of energy
– Gravitational potential energy– Decay of radioactive elements
The Convecting Mantle –cont.
• Movement– Heat moves to cooler regions– Convection cells in mantle– Very slow
• 200 million years for one cycle
Mantle Convection
Science in the Making
• Reactions in plate tectonics• Pioneer expedition
Plate Boundaries
• Three main boundary types– Divergent– Convergent– Transform
Divergent Boundaries
• Characteristics– Volcanoes
• Chain of mountains
– Earthquakes
Divergent Plate Boundary
Divergent Boundaries –cont.
• Seafloor spreading– Plates pushed apart– Old spreading centers
• Located in middle of ocean
– New spreading centers• May begin anywhere
Africa’s Great Rift Valley
Convergent Plate Boundaries
• Types– Oceanic-oceanic
• Subduction zone– Deep oceanic trench– Island arc
Ocean-Ocean Plate
Convergent Plate Boundaries-cont.
– Continental-continental• High, jagged mountain chain
Continental-Continental Plate
Convergent Plate Boundaries-cont.
– Continental-oceanic• Subduction zone
– Deep oceanic trench– Coastal mountain range
Ocean-Continental Plate
Transform Plate Boundary
• Two plates move past each other– NOT smooth– Earthquakes as a result of movement
Transform Plate Boundary
The Science of Life
• Upright posture• Contributions by Richard Leakey
The Geological History of North America
• Northeastern Canada and Greenland– Several billion years old
• Western US– Terranes
• Added to continent over time
• Appalachian Mountains– Formed 450-300 million years ago– Continental-continental convergence
zone
The Geological History of North America – cont.
• Rocky Mountains– 60 million years ago– Warping, folding and fracturing of
continent
• The Colorado Plateau– Gentle uplift
• The Sierra Nevada– Molten rock pushed up sediments
US Mountains
Another Look at Volcanoes and Earthquakes
• Plates and volcanism– Divergent plate boundaries– Convergent plate boundaries
• Subduction zones
– Hotspots• Source stationary, plates move• Chain of volcanoes
• Earthquakes– At plate boundaries or elsewhere
Volcanoes Form Above a Subduction Zone
The Hawaiian Islands
Seismology: Exploring Earth’s Interior with Earthquakes
• Seismology– Study of sound vibrations within earth– Used to determine earth’s inner structure
• Seismic waves– Compressional or longitudinal– Transverse or shear waves
• Earthquake predictions
Seismic Waves Passing through Earth
The Ongoing Process of Science
• Seismic tomography
Technology
• The design of earthquake-resistant buildings