Ch17

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