1

Plate Tectonics5 November 2015

Chapter 17

Great Idea:The entire Earth is still changing, due to the slow

convection of soft, hot rocks deep within the planet.

2

Chapter Outline

• The Dynamic Earth• Plate Tectonics: A Unifying View of

Earth• Another Look at Volcanoes and

Earthquakes

3

The Dynamic Earth

4

The Dynamic Earth

• Small-scale changes– Construction site

• Erosion by rain

• Large-scale changes– Volcanoes – Earthquakes – Erosion

5

The Case of the Disappearing Mountains

• Erosion– Few hundred million years– Mountains continually forming

• Earth’s surface is not static

6

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

7

The Movement of the Continents

• F. Bacon– Continents like a

puzzle

• Wegener– Continental Drift

• Continents in motion

• Current Evidence– Ocean Floors– Magnetic

Reversals– Rock Ages

8

Ocean Floors

• Mapping– Ocean floor

dynamic• Canyons,

mountains• Mid-Atlantic Ridge

– Earthquakes, volcanoes, lava flows

9

Magnetic Reversals

• Earth’s magnetic field– Changes periodically

• Magnetite– Crystals in lava align to magnetic field

• Paleomagnetism• Seafloor Spreading

– New rock comes to surface

10

Magnetic Reversals

11

Rock Ages

• Radioactive Isotopes– Rocks near Mid-Atlantic Ridge younger– Rocks farther away older

12

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

13

Plate Tectonics: A Unifying

View of Earth

14

Plate Tectonics• Plate Tectonics

– Large-scale surface features

– Related phenomena• 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)

• Earth’s surface– ¼ continent, ¾ water

15

Earth’s Plates

16

The Convecting Mantle• Mantle convection

– Motion driven by Earth’s interior heat energy• Sources of energy

– Gravitational potential energy– Decay of radioactive elements

• Movement– Heat moves to cooler regions– Convection cells in mantle– Very slow

• 200 million years for one cycle

17

Plate Boundaries

• Three Main Boundary Types– Divergent– Convergent– Transform

18

Divergent Boundaries

• Characteristics– Volcanoes

• Chain of mountains

– Earthquakes

• Seafloor spreading– Plates pushed apart– Old spreading centers

• Located in middle of ocean

– New spreading centers• May begin anywhere

19

Convergent Plate Boundaries

• Types– Oceanic-oceanic

• Subduction zone– Deep oceanic trench– Island arc

– Continental-continental• High, jagged mountain

chain

– Continental-oceanic• Subduction zone

– Deep oceanic trench– Coastal mountain range

20

Transform Plate Boundary

• Two plates move past each other– NOT smooth– Earthquakes as a result of movement

21

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

• 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

22

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

23

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• P-waves (pressure waves)

– Transverse or shear waves• S-waves

24

Seismology: Exploring Earth’s Interior with Earthquakes

25

• Plate tectonics is involved in the formation of the three major categories of rocks– Igneous Rock

• cooled from molten material

– Sedimentary Rock • Layered eroded

material formed by the action of wind, water, or ice

– Metamorphic Rock • Rock that has

been altered in the solid state by extreme heat and pressure