Plate Tectonics

Chapter 3

How Does The Earth Change With Depth

• Earth’s Crust – Outermost layer. – Continental Crust - 35-40 km in thickness– Oceanic Crust – exists beneath oceans.

Average of 7 km thick. – Mantle – extends from base of crust down

2,900 km.– Outer Core – Molten iron and nickel.

• 2900 to 5150 km

– Inner Core – Solid iron and nickel.

Layers of the earth

Some Layers Are Stronger Than Others

• Earth’s layers are defined by how easily they are affected by force.

• Lithosphere vs. Asthenosphere – Lithosphere – Uppermost portion of mantle

and crust. Relatively strong layer• From the Greek word, stone

– Asthenosphere – Lower mantle the soft, weak zone over which the lithosphere moves.

• From the Greek word, not strong

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Some Layers Are Stronger than Others

Oceanic crustOceanic crust

Continental crustContinental crust

Uppermost mantleUppermost mantle

Lithosphere Lithosphere (stronger)(stronger)

Asthenosphere (weaker)Asthenosphere (weaker)

Asthenosphere: hot and weak; mostly solidAsthenosphere: hot and weak; mostly solid

Why Are Some Regions HigherIn Elevation than Others?

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Observe the relationship between the height of each block and its thickness relative to other blocks

Dense materials are lower

Thick blocks higher than thin blocks

Relationship between crustal thicknessand elevation: isostasy

The Plate Tectonic Theory

• Describes the formation, motion and interaction of the plates.

• Continents are embedded in lithospheric plates.

• As plates move, continents move.

Divergent Boundaries

• 2 plates separating.

• Caused the formation of the Mid-Ocean Ridge.

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Stretching and faulting form rift Stretching and faulting form rift

Initial uplift from rising mantleInitial uplift from rising mantle

Can lead to seafloor spreading Can lead to seafloor spreading and new ocean basinand new ocean basin

Ocean widens with spreadingOcean widens with spreading

Continents rift apart

Sketch a mid-ocean ridge Sketch a mid-ocean ridge

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Magma rises Magma rises through fracturesthrough fractures

Narrow trough, or Narrow trough, or riftrift

Asthenosphere Asthenosphere rises and meltsrises and melts

Magma erupts Magma erupts or solidifies at or solidifies at depth; forms depth; forms new oceanic new oceanic crustcrust

Ridge high (hot rocks and Ridge high (hot rocks and thin lithosphere)thin lithosphere)

Convergent Boundaries

• When 2 plates move toward each other

• 2 types – Subduction – When an

oceanic plate plunges beneath another plate

– Collision – 2 continental plates collide.

• Forms mountain ranges

Subduction BoundariesOcean-Ocean subduction

• When 2 oceanic plates converge and one plunges below the other..– Causes formation

of volcanic islands.

Subduction boundariesOcean-Continental

• Form when oceanic plate collides with continental plate. – Oceanic plate ALWAYS

plunges because it’s more dense.

– Forms a trench.• Due to friction.

– Forms volcanic mountains inland.

Transform Boundaries

• 2 plates sliding against each other.– Causes earthquakes – Transform boundaries

link other types of plate boundaries such as a mid-ocean ridge and a trench.

• Location of earthquakes, volcanoes and mountain ranges.

• Magnetism and age of ocean floor. – The change in magnetic poles is recorded in

igneous rock.

• The shape of the continents

• Mesosaurus fossils in S. America and Africa.

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Evidence of Plate Tectonics

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Compare the distribution of earthquakes (yellow), Compare the distribution of earthquakes (yellow), volcanoes (orange), and high elevations (brown)volcanoes (orange), and high elevations (brown)

EQ, volcanism, EQ, volcanism, or mtn. building = or mtn. building = tectonic activitytectonic activity

Belts of tectonic Belts of tectonic activity divide activity divide lithosphere into lithosphere into tectonic platestectonic plates

Magnetism and the age of the earth’s floor

• Mid-Ocean Ridge (MOR)- a long chain of volcanic mountains on the ocean floor with a deep central valley.– Scientists found rock on

either side of MOR were identical.

– The center of the ridge always showed current magnetic orientation.

Evolution of South America

Observe the Observe the evolution of evolution of South America, South America, beginning with beginning with continental rifting continental rifting away from South away from South AmericaAmerica

Middle Middle MesozoicMesozoic(140 m.y ago)(140 m.y ago)

Late Late MesozoicMesozoic(100 m.y ago)(100 m.y ago)

PresentPresent

Shape of the Continents/ Mesosaurus

How Do Plates Move?

• Movement requires a driving force overpowering a resisting force. – Question: What forces

drive tectonic plates?

• Ridge Push • Slab Pull

Ridge Push/Slab PullRidge pushRidge push

Slab pullSlab pull Other forces, such as Other forces, such as convection in mantleconvection in mantle

Ridge Push/Slab Pull

• Ridge Push – MOR is higher than the ocean floor because the lithosphere near the ridge is thinner and hotter and gravity causes the plate to slide away and outward.

• Slab Pull – Subducting oceanic lithosphere is denser than the surrounding asthenosphere. Gravity pull the plate downwards into the asthenosphere. – **Strongest driving force**

Plate moves over a hot spotPlate moves over a hot spot

Lines of islands Lines of islands and seamountsand seamounts

Volcano Volcano forms forms over hot over hot spotspotPlate subsides Plate subsides

as cools, so as cools, so islands become islands become seamountsseamounts

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Volcanism shuts Volcanism shuts off as area off as area moves awaymoves away

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Observe the Observe the features features around South around South AmericaAmerica

Envision a Envision a cross section cross section from west of from west of

South South America America

to the Mid-to the Mid-Atlantic Atlantic

RidgeRidge

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Compare this cross section with the one Compare this cross section with the one you envisionedyou envisioned

Eastern edge of Eastern edge of continent not a continent not a plate boundaryplate boundary

Spreading along Spreading along mid-ocean ridgemid-ocean ridge

Subduction beneath Subduction beneath western edgewestern edge

Andes over subduction zone; Andes over subduction zone; trench offshoretrench offshore

Earthquakes

• An earthquake occurs when mechanical energy is released and transmitted through rock as vibrations called seismic waves.

• Hypocenter (focus) – point where earthquake is generated. –Usually depths less than 100 km.

Earthquakes continued

• Epicenter – point on Earth’s surface directly above hypocenter. (p. 332)

EpicenterEpicenter

HypocenterHypocenter

Causes of Earthquakes

• Caused by movement along faults.

• Three types (p. 332)

• Normal Faults – Rocks above fault (hanging wall) move down against the lower rocks (foot wall).

• Occurs mostly at divergent plate boundaries.

Causes of Earthquakes

• Reverse and Thrust Faults – The hanging wall moves up against the foot wall.

• Happens at convergent boundaries.

Causes of Earthquakes

• Strike-Slip Faults – The two sides slip horizontally past each other.

• Occurs at transform boundaries.

Build Up and Release of Stress

• A build up of stress caused by friction created by sliding plates is released when the stress equals the fault strength. (P. 335)

• The stress is immediately released and decreases to the original level.

Earthquake Waves

• Earthquakes travel in seismic waves.

• Seismic is the Greek word for earthquake.

• There are 2 types.

– Body waves – travel inside the earth.

– Surface waves – travel on the surface of the earth.

• Body waves become surface waves.

Wave Shape

• Crest – the top of a wave

• Trough – the bottom of the wave

Surface Waves

• Vertical Surface Wave – Travel up and down.

• Horizontal Surface Waves – Travel perpendicular to the direction of travel.

Body Waves

• Two types• Primary (p-waves) – Travel in the same

direction in which it starts. (like a slinky)–Travels through liquids and solids

because the particles are close enough.

–Faster!

Body Waves

• Secondary (s-waves) – Travel up and down, they sheer the rock. (like a snake)

–Cannot travel through liquids because they’re not rigid. Only through solids.

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Surface wavesSurface waves

Secondary body Secondary body wave (S-wave) wave (S-wave) shears materialshears material

Primary body Primary body wave (P-wave) wave (P-wave) compresses compresses material; fastestmaterial; fastest

Vertical surface waveVertical surface wavecompresses materialscompresses materials

Horizontal Horizontal surface wave surface wave shears materialshears material

Body wavesBody waves

Recording Seismic Waves

• Seismometers are used to record ground motion during earthquakes

• They also calculate the hypocenter and magnitude.

Reading a Seismograph

• Seismogram plots vibrations vs. time. Time is marked at regular intervals to determine arrival of first p and s waves. (p. 339)

Waves on seismogramWaves on seismogram

SeismogramSeismogram

Amplitude vs. Period

• Seismic waves are characterized by wave amplitude – How much the ground moves

• And…the time it takes for the wave to pass by (the period)

Determining Location

• Seismograph stations in different locations record the P-S interval.

• The difference in P-S interval shows the distance of the earthquake from the seismograph station. – The longer the interval, the farther the

earthquake.

p. 340 in your book

Estimating Station Distance

• The P-S interval is mathematically related to the distance from the epicenter using a time-travel curve. (p. 340)

• Takes into account the materials the wave passes through.

Triangulating the Epicenter

• Distance from each station can be compared graphically to find the epicenter.

• Circle is drawn around each station with a radius equal to the distance calculated from the time-travel curve

• Intersection of circles is the

epicenter

Measuring Magnitude

• Richter Scale – Based off distance and amplitude.

Mercalli Scale

• The modified Mercalli intensity scale (MMI)

• Describes the effects of an earthquake.

• On a scale from I to XII.

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Hebgen Lake, 1959Hebgen Lake, 1959Alaska, 1964Alaska, 1964

San Francisco, 1906San Francisco, 1906

Northridge, 1994Northridge, 1994

Charleston, 1886Charleston, 1886

Mexico City, 1985Mexico City, 1985