plate tectonics chapter 3. how does the earth change with depth earth’s crust – outermost layer....
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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.
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.
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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
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.
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.
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
Mercalli Scale
• The modified Mercalli intensity scale (MMI)
• Describes the effects of an earthquake.
• On a scale from I to XII.