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Earth ScienceEarth Science
Chapter 4Chapter 4
The Dynamic CrustThe Dynamic Crust
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Earth’s Crust and InteriorEarth’s Crust and Interior
• Earthquake waves help determine properties of Earth’s interior such as:– thickness– composition – temperature – density– pressure
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Earth’s Crust and InteriorEarth’s Crust and Interior
• The speed of earthquake waves change abruptly at the interfaces between layers of the Earth.– Waves speed up due to increasing density.– Example: speed increases at the interface
between the crust and asthenosphere.• This interface is the Mohorovicic
discontinuity, or “Moho” interface.
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EarthquakesEarthquakes
• An earthquake is the shaking of Earth’s crust caused by rapid moving of rocks. – Point beneath Earth’s surface where an
earthquake originates is the focus– Point on Earth’s surface directly above the
focus is called the epicenter
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EarthquakesEarthquakes
• When an earthquake occurs, it generates energy waves, called seismic waves.– Seismograph – instrument used to
detect and record earthquake waves
– Seismogram –recording of earthquake waves
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EarthquakesEarthquakes
• Main types of earthquake waves:– P-waves (primary waves)
• Particles vibrate in same direction as wave• Travel fast through solids, liquids & gasses
– S-waves (secondary waves)• Particles vibrate at right angles to wave• Travel slower than p-waves• Only travel through solids
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EarthquakesEarthquakes
• Since the outer core is liquid, S-waves do not travel through it and P-waves refract. – This results in a shadow zone where no
seismic waves are received.
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EarthquakesEarthquakes
• P-waves travel fast and arrive at a seismic station ahead of S-waves.
• The difference in travel times can be used to determine the distance from a seismograph station to the epicenter of an earthquake.
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EarthquakesEarthquakes
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EarthquakesEarthquakes
• To determine the exact epicenter data from three seismic stations must be used to construct three circles of possible locations.
• The point where all three circles intersect is the epicenter.
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EarthquakesEarthquakes
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EarthquakesEarthquakes
• The intensity and energy of an earthquake are measured on two different scales.– Richter Scale – measure of energy released
• Scale from 1 to 10 • Each step increases 10 times in magnitude
– Modified Mercalli Scale – measure of effect• Scale uses Roman numerals from I to XII • Higher numbers indicate greater damage
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Evidence for Crustal MovementEvidence for Crustal Movement
• Zones of frequent crustal activity can be located on the Earth’s surface.
• Most earthquakes, volcanoes, ridges, trenches, and mountains occur here.
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Evidence for Crustal MovementEvidence for Crustal Movement
• Continental Drift– Continental land masses have been moving
across the Earth’s surface for millions of years– Evidence supporting continental drift
• Continents look like they fit together• Similar rocks, minerals, and fossils are
found on different continents in areas that look like they once fit together.
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Evidence for Crustal MovementEvidence for Crustal Movement
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Evidence for Crustal MovementEvidence for Crustal Movement
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Evidence for Crustal MovementEvidence for Crustal Movement
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Evidence for Crustal MovementEvidence for Crustal Movement
• Plate Tectonics – Earth’s crust is divided into several sections
(plates) that move across Earth’s surface.
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Evidence for Crustal MovementEvidence for Crustal Movement6 min video
• Plates move due to convection cells produced by unequal heating of the fluids within the asthenosphere.
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Evidence for Crustal MovementEvidence for Crustal Movement
• There are three types of plate interactions– Divergent boundaries– Convergent boundaries– Transform boundaries
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Evidence for Crustal MovementEvidence for Crustal Movement
• Divergent Boundaries– Plates move apart from each other– New crust is formed here – Most commonly found at mid-ocean ridges
and develop deep rift valleys.
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Evidence for Crustal MovementEvidence for Crustal Movement
• Convergent Boundaries– Plates move toward each other– When 2 continental plates meet mountains
are formed.• Himalayan Mountains formed like this
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Evidence for Crustal MovementEvidence for Crustal Movement
– When an ocean plate and a continental plate converge subduction occurs. • Denser ocean plate slides under the less
dense continental plate.
• Ocean trench develops at point of subduction.• Volcanoes and mountains form along the
edge of the continental plate.
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Evidence for Crustal MovementEvidence for Crustal Movement
• Transform Boundaries– Plates slide horizontally past one another– Tension builds as plates slide and when
tension is released earthquakes occur.• San Andreas Fault is an example
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Evidence for Crustal MovementEvidence for Crustal Movement
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Evidence for Crustal MovementEvidence for Crustal Movement
• The ocean floor appears to be spreading based on two major pieces of evidence.
1. The age of igneous rocks increases as the distance from the mid-ocean ridges increases.
2. Parallel strips of igneous rocks on either side of the mid ocean ridge show periodic reversals of magnetic polarity.
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Evidence for Crustal MovementEvidence for Crustal Movement
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Evidence for Crustal MovementEvidence for Crustal Movement
• Deformed rock strata– Sedimentary rocks normally form in horizontal
layers.– Sedimentary layers that are not horizontal
provides evidence of change.
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Evidence for Crustal MovementEvidence for Crustal Movement
Syncline Anticline
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Evidence for Crustal MovementEvidence for Crustal Movement
• Displaced rock strata occurs along faults (cracks in rock formations).– The 3 types of faults are:
• Normal• Reverse• Transform or strike-slip
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Evidence for Crustal MovementEvidence for Crustal Movement
Normal Fault
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Evidence for Crustal MovementEvidence for Crustal Movement
Reverse Fault
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Evidence for Crustal MovementEvidence for Crustal Movement
Transform or Strike-slip Fault
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Evidence for Crustal MovementEvidence for Crustal Movement
• Displaced marine fossils are sometimes found high up in the mountains.
• Benchmarks labeled with date and elevation are set into ground and checked periodically.