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Earth Science. Chapter 4 The Dynamic Crust. Earth’s Crust and Interior. Earthquake waves help determine properties of Earth’s interior such as: thickness composition temperature density pressure. Earth’s Crust and Interior.
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Earth Science Chapter 4 The Dynamic Crust
Earth’s Crust and Interior • Earthquake waves help determine properties of Earth’s interior such as: • thickness • composition • temperature • density • pressure
Earth’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.
Earthquakes • 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 2
Earthquakes • 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
Earthquakes • 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
Earthquakes • 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.
Earthquakes • 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.
Earthquakes • 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.
Earthquakes • 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
Evidence 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.
Evidence 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.
Evidence for Crustal Movement • Plate Tectonics • Earth’s crust is divided into several sections (plates) that move across Earth’s surface. 2
Evidence for Crustal Movement 6 min video • Plates move due to convection cells produced by unequal heating of the fluids within the asthenosphere.
Evidence for Crustal Movement • There are three types of plate interactions • Divergent boundaries • Convergent boundaries • Transform boundaries
Evidence 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.
Evidence for Crustal Movement • Convergent Boundaries • Plates move toward each other • When 2 continental plates meet mountains are formed. • Himalayan Mountains formed like this
Evidence 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.
Evidence 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
Evidence for Crustal Movement • The ocean floor appears to be spreading based on two major pieces of evidence. • The age of igneous rocks increases as the distance from the mid-ocean ridges increases. • Parallel strips of igneous rocks on either side of the mid ocean ridge show periodic reversals of magnetic polarity.
Evidence for Crustal Movement • Deformed rock strata • Sedimentary rocks normally form in horizontal layers. • Sedimentary layers that are not horizontal provides evidence of change.
Evidence for Crustal Movement Syncline Anticline
Evidence 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
Evidence for Crustal Movement Normal Fault
Evidence for Crustal Movement Reverse Fault
Evidence for Crustal Movement Transform or Strike-slip Fault
Evidence 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.