Earth Science Chapter 4 The Dynamic Crust. Earth’s Crust and Interior Earthquake waves help determine properties of Earth’s interior such as: –thickness.

Earth ScienceEarth Science

Chapter 4Chapter 4

The Dynamic CrustThe Dynamic Crust

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

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.

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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• 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


• 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


• 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.


• 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.



• 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.



• 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 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.


• 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.





• 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.


• There are three types of plate interactions– Divergent boundaries– Convergent boundaries– Transform boundaries


• 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.


• Convergent Boundaries– Plates move toward each other– When 2 continental plates meet mountains

are formed.• Himalayan Mountains formed like this


– 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.


• 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



• 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.



• Deformed rock strata– Sedimentary rocks normally form in horizontal

layers.– Sedimentary layers that are not horizontal

provides evidence of change.


Syncline Anticline


• Displaced rock strata occurs along faults (cracks in rock formations).– The 3 types of faults are:

• Normal• Reverse• Transform or strike-slip


Normal Fault


Reverse Fault


Transform or Strike-slip Fault


• Displaced marine fossils are sometimes found high up in the mountains.

• Benchmarks labeled with date and elevation are set into ground and checked periodically.

Earth Science Chapter 4 The Dynamic Crust. Earth’s Crust and Interior Earthquake waves help determine properties of Earth’s interior such as: –thickness.

Documents

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