Earth Science Chapter 4 The Dynamic Crust. Earth’s Crust and Interior Earthquake waves help...

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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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

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

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.

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.

EarthquakesEarthquakes

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.

EarthquakesEarthquakes

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

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.

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.

Evidence for Crustal MovementEvidence for Crustal Movement

Evidence for Crustal MovementEvidence for Crustal Movement

Evidence for Crustal MovementEvidence for Crustal Movement

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.

Evidence for Crustal MovementEvidence for Crustal Movement

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

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.

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

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.

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

Evidence for Crustal MovementEvidence for Crustal Movement

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.

Evidence for Crustal MovementEvidence for Crustal Movement

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.

Evidence for Crustal MovementEvidence for Crustal Movement

Syncline Anticline

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

Evidence for Crustal MovementEvidence for Crustal Movement

Normal Fault

Evidence for Crustal MovementEvidence for Crustal Movement

Reverse Fault

Evidence for Crustal MovementEvidence for Crustal Movement

Transform or Strike-slip Fault

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.