section 1: earth’s crust in motion how do stress forces affect rock?
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Section 1: Earth’s Crust in Motion How Do Stress Forces Affect Rock?. The movement of earth’s plates creates powerful forces that squeeze or pull the rock in the crust – these forces are examples of stress Stress – a force that acts on rock to change its shape or volume - PowerPoint PPT PresentationTRANSCRIPT
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Section 1: Earth’s Crust in MotionHow Do Stress Forces Affect Rock?
• The movement of earth’s plates creates powerful forces that squeeze or pull the rock in the crust – these forces are examples of stress
• Stress – a force that acts on rock to change its shape or volume
• An earthquake is the shaking and trembling that results from the movement of rock beneath earth’s surface
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How Does Stress Affect the Earth’s Crust?
• Deformation – any change in the volume or shape or earth’s crust
• Three kinds of stress in the crust:
–Shearing – stress that pushes a mass of rock in two opposite directions
–Tension – pulls on the crust, stretching rock so that it becomes thinner in the middle like warm bubble gum
–Compression – squeezes rock until it folds or breaks like a giant trash compactor
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What Is a Fault?
•Fault – a break
in earth’s crust where slabs of crust slip past each other; These usually occur at plate boundaries
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a. Most faults lie between the surface and a depth of 70 kilometers
b. Focus- point below Earth’s surface where rocks break and move
c. Epicenter- point above Earth’s surface directly above the focus
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What Kind of Faults Are There?
• Three Kinds:
–Normal Faults–Reverse Faults–Strike-slip faults
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What Are Strike-slip Faults?
• Strike-slip faults
–Shearing forces cause
rocks to slip past each other
sideways with little up
and down Motion;
– Ex. San Andreas fault in California
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What Are Normal Faults?
• Normal faults
–Tension forces cause the rocks to form
the fault at an angle– One block is above the fault
–Hanging wall – the half of the fault that lies above
–Footwall – the half of the fault that lies below
– Ex. Rio Grande rift valley
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Normal Fault
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What Are Reverse Faults?
• Reverse faults
–compression forces cause the rocks to move
towards each other
– Same structure as normal fault but the blocks move in opposite direction; hanging wall move up
– Ex. Appalachian Mountains and Mount Gould in Glacier National Park
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Reverse Fault
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What type of fault?What type of fault?
A miner walks on the foot wall and looks up at the hanging wall!
A B
Normal Fault Reverse Fault
Hanging wall moves down
Hanging wall moves up
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Section 2: Measuring QuakesHow Does the Energy of an Earthquake Travel Through Earth?
• Earthquakes – most begin in the lithosphere
• Focus – the point beneath the earth’s surface where rock that is under stress breaks, triggering an earthquake
• Epicenter – the point on the earth’s surface directly above the focus
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What Are Seismic Waves?
• Seismic Waves – vibrations that travel through Earth carrying the energy released during an earthquake – They move like ripples on a pond– They carry the energy of an earthquake
away from the focus, through Earth’s interior, and across the surface
– The energy is greatest the the
Epicenter
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What Are the Different Kinds of Seismic Waves?
• Three categories:
–P waves–S waves –Surface waves
• P waves and S waves are sent out from
the focus; Surface waves develop
when the waves reach the surface
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What Are P Waves?
• P waves are primary waves
– The first waves to arrive
– Earthquake waves that
compress and expand the ground like an accordion
– Cause buildings to contract and expand
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What Are S Waves?
• S waves are secondary waves– Earthquake waves that vibrate from side
to side as well as up and down– These waves shake the ground
back and forth– Shake structures violently
–Cannot move through liquids
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What Are Surface Waves?
• When P waves and S waves reach the surface some are transformed into surface waves– Surface waves move more
slowly than P waves and S waves
– Produce the most severe ground movements
– Can make the ground roll like ocean waves or shake buildings from side to side
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How Do Scientists Detect Seismic Waves?
• Seismograph – records
the ground movements caused by seismic waves as they move through the Earth
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How Do Scientists Measure Earthquakes?
• There are at least 20 different measures for rating earthquakes, three are:
–Mercalli–Richter–Moment Magnitude
•Magnitude – a measurement of earthquake strength based on seismic waves
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What Is the Mercalli Scale?
• Rated earthquakes according to
their intensity
–Intensity: strength of ground motion in a given place
• Not a precise measurement• Describes how earthquakes
affect people, buildings, and the land surface
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What Is the Richter Scale?
• A rating of the size of
seismic waves as measured by a particular type of seismograph
• Accurate measurements for small, nearby earthquakes not large, distant earthquakes
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What Is the Moment Magnitude?
• A rating system that
estimates the total energy released by an earthquake
• Can be used to rate earthquakes of all sizes, near or far
• Below 5.0 – little damage• Above 5.0 – great destruction
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Earthquake damage in Charleston
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February 21, 1916 Asheville, NC - the most intense earthquake in NC history, measuring a 6 on the Mercalli scale.
June 5, 1998 an earthquake in Moorseville, NC measured 3.2 on the Richter scale, but there was no reported damage.
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Mantle- directly above the outer core
1. Plasticity is the property of a solid with the ability to flow like a liquid at very, very slow rates.
a. High temperature and pressure allow the rock to flow like a liquid
b. This also allows the plates of Earth (lithosphere) to move on the mantle.
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MANTLE
The mantle is composed of silicon, oxygen, iron and magnesium.
3. Moho- boundary between the crust and the mantle.
a. Change in the speed of seismic waves moving through the Earth led to its discovery.
b. Discovered in 1909 by a Yugoslav scientist, Andrija Mohorovicic.
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How Do Scientists Locate the Epicenter?
• Geologists use seismic waves– P waves arrive first– S waves arrive close behind
– Scientist measure the
difference in arrival times • The farther away an earthquake is the
greater the time between their arrival– Scientists draw three circles using data
from seismographs set at different stations to see where they intersect – the epicenter