section 1: earth’s crust in motion how do stress forces affect rock? the movement of earth’s...
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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
How Does Stress Effect the Earth’s Crust?
Deformation – any change in the volume or shape of 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
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
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
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
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
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
How Are Mountains Effected by These Forces?
Fault-block mountains – normal faults uplift a block of rock
Folding – bends in the rock that form when compression shortens and thickens part of the earth’s crust. Ex. Himalayas
How Are Mountains Effected by These Forces? (Continued)
Anticlines – a fold upward into an arch
Syncline – a fold downward into an arch
Plateaus – a large area of flat land elevated high above sea level
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
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 at the
Epicenter
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
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
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
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
How Do Scientists Detect Seismic Waves?
Seismograph – records
the ground movements caused by seismic waves as they move through the Earth
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
What Is the Mercalli Scale?
Rated earthquakes according to
their intensity
–Intensity: strength of ground motion in a given place
Not a precise measurementDescribes how earthquakes
affect people, buildings, and the land surface
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
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
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