4-1 Notes Forces in Earth’s Crust (front side)
Stress – a force that acts on rock to change its shape or volume
There are 3 types of stress:
1. tension - pulls on the crust and thins rock in the middle
-occurs when 2 plates pull apart (divergent boundaries)
2. compression – squeeze rock until it folds or breaks
-occurs where 2 plates come together (convergent boundaries)
3. shearing – pushes a mass of rock in 2 opposite directions
-occurs where 2 plates slip past each other (transform boundary)
folds – are bends in rock when compression shortens and thickens Earth’s crust without
breaking it
-mountains can occur when 2 plates collide without breaking
ex
Appalachian Mountains
ex
Himalayas
-anticline – upward fold in rock without breaking it (arches up)
-syncline – downward fold in rock without breaking it (dip down)
4-1 Notes (back side)
fault – break in the rock of the crust where rock surfaces slip past each other
-forms when enough stress builds up in rock and it breaks
-hanging wall – block of rock that sits over the fault
-footwall – block of rock that is under the fault
There are 3 main types of faults:
1. normal fault – tension causes the hanging wall to slip downward
-occur where 2 plates diverge (pull apart)
ex
New Mexico
2. reverse fault – compression causes the hanging wall to move upward
-occur where 2 plates converge (come together)
ex
Rocky Mountains
-thrust fault – a special type of reverse fault with a low angled crack
-compression causes the hanging wall to slide up and over
-mix up the rock layers
3. strike-slip fault – shearing causes the rock on either side of the fault to slip past each
other sideways with little or not up or down motion
-occur where transform boundaries are located
ex
San Andreas fault in CA
fault-block mountains – by tension acting on 2 faults causing the valleys on the sides to drop
down and a mountain or plateau is created in the middle
-valleys can also form by tension acting on 2 faults causing the
middle section to slide downward to create a valley in the middle
law of superposition – states that in horizontal rock layers each layer is older that the layer
above it and younger that the layer below it
4-2 Notes Earthquakes and Seismic Waves (front side)
Thousands of earthquakes are detected daily.
Plate movement produce stress in Earth’s crust adding energy to rock and forming faults.
Stress increases along a fault until the rock slips or breaks, causing an earthquake (shaking and
trembling that results from the movement of rock beneath Earth’s surface)
seismic waves – vibrations, similar to sound waves, carry energy released by an earthquake
-race out in every direction from the earthquake’s focus (area beneath Earth’s
surface where rock that was under stress begins to break or move)
There are 3 main categories of seismic waves:
1. p-waves (primary waves) –first sent out from the focus
-compress and expand like an accordion
2. s-waves (secondary waves) – sent out from the focus after p-waves
-vibrations are at 900 angle to the direction to the wave
-vibrates side to side or up and down
3. surface waves - develop where p-waves and s-waves reach the surface
-largest disturbance
-slower than p-waves and s-waves
-severe ground movement like ocean waves
epicenter- point on the surface of the Earth directly above the focus
4-2 Notes (back side)
seismograph – a device that records ground movements caused by seismic waves as they
move through Earth
2 ways to monitor earthquakes are by measuring the seismic waves they produce:
1. Modified Mercalli scale – rates the amount of earthquake damage or shaking that is
felt using people’s observations without instruments
2. rating the magnitude (measurement of an earthquake’s strength based on seismic
waves and movement along faults) or size of an
earthquake measured on the seismograph
Richter scale – uses a single number for the earthquakes magnitude (strength)
-each number is ten times the previous strength
moment magnitude scale – rate the total energy an earthquake releases
-each point increase in magnitude represents the
release of approximately 32 times more energy
-below 5 cause minimal damage, above 6 is great damage
Geologists use seismic wave to locate an earthquake’s epicenter.
To locate the epicenter at least 3 seismographs are need.
The difference in arrival times of p-waves and s-waves measures how far the earthquake is
from the seismograph.
4-3 Notes Monitoring Earthquakes
Seismic waves cause a simple seismograph’s drum to vibrate, which in turn causes the pen to
record the drum’s vibrations.
seismogram – recorded pattern of lines of an earthquake’s seismic waves on a seismograph
-the greater the height of the lines drawn the more severe the earthquake
aftershock – smaller earthquake(s) that occurs hours, days, weeks or even months after a
larger earthquake
Geologists still cannot predict when and where an earthquake might happen.
Earthquakes often occur along plate boundaries
-earthquake risk depends on how close a given location is to a plate boundary
-The Pacific Plate and the North American plate meet along the San Andreas fault
causing frequent earthquakes.
-The Ring of Fire is where many of the world’s earthquakes and volcanoes occur
because there are many plate boundaries around the Pacific Ocean.