Download Chapter 5 Complete Notes and Questions

Chapter 5 Notes and Questions
Grade 8 Science
BIG QUESTION: Why do earthquakes occur more often in some places than in others?
• 5.1 Forces in Earth’s Crust
A. How Does Stress Change Earth’s Crust?
a. Think back to the movement of the plates. The pieces of crust can be strong or weak, causing
bends and fold.
b. Stress is a force that acts on rock to change its shape or volume. This is expressed in unit per
area.
c. As force increases, stress increases. Force is an increase in energy
d. Kinds of Stress
i. Types of stress act on rocks over millions of years to change the shape and volume of
rock.
1. Tension: plates pull apart, crust becomes thinner in the middle
2. Compression: plates come together, plates squeeze
3. Shearing: plates slip past each other, grind past each other
B. Earthquake Risk Around the World
a. Most earthquake activity occurs near the Ring of Fire in the Pacific Ocean
i. West coast of Central America
ii. West coast of South America
iii. India, China and Pakistan due to the Indo-Australian Plate colliding with the Eurasian
Plate.
iv. Common where the Eurasian Plate meets the Arabian and African Plates
C. How Do Faults Form?
a. When enough stress (force) builds up, the rock will break and a fault will form.
i. A hanging wall is a piece of rock that sits over a fault.
ii. A footwall is a piece of rock that lies under the fault.
b. Types of Faults
i. Normal Fault: faults cut at an angle so one block of rock sits over the fault and one sits
under. Occur where two plates diverge, or pull apart. The hanging wall slips down.
ii. Reverse Fault: same structure as a normal fault. Hanging wall moves up.
iii. Strike-Slip Fault: rocks on either side of the fault slip past each other. Think of a
transform boundary!
D. How Does Plate Movement Create New Landforms?
a. Over millions of years, the forces of plate movement can change a flat plain into features such as
anticlines and synclines, folded mountains, fault-block mountains, and plateaus.
b. Folding Earth’s Crust
i. How Folds Form
1. Compression shortens and thickens Earth’s crust
ii. How Anticlines and Synclines Form
1. Compression forces fold crust upward into an arch (anticline) and downward
into a V-shape (syncline)
iii. How Folded Mountains Form
1. Collision of two plates can cause compression and folding of the crust over a
wide area
c. Stretching Earth’s Crust
i. Mountains that are separated by broad valleys or basins have been formed from tension
in Earth’s crust that cause faulting.
ii. Footwalls move outward and hanging walls slip downward.
d. Uplifting Earth’s Crust
i. The forces that raise mountains can also raise plateaus.
ii. A plateau is made of many flat layers and is wider than it is tall.
iii. Famous Plateau: Four Corners in Arizona, Utah, Colorado and New Mexico.
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Chapter 5 Notes and Questions
Grade 8 Science
Questions:
1.
How do forces acting on rock cause tension?
2.
How do forces acting on rock cause compression?
3.
How do forces acting on rock cause shearing?
4.
What is stress?
5.
How is stress stored in a rock released?
6.
What do you think happens at places where rock under stress breaks?
7.
What kind of stress causes a normal fault?
8.
What kind of stress causes a reverse fault?
9.
What kind of stress causes a strike-slip fault?
10. How does the hanging wall move at a normal fault?
11. How does the hanging wall move at a reverse fault?
12. Many people believe that California is going to fall into the Pacific Ocean as a result of movement along
the San Andreas Fault. However, this fault is a strike-slip fault, so the movement of the rock is lateral, not
up or down. Part of California is actually moving north. What kind of fault would the San Andreas fault
have to be in order for California to slide into the Pacific Ocean?
13. How can a fault change the level of the land surface?
14. Give three ways that Earth’s crust can be changed over time.
15. What kind of stress causes folding of the crust?
16. What are three mountain ranges caused by folding?
17. When a mountain is cut to make way for a highway, you can often see different kinds of rocks in the cut
surface. Why is this possible?
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Chapter 5 Notes and Questions
Grade 8 Science
5.2 Earthquakes and Seismic Waves
E.
F.
When you focus on something you are looking at a specific point. The focus of an earthquake is the point
at which a rock beings to move, causing an earthquake.
a. An earthquake is the shaking and trembling that results from movement of rock beneath Earth’s
surface.
i. Some can be unnoticed, some can cause the earth to break open, shift mountains and
cause great damage.
b. What are Earthquakes? Video
c. Cause of Earthquakes
i. Forces of plate movement cause earthquakes. Remember forces are transfers of energy.
When the energy builds so high, an earthquake occurs.
ii. Seismic waves are vibrations that are similar to sound waves. They travel through Earth
carrying energy released by an earthquake.
Types of Seismic Waves
a. Waves race out from the focus of an earthquake.
b. The point on the surface directly above the focus is the epicenter.
c. Three types of seismic waves:
st
i. P Waves: 1 waves to arrive, compress and expand ground like an accordion, travel
through liquids and solids; damage buildings
nd
ii. S Waves: 2 waves to arrive, vibrate, only travel through liquids, move left and right
(think of swaying)
iii. Surface Waves: move more slowly but produce ground movements that roll like ocean
waves.
G. How Are Earthquakes Measured?
a. The amount of earthquake damage or shaking that is felt is rated using the Modified Mercalli
Scale. The magnitude, or size, of an earthquake is measured on a seismograph using the Richter
scale or moment magnitude scale.
b. A seismograph is an instrument that records and measures an earthquake’s seismic waves.
i. The Modified Mercalli Scale
1. Rates the amount of shaking (damange) from an earthquake.
2. Rated by observations, not instruments
Rank
Description
I-III
People notice vibrations like those form a passing
truck. Unstable objects disturbed.
IV-VI
Some windows break. Plaster may fall.
VII-IX
Moderate to heavy damage. Buildings jolted off
foundations.
X-XII
Great destruction. Cracks appear in ground. Waves
seen on surface.
ii. The Richter Scale
1. The magnitude is a single number that geologists assign to an earthquake
based on the earthquake’s size.
2. Uses seismograph
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Chapter 5 Notes and Questions
Grade 8 Science
iii. The Moment Magnitude Scale
1. Used to rate the total energy an earthquake releases.
2. Use seismographs
c.
d.
Comparing Magnitudes
i. Each one-point increase in magnitude represents the release of about 32 times more
energy.
ii. Magnitudes above 6 can cause great damage.
iii. Magnitudes of 8 or above are rare and the most powerful.
How is an Epicenter Located?
i. Geologists use seismic waves to locate an earthquake’s epicenter.
ii. Use seismographs from all over the world.
iii. Observe the arrival of the P and S waves.
iv. Circles are drawn around three seismograph stations to locate the epicenter of an
earthquake. The radius of each circle is the distance from that seismograph to the
epicenter. The point where the three circles intersect is the location of the epicenter.
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Chapter 5 Notes and Questions
Grade 8 Science
Questions:
1.
Where does the energy in an earthquake wave come from?
2.
What releases the energy in an earthquake?
3.
What causes rock to break and move at a fault?
4.
What is the result of the breaking and moving?
5.
What type of wave cannot travel through a liquid?
6.
How are surface waves different from P and S waves?
7.
Which scales are based on a seismograph?
8.
What is a seismograph?
9.
What does the Modified Mercalli Scale indicate?
10. Which scale measures the amount of energy released by an earthquake?
11. On which scale would an earthquake’s strength very from one place to another? Why?
12. What are the two kinds of seismic waves that travel outward from an earthquake’s focus?
13. What device records seismic waves?
14. Why do you think we need three seismograph locations as opposed to two when trying to locate the
epicenter of an earthquake?
15. Have you ever listened for thunder after seeing lightning? Both light and sound travel as waves, but they
travel at different rates. Why do you hear thunder after you see lightning?
16. How is the relationship between thunder and lightning similar to the behavior of S and P waves?
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Chapter 5 Notes and Questions
Grade 8 Science
5.3 Monitoring Earthquakes
H. Seismic waves cause a simple seismograph’s drum to vibrate, which in turn cause the pen to record the
drum’s vibrations.
a. Measuring Seismic Waves
i. The pen of the seismograph moves and the paper stays stationary. Since the pen is
suspended, the movement of the earthquake causes the seismograph to move because
it is anchored to the ground and vibrates when the seismic wave arrives.
ii. How Seismographs Work Video
iii. Reading a Seismogram
1. When earthquakes seismic waves reach a seismograph, the vibrations are
recorded.
a. P waves arrive first and the fastest
b. S waves arrive shortly after the P waves
c. Surface Waves produce the largest disturbances on the seismogram.
d. An AFTERSHOCK is a smaller earthquake that occurs after a larger
earthquake.
b.
I.
What Patterns Do Seismographic Data Reveal?
i. From past seismographic data, geologists have created maps of where earthquakes
occur around the world. The maps show that earthquakes often occur along plate
boundaries.
ii. Earthquake risk largely depends on how close a given location is to a plate boundary.
1. At risk: California, Washington, Alaska
So, why do earthquakes occur more often in some places than in others?
a. Earthquakes often occur along plate boundaries, where stress caused by plate movement stores
energy in rock that makes up the crust.
Questions:
1.
What part of the seismograph does not vibrate during an earthquake?
2.
What does the straight line on a seismogram indicate?
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Chapter 5 Notes and Questions
Grade 8 Science
3.
What does the first set of disturbances on the seismogram indicate?
4.
What does the second set of disturbances on the seismogram indicate?
5.
What does the third set of disturbances on the seismogram indicate?
6.
Where do most earthquakes occur?
7.
What region of the United States has had the most earthquakes?
8.
Where would you expect an earthquake to occur in the United States?
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