Guided Notes for Guided Notes for Forces Within Forces Within EarthEarthSection 19.1Section 19.1

1) Earthquakes are 1) Earthquakes are natural vibrations natural vibrations of the groundof the ground caused by caused by movement movement along gigantic fractures in the along gigantic fractures in the Earth’s crust.Earth’s crust.

2) Most earthquakes occur when 2) Most earthquakes occur when rocks fracture deep within Earth’s rocks fracture deep within Earth’s crustcrust. Fractures form when stress . Fractures form when stress exceeds exceeds the strength of the rocks the strength of the rocks involved.involved.

3 Kinds of Stress That Act 3 Kinds of Stress That Act on Rockson Rocks

Compression: stress that decreases Compression: stress that decreases the volume of a material the volume of a material pushing pushing togethertogether

Tension: stress that pulls a material Tension: stress that pulls a material apartapart

Shear: stress that causes a material Shear: stress that causes a material to twistto twist

4) The deformation of materials in 4) The deformation of materials in response to stress is called response to stress is called strainstrain..

5) Elastic strain causes 5) Elastic strain causes a material to a material to bend and stretchbend and stretch, and can be , and can be demonstrated by demonstrated by applying tension to applying tension to a rubber banda rubber band. Elastic strain is . Elastic strain is proportional to proportional to stressstress, and thus, , and thus, if if the stress is reduced to zero, the the stress is reduced to zero, the strain disappears.strain disappears.

6) When stress exceeds a certain 6) When stress exceeds a certain value, a material undergoes value, a material undergoes ductile ductile deformationdeformation. This type of strain . This type of strain produces produces permanent deformationpermanent deformation, , which means that which means that the material is the material is deformed even if the stress is deformed even if the stress is reduced to zero.reduced to zero.

7) Most rocks are brittle 7) Most rocks are brittle under the under the relatively low temperatures that exist relatively low temperatures that exist in the crustin the crust, but become ductile , but become ductile at at the higher temperatures at greater the higher temperatures at greater depthsdepths..

8) Rocks that make up Earth’s crust 8) Rocks that make up Earth’s crust fail when fail when stress is applied too stress is applied too quicklyquickly, or , or when stress is too great.when stress is too great.

9) Define Fault and Fault 9) Define Fault and Fault PlanePlane

FaultFault: the fracture or system of fractures : the fracture or system of fractures that occur as a result of stress and along that occur as a result of stress and along which movement occurswhich movement occurs

Fault PlaneFault Plane: the surface along which : the surface along which movement takes placemovement takes place

The 3 Types of FaultsThe 3 Types of Faults

Reverse FaultsReverse Faults Form because of horizontal compressionForm because of horizontal compression Result in a horizontal shortening of the Result in a horizontal shortening of the

crustcrust

The 3 Types of FaultsThe 3 Types of Faults

Normal FaultsNormal Faults Caused by tensionCaused by tension Movement is both vertical and horizontalMovement is both vertical and horizontal

The 3 Types of FaultsThe 3 Types of FaultsStrike-Slip FaultsStrike-Slip Faults

Caused by horizontal shearCaused by horizontal shear Movement is horizontalMovement is horizontal

11) Most earthquakes are caused by 11) Most earthquakes are caused by movements along faultsmovements along faults. Irregular . Irregular surfaces in rocks can surfaces in rocks can snag and locksnag and lock. . As stress continues to build in these As stress continues to build in these rocks, rocks, they reach their elastic limit, they reach their elastic limit, break, and produce an earthquake.break, and produce an earthquake.

12) The vibrations of the ground 12) The vibrations of the ground during an earthquake are called during an earthquake are called seismic wavesseismic waves. Every earthquake . Every earthquake generates generates 3 types of seismic waves.3 types of seismic waves.

3 Types of Seismic Waves3 Types of Seismic Waves

Primary WavesPrimary Waves These waves squeeze and pull rocks in These waves squeeze and pull rocks in

the same direction that the wave travels.the same direction that the wave travels.

3 Types of Seismic Waves3 Types of Seismic Waves

Secondary WavesSecondary Waves Can cause rocks to move at right angles Can cause rocks to move at right angles

to the direction of travel of the waveto the direction of travel of the wave

3 Types of Seismic Waves3 Types of Seismic Waves

Surface WavesSurface Waves Cause rocks to move both up-and-down Cause rocks to move both up-and-down

and side-to-side as the wave passes and side-to-side as the wave passes throughthrough

Define Focus and Define Focus and EpicenterEpicenter

Focus:Focus: the point within the earth’s crust the point within the earth’s crust where an earthquake originateswhere an earthquake originates

Epicenter:Epicenter: the point on the earth’s the point on the earth’s surface directly above the focussurface directly above the focus