Convection currents in the Earth's mantle and the movement of tectonic plates apply stress to the crust, causing it to break along faults or fold. This movement is responsible for earthquakes and volcanic activity, often occurring at plate boundaries. When stress exceeds rock strength, it releases suddenly in earthquakes. Seismic waves from quakes travel through the Earth and can be recorded on seismographs. Magma rises from hot spots in the mantle and at plate boundaries, fueling volcanic eruptions at the surface that pose hazards through lava flows, ash falls, and pyroclastic flows.

2. Earth’s Crust in Motion
•Convection
currents in
the mantle
and the
movement of
plates push
and pull the
earth’s
crust.
•This is

3. Earth’s Crust in Motion
•Stress acts on rock
changing its shape.
Types of Stress:
1.Stress that pushes a mass
of rock in two opposite
directions is called shearing .
•A lot of shearing stress
happens at the faults of
transform boundaries.

4. Types of Stress
2. Tension pulls on the
crust, stretching rock so
that it becomes thinner in
the middle.
•A lot of tension stress
happens at divergent
boundaries.

5. Types of Stress
3. Compression
squeezes rock until it
folds or breaks.
•A lot of compression
stress happens at
convergent boundaries.

6. Plate Boundaries and Faults
•When enough stress
builds up in a rock,
sometimes it will
eventually break or
crack, creating a fault.
•Plate boundaries—
where most stress
occurs– often have
really large faults but
faults can be in the
middle of plates too.
Ex. San Andreas
Fault (transform

7. Folding and Folded Mountains
•Sometimes instead of
causing faulting, the
force of compression
can cause rocks to
bend and fold.
•The Himalayas &
Appalachians are
folded mountain ranges.

8. Section 6-2
Earthquakes and
Seismic Waves

9. How an Earthquake Occurs
•When rocks along a fault are put under stress, they
may bend and change shape but initially resist
moving along the fault.
•Eventually, the rock slips suddenly and releases
this built up stress causing vibrations which we
know as earthquakes.
•Often the area where the rocks slips along the fault
happens below the earth’s surface. This is called
the Focus.
•Epicenter: the point on the earth’s surface directly
above the focus.

10. •Earthquakes often occur at transform
boundaries such as the San Andreas Fault
or convergent boundaries at subduction
zones such as the Cascadia Fault in the
Pacific Northwest.

11. Seismic Waves
•Seismic waves carry energy from an
earthquake away from the focus,
through Earth’s interior, and across
the surface.

12. Types of Seismic Waves
1. P or “primary” waves are the felt first in an
earthquake because they are the fastest. They
can travel through solids and liquids.
2. S or “secondary” waves are felt second in an
earthquake. They are slower than P waves and
unable to travel through liquids.

13. Types of Seismic Waves
3. Surface waves are felt last in an
earthquake because they are the slowest.
They only travel through the crust. Despite
being the slowest, they cause the most
damage and make the ground roll like
ocean waves!

14. Why Can’t S Waves Travel
All the Way Through the Earth?

15. The Seismograph
•A seismograph is an
instrument that records
the vibrations of an
earthquake to produce a
seismogram.
•Modern seismographs
are electronic and can
sense and record ground
movements that are very
small and far away!

16. http://www.youtube.com/watc
h?v=VSgB1IWr6O4

17. Magnitude
•Magnitude is a number from 1-10 that
geologists assign to an earthquake based on
the earthquake’s size.
•The magnitude of an earthquake is rated by
the following:
1.The highest wave on the seismogram
determines the magnitude.
2.How much movement occurred along the
fault.
3.The strength of the rocks that slipped
along the fault.

18. Moment Magnitude (Updated Richter) Scale
of Earthquake Energy:
Each number releases 32 times more energy
than the previous number!

19. Comparing Magnitudes
•Each one-point
increase in
magnitude
represents a release
of 32 times more
energy!
•An earthquake with
a magnitude 6
releases 32 times
more energy than a
magnitude 5, and
1024 (32x32) more

20. New Madrid Fault Zone
• Although most
Earthquakes
happen at plate
boundaries,
stress can create
fault lines in the
middle of plates
too such as the
New Madrid Fault
http://abcnews.go.com/GMA/v
Line .
ideo/fault-lines-america-
9592211

21. Section 6-3
Monitoring
Earthquakes
and
Section 6-4
Earthquake Safety

22. Instruments That Monitor Faults and
Help Predict Earthquakes
•In trying to predict earthquakes, geologists have
various developed instruments to measure changes in
elevation, tilting of the land surface, and ground
movements along faults. The most important of these
instruments is GPS.
•If any of these changes occur, they indicate stress
is building up (or rather, built up stress could soon be
released!)

23. Earthquake Risk
•Geologists can determine
earthquake risk by locating where
faults are active and where past
earthquakes have occurred.

24. How Earthquakes Cause Damage
•Plate movements on the ocean floor can
cause earthquakes and push water out of
the way forming a tsunami, a series of
waves each on larger than the next.
•A tsunami spreads out from an
earthquake's epicenter and speeds across
the ocean.
•Tsunamis can go undetected because the
waves only get large as the water gets
shallow!
Subduction Zone

25. How Earthquakes Cause Damage
•Liquefaction occurs when
shaking turns loose, moist soil
into liquid mud and cause
buildings to sink!
•An aftershock is an earthquake
that occurs after the initial
earthquake.

26. Designing Safer Buildings
•To reduce
earthquake
damage, new
buildings
must be made
stronger and
more flexible.
•Buildings on
soft soil need
to be
anchored to
bedrock to
prevent
liquefaction.

27. Designing Safer Buildings
•Base Isolated buildings “isolate” a
building from its foundation and
“smooth out” the earthquake with
huge springs.

28. What if you are in an Earthquake?
•Drop, cover, and hold on to
something sturdy if possible.
•If you live in an earthquake prone
area, have an earthquake kit with
food, water, and first aid ready.
http://www.youtube.com/watc
h?v=Io4QH8I04h4

30. 7-1 Volcanoes and
Plate Boundaries

31. Volcanoes and Plate Boundaries
•Volcanoes are weak spots in Earth’s Crust
where magma comes to the surface.
•Like Earthquakes, Volcanic belts often form
along the boundaries of Earth’s plates. (ex.
Pacific Ring of Fire)

32. Volcanoes and Plate Boundaries
•Volcanoes often form where two oceanic
plates collide, where an oceanic plate
collides with a continental plate, and of
course at mid-ocean ridges.

33. Hot Spot Volcanoes
•Sometimes, like in Hawaii, volcanoes can occur in
the middle of a tectonic plate!
•Scientists think that sometimes hot rock will rise in
the mantle, then melt forming a pocket of magma in
the mantle called a hot spot. Hot spots can be
anywhere, including the middle of plates.
•A volcano forms above a hot spot when magma
erupts through the crust and reaches the surface.

34. Hot Spot Volcanoes
• Many of these eruptions will cause the volcanoes
to get bigger and bigger.
• Eventually they will rise above sea level to form
islands.
• Over time, the plates will move this volcanic
island away from the hot spot (the hot spot
remains in the same place in the mantle) and a
new volcanic island will begin to form.

35. New Island Formation
http://education.sdsc.edu/opti
puter/flash/hotSpots.htm

36. Loihi will be above sea level in about
18,000 years!

38. • Yellowstone National Park in Wyoming is
actually a “Super Volcano” and is located
above a hot spot in the middle of the North
American Plate.

40. Section 7-2 Properties of
Magma
Section 7-3 Volcanic
Eruptions

41. Parts of a
Volcano…

42. How a Volcano Erupts
•As magma rises, less
weight is on it so
pressure on it decreases.
•Gases in the magma
now have room to expand
and spread out. The
force of the gases
expanding pushes
magma from the magma
chamber through the
pipe until it flows or
erupts out of the vent
(and crater) or side vent.

43. “Quiet” Eruptions
•Some magma has low silica (quartz) so it is fairly thin
and can flow easily. This means is has a low resistance
to flow or a low viscosity.
•Therefore it will have a “quiet” eruption because the
magma can easily flow out of the vent and crater.
“Quiet” eruptions still cause a lot of damage!
•When this lava cools, it often forms rocks like basalt.
•Ex. Mt. Kilauea Hawaii (Kilauea means “much
spreading.” )

44. Explosive
Eruptions
•Some magma has a high silica content
(quartz) so it is thick and flows very slowly, or
has a high viscosity.
•Magma is so thick it gets stuck in the
volcano’s pipe, pressure builds up and then it
explodes!
•When this lava cools it often forms high silica
rocks like pumice, rhyolite, and obsidian!
•Ex. Mt. St. Helens

45. What Erupts from a Volcano?
1. Lava Flow- The river of lava that flows down the
volcano and over the land.
2. Pyroclastic material
If an explosive eruption occurs, there is usually a large
fast moving cloud of gases, ash, cinders and bombs
called pyroclastic flow.
• Rock fragments created by eruptions
•lava explodes from the volcano and solidifies in
the air forming the pyroclastic materials.
Volcanic bombs
Volcanic cinders Volcanic ash

46. Pyroclastic flow
Pyroclastic flow
Mt. Augustine, Alaska
Mt. St. Helens (1980)
(1996)

47. U.S. Geological Survey scientist examines pumice
blocks at the edge of a pyroclastic flow from the
May 18, 1980 eruption.

48. Volcanic Hazards
•As lava flows, it buries
everything in its path!
•Poisonous gases are
released
•Pyroclastic materials like
ash, cinders and bombs can
bury entire towns!
•When volcanic ash gets wet
it can create massive
mudslides.
•After the Mt. Pinatubo
eruption in 1991 in the Even in Hawaii, sulfur gases
Philippines, the global have caused
evacuations
temperature was reduced by
1 degree F because the ash
and gases blocked sunlight.

51. Lives of
•Volcanoes may be active, dormant (sleeping) or
Volcanoes
extinct based on the activity or non-activity that
they exhibit.
•How might geologists determine what stage a
volcano is in?
•Past eruptions, monitoring the volcanoes for
surface changes (using tiltmeters), monitoring
small earthquakes triggered by the movement of
magma, and changes in temperature.
Active Dormant Extinct

52. •Tiltmeters, like this one installed on Mount St. Helens,
can signal impending eruptions by detecting changes in
slope caused by magma moving beneath a volcano.

53. Volcanic Landforms
•A caldera forms
when an volcano’s
magma chamber
empties and the
roof of the
chamber collapses.
The result is a
large, bowl-shaped
caldera.
•Future eruptions
can create new
volcanic cones in
the caldera.

54. Volcanic Landforms
•Examples: Crater Lake, Oregon and
Yellowstone National Park.
Crater Lake, Oregon
(or more accurately, Caldera Lake)