Transcript EarthQuakes & Volcanoes (eg Shake and Bake)

EarthQuakes & Volcanoes
(e.g. Shake and Bake)
• The most visible
manifestations of the
earth’s internal heat are
earthquakes and
volcanoes.
• There is an interesting
relationship between the
two that led researchers
in the 1960’s to overturn
our view of the Earth
Igneous Rocks
• Formed when
minerals and other
rocks melt
• Are classified
according to their
chemistry and texture
• Bowen’s Reaction
Series Explains all
there is to know
Texture of Igneous Rocks
• Texture describes
how the minerals fit
together in a rock
and their size
• Fine-grained are
usually volcanic
rocks
• Coarse grained are
commonly plutonic
rocks.
Definitions
• Plutonic = Intrusive (magma stays
underground)
• Volcanic = Extrusive (magma flows to the
surface)
• Lava = Magma that has reached the
surface.
Geothermal Gradient
• Why do rocks melt?
• Because the earth is hot inside!
• The change in temperature with increasing
depth is called the Geothermal gradient.
Geothermal Gradient
-10
Depth (km)
-20
-30
-40
-50
-60
0
100
200
300
400
500
600
700
Temperature (C)
800
900 1000 1100 1200
What Happens When Rocks
Melt?
• They expand which causes….
• The density to decrease (because
you have the same amount of rock
in a larger volume) which
causes…..
• The melted rock to rise toward the
surface!
Volcanic eruptions
• We have discussed why
volcanic material (magma)
rises to the surface.
• There are several
different types of volcanic
eruptions that are related
to the composition of the
magma.
• Volcanoes are found all
over the world, but tend
to occur along linear belts.
Locations of Quakes/Volcanoes
Earthquakes/Volcanoes Occur along Linear Belts. The most famous
of these regions is called the “Pacific Ring of Fire”
Locations of Volcanoes
Types of Volcanoes/Locations
• Rhyolitic or Andesitic Volcanoes: Concentrated near the
boundaries between oceans and continents.
• Basaltic Volcanoes: Concentrated in the middle of the
Oceans (Mid-Atlantic Ridge).
Types of Volcanoes
Strato or Composite Cones
•
Strato Volcanoes: Made up of
alternating layers of rhyolitic to
andesitic flows and ashfall
deposits.
•
Top picture is Mt. St. Helens in
Washington and the bottom is
Mt. Fuji in Japan.
Types of Volcanoes
Shield Volcanoes
Shield Volcanoes tend to be
very large, but not as steep or
tall (with respect to the land
surface) as Composite
volcanoes.
Shield volcanoes tend to be
dominated by basaltic
volcanism.
Eruption of Kiluaea, Hawaii
Types of Volcanoes
Cinder Cones
• Cinder cones are the
simplest type of volcano.
They are built from
particles and blobs of
congealed lava ejected
from a single vent.
• They tend to be much
smaller than the other
types of volcanoes.
Mt. Capulin, New Mexico
Frequency of Eruptions
• Volcanic activity on Earth is nearly
constant.
• This activity, more than anything else,
indicates the dynamic hot nature of the
planet.
Predicting Volcanic Eruptions
• Predicting volcanic
eruptions has become
increasingly more
accurate.
• Two Good Books
– Surviving Galeras by
Stanley Williams
– No Apparent Danger
by Victoria Bruce
Bowen’s Reaction Series (Chemistry of
Igneous Rocks
Additional Notes on Bowen’s
• As SiO2 increases density decreases.
• As SiO2 increases melting temperature
decreases.
• As SiO2 increases crystallization
temperature decreases.
• As SiO2 increases viscosity increases.
• As SiO2 increases explosive potential
increases.
More Interesting Trivia
•
•
•
•
Why are continents, continents?
Why are oceans, oceans?
Answer:
Continental Crust: Thick and Light
(2.7)
• Oceanic Crust: Thin and Heavy
(2.9)
EarthQuakes
• Earthquakes, like
volcanoes, reflect the
dynamic nature of the
planet earth.
• Earthquakes, like
volcanoes, tend to occur
along linear belts.
• There are different
types of earthquakes in
different regions.
Details of Earthquakes
•
•
•
•
What causes an earthquake?
Where did the earthquake occur?
How big was the earthquake?
What type of fault generated the
earthquake?
• How frequently do earthquakes occur?
• Can Earthquakes be predicted?
What Causes an Earthquake?
• The simple answer is the buildup of stress
in solid rock.
• Elastic Rebound Theory states that stress
will build to a critical level and then the
rock will break suddenly causing an
earthquake.
• Earthquakes occur on faults which are
breaks in the earth’s crust along which
movement occurs.
Time
Earthquake
Earthquake
Earthquake
Stress
Critical Stress
Where did the Earthquake Occur?
• We need the following information to accurately
locate an earthquake.
• Time that P and S waves arrive at 3 earthquake
monitoring stations.
• P-waves are also known as ‘primary’ or ‘push’ waves
because they are the first to arrive at a seismic
station. P-waves can travel through gas, solid or liquid.
• S-waves are known as ‘shear’ waves or ‘secondary’
waves because they are normally the second type of
wave to arrive at a seismic station.
Seismogram
2 minutes
0
P-wave
S-wave
Time-------->
Time (minutes)
S-Wave
P-Wave
2 minutes
500 km
Distance
Locating an Earthquake
Difference in P-S gives an estimate of
distance, but not direction. In essence,
we end up with a ‘circle of possible
locations’ for each station.
A
B
750
750
Here?
Here?
500
A
500
B
750
750
500
A
500
300
C
B
750
750
Focus-point of rupture on fault
Epicenter: Point on the earth’s
Surface directly above the focus
500
A
500
300
C
How Big?
• The size of an earthquake is called its
magnitude.
• Originally developed by Charles Richter, there
are now many different magnitude scales.
• Most of these depend on Distance to the quake,
amplitude of a particular wave and period of that
wave.
• Station corrections are added to account for the
crust in the region of the seismometer.
Magnitudes
Amplitude
Period
Sample Formula would look
like:
Mag= log (A/T) + log D + 3.2
Where A=amplitude (mm)
T= period (seconds)
D=Distance
And the last number is a
station correction.
• Because the scale is logarithmic, each
single digit increase in magnitude
represents a 10x increase in the amplitude
of the wave.
• Perhaps more importantly, each single
digit increase in magnitude represents a
32x increase in the amount of energy
released by a quake.
What type of fault?
• The type of fault depends on the type of stress
generating the earthquake.
• Normal fault= Extensional vertical stress (pullapart)
• Reverse fault= Compressional vertical stress
(push-together)
• Strike-slip fault=sliding horizontal stress
Hanging Wall:
Miners hang
their lamps on
it. Foot wall is
the side of the
fault that
miners could
walk on.
Left-lateral motion: Observers
standing on opposite sides of the
fault would see each other moving to
their left.
Right-lateral motion: Observers
standing on opposite sides of the
fault would see each other moving to
their right.
Focal Mechanism Maps
• Often called ‘beach balls’ they give geologists a
way to determine fault type remotely.
• Based on the first motion of the P-wave (either
up or down.
• If the first motion of the P-wave is ‘up’, then this
signifies upward movement of the ground
surface beneath the seismograph.
• If the first motion of the P-wave is ‘down’, then
this signifies downward movement of the ground
surface beneath the seismograph
In this case, which direction is the movement on the
fault?
C
A
D
B
Fault
Down
C
Up
A
D
Up
B
Down
Strike-Slip Fault Plane Solution---In this case a right lateral fault.
Down
C
Up
A
D
Up
B
Down
Normal Fault Focal Mechanism
Pushes
Pulls
Pushes
Reverse Fault Focal Mechanism
Pulls
Pushes
Pulls
Relationship between Faults/Volcanoes
• Normal faulting is associated with basaltic
volcanism.
• Reverse faulting is associated with
andesitic and rhyolitic volcanism.
• Strike-slip faulting is amagmatic.
How Often?
• Earthquakes occur on a daily basis.
• In your homeworks, you will learn more
about the frequency-energy relationships
of earthquakes.
• Can Earthquakes be predicted?
Earth Structure From Earthquakes
• Recall:
– P-Waves are fastest and can travel through
all media
– S-Waves cannot travel through liquid or gas.
Snell’s Law
• As a wave (represented by its ray path)
moves through layers of different density it
will bend.
Case 2: Layer 2 is ‘slower’ than Layer 1
V2 >< V1
Case 1: Layer 2 is ‘faster’ than Layer 1
The Real Earth
S-Wave Shadow Zone
Can Earthquakes Be Predicted?
NO!
• I should say, not yet.
• Anomalous Features associated with
quakes includes:
• Cockroach scattering.
• Lights
• Gravity Changes
• Animal Behavior
• Velocity changes of P & S waves
EarthQuakes and Earthquake Damage
Earthquake damage depends on many variables. For example:
How Deep was the quake? The deeper the quake, the less likely
to see damage at the surface.
How Big was the quake? Larger magnitudes cause more
damage.
What time did the earthquake occur? Crowded buildings,
streets etc may result in more death.
What are building codes? Places such as California and Japan
have rigid earthquake building codes so a mag 6 quake is less
likely to cause severe damage there than in a
highly populated
region with poor/no building codes.
Number & Size of aftershocks- Large aftershocks can
destroy damaged buildings.
Tsunami’s: What are they?
Tsunami is an wave generated most commonly by earthquakes; however,
Tsunami’s can also form when large chunks of ice calve off into the ocean
Or massive landslides fall into the sea. They are most commonly the result
Of earthquakes.
Tsunami’s are sometimes called ‘tidal waves’ or seismic sea waves.
Why are they different?
Sumatra Earthquake
Dec 26, 2004