Transcript Volcanoes_Earthquakes

Volcanoes
Definition
- A vent at which lava, pyroclastic debris (ash
and fragments of previously solidified rock),
and gas erupt.
- Eruption may build a mountain around the
vent. (Mountain is also called a volcano).
- Anatomy of a volcano:
- magma chamber at depth
- a vent to the surface.
Where do Volcanoes Occur?
- Mid-ocean ridge
- we don't see them
- Convergent Margins
- the majority of major volcanoes;
- e.g., the Andes, Japan, Aleutians
- Rifts
- e.g., East African rift
- Hot spots
- e.g., Hawaii; Yellowstone Park
Nature of lava
• Characteristics of lava depend on its
temperature and viscosity.
– viscosity is a measure of the ability of a
fluid to flow;
• high viscosity means sticky and slow
• low viscosity means watery and fast
• The higher the temperature of a lava, the
lower its viscosity (i.e., the easier it flows)
• The greater the gas content, the lower the
viscosity. Frothy lava flows more easily.
Nature of Lava
• Mafic lava (rich in Mg and Fe, low in silica)
is also at high temperature (1100°C). 
tends to flow very easily, so basalt flows in
widespread thin sheets.
• Silicic lava is also at low temperature
(750°C).  lavas are very sticky and plug
up volcanoes, unless they are very gassy.
Types of Eruptions
1. Lava dominated eruptions:
- Characteristic of low
viscosity eruptions.
Basaltic composition.
- Fountains out easily. You
will see lava fountains,
lakes and rivers, that flow
long distances.
Types of Eruptions
2. Explosive eruptions:
– Characteristic of silicic volcanoes.
Sticky, very viscous lava clogs up
the vent. Gases make bubbles.
Gas pressure builds up and the
volcano explodes.
– Result = cloud of ash that
blankets countryside or an ash
flow that rushes down the
volcano side at 100 km/h,
incinerating everything in its
path.
Types of Eruptions
3. Explosions when water gets into magma
chamber and turns to steam
4. Explosions when part of the mountain slips
away and pressure is released
Kilauea caldera
Mt. Saint Helen’s
Types of Volcanoes
• The fissure volcano: Is a long crack in the earth's
surface through which magma erupts. These cracks
may form as two tectonic plates pull apart.
• The shield volcano: This is a broad, shallow volcanic
cone, which arises because the running lava, which
is fluid and hot, cools slowly.
• The dome volcano: This one has a steep, convex
slope from thick, fast-cooling lava
Types of Volcanoes
• The ash-cinder volcano: Throws out - besides lava much ash into the air. Through this the volcanic cone is
built up from alternate layers of ash and cinder.
• The composite volcano: These are also built up from
alternate layers of lava and ash but, besides its main
crater, it has many little craters on its slope.
• The caldera volcano: An older volcano with a large
crater which can be 62 miles(100km) wide. In this
crater many little new craters are formed
Types of Volcanic Hazards
• Lava Flows: Basaltic (lava-dominated) eruptions may
emit flows that can bury highways and towns.
• Ash falls and ash flows: Ash falls can bury the
countryside. Ash flows can incinerate towns.
• Explosive blast: The blast of an exploding volcano can
flatten everything within range.
• Lahars: These are fast-moving slurries made from a
mixture of water and ash (the water may come from
melting snow on the volcano during the eruption).
Lahars rush down river valleys and wash away or bury
everything in their path.
Predicting Eruptions & Decreasing Consequences
• Geologists are able to determine if a volcano is about to
erupt, based on several clues:
(1) Change in shape of the volcano
(2) Change in heat flow
(3) Increase in local seismicity
• If there is enough warning, the region can be evacuated.
People must also get out of river valleys that could fill with
lahars.
• NOTE: Volcano prediction is much more possible than
earthquake prediction. All we can do with earthquakes is
estimate recurrence interval (statistically average time
between events). We cannot exactly say when or where an
earthquake will occur. But we can with a volcano.
STOP HERE FOR LECTURE 1
Earthquakes
Definition
= Vibration of earth produced by rapid release
of energy (seismic waves) with radiate in all
directions from the source (focus)
• Earthquakes don't occur randomly. Occur on
faults or fractures within the earth
• Explained by plate tectonics. Most occur on
plate boundaries. Sometimes in plate interiors
if enough stress is built up
Key Terms
• Focus
• Epicenter
Measuring Earthquakes
• Seismometer
• Seismograph
• Seismogram
Mechanism
• Not known until after 1906 quake in San Francisco
• Elastic rebound theory
– Forces bend rock on either side of fault, rock strains
ever so slowly, then weakest point breaks. Break sends
out shock waves, which migrate outwards from the
original break, causing shaking. Stress is released
– Aftershocks are adjustments to that change in stress.
They are always less strong than the main shock, but
they may cause more damage to already weakened
structures.
What Causes shaking?
• Earthquake waves = seismic waves. Recorded
on seismometers on seismographs.
• Seismic waves are important for 2 things
– locating earthquakes - know how to do this using
P&S travel time curve
– describing the Earth's interior
Types of Seismic Waves
• Body Waves
– P-Waves (push-pull wave)
• vibrates parallel to the direction of wave propagation.
• Fast travel: 4-7 km/sec (15,000 mph)
– S-Waves (up and down waves /shear)
• vibrates perpendicular to the direction of wave propagation.
• Slower than P wave: 2-5 km/secs
• Surface waves - Very slow waves. Cause a lot of damage, rolling
feeling at end of earthquake
– Rayleigh waves - like waves on the ocean.
– Love waves - side to side waves.
How strong is an Earthquake?
• Intensity
–modified Mercalli scale
• Magnitude
–Richter scale
–other magnitude scales
Locating an Earthquake
• relationship of P and S waves
• Travel Time Curve = Distance
• Triangulation to locate Epicenter
Earthquake belts
• convergent margins, divergent margins,
transform margins
– Big quakes in convergent margins
– Small ones in divergent
– Transform vary in quake size
What effects do Earthquakes have?
•
•
•
•
Ground motion - displacement
Fires
Landslides
Tsunamis
– Seismic Sea wave
– " Tidal Wave" - (NOT)