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Earthquakes and Volcanoes Study Guide
1. What are earthquakes? What causes them? Where do they occur?
Earthquakes are waves of energy released from breaking/shifting rocks at fault lines.
2. What are seismic waves? Compare and contrast primary and secondary waves. What does a larger
time lag between primary and secondary waves mean in terms of distance to the epicenter of the
earthquake? What does a shorter time lag mean?
Seismic waves are waves of energy released from the fault, and come in two varieties: primary and
secondary. Primary waves (P-waves) are compression waves (in a straight line) that are faster than
S-waves, so they hit first. Secondary waves (s-waves) have an up-and-down motion that travels
slower, but does more damage to standing structures.
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Longer time lag means you are farther from the epicenter. Shorter time lag between waves means
the waves are nearly on top of each other, meaning you are nearer to the epicenter.
What does a seismograph measure? What is magnitude? How is information from seismographs
used to determine the epicenter of an earthquake?
Seismographs measure magnitude. Magnitude is the energy (size) of an earthquake. The largest
reading on a seismograph tells us we are at or near the epicenter.
What does the Richter scale measure? What does the modified Mercalli scale measure?
Richter scale measures magnitude on a scale from 1.0 – 10.0. Modified Mercalli scale measures
damage of an earthquake from I – XII (1-12).
What are two reasons why a 7.2M earthquake might cause less damage than a 5.1 earthquake?
 Location (ocean, far from cities)
 Depth (shallow = more damage)
 Preparedness (Indiana v. California)
Compare and contrast focus and epicenter.
Focus – point INSIDE THE EARTH where the earthquake starts
Epicenter – point directly above the focus ON THE EARTH’S SURFACE that is hardest hit by the
earthquake
What is a tsunami? What causes tsunamis?
A tsunami is a seismic sea wave caused by friction between the ocean floor and the building wave.
The cutoff for massive tsunamis is a 7.5M earthquake.
Composite volcanoes (stratovolcanoes): where are they formed? What size and shape are they?
How do they erupt? What do they erupt? What is their lava made of?
 Convergent subduction zones
 Medium-sized, cone-shaped (steep slope)
 Explosive eruptions
 Thick, sticky silica-based lava
Shield volcanoes: where are they formed? What size and shape are they? How do they erupt?
What do they erupt? What is their lava made of?
 Divergent boundaries
 Large, dome-shaped (gentle slope)
 Small, constant eruptions
 Runny, iron- and magnesium-based lava
Cinder cone volcanoes: what size and shape are they? How do they erupt? What do they erupt?
 Small, cone-shaped
 Explosive bursts of hot gases, ash, and cinders
11. Compare and contrast lava flow that is rich in silica with lava low in silica. How does the presence of
silica change how the lava flows? Where does the silica come from? How does the silica get in the
magma?
Rich in silica – slow, thick flow that doesn’t travel far before cooling
Low in silica – thin, runny flow that spreads out and cools in thin layers
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Silica comes from the melting of tectonic plates (convergent subduction zones).
How does how the lava flows affect the shape and size of each type of volcano? For example, a
composite volcano is steep sloped. Why?
See above – thick lava builds up, not out, forming a short cone. Runny lava, like water, flows long
distances, spreading a thin layer over the foundation and forming a tall, wide mountain.
Why can volcanoes occur at convergent and divergent boundaries, but not at transform boundaries?
Convergent and divergent boundaries allow for magma to seep up to the crust. Transform
boundaries do not melt crust or open up to the mantle, so there is no “fuel” for volcanoes.
What is the force that moves tectonic plates?
Convection currents
Lava from volcanoes moves slowly enough for people to get away from it. What are three reasons
why volcanic eruptions are still dangerous?
Mudslides, avalanches, pyroclastic flow, flooding, falling tephra, heat, poisonous gases, ash, flying
debris, etc.
When does a volcano become extinct, which means that it will never erupt again?
Volcanoes become extinct after they lose their magma source (eg, Hawaii – islands move off the hot
spot). We judge volcanoes “extinct” after 10,000 years of no eruptions, but these judgments are not
always correct.