1. Sec Continued

2. Earthquakes
Earthquakes often form from the friction between moving tectonic plates.
accounts for 95 % of all earthquakes
The Juan de Fuca convergent plate boundary west of
Vancouver Island has many earthquakes.
Large earthquakes hit this region every years.

3. Describing Earthquakes
Very difficult to predict their timing, exact location, & strength.
Pressure builds-up underground, over very long periods of time.
Focus = where the pressure is finally released.
Epicentre = the point on the surface directly above the focus

4. Depth of Earthquakes Earthquake Waves
occur at various depths, depending on the plates involved
Those at the surface tend to cause more damage.
Earthquake Waves
Seismology is the study of seismic waves.
waves behave differently in different Earth layers
reveal the source & strength of an earthquake.
can help us learn about the composition & distances of the Earth’s interior.
See page 528

5. Types of Earthquake Waves
See Table 12.3 p. 529

6. Seismic Waves Seismometers measure seismic wave energy.
Early seismometers just measured whether the ground shook.
Some seismometers measure horizontal movement, others vertical movement.
Seismogram is produced.
shows when an earthquake started
how long it lasted
Magnitude (1 increase in magnitude = 10X stronger)
Ex) Magnitude 6 earthquake is 100X more powerful than a 4.
Since seismic waves travel at different speeds, a distance-time graph can reveal the focus.

7. Since seismic waves travel at different speeds, a distance-time graph can reveal the focus.

8. Volcanoes The movement of tectonic plates can cause volcano formation.
Composite volcanoes – found along plate boundaries
Layers of ash and thick lava form a tall cone.
As magma reaches the surface, it cools, hardens,
& traps gases below.
Pressure builds
Eventually, an eruption
Mount St. Helens is a composite volcano.
See pages

9. 2. Shield Volcanoes 3. Rift eruptions
–not found at plate boundaries but instead form over hot spots
Thin magma/lava flows out from a hot spot & forms a low, wide cone.
Ex) The Hawaiian Islands (chain of shield volcanoes)
3. Rift eruptions
– occur along long cracks in the lithosphere
are not explosive, but they release massive amounts of lava
Take the Section 12.2 Quiz