Outline for class: Thursday, 18 February
Topic: Rates of plates and quakes
In Class Activities:
- News articles of the day: SF Chronicle articles about the
Mavericks surfing site and otters along the coast.
- Characteristics of transform boundaries: (1) no volcanoes
form; (2) earthquakes have shallow depths; (3) examples: transforms
that offset the oceanic ridges and the San Andreas fault.
- The San Andreas fault is the boundary between the Pacific
and North American plates. The Pacific plate is moving toward
the northwest relative to the North American plate. At SFSU we
are located on the North American plate. When earthquakes occur
on plate boundary faults (for example, the San Andreas or Hayward
faults), we feel them!
- Hot spots are anomalously hot areas in the asthenosphere
that cause lots of magma to rise to the surface and form volcanoes.
The Hawaiian Island chain is one example. The islands get younger
toward the southeast and inform us that the Pacific plate is
moving toward the northwest. The bend observed in the Hawaiian-Emperor
Seamount chain occured about 40 million years ago and tells us
that the direction of the Pacific plate changed at that time.
- The rate of Pacific plate motion (plates) can be calculated
from the age of the volcanoes that form the Hawaiian Islands
and the distance that lies between the oldest island (Kauai)
and the youngest island (Hawaii), that is forming now. Based
on these data, the Pacific plate moves at an average rate of
about 10 cm/yr. Like Muni streetcars, the plates stop and go.
For example, in 1906 there was about 5 meters of motion along
our segment of the San Andreas fault. The fault since then has
not moved at all. When sufficient pressure has built up, another
large earthquake is likely to occur.
- We can estimate how often earthquakes will occur along along
the San Andreas fault by using the average rate of plate motion
(for example, 10 cm.yr) and the amount of motion that occurs
in each quake (for example, 5 meters).
- Question for 3x5 card: Why do we observe so much symmetry
(for example, seafloor age, sediment thickness) on each side
of the oceanic ridges?