Outline for class: Tuesday, 16 February
Topic: Plate Edges: where the action is
In Class Activities:
- Preview of Exercise 3 images
- Review of Exercise 2 part about earthquake intensity: which
locations are expected to have a high intensity of shaking and
why; which locations are expected to hav a low intensity of shaking
and why
- The travel time of earthquakes gives us information about
the earth's interior.
- The lithosphere is the hard, brittle outer layer of Earth
where earthquakes occur. The aesthenosphere is the soft, toothpaste-like
layer beneath the lithosphere that is very hot and partially
melted. The melted parts rise upward to the surface to form volcanoes.
- Less dense materials float over more dense materials. For
example, the lithosphere is less dense than the asthenosphere
and floats on top of it. Ocean crust is more dense than continental
crust and sinks beneath it in the oceanic trenches.
- Earthquakes occur in the lithosphere and most occur at lithospheric
plate boundaries.
- Types of plate boundaries (see Table 3.1 in the textbook).
- Characteristics of divergent plate boundaries: (1) plates
diverge at mid-ocean ridges; (2) the ridges are high features
on the seafloor because it is very hot; (3) there are volcanoes;
(4) earthquakes have shallow-depths because the lithosphere is
thin at the ridges; (5) ocean crust is created at the mid-ocean
ridges; (6) ocean crustal ages are youngest at the ridges and
get progressively older away from the ridges; (7) examples are
the East Pacific Ridge (or Rise) in the Pacific Ocean and the
Mid-Atlantic Ridge in the Atlantic Ocean.
- Characteristics of convergent plate boundaries (ocean-continent
types): (1) ocean crust is destroyed at subduction zone trenches,
where convergence causes one plate to sink beneath the other
plate into the asthenosphere (in continent-ocean examples, the
ocean plate always goes down; in ocean-ocean examples, the densest,
usually the oldest, ocean plate goes down); (2) volcanic mountain
ranges form adjacent to the trench because some of the downgoing
oceanic plate melts and the magma rises back up to the earth's
surface; (3) earthquakes range from shallow (next to the trench)
to deep (away from the trench). An example of the ocean-continent
type is the Peru-Chili trench (west coast of South America).
- Characteristics of convergent plate boundaries (ocean-ocean
type): (1) ocean crust is destroyed at subduction zone trenches,
where convergence causes one plate to sink beneath the other
plate into the asthenosphere (the densest, usually the oldest,
ocean plate goes down); (2) volcanic mountain ranges form adjacent
to the trench because some of the downgoing oceanic plate melts
and the magma rises back up to the earth's surface; (3) earthquakes
range from shallow (next to the trench) to deep (away from the
trench); (4) Examples: Marianas trench and Aleutian trench.
- Characteristics of convergent plate boundaries (continent-continent
type): (1) huge mountains ranges are created where the plates
converge because neither plate will sink into the asthenosphere
(continental crust is too buoyant); (2) volcanoes generally do
not form because no crust in sinking into the asthenosphere to
melt; (3) earthquakes are generally shallow (to moderate) depths.
An example is the Himalayan Mountains.
- Continental drift and plate tectonics video: see questions
in course reader.
- Question for 3x5 card: why are earthquakes deep at trenches
and shallow at oceanic ridges?