Tectonic movements apply stress (force) to rocks
The rocks undergo strain (deform)
Structures produced
from strain: folds joints
faults
Stress can bend rock
layers (ductile strain)
Figure 15.1 Folded rocks, but any faults also?
Joint a fracture or crack in rock with no
relative movement of the pieces
Fault a fracture in rock with
an offset
of the pieces
Joints Figure 15.18
in the textbook regional joint
pattern created by uplift of Colorado Plateau
Figure 15.19 joints caused by different stresses
Conditions that favor ductile deformation:
slow rate of strain
high pressure
high temperature
Rocks near the Earth surface are usually cold and brittle
Figure
16.2 model for elastic rebound
Figure
15.20 Do these look like strong rocks?
A new idea: faults are weak
Only a little extra force is needed to cause a fault rupture (earthquake)
Box 16.3 page 407
Last major quakes:
1906 mag. 8.25 San Francisco
1857 mag. ~8 Los Angeles area
mostly mag. 7s
since then
Body waves spherical waves through the body of the Earth
Surface waves responsible for most damage
Love wave and
Rayleigh wave
Figures
16.1, 16.3, 16.15, 16.17, 16.18
the distribution of earthquakes is not random
Circum-Pacific belt Mediterranean-Himalayan belt
Figure 16.22
Earthquakes at plate boundaries
:
Deep at trenches Shallow at mid-ocean ridges
Figure 19.12
Convergent boundaries (moving together)
trench systems, mountain ranges
Divergent boundaries (moving apart)
mid-ocean ridges, continental rifting
Transform boundaries (moving laterally)
large-scale fault systems
Figure 16.23 A zone of inclined seismic activity (earthquakes) associated with a trench
Oceanic Oceanic
the older, denser crust goes down
forms a volcanic island arc
Oceanic Continental
oceanic crust goes under
mountain range with volcanoes
Continental Continental
collision, regional uplift, suturing
major
mountain range Himalayas
Figure 16.28 Oceanic crust being subducted under continental crust
Examples: NW United
States, Peru-Chile
Figure 16.28 India colliding with Asia
An earlier collision: Africa and North America
Figure 16.26 Divergent boundaries
Many shallow earthquakes associated with
injections of magma
Divergent
plate boundaries continental rifting
Initial rifting (break-up) and spreading of continental crust will produce an ocean basin
Example: San Andreas fault system
Volcanoes
associated with subduction zones:
Volcanic island arcs landward of trenches
Volcanoes in coastal mountain ranges landward of trenches
NOT associated with subduction:
Volcanoes associated with hot spots or mantle plumes
Volcanoes associated with rifting
Figure 16.29 Deep quakes on the descending plate
Base of the asthenosphere is an obstacle
Variations: an older, colder, denser plate penetrates the base of the asthenosphere
For more information, read In Greater Depth: A CAT
Scan of the Mantle
Box 17.2,
page 419
Some descending plates reach the bottom of the mantle and are recycled
Other possible causes for deep quakes:
Extreme pressure
causes minerals to collapse
Figure 16.32 Variations: Subduction is not always straight
Example: oblique subduction at New Zealand trench
Different
pieces of the plate may subduct differently because of different physical properties from the time of crust formation
To see the figures that I used in class:
http://www.ldeo.columbia.edu/margins/IZUBonin.html
Figures 19.21, 19.22, 19.23
Initial phase of
rifting a mantle plume rises beneath a continent
Crust thins and a
rift valley forms
Forming a juvenile ocean basin
Rift widens, floods
with seawater
A spreading center
develops, forming oceanic crust
D
evelops
into a mature ocean basin
Oceanic crust formed
at mid-ocean ridge
Wide continental
margins, deep basin
Modern examples:
Initial rifting East African Rift Valley
Juvenile ocean basin Red Sea
Mature ocean basin Atlantic Ocean
Produced by strain:
folds joints faults
Elastic ductile brittle
Conditions that favor folding
Elastic rebound theory
Another idea faults are weak
Seismic waves body waves, surface waves
Global distribution of earthquakes, and volcanoes
Plate boundaries:
Convergent divergent transform
Benioff zone
Trench systems
Volcanic island arc
Coastal mountain range with volcanoes
Continental collisions
Mid-ocean ridges spreading centers
Continental rifting
East African Rift Valley
Red Sea
Atlantic Ocean