1. Evidence for plate motions
Measurement of plate motions
Makeup of the crustal plates
Plate boundaries
Environmental Geology
jbartlett@national.edu

2. Your news items: ripped from the headlines
Thanks
Shedavia!
VERY large planet
VERY strange configuration

3. Tectonics is the study of large-scale movement and deformation of the earth’s
outer layers
A small number of rigid pieces = plates form ocean basins & continents
These move on top of flowing earth materials beneath
Plate tectonics studies the interaction of crust plates over a weak or partly
molten layer in the earth’s upper mantle

4. Magnetic stripes occur in ocean plates
As new crust is formed, magnetic field
occasionally reverses and changes cooling
rocks
These stripes are great
evidence for new crust
formation pushing
ocean crust out from
the center – thus called
“divergent” boundaries

5. Different boundaries
show up on the
earth‟s surface
Midocean ridges:
Divergent plates
Coastal mountains:
Convergent plates
In between:
Transform
boundaries

6. Plate Boundaries
Divergent Plate Boundary
Lithospheric plates move apart;
form oceanic ridges
Rising magma forms oceanic
ridges and new oceanic crust
Forces plates apart
Convergent Plate Boundary
Lithospheric plates move
together
Heavy plate subducts
Mountains, quakes, volcanoes
Transform Boundaries – short segments of a ridge
Transform faults offset ridge, move „sideways‟
San Andreas Fault –

7. Plate Boundaries
Convergent Plate Boundary
Lithospheric plates move
together
Heavy plate subducts
Mountains, quakes, volcanoes
Divergent Plate Boundary
Lithospheric plates move apart; form
oceanic ridges
Rising magma forms oceanic ridges
and new oceanic crust
Forces plates apart
Transform Boundaries – short segments of a ridge
Transform faults offset ridge, move „sideways‟
San Andreas Fault –

8. Relative plate motions
Direction and speed vary a lot over the world
Longer arrow = faster rate
5cm.yr = ~1 mile
per 32000 years

9. Faults: earthquake sites
Earthquake process
Earthquake hazards
Environmental Geology
jbartlett@national.edu

10. Faults are planes where rocks can
slip past each other
Three different types are based on
three different motions
Friction between rocks against
slipping generates elastic
deformation and builds up energy
before failure
When the stress exceeds the
friction (or rupture strength of the
rock), a sudden movement occurs
to release the stress
Normal fault

11. 
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Energy releases from a dynamic earth occur
along faults
Earth’s crust moves very slowly
over time enough stress builds up and a brittle
release occurs – an earthquake
Stress is suddenly released and transferred
Actual site of the first
movement along a fault is the
focus (or hypocenter)
Actual point on the earth’s
surface directly above the
focus is the epicenter

13. 


Energy is released during an earthquake
As the waves of energy are transmitted through the rock, this
energy with be felt by people at the surface
Magnitude – the amount of ground motion related to an
earthquake
Intensity – effect on humans, and their structures, caused by the
energy released by an earthquake

14. 


Measured by a seismograph
Richter magnitude scale most common
Richter scale is logarithmic
• An earthquake of magnitude 4 causes 10 times more ground
movement as one of magnitude 3
• The energy released by an earthquake of magnitude 4 releases
about 30 times more energy than an earthquake of magnitude 3

15. 



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Shaking is the most obvious and violent effect, but is highly variable and
depends on initial conditions at location
Large magnitudes may have small or large human impact
Tsunamis are seismic sea waves. When an undersea or near-shore
earthquake occurs, sudden movement of the sea floor may set up
waves traveling away from that spot, hitting shore with devastating effect
Fire is caused by broken gas lines and infrastructure
Power outages, water disruption are major impacts
Famine and disease have been major historical impacts

16. Magnitude 7.7 , 15 km deep
Strike slip fault
~300,000 people involved

17. Like much of geology, prediction is
difficult on human scales
Models generate probabilities but
not predictions
Current focus looks at indicators
like „seismic gaps‟
Active hazard areas can go
sometime soon – like in the next
100, 1000 or 10,000 years!

18. 
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Predicting ground shaking and movement along the fault – the
obvious hazards
Considering both structures and bedrocks
Designing “earthquake-resistant” buildings
Knowing the characteristics of the earthquakes in a particular
region
The best building codes are typically applied only to new
construction
Liquefaction and Landslides can be a serious secondary
earthquake hazard in hilly areas

19. Associated with boundaries
Variety of compositions
Variety of styles
Volcanic hazards
Environmental Geology
jbartlett@national.edu

20. Associated with
divergent or
convergent plate
boundaries
Different locations
mean different
composition & type
of volcanoes
Iron rich mantle stuff
Subducted melting stuff

21. 

Mafic magmas
produce dark, dense ,
liquid lavas
• Relatively calm,
low drama events
• Ocean crust
Felsic magmas
produce light colored,
sticky lavas
• Stiffness causes
violent eruptions
• Very dangerous
• Continental crust
Composition

Depth
Felsic
Intermediate
low iron
light toned
Mafic
high iron
dark toned
Volcanic:
Surface,
fine
grained
Rhyolite
Andesite
Basalt
Plutonic:
Deep,
coarse
grained
Granite
Diorite
Gabbro
Make up
continental
crust
Make up
ocean crust

22. Magma composition mafic  felsic affects
the lava properties
Silica is very stiff, tends to stick and then
explode
Continental environments tend to have water
and volatiles melted along with magma
Explosive versus calm eruptions

23. 
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Lava, not the principal hazard! Actually not life-threatening generally
Airborne/pyroclastic flows, way more dangerous than lava flows
Ash falls from eruptions can be much more devastating than lava
Gas, steam and poisons can also spew from volcanoes
Lahars, a volcanic ash and water mudflow
Pulaweh,
Indonesia

24. Ash cloud goes up and up into
stratosphere
Can circle for years or decades, blocking
sunlight
With enough blockage, global
temperature can be altered with huge
economic effects

25. Very large eruptions have happened geologically recently
Results:
Global „volcanic winter‟ ~10 years
Enhanced cooling ~1000 years
Effect on humans… dramatic
6 supervolcanoes of >1000 km3
In the recent past
{toba}
Historic eruption:
Mt Tambora, 1815,
~160 km3
Result:
„year without summer‟

26. 

Far from plate boundaries or magma
plumes
We are “near” one major risk:
Yellowstone, site of major potential
supervolcano
BBC: 12/10/13
Large Yellowstone magma
chamber may be 3 times
larger
than
estimated
before, so these maps
may underestimate effect
US Geologic Survey: “Thick ash deposits would bury vast areas of the United
States… injection of huge volumes of volcanic gases into the atmosphere
could drastically affect global climate.
Fortunately, the Yellowstone volcanic system shows no signs that it is headed
toward such an eruption in the near future”