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The Geosphere (Solid Earth)
Super continent “Pangea” 200 millions years ago (200 MYA)
145 MYA Northern hemisphere and Southern Hemisphere
continents, Laurasia and Gondwanaland.
Longitudinal waves P-waves. Like sound waves or acoustic wave.
S-Waves or shear waves travel slower than P-waves. The motion
Of the wave disturbance is perpendicular to the wave’s motion.
If one know the speeds of the P-wave and S-wave (Vp and Vs) ,
then the difference between P-wave arrival (Tp) and S (Ts)wave
arrival can be used to figure out the distance to the Earth quakes.
D= Vp Tp
D= Vs Ts
D
(TS  TP )
 1  1 
 V

V
P
 S
3 stations need to locate the focus of the Earth quake.
Know the difference
between
Epicenter and hypocenter.
Where’s the epicenter? Two seismic stations is not enough.
Three will do as long as they are not on the same line.
Where’s the epicenter? Point A
Two seismic stations is not enough.
Three will do as long as they are not on the same line.
Wave fronts are represented as rays
Rays are drawn perpendicular to the wave fronts and show the
direction of travel for the wave front.
Refraction
Typically in the earth as one goes deeper the wave velocity V
increases so V1<V2 is
A common refraction. However, when Oil, Natural gas, or water is
embedded in the rock, especially sandstone formations, the wave
Wave speed changes constantly with
depth giving a curved wave
trajectory.
• P waves can travel through liquids and
solids
• S- waves (transverse waves) can only travel
through solids.
Seismic Tomography
P-wave shadow zone at 4 and 8 o’clock reveal
a liquid outer core.
Credit:
A. Dziewonski/IRIS Consortium
http://www.tectonics.caltech.edu/outreach/animations/mase/mase_tomography.html
The lithosphere is hard / brittle
The asthenosphere can deform like
stiff putty.
Earth’s Radius 6400 km
Depth to inner core 5000 km
Depth to outer core 3000 km
Depth to mantle 10 to 50 km (oceanic crust 10 km Continental
crust 50 km)
Oceanic crust is relatively young (80 million years) whereas
continental crust can be billions of year old.
Earth Radius 6400 km
Depth to inner core 5000 km
Depth to outer core 3000 km
Depth to lower 1000 km Mantle
There are approximately 12 lithospheric plates on Earth’s surface.
Most scientists believe that convection within Earth’s lower and upper
Mantle is the major driving force linked to plate motion. Convection is
driven my temperature differences/density differences and gravity.
Juan de Fuca plate. Spreading and subduction
Mt St Helens is part of the Cascade Arc Volcanoes
Hot spot
volcanoes
Hawaiian Islands
Hot Spots and Mantle plumes reaching the surface. Hot spots tend to
be relatively stationary. As an oceanic plate moves over a hot spot new
islands can be formed. Most think that the hawiian islands are a result
of motion over a hot spot.
Plate motion at trenches can be up to 10 to 12 cm year.
Continental crust is very old
(typically over a billion years
old) because it does not get
recycled frequently. Oceanic
crust is typically 100 million
years old on average (fairly
young). Oceanic crust is
continually recycled.