I.Review from Last Class

●  brief  history  of  Plate  Tectonics  and  how  it  came to  be  (using the  steps  of  the Scientific    Method)

·     the   two   hypotheses that  combined  to   become  the  Theory  of  Plate  Tectonics

o   what were these?

1.

2.

●   can   you   recall   the   5   steps   of   the   Scientific   Method   without   looking   back   at  the notes?

II.Today:Expand      Upon      Plate     Tectonics

● three  types  of  plate  boundaries:

o           transform,divergent,convergent

·     major compositional zones  of the  Earth

o    crust,mantle,outer    core,inner    core

●  major mechanical zones  of  the  upper  Earth

o         lithosphere,asthenosphere

●  how  do  we  know  the  interior  structure  of  the  Earth?

o  by  examining  seismic  waves  as  they   move  through  the   interior

● three  types  of  plate  boundaries:

o divergent

plates   spreading   apart

generally  produce small earthquakes

▶ example:ocean floors

o convergent

plates  come  together

■  called:subduction      zones

■  increased   volcanic   activity

mostly  produce large earthquakes

example:South      America

o transform

■  plates slide past one another generally produce moderate earthquakes

example:San Andreas fault

●  how do we know the structure of the Earth?

o  by  using  seismology

study of seismic waves as they move through the Earth created by EQ's

can be VERY destructive

but also used to “image”the structure of the Earth

will look at this more when we examine earthquake (EQ)disasters

Ⅲ.Seismology

● definition:the branch of geophysics concerned with the study and analysis of EQs and the science of the energy they produce

●  seismic waves:

o energy travels in the form of waves (causing particle motion)radiating out from a fault rupture (break)

o  two  main  types:

■  body waves

■ surface waves

o type  1:Body Waves:

■  move through the volume of the Earth

■ two   sub-types:P-waves,S-waves

■  P(primary)waves:

compressional particle motion

fastest average speeds in the crust(~6 km/s)

behave similar to sound waves in air

can pass through solid,liquid and gas

■  S(secondary/shear)waves:

second fastest wave(~3.5  km/s)

▶ shearing (side to side movement)particle motion

more damaging to structures because of the shearing ground motion can only passes through solids

o type 2:Surface Waves:

confined to the very upper surface of the crust

■  are slower than both P&S waves

can be very damaging to structures

■  two   main  sub-types:Love-waves,Rayleigh-waves

■  Love waves:

similar to the motion of S-waves

■  Rayleigh waves

Retrograde (reverse)rotating particle motion

o we willdiscuss how we can use these waves along with a seismogram(below) to estimate the distance to an EQ epicenter

■ this will help us to understand earthquake locations and disasters in the coming weeks!

notes:

Copyright e The MeGraw-Hill Companies,Ine.Permission required for reproduction or display.

Minutes

Seismic record of an EQ

·location of EQ epicenters  (at subduction zones)

o related to the dip angle of the ocean crust being subducted

o this location is further from the subduction zone(more inland)for shallow dipping ocean crust

o conversely,they are closer to the subduction zone (and to the shore line)for steeper dipping ocean crust

·    generalized structure of the Earth

o how do we know what is below our feet?

we have only drilled down several kilometers

never through the crust:

5 km thick (ocean crust)and 35 km thick (continental crust)

by comparison,the Earth has a radius of 6,370 km!

o examining the seismic waves as they pass through the Earth

waves change speeds when they travel through different rock compositions, densities,and  solid/liquid

■  seismologists can model these changes and estimate:

the rock types

rock layer thicknesses

their state (a solid or liquid)

Velocity (km/sec)

vellow    arrow:"low-velocity"zone(small   amount   of    liquid)-both   P    and   S   waves    slow red    ellipse:outer   core(all   liquid)-P   waves   slow   dramatically    &S   waves   stop

● structure of the Earth's interior:

o  major compositional zones of the Earth

■ crust

two types:oceanic and continental

1.continental   crust

thicker (30-50 km)

less dense (granitic rocks)

▶ older (some regions up to~3 billion years old)

2.oceanic   crust

▶    thinner(~5km)

more dense(gabbroic  rocks)

much younger(only up to~200 million years old)

■ mantle

extends from the crust boundary down to the outer core

~2900 km

some indirect sampling when material(called kimberlites)is brought to the surface from very deep rooted,explosive volcanoes

most information comes from seismic profiles

-relatively  uniform  composition  of  rock

-pressure/temperature  increases  with  depth

-low-velocity  zone  (upper  80-300  km)in  upper  mantle

-has a few  percent  of molten  material

- critical: this zone allows Plate Tectonics to happen

core

two zones that are VERY different

enriched in metal compared to the crust/mantle

much more

1.outer   core

liquid (caused by the very high temperatures)

rotation of this metallic liquid (convection)creates the Earth's magnetic field

2.inner   core

solid(even higher pressures overcome the very high temperatures)

o  major mechanical zones of the upper Earth upper layer (lithosphere)

defines a tectonic plate

brittle behavior

-does not flow and therefore fractures when stress is applied (such as an EQ)

all of the crust and part of the upper mantle (both compositional zones) define the lithosphere (a mechanical zone)

asthenosphere below this

all mantle material

plastic behavior-flows when stress is applied

small amount of partial melting

-the “low velocity”zone because seismic waves slow down through this partial melt zone