THE DYNAMIC EARTH

1
THE DYNAMIC EARTH

• A Study of Plate Tectonics
• And
• Earthquakes

2
What is Plate Tectonics?

• It is the current theory explaining the motions
  of Earths crust
• It is a combination of two earlier ideas
• Continental drift and
• Sea floor spreading
• http//www.enchantedlearning.com/subjects/astronom
  y/planets/earth/Continents.shtml

3
What is Continental Drift?

• In 1912 Alfred Wegener proposed the idea that the
  continents were broken fragments originated from
  a single giant landmass called Pangaea
• Evidence
• The puzzle like fit of the continents as viewed
  on maps
• The same rocks and fossils found on two
  continents that are now separated by oceans
• Glacier evidence in locations that are now warm
• Wegener did not have the scientific capability to
  fully prove his ideas but it wasnt long after
  his death that scientific evidence arose to
  support his proposals.

4
What is sea-floor spreading?

• It is the process in which new ocean crust is
  created.
• Mid ocean ridges are the location of new ocean
  crust
• Ocean crust is older, thinner, denser, and basalt
  (mafic)
• Continental crust is older, thicker, less dense,
  and granite (felsic)
• Patterns of Magnetism
• Records of the magnetism of the ocean floor
  reveal a pattern which shows the changes in
  polarity.
• Polarity is the direction of the magnetic field
  of Earth
• It has reversed several times
• Rocks with s minerals will line up in the
  direction of the magnetic poles

5
Plate Tectonics

• The Earth is made up of tectonic plates (breaks
  in earths crust) surrounded by boundaries.
  http//vulcan.wr.usgs.gov/Graphics/framework2.html
  
• These boundaries are zones of crustal activity
  where most earthquakes and volcanoes occur.
• The motion of the plates varies by the boundaries
  surrounding them
• Divergent boundaries where plates are moving
  apart from one another such as at mid ocean
  ridges
• Convergent boundaries where plates are
  colliding or diving under one another such as at
  trenches
• Transform boundaries where plates are sliding
  past one another such as at faults

6
Divergent Boundaries

• Where plates are moving apart
• New crust is created
• Mid ocean ridges or rifts
• Forming under water chain of mountains
• May be sites of small, shallow earthquakes

7
Convergent Boundaries

• Where plates are diving under (subduction) or
  colliding with one another
• Crust is being destroyed
• Sites of volcanoes and earthquakes
• Form different landforms depending on type of
  convergence
• Ocean-ocean
• Ocean-continent
• Continent-continent

8
Ocean-Ocean Convergent Boundary

• Where two ocean plates meet and one dives
  (subducts) under the other
• Creates a chain of volcanic islands (island arcs)
• Example - Japan

9
Ocean-Continent Convergent Boundary

• Where an ocean plate dives (subducts) under a
  continental plate.
• Creates a chain of coastal volcanic mountains
• Examples the Cascades in the US
• or the Andes in South America

10
Continent-Continent Convergent Boundary

• Where two continental plates collide with one
  another
• Creates high mountains
• Example the Himalayas

11
What about the Hawaiian Islands

• The Hawaiian Islands are volcanic islands yet
  they are not or near a plate boundary
• They are in the middle of the Pacific Plate
• They are located over a HOT SPOT

12
Transform Boundaries

• Where plates are sliding past one another
• Crust is not created or destroyed
• Sites of many major earthquakes
• Examples San Andreas Fault in California

13
What Drives the Tectonic Plates?

• Convection current in the Mantle cause the plates
  on Earth to move.
• Convection currents result from the changes in
  temperature and density of the magma
• Hot magma rises in the mantle at a divergent
  boundary where it oozes out or
• It slides under the crust pulling the plates
  apart
• At a convergent boundary the cooler magma begins
  to sink and pulls the crust downward (subduction).

14
Activity at Plate Boundaries

• Plate boundaries are areas of movement of Earths
  crust
• When the Earths crust moves major events occur
• Earthquakes, volcanoes, tsunamis, mountain
  building

15
What are Earthquakes?

• An earthquake is a sudden movement of Earths
  crust that releases energy
• Earthquakes generally occur at faults
• Regions where there are breaks in Earths crust
  and plates slide past one another
• http//www.crustal.ucsb.edu/ics/understanding/
• http//earthquake.usgs.gov/recenteqs/

16
What Happens During an Earthquake?

• When the earth moves a sudden release of stored
  energy occurs
• Focus location of earthquake inside the crust
• Epicenter location of earthquake on earths
  surface directly above the focus
• The energy travels in the form of waves
  throughout the earth

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Types of Earthquake Waves

• There are two major types of earthquake waves
• P-Waves primary waves
• Travel fastest
• Travel through solid and liquid
• Known as compression waves
• They move back and forth
• S-Waves secondary waves
• Travel slower
• Travel through solids ONLY
• Known as shear waves
• They move side to side

18
Surface Waves

• Another type of earthquake wave is a surface wave
  (L-Waves)
• These waves are the slowest but they are the most
  destructive.
• They shake the surface of the earth up and down.

19
What are the Characteristics of Earths Interior?

• From the ESRT, you can gather all the information
  needed to describe the layers of the Earth
• Temperature
• Pressure
• Density
• Depth
• The boundaries (interfaces) of each of the layers
  can also be determined.
• In general you will find that as the depth in the
  Earth increases so does the
• Temperature
• Pressure
• And density
• Graphically,
• it is a direct relationship.

20
What Do Earthquake Waves Tell us About the
Interior of the Earth?

• Earthquakes and the study of the waves that
  travel through the earth help us draw conclusions
  (inferences) about the interior of the Earth.
• The waves change direction and speed as they
  travel through the Earth
• The speed increases as the density increases
• From this we know that there are different layers
• S-Waves do not travel through liquids
• Directly opposite the location of the epicenter
  the stations receive p-waves only
• From this it is concluded that the outer core of
  the earth is liquid

21
The Shadow Zone

• The shadow zone is a belt that extends around the
  Earth
• No p or s waves around at locations in the shadow
  zone
• Its location is dependent on the location of the
  epicenter.

22
How are Earthquakes Measured?

• There are two scales that record earthquake
  activity
• The Mercalli Scale which measures the observable
  damage (intensity) from the earthquake
• Scale is I to XII
• The Richter Scale which measures magnitude or
  strength using a seismograph
• Scale is 1 to 10 with each unit increasing 10X
• Example an earthquake of magnitude 7 is 100 times
  stronger than a magnitude 5 (10x10)

23
How Do You Find the Epicenter of an Earthquake?

• Find the arrival time of the p-wave and s-wave.
• Subtract p s to get the difference
• Use the difference and the time travel in the
  ESRT to find the distance to the epicenter
• Do this for three seismograms
• Draw a circle around each point, where all three
  intersect is the epicenter.
• http//www.geo.mtu.edu/UPSeis/locating.html

24
How Do You Find the Origin of an Earthquake?

• The origin is the actual time that the earthquake
  occurred.
• For any seismic station the origin time will be
  the same
• Origin time is equal to the p-arrival time the
  p travel time.

25
References

• Textbook
• Chapters 7, 8, and 9