Identifying plate margins
Step 1
Locating plate margins on a map of the world is best done by plotting recent earthquake
distribution since plate margins are all highly active seismic zones. Use the database from
the United States Geological Survey to generate a map showing earthquake locations in the
recent past.
http://neic.usgs.gov/neis/epic/epic_global.html
Select the Generate Map output.
Earthquake Hazards Program: Global Earthquake Search
http://neic.usgs.gov/neis/epic/epic_global.html
Step 2
Scroll down and select the optional search parameters.
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Earthquake Hazards Program: Global Earthquake Search
http://neic.usgs.gov/neis/epic/epic_global.html
Select only a period of months. The output produced will show the concentration of earthquakes
occur in zones or along lines on a world map. These lines of high levels of seismic activity are plate
boundaries.
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NEIC: Earthquake Search Results
http://neic.usgs.gov/cgibin/epic/epic.cgi?SEARCHMETHOD=1&FILEFORMAT=5&SEARCHRANGE=HH&SYEAR=2009&SMONTH=1&SDAY=
1&EYEAR=2009&EMONTH=07&EDAY=1&LMAG=2&UMAG=10&NDEP1=&NDEP2=&IO1=&IO2=&SLAT2=0.0&SLAT1=0.0&SLO
N2=0.0&SLON1
=0.0&CLAT=0.0&CLON=0.0&CRAD=0&SUBMIT=Submit+Search
Note the legend on the output map that shows the depth of earthquake activity is
represented as a bar graph on the side of the output.
Where earthquake depths are indicated as consistently at 33 km or shallower then that zone
is a spreading zone or midocean ridge. These zones are where new crust is made or are
divergent margins.
Where earthquakes are shown as predominantly deeper that 70 kilometres then this is an
indication of a convergent margin or zone where crust is destroyed often called a subduction
zone.
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Step 3
Perform a new search to show only subduction zones by changing the search parameters to
search only for earthquakes deeper than 120 km during the same time frame.
Earthquake Hazards Program: Global Earthquake Search
http://neic.usgs.gov/neis/epic/epic_global.html
The search output will show clearly the major subduction zones active on the Earth's
surface.
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NEIC: Earthquake Search Results
http://neic.usgs.gov/cgibin/epic/epic.cgi?SEARCHMETHOD=1&FILEFORMAT=5&SEARCHRANGE=HH&SYEAR=2009&SMONTH=
1&SDAY=1&EYEAR=2009&EMONTH=07&EDAY=1&LMAG=2&UMAG=10&NDEP1=100&NDEP2=800&IO1=&IO2=&SLAT2=0.0&
SLAT1
=0.0&SLON2=0.0&SLON1=0.0&CLAT=0.0&CLON=0.0&CRAD=0&SUBMIT=Submit+Search
Assessment
1. Download a blank map of the world, and on it identify and draw zones on the Earth’s
surface where you believe the earthquake evidence indicates the existence of a plate
boundary. Label each boundary with an indication as to whether it is a constructive
plate boundary (where new crust is being produced), a destructive plate boundary
(where old crust is being subducted) or a conservative plate boundary (where one
plate is sliding past another but crust is neither being created nor destroyed).
2. Look closely at the distribution of earthquakes on the African continent. Describe any
evidence that you might see that suggests that Africa may have a spreading zone
developing in it.
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3. Look closely at the map of the world showing the distribution of earthquakes and
determine where most of the world’s subduction zones are located. Justify why you
believe that to be the case based on the evidence of earthquakes.
Follow up exercise
Look up a map of the world showing the topography of the landmasses and that of the
world’s ocean floor.
Describe the characteristics of surface topography at divergent margins particularly those
located within oceans and at then at convergent margins within oceans or at continent
margins.
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