Chapter 8 Plate Tectonics

208 CHAPTER 8: PLATE TECTONICS
Hawaiian hot spot below the lithosphere has been supplying
magma for at least the past 60 million years. During this
time, the Pacific Plate has moved thousands of kilometers to
the northwest. Even at a pace of a just few centimeters per
year, over a period of millions of years the lithospheric plate
can move great distances.
ACTIVITY 8-3 ZONES OF CRUSTAL ACTIVITY
You can find worldwide lists of active volcanoes and earthquakes,
which provide latitude and longitude, on the Internet. Use this information
to plot the position of at least 25 volcanoes or earthquake
epicenters (choose one) on an outline map of the world.
Compare the location of the features on your map with the features
that others have plotted from different lists. Can you see
similarities? Do the maps show any kind of pattern? Why do earthquakes
and volcanoes often occur in the same places?
Locating Plates and Plate Boundaries
A tectonic plate moves over Earth’s surface as a single unit.
The rigid nature of the plates generally transfers force applied
anywhere in the plate to the edges of the plates. Although
large earthquakes can and do occur within the plates, earthquake
epicenters are more common where one plate meets another.
When seismologists were able to record and locate
earthquakes all over the globe, they noticed that there are distinct
zones of crustal activity that stretch around the world.
These zones of earthquake activity are also regions in which
there are many volcanic eruptions and where tectonic mountain
building is occurring. Compare Figure 8-9 with the map of
plate boundaries in the Earth Science Reference Tables.
At the ocean ridges where new crust is forming, the plates
are relatively thin. Deep earthquakes do not occur in this region
because the upwelling material is relatively plastic, so it
tends to flow and bend rather than break suddenly. Although
there are many earthquakes at the mid-ocean ridges, their