Activity 1: Transform Faults and Fracture Zones
 
Plate motions at midocean ridges are relatively simple; i.e., plates move away from the ridges as new crust is created. In contrast, the behavior of plates at transform faults is much more complex. Recognition of the nature of this motion marked an important development in the theory of plate tectonics.
However, a closer examination of the midocean ridge system shows that ridges are offset by a series of linear steps (Fig. 1). These offsets or transform faults occur when the ridge first opens and do not get any longer with time. Along transform faults, the two lithospheric plates move past each other in opposite directions, producing shallow-level earthquakes (Fig. 2).
 



Fig. 1: The midocean ridge system and some of its many transform faults.

 
Linear structures, fracture zones, extend out from the transform faults and past the ridges into the ocean basins. Because they are outside the zone of sea-floor spreading, plates on either side of a fracture zone move at the same rate (Fig. 2). Thus fracture zones are aseismic, lacking earthquakes, but there is a significant difference in age of the ocean floor across fracture zones. Fracture zones are often characterized by steep scarps that provide a means of sampling the deeper section of the ocean floor.
 
Transform Fault - Fractrue Zone
Fig. 2: Transform fault motion and fracture zone behavior.  Between the ridge segments, where the transform fault occurs, the two lithospheric plates move past each other. This differential movement produces shallow earthquakes. Outside the ridge crests along the fracture zones, the two plates move at the same speed and in the same direction. Thus there are no earthquakes along fracture zones.
  

 

A simple paper model elegantly illustrates the important mechanics of transform faults and fracture zones.  More...
 
To complete Activity 1, select one of these files:word.gif (252 bytes)
   

 

Copyright Houghton Mifflin Company. All Rights Reserved.
Terms and Conditions of Use, Privacy Statement, and Trademark Information