ra plates beneath the North American plate. These two plates "spread away" from the East Pacific Rise, and are separated from each other by a transform fault known as the Rivera fracture zone (Johnson, 1991). This fracture zone results from the different rates of motion of the two plates-6 cm per year for the Cocos plate and 1 cm per year for the Rivera plate (Johnson, 1991).
             Fig. 3. A very generalized map of central Mexico showing the Trans Mexican Volcanic Belt, the Michoacán-Guanajuato Volcanic Field, the Chichinautzin Formation and various volcanoes located in the Trans-Mexico Volcano Belt (from Newton, 1998)
             The volcanoes in the Popocate'petl area are the nearest of all the volcanoes to the Middle American Trench. The Middle American Trench occurs where the Coccos and the Rivera plates are subducting. The reason the distance from the trench to the volcanoes in chain (the arc trench gap) varies so much (f
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