Clarion Island is the westernmost of the four islands of the Revillagigedo group in the east Pacific. The Clarion Fracture Zone was named for this island, which is the first subaerial expression of the fracture zone as it approaches the Mexican mainland. The petrography and submarine topography indicate that the island is the summit of a submarine volcano. Five lithologic units recognized on Clarion Island appear to represent distinct eruptive or depositional phases in its evolution. The earliest eruptions took place along east-trending fractures which may be related to movements on the Clarion Fracture Zone, while later eruptions issued from north- or northeast-trending fractures, suggesting a late Cenozoic change in the regional stress pattern.

The volcanic rocks are all members of the alkalic basalt-trachyte association typical of oceanic islands. Variation from the average of low-potash tholeiite from the East Pacific Rise through a well-defined basalt sequence (the Gallegos basalt) can be expressed by the equations  
suggesting an ultimate tholeiitic parentage for the alkalic basalt. Fractional crystallization of low-potash tholeiite, and possibly volatile diffusion of alkalis, are considered to be the major causes of differentiation.

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