Quaternary volcanism in the western conterminous United States
This discussion of Quaternary volcanism in the western conterminous United States in part is similar to our earlier paper (Smith and Luedke, 1984) concerned with the late Cenozoic volcanism for the same region. One important difference between the two papers is the much shorter geologic time interval covered here (about the last 1.6 m.y.); in contrast, our earlier paper considered volcanism for a time span covering about the last 16 m.y. Both this and our earlier paper are based on a comprehensive study utilizing hundreds of references and five maps compiled and published in the U.S. Geological Survey Miscellaneous Investigations Series (Maps I-1091-A through E) at the scale of 1:1,000,000 (Luedke and Smith, 1978a, 1978b, 1981, 1982, and 1983). For compilation purposes, the volcanic rocks were grouped into five major types based primarily on their known or assumed silica content; the ages of the volcanic rocks arbitrarily were divided into three time frames of about 5 m.y. each, and the distribution and extent of volcanic fields were shown with and without vents. The information from those five maps, excluding vent data, was combined into a composite map at the scale of 1:2,500,000 (Luedke and Smith, 1984) thereby providing an overall data base for late Cenozoic volcanism.
It is not the intent of this brief review of Quaternary volcanism to discuss major volcano-tectonic or magmatic events, particularly considering the relatively short geologic time period. Such events, well