Initial Publication Date: October 9, 2006
Introduction
The origin of volcanism in the western United States has been a topic of considerable research interest for many years. Isotopic data have provided some of the most important information used to test petrogenetic models for origin of the igneous activity in this region.Why mafic rocks?
Basalt flows, Black Rock Desert, Utah
Image produced with Google Earth
Young volcanic activity in the western U.S. spans a large compositional range from mafic (basaltic) to silicic (rhyolitic). However, different magma types within this compositional spectrum may have very different origins. While basaltic magma originates from portions of the Earth's mantle, magmas with higher SiO2 contents like andesite and rhyolite usually have more complicated origins and cannot be melted directly from mantle sources. Instead, andesitic to rhyolitic magmas require additional fractionation of mantle-derived basalt or secondary partial melting of older mafic rocks. Still, many petrologists believe that higher-silica rocks like andesites and rhyolites are upper-crustal manifestations of deeper (mantle)-derived basaltic magmatism. For this reason, a study of the mafic magmas (basalts with SiO2 < 55 wt.%) will yield clues to the ultimate causes of most of the volcanism which has affected the western U.S. This is the reason we focus on mafic rocks for this exercise.
Image produced with Google Earth