way to the surface. The more silica-rich lavas incorporated more melted crust and are described as evolved magmas. The hawaiites have unusual trace element compositions, suggesting they originated in the remnants of the Farallon plate known to lie below western North America. Eruptions came in three phases. The first and most voluminous phase lasted from about 2.7 to 2.6 million years ago. The lavas erupted during this phase were mostly basalt and basaltic andesite. Because of their low silica content, these lavas were highly fluid, and spread out to form most of the plateau. The eruptive centers, which included Cerro Montoso, Cerro Colorado, and Tetilla Peak, are mostly large composite cinder cones. The second phase, which lasted from about 2.5 to 2.2 million years ago, was more prolonged but much less voluminous and included a wide range of compositions, including andesite that makes up much of the higher terrain of the Caja del Rio. The final phase, from 1.5 to 1.1 million years ago, was restricted to a small area in lower White Rock Canyon east of the Rio Grande. These flows were basaltic andesite and dacite, and all overlie the lower Bandelier Tuff (Otowi Member) and the youngest overlies the upper Bandelier Tuff (Tsherige Member). One of the scoria cones of the Caja del Rio, the Cienega volcano (), has been quarried since 1919 for cinder. The quarry has exposed part of the deep plumbing of the volcano, which has been studied for clues to the nature of magmatic plumbing systems under cinder cones. The volcano itself has been radiometrically dated as 2.73 ± 0.06 million years old and erupted through three vents in at least two stages. Basalt flows on the west side of White Rock Canyon, across from the Caja, are considered geologically part of the Caja del Rio volcanic field. These are mostly composed of tholeiitic basalt, which is similar to the basalt erupted at mid-ocean ridges. Many of the flows of the Caja del Rio are covered with a mantle of sand, probably originating in the Rio Grande floodplain to the southwest and brought in by prevailing winds. The Caja is not the only volcanic feature in this part of New Mexico. Approximately 20 miles (32 km) to its northwest is the Valles Caldera, a spectacular caldera which lies at the heart of the Jemez Mountains. This mountain range has been created by a series