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 of eruptions since the Miocene. The ages of the first eruptions are difficult to determine, since the older rocks have been almost entirely buried by the material from younger eruptions. The oldest exposed volcanic rocks in the vicinity are approximately 16 million years old. The Jemez area experienced an intense pulse of basalt volcanism between 9 and 11 million years ago. The lavas from this cycle had little silica, and originated in the mantle. Additional pulses of volcanism occurred between 7-10 million years ago, between 6-7 million years ago, between 3-6 million years ago, and between 2-3 million years ago. The last of these pulses is believed to have created the Cerros del Rio volcanic field, which covers the entire Caja del Rio Plateau.