The San Juan volcanic field is a vast volcanic landscape in the San Juan Mountains of southwestern Colorado and the largest remaining part of a once much broader composite volcanic field in the southern Rocky Mountains. Activity began about 40 to 35 million years ago and peaked around 35 to 30 million years ago, shifting from numerous composite volcanoes to explosive eruptions, ash flows, and widespread caldera collapse. About fifteen calderas are known today. Geologists identify an earlier Oligocene phase dominated by intermediate volcanic rocks and a later Miocene-to-Pliocene bimodal phase. This volcanic history also created a major mining district rich in lead, zinc, copper, gold, and silver.