Valles Caldera from Space - Valles Caldera -Jemez Spring Mountains NM - The Valles Caldera is one of three active calderas in the United States. It encircles a field of volcanoes whose resurgent domes partition the 22-kilometer-wide caldera into five sections, or valles, which means valleys without trees in Spanish. The largest of these, Valle Grande, is almost 10 kilometres long and six kilometres wide. A magma chamber seethes five kilometers below the idyllic grasslands that shroud the surface of the Valle Grande. The eruptions formed the caldera roughly 1.2 million years ago, when the volcanic field expelled more than 750 cubic kilometres of ash and lava. Ash deposits contributed tuft to the surrounding Jemez Mountains, and the landscape sank back in on itself to form the vast bowl of the Valles Caldera.