Jemez Volcanic Field & The Valles Caldera

The Jemez Volcanic Field, which is located at the junction of the Jemez Lineament and the Rio Grande Rift, started life some 14 million years ago (14 mA). It was a normal volcanic field composed of many small eruptive cones. It is generally thought that there has never been a single, large volcanic cone or mountain like Mt St Helens. 

Over the course of 12 million years, continual extrusions of magma formed a complex landscape of domes, dikes, lava flows, ridges and valleys. The extrusions were fundamentally basalt ranging into more intermediate and silicic lavas such as dacite, prior to the catastrophic, rhyolitic, caldera forming eruptions. 

The unique location of the field caused it to erupt for the longest time and it is, in fact, one of the longest lived volcanic fields with the most recent eruptions occurring 50-60,000 years ago. It has produced the largest amount of erupted volcanic material anywhere along the Jemez Lineament. 

According to the New Mexico Natural History Museum, the continual eruptions caused the individual magma chambers of the numerous small cones to merge into one large chamber, which was the source for at least two caldera forming eruptions: Toledo Caldera (Valles I) and Valles Caldera (Valles II). It is believed there was a third eruption which preceded the Toledo one by some 400,000 years.  resulting in three members of Bandelier tuff: La Cueva, Otowi and Tshirege. 

The La Cueva member was formerly known as the San Diego Canyon ingnimbrites to the southwest of the caldera. It has been dated at around 1.8 mA. Source vents for this member and any other evidence for a caldera forming eruption was obliterated by the Toledo caldera forming eruption. 

The Toledo Caldera eruption (Valles I) occurred about 1.6 mA. It erupted the Otowi member of the  Bandelier tuff, which is hundreds of meters thick in some locations. This member includes the Guaje  pumice beds (Bailey & others, 1969) and was laid down in 2 eruptive sequences. 1st the Guaje pumice beds followed by ash flow deposits aka pyroclastic flows. The Toledo caldera is thought to be approximately the same size as the current Valles II caldera, however, it was destroyed during the Valles caldera eruption. 

Tshirege member: tuff over ash fall

The Valles Caldera eruption (Valles II) occurred about 1.2 mA and erupted the Tshirege member of the Bandelier tuff. The eruption spewed out >400 cubic km of ash, mostly in pyroclastic flows, which traveled up to 35 km from the source. Ash from the eruption has been identified as far east as Kansas & Iowa. The Tshirege member, like the Otowi member, was laid down in two eruptive sequences: 1) Tsankawi pumice bed and 2) ash flow deposits, which form the major portion of the canyon cliffs. By the way, Bandelier tuff can be either welded or unwelded. 

The Jemez Volcanic field and the Valles caldera are considered active.

For more info & info much better than what I've put together can be found at the following links.

Here's a YouTube video, a short clip from the documentary, Valles Caldera: The Science describing the formation of the caldera.

Valles Caldera Story Map