Hydromagmatic deposits near Albuquerque

These are some photographs I recently took of hydromagmatic deposits along Interstate 25 near Albuquerque, about 1 mile southwest of the Isleta exit. Hydromagmatic deposits are commonly formed when ascending magma, typically of mafic composition, encounters shallow groundwater in alluvium. The several-hundred degree temperature of the magma induces the groundwater to flash into steam, and the resulting explosion hurls the relatively loose alluvial sediment, plus pieces of the magma (ash, lapilli), into the air. This mix of ejecta then falls back down relatively close to the ascending magma.

Hydromagmatic deposits can be seen along the entire Rio Grande rift, from Taos south to the southern border. These deposits are characterized by being extremely well-bedded (Photo 1), commonly ledge- forming, and often capped by basalt flows that represent slightly younger phases of the same eruptive/magmatic event (the basalt is the black rock capping the 10 m-tall exposure of Photo 2). The beds are laminated to very thin (up to 3 cm thick), tabular to wavy, and drape underlying paleo-topography. The texture of the sediment in the exposure I visited is mostly silt to medium sand, with lesser coarser sand (Photo 2), and this is relatively representative of other exposures in the rift. An individual lamina could be well sorted to poorly sorted. Vertical grading wasn’t obvious in most lamina, although sometimes either reverse grading or normal grading was observed. The amount of basaltic detritus was variable, and where basalt ash or lapilli was more abundant the outcrop is darker (Photo 3). Although I didn’t see them here, in many places particularly big pieces of basalt, called bombs, can be seen whose weight and falling impact have caused sagging of underling bedding. Another common feature are mm-size concretions of the fine sediment (Photo 4, left of finger in the shade), which are called concretionary lapilli.

The well-defined, thin-bedded nature of these deposits, and the overall range of clast/grain sizes, reflects multiple (probably hundreds in this exposure) of individual steam-forming blasts. Ejecta can fall relatively vertically or sideways. Powerful, laterally-orientated blasts can cause cross-stratification (Photo 5).

So look for these deposits in areas where basalt outcrops, especially underneath the black-rock basalts that are common in the Albuquerque area and near the newly created Caja del Rio national monument (west of Santa Fe). The age of these basalts range from about 100,000 to 2.5 million years old. In addition to admiring the well-preserved bedforms, it’s fun to imagine the repetitive explosive eruptions causing chaos for all the Pleistocene creatures living in the Rio Grande valley at the time.