Pyroclastic Deposits of the Jemez volcanic center at Tent Rocks National Monument, New Mexico

New Mexico is full of young (geologically) volcanic rocks because a large feature known as the Rio Grande rift runs north-south down the middle of it. This rift is formed because the crust on either side is pulling apart, and many rift trigger the production of magma at their roots. A major feature in this rift is the Jemez "super" volcano, which can be seen in the image below from NASA Visble Earth below. The large central and roughly circular depression is the 'throat' of the volcano and is roughly 13.7 miles across. The flanks of the volcano, down which very large pyroclastic flows once swept, surround the caldera and extend past the edge of the image to the east. Both the town of Los Alamos and Bandelier National Monument lie on the east flank. The caldera formed a little over 1 million years ago, but much younger associated eruptions exist, and a geophysical anomaly underneath can be interpreted as magma that provides the possibility of future eruptions. The Rio Grande River is cutting the lower right (SE) corner and near there is Tent Rocks National Monument, a sacred site to Native Americans and a excellent place to see the eroded remnants of a variety of pyroclastic flows. The photographs below are from this site.

Pyroclastic deposits are the result of more explosive volcanic activity and the resulting deposits are layered and distinctly fragmental. Volcanic ash is perhaps the best known pyroclastic deposit, but there is a great variety of other types, including volcanic debris flows that sweep down the volcano flanks. Below you can see the layered character of some of these deposits at Tent Rock National Monument.

Because the large blocks of harder volcanic rock in these relatively soft and easily eroded pyroclastic deposits shelter the underlying rock from erosion, tee-pee forms result, which led to the name for this national monument. These can be seen in the above and below photos. Below is a close-up of some of these tent forms. Note the large blocks of rock "suspended" in the finer pyroclastic material visible on the left side. How did these blocks get transported here? Multiple possibilities exist, all related to explosive volcanism.

In some places erosion precedes to the point where pillars form instead of 'tents' as can be seen below. In other places similar features have been likened to mushrooms.

Below is visible some of the well bedded pyroclastic material. Some of these layers may be the result of lahars, which are mudflows that orginate on the flanks of many volcanoes. These can occur during an eruption, or after due to rains falling on and mobilizing the loose volcanic debris. Other whiter layers without larger fragments here are likely "air fall" deposits, and other more massive and coarse deposits are likely the result of volcanic debris flows.

A close-up of one of the deposits below displays the fragemental character. Many of the fragments here are pumice, very light volcanic rock so full of gas bubble pockets that they are very light. Pumice is typically light color reflecting the more Si rich character of the lava it formed from.

When more Si rich lava cools quickly under the right condition it can form glass, known as obsidian. In the tent rock deposits can be found rounded pieces of such volcanic glass, which are commonly known as Apache tears. Since collecting is prohibited here only this image was taken from the site.