PEEC logo
powered by FreeFind

Geology Guide

Rock Formations of the Pajarito Plateau

By Shari Kelley and Kirt Kempter

Most of the rocks that make up the Pajarito Plateau are volcanic rocks, which are the products of volcanic eruption. The volcanic rocks lie on top of sedimentary rocks, rocks that are eroded from older pre-existing rocks, transported by water, wind, or ice, and then deposited. The sedimentary rocks are important aquifers for the communities of Los Alamos and White Rock.

The rocks on the Pajarito Plateau, from youngest to oldest, are as follows.


El Cajete Pumice

Pumice is a white volcanic rock that is full of gas bubbles. Pumice will float on water because it is full of gas bubbles. The El Cajete pumice can be distinguished from older pumice deposits because it contains small crystals of a black, flaky mineral called biotite. El Cajete pumice originated from an eruption in the southwestern Jemez Mountains, south of Redondo Peak, about 50,000 to 60,000 years ago. These pumice deposits are very thick in the southern and southeastern Jemez Mountains, but only two small patches of this unit, both in Pueblo Canyon, are present near Los Alamos. The El Cajete pumice is common on the mesa tops to the west of the Bandelier National Monument headquarters.

Golf Course Alluvium
Click image to enlarge

"Golf Course" Alluvium

This rock unit is made up of pumice, sand, and gravel derived from inside the Valles Caldera to the west of Los Alamos. This particular pumice does not contain crystals that can be seen with a hand lens; it is what geologists call aphyric (without crystals). This pumice is 1.2 million years old and likely came from the Cerro del Medio dome inside the Valles Caldera. The prevailing winds, which blow from southwest to northeast, blew the pumice from the caldera to Los Alamos. Consequently, the "Golf Course" alluvium is younger than 1.2 million years old. This alluvium underlies Western Area, North Community, Ponderosa Estates, the area west of downtown, and the Golf Course in the town site of Los Alamos. One of the best exposures of this unit in Los Alamos is a road cut by Caballo Peak Apartments on the south side of Canyon road.

Tshirege member of the Bandelier Tuff
Tsankawi Pumice
Click image to enlarge

Tshirege member of the Bandelier Tuff
Tshirege Tuff
Click image to enlarge

Tshirege member of the Bandelier Tuff

Tuff is a rock that contains broken pieces of volcanic glass (ash), pumice, crystals, and rock fragments. Tuff forms during violent volcanic eruptions that involve magma (molten rock) with a high silica and gas content. Magmas with a high silica content are viscous or sticky, so gas pressures can build up, causing an explosion. Often, during the first phases of a volcanic eruption, gas and magma combine in the vent of a volcano to form pumice, which will shoot way up into the air and rain down as a blanket around the volcano. Later, during the explosive phase of an eruption, a mixture of hot gas, ash, pumice, and rock fragments will be blown from the volcanic center and roll at high speed down the sides of volcano, forming a glowing, hot cloud. Eventually, the roiling mixture of ash, pumice, and rock fragments will loose speed, come to rest, and the heat of the mixture will cause the rock to become indurated (hardened).

The Tshirege member of the Bandelier Tuff was erupted 1.25 million years ago during the collapse of the Valles Caldera. A caldera is a special type of volcanic eruption where the volumes of material erupted are so large that the volcano collapses into the emptied magma chamber and forms a large circular crater. The eruption began by shooting pumice far into the sky, forming a layer of Tsankawi pumice, the name applied to the pumice associated with the Tshirege eruption. The wind must not have been blowing very hard on the day that the Tsankawi pumice was erupted, because the pumice layer that was deposited by this eruption is uniformly about 1 to 2 m thick around the Valles Caldera. In contrast to the Cerro del Medio pumice, which is aphyric (no crystals), the Tsankawi pumice contains abundant quartz and sanidine crystals. The exact duration of the eruption is not known, but it probably happened over the course of weeks to months because at least five different cooling units have been identified. The town site of Los Alamos, Los Alamos National Laboratory, and portions of White Rock are built on the Tshirege tuff.

Cerro Toledo interval
Click image to enlarge
Cerro Toledo interval
Click image to enlarge

Cerro Toledo interval

Prior to the eruption of the Tshirege Tuff, an earlier eruption formed the Otowi Tuff (see below). Between the big tuff eruptions, volcanic centers in the caldera related to the Otowi Tuff were active, producing pumice. The prevailing winds, which blow from southwest to northeast, blew the pumice from the caldera to Los Alamos. These pumices, which are aphyric, are interbedded with sand and rare gravel that formed as the result of erosion during the ~400,000 years between the big eruptions.

Otowi member of the Bandelier Tuff
Click image to enlarge

Otowi member of the Bandelier Tuff

The Otowi member of the Bandelier Tuff was erupted during caldera collapse 1.6 million years ago. Like the Tsankawi eruption, this eruption began by emitting pumice, but the wind must have been blowing very hard from the west that day. The Guaje pumice, the name applied to the pumice associated with the Otowi eruption, is absent on the south, west, and north sides of the Jemez Mountains, but this pumice is 3 to 5 m thick on the Pajarito Plateau northeast of Los Alamos, where the pumice is mined. Like the Tsankawi pumice, the Guaje pumice contains abundant crystals of quartz and sanidine. The Otowi tuff can be distinguished from the Tshirege tuff in many places by its relatively higher content of black rock fragments. The distribution of the Otowi Tuff on the north side of Los Alamos seems to have been controlled by topography that existed before the Otowi eruption. The older Tschicoma lava domes that form the skyline of Los Alamos (see below) appear to have deflected the Otowi tuff so that this unit was not deposited north of Los Alamos. For example, Tshirege tuff sits directly on older rocks in Rendija Canyon, the northernmost Canyon in Los Alamos, to the west of the shooting range. Near the east end of the shooting range, a thin section of Otowi appears. This section of Otowi thickens dramatically in Barrancas and Bayo canyons toward the south. The Otowi tuff is not present at all in Guaje Canyon immediately north of Los Alamos.

Tschicoma Rendija
Tschicoma Rendija Lava
Click image to enlarge

Tschicoma dacite and rhyodacite lava flows

The mountains that form the skyline of Los Alamos, the Sierra de los Valles, are made up of lava domes and flows of the Tschicoma Formation. The lavas of the Tschicoma Formation around Los Alamos are typically rhyodacitic to dacitic in composition, which means that the lavas have silica contents between those of basalts (low silica) and rhyolites (high silica). Most flows traveled <10 km from their source and have steep sided flow fronts. The flows often exhibit block-and-ash features, where oversteepened lava flanks cool and collapse into a pile of rubble during flowage. A beautiful example of a block-and-ash flow can be seen in the water gap in Rendija Canyon just to the east of the cemetery in Los Alamos.

Three types of Tschicoma lavas, each coming from a different volcanic center, have been identified near Los Alamos:

One center is located on Caballo Mountain to the northwest of Los Alamos. This lava has a streaky appearance, and is lavender to red on weathered surfaces. Phenocrysts (well-developed crystals in a fine-grained matrix) of plagioclase, pyroxene, and sparse biotite and hornblende can be seen using a hand lens. At least three separate flows occur; the youngest contains more abundant hornblende. These flows extended in two main directions away from the source on Caballo Mountain: one going south-southeast of Caballo Mountain, capping the north canyon wall of Guaje Canyon, and one flowing to the east-northeast of Caballo Mountain, extending to just west of Pine Springs.

Another center is Pajarito Mountain, locally known as the "Ski Hill." This lava is light to medium gray to reddish. Phenocrysts of plagioclase, hypersthene, and pyroxene can be seen with a hand lens. At least two major flows are observed in Los Alamos Canyon, overlying the Rendija lavas described below. Age dates range from 3.4 to 2.9 Ma.

The rhyodacitic Rendija lavas form the majority of Sierra de Los Valles; however, the source of these voluminous flows is uncertain. The vent may have been destroyed when the Valles Caldera formed. These lavas are very porphyritic, with phenocrysts of plagioclase (up to 2 cm across), quartz, and trace amounts of biotite, pyroxene, and hornblende. These lavas are massive, with associated block and ash flows. The block and ash flow in Rendija Canyon mentioned earlier is in the Rendija lava. Guaje Mountain is composed of Rendija lava. The Rendija lavas north of Guaje Canyon contain moderate amounts of biotite.


Puye Formation

The Puye Formation is made up of sand and gravel washed off of the Sierra de los Valles prior to the eruption of the Bandelier Tuff. The gravel is made up mostly of rounded boulders and pebbles made of Tschicoma lavas. Some of the blocks are huge - the size of a car or small house, but most of the boulders are softball to basketball-sized. This unit is especially well exposed in Rendija Canyon east of the shooting range, but it is also exposed along Highway 502 near the Totavi gas station. The Puye Formation underlies most of the Pajarito Plateau and is an important aquifer.


Basalt of White Rock Canyon

Basalt is a black, fine-grained, dense lava that contains relatively little silica. Basalt, because of its high eruption temperature and low viscosity, has a tendency to flow long distances from its source. The tops of basalt flows are typically vesicular (contain abundant gas bubbles). Most of the basalt exposed in White Rock Canyon originated from vents located in the Cerros del Rio volcanic field located east of the Rio Grande. The explosive interaction of hot basalt with wet sediment underlying the basalt can be seen along the Frijoles trail in Bandelier National Monument.


Older Jemez Volcanic Rocks

In a few places, 8 to 9 million year old basaltic to andesitic lavas are preserved in the deep canyons of the Pajarito Plateau. Old basalt is found in Los Alamos Canyon to the west of the reservoir, and old andesite is found near Pine Springs north of Los Alamos.


Santa Fe Group

The Santa Fe Group is composed of sand and gravel that is largely derived from mountainous areas to the northwest, north, and northeast of the Jemez Mountains. In some places, ancestral Rio Grande or Rio Chama river gravels with well-rounded quartzite, granite, Pedernal chert, or volcanic pebbles are preserved. The Santa Group is also an important aquifer. The Santa Fe Group is the pink sediment beneath the gray Puye Formation along Highway 502 near the Totavi gas station. Its pink color is largely derived from the pink mineral orthoclase, an important component of the Sangre de Cristo range.

Additional Geology Resources