PEEC Amateur Naturalist: Part 1 The Pajarito Plateau – Setting the Stage

PEEC Amateur Naturalist: Part 1 The Pajarito Plateau – Setting the Stage
Particular thanks to Patrick Rowe

The Pajarito Plateau can be described with four words: canyons, mesas, mountains and skies. It is the geology represented by these four words that makes the Pajarito Plateau so appealing and interesting. What are the geologic events that created these features?

To understand how the Pajarito Plateau was created we have to make a short journey back to the time of the dinosaurs. Between approximately 100 to 60 million years ago, a large inland sea split the continent of North America into two landmasses. The Laramidia landmass was to the west and the Appalachia landmass was to the east. The inland sea covered much of what is now New Mexico. Sediment eroding from Laramidia and Appalachia created a great and relatively flat floodplain.

87–83 million years ago                           6258 million years ago                         

Source: Ronald Blakey at

Between 70 and 80 million years ago the great floodplains began to be disturbed by linear up-warps paralleled by flanking down-warps or depressions. Envision a rug that has been pushed from an edge to form a rolling pattern. These early mountain-building tectonic features were developed along the entire length of the New Mexico Rocky Mountains. The area now covered by the Pajarito Plateau was part the Rio Grande Depression. This plain stretched from the Sangre de Cristo Range on the east to the Nacimiento uplands on the west. The depressions filled with the products of erosion from the uplifts.

Nacimiento uplands                                                   Sierras de los Valles Mountains

Source: Google Maps

Volcanism in the Jemez area began with widespread eruptions of basalts and continued with successive effusions of other volcanic rock layers over an area approximately forty miles by 40 miles. Some eruptions involved flowing lava as is seen in Hawaii while others were more explosive as occurred with Mount St. Helens in Washington State. These basalts locally attained thickness of up to 2,000 feet and individuals flows may be as thick as 800 feet. These flows built up the Jemez Plateau similar to a layer cake.

While there were some earlier volcanic eruptions, a set of volcanic eruptions occurring between 2.9 and 5.3 million years ago created a set of mountains that now are called the Sierras de los Valles. The peaks of these mountains form the western horizon for Los Alamos. Pajarito and Caballo mountains are part of the Sierra de los Valles. 

Editor’s note: The creation of the Sierra de los Valles set the stage for the creation of the Pajarito Plateau. The Valles Caldera would be created between the Nacimiento and Sierra de los Valles mountains. This will be described in Part 2 of this special series.

LOS ALAMOS website support locally by OviNuppi Systems