Focus Question: How has New Mexico changed over geologic time?
Description/Background Information:

New Mexico shows a great diversity in its geologic history. Rocks from each of the four eras can be found exposed within the state. Through the study of these rocks, geologists have put together a chronicle of our state's history as it has changed over time.

Precambrian igneous and metamorphic rocks of more than 1.5 billion years old have been found in New Mexico. In general, these rocks form the core of mountain ranges along the east side of the Rio Grande river. The oldest of the Precambrian rocks are found in the northern part of the state in the Brazos, Taos and Nacimiento mountains, dating 1.8 billion years old. These Precambrian rock formations most likely have been buried and later exposed by the uplift of mountain chains and the erosion of sediments that had once covered these formations. A picture of ancient New Mexico during the Precambrian is one of volcanic activity and mountain building, with periodic episodes of submersion under the sea. After millions of years of erosion, only the remnants of these old mountain systems remain. Most have since been affected by a new round of mountain-building followed by further erosion.

During the Paleozoic, most of the North America was part of a large landmass called Pangaea. The state was covered by a vast shallow sea, from which large deposits of limestone, sandstone and shale accumulated. These limestone beds can now be found in many areas of New Mexico, often containing small marine invertebrate fossils such as brachiopods, crinoids and trilobites. During the later part of the Paleozoic, the ancestral Rocky Mountains formed, uplifting the central and northern part of the state. Toward the southern part of New Mexico, a great barrier reef developed. As the reef was cut off from the sea, the evaporation of the water left deposits of salt, potash and gypsum that can be found today.

In the Mesozoic era, New Mexico saw a drying trend and dinosaurs roamed much of the state. Many of the rocks of the northern part of the state, including the colorful red, green, gray, brown, and white sandstone and shale, were deposited by rivers that flowed toward the seas. Later in the era, the inland sea once again returned and New Mexico was on the western shore of a great shallow ocean covering most of the central United States. North America broke away from the remaining part of the Pangaea landmass and started its drift to the west, eventually colliding with the Farallon plate.

The Cenozoic is characterized by tremendous changes in the New Mexico landscape, in response to this collision of the North American and Farallon Plates. The Rocky Mountains of today rose and volcanoes produced a thick layer of igneous rock that covered much of New Mexico. In addition, stresses in the crust produced the large central depression that divides the state in half, called the Rio Grande rift. Within the last 10 million years, New Mexico has seen tremendous volcanic eruptions, such as those that produced the Jemez Mountains, Capullin Peak and Mt. Taylor. In addition, the continuous erosion and deposition of sediments in recent times have produced many of the more scenic landscapes of the state, such as its colorful mesas, badlands and deserts.

Resources/Activities:
USGS: Historical Perspective (Plate Tectonics)
Places and Processes: Analyzing the Role of Physical Processes in Shaping Places