Focus Questions:
What is a volcano?
Is there a history of volcanic activity in New Mexico?

Description/Background Information:
General Volcanic Overview-
Molten rock that forms below the Earth’s surface is referred to as magma. It forms wherever temperatures and pressures are high enough to melt rock. Once molten, this rock has a tendency to rise upward due to the fact that is has a greater volume and slightly lower density than surrounding rock. If the molten rock can reach the Earth’s surface, it erupts through openings or fissures in the crust to produce a volcano. These openings may erupt the molten rock (now called lava at the earth’s surface) or blow out fragments of solidified rock (called pyroclasts) to the surrounding area, producing a raised feature that is typically recognized as the volcano.

The shape of volcanoes and the types of volcanic eruptions are controlled by the chemical and physical properties of the magma including:
  1. Silica content: Silica is the major ingredient in all magmas. Molten rock with relatively higher silica content is thick, light-colored, and viscous. These magmas are called felsic magmas. Molten rock with relatively lower silica contents tend to be thinner, darker and have a lower viscosity. These are referred to as mafic magmas. In general, very violent and potentially destructive volcanic eruptions are associated with felsic magmas, especially if they have appreciable amounts of dissolved gas in the magma (see below). Also, volcanoes that form from felsic magmas tend to have steep slopes and have the classic cone shape associated with volcanoes. These volcanoes are called strato- or composite volcanoes. Shield volcanoes form from flow of low viscosity mafic magmas and generally have shallow slopes. Volcanic eruptions associated with mafic magmas can be very violent adjacent to the source (the very common lava fountains found in Hawaii) but generally do not have the very violent nature associated with felsic magma.
  2. Gas content: Gases such as water vapor, carbon dioxide and others are often dissolved in the magma. As magma comes closer to the surface and pressure decreases dissolved gases in the molten rock are released. Rapid expansion of the gases due to pressure release can result in explosive eruptions as compared to magma with small amounts of gases.
By understanding the type of magma or lava associated with a particular volcano, the gas content and the geologic history of a volcano, geologists can predict the eruption style.

New Mexico Volcanism-
New Mexico is located on the boundary between two distinct geologic features. The eastern side of the state is located on the High Plains, an area of little geologic activity with rare earthquakes and no volcanic activity. The western side, extending all the way to the Pacific Ocean, is geologically active. Mountains, volcanoes, and earthquakes are more common in this part of the state. The Rio Grande Rift, a large-scale fracture in Earthís crust that extends from northern Colorado to Mexico and forms a valley through which the Rio Grande River flows, is the division between these two very distinct zones. The Rift is the site of major spreading action in the Earthís crust due to the motion of crustal plates that have helped to form the western part of the United States. When the earthís crust is pulled apart, it becomes thin and allows heat from the Earthís mantle to rise to the surface. As a result of this action, you find locations for volcanic and geothermal activity spread throughout the western part of New Mexico.

Many locations in the state show the results of this recent volcanic activity. The most recent of these are the lava flows at the Valley of Fires State Park near Carrizozo and McCartyís flows near Grants, each less than 1,000 years old. In addition, a very large complex forming the Jemez Mountains shows the tremendous effects of volcanic activity within the state. Approximately 1 million years ago, a series of massive eruptions produced the structure known today as the Valles Caldera. This structure, measuring 14 miles across, was produced as large volumes of ash and dust were erupted into the air, scattering material as far east as Kansas!

Other prominent volcanic features in the state include Mt. Taylor, a composite volcano that last erupted approximately 2 million years ago; Capulin Peak, a lava dome that is about 4,500 years old; Sierra Grande, a shield volcano and one of the largest in New Mexico; and the Albuquerque volcanoes, a series of small cinder cones that are between 150,000 and 200,000 years old.

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