Amateur Naturalist: Small And Large Geologic Bands
Los Alamos
Summer provides a good time for exploring and observing many aspects of the natural world around us. There are variations between the large and the small in geology. The canyons in the Pajarito Plateau provide parallel patterns on a large scale.
Picture 1 below is of Pueblo Canyon. It is about 3.5 miles from it upper to lower end of Los Alamos mesa. It is nearly a half mile wide at this lower end and 400 feet deep from mesa top to canyon bottom.
There are parallel layers that form the wall of Pueblo Canyon. These layers are called members. These are named the Tshirege member, the Otowi member and the La Cueva member.
The Tshirege Member is at the top and can be divided into four units. Picture created by lava that flowed repeatedly across the land or was thrown through the sky as a result of the volcanic eruptions from the Valles Caldera.
There was a major eruption 1.2 million years ago which expelled an estimated 95 cubic miles of material.
The rim of the Valles Caldera can be seen at the horizon in Picture 1.
Picture 1: Layers of lava from eruptions compose the walls of Pueblo Canyon. Photo by Kent G. Budge
The middle Otowi Member is below the Tshirege member and is the result of an eruption of the Valles Calera. The La Cueva member is from 1.8 million years ago. This member also is a result of eruptions and now is largely buried beneath the Tshirege and Otowi members.
Picture 2: The four units of the Tshirege Member form most the canyon wall. The Otowi Member is referred to as the Cerro Toledo Formation in this picture. The Cueva member is hidden below the Cerro Toledo Formation. Photo by Robert Dryja
The Valles Caldera is thirteen miles in diameter. The lava that flowed from it edge extended to the Rio Grande River. It is twenty miles from its center to the Rio Grande River. The Valles Caldera therefore represents an upper end for size in volcanology.
It is impressive that Pajarito Plateau did not have canyons in it when first created by the lava flows from the Valles Caldera. It is equally impressive to realize that a four hundred foot deep canyon involved the eroding of columns of solid rock into small rocks that could be washed away by rain and melting snow. Rocks found in the canyons represent the lower end for size.
Lava is composed of minerals that are expelled in a volcanic eruption in various ways. Lava also may be more or less viscous depending on its starting temperature. High temperature lava is more liquid to start. Lava that contains a large portion of minerals may develop large crystals as it slowly cools. Lava that is composed of mica, silica or quartz creates a type of volcanic rock called rhyolite. This rhyolite may be very attractive because of the large crystals in it. Picture 3 shows that the crystals may occur as bands.
Picture 4 shows a rhyolite that is composed of orange and red colored bands but does not have visible crystalline minerals. Even if not shiny, it still it is interesting see while exploring a canyon.
Picture 3: This rock may have been within a lava flow that cooled slowly. Large quartz crystals therefore emerged. Other kinds of minerals remained as small black crystals. Photo by Robert Dryja
