Picture 1: A layer of lava below the cap has divided into thinner layers due to the way it cooled when a liquid. Freezing water between the developing thin layers pushed them further apart. Photo by Robert Dryja
By Robert Dryja
Los Alamos is located on the Pajarito Plateau. This is an area that was created as part of volcanic eruptions from what is now known as the Valles Caldera. A major eruption occurred approximately 1.2 million years ago when softer forms of lava called tuff spread out for miles. Imagine the Rio Grande River back then being blocked by lava. A lake was created. The road that crosses the Rio Grande River today would have been under about 200 feet of lake water.
Although there were several volcanic eruptions, the resulting rock formations could vary in many ways. Lava may have flowed initially as a single thick hot layer but then developed thinner layers as it cooled. The surface of a thick layer of lava in open air would cool sooner. This surface area would shrink as part of the cooling and separate slightly from the still hot liquid lava below. This process could repeat itself, creating multiple layers, (see picture 1).
Different kinds of rock texture and thickness could emerge in a particular area even when the overall area was created volcanically. These variations can be seen on a large scale when looking at the sides of a canyon that is hundreds of feet deep. White Rock Canyon for example is up to a thousand feet deep. The Rio Grande River has slowly eroded its way down, draining the lake that been created initially when the volcano had erupted.
The canyon walls have four distinctive layers. Water and wind erosion over thousands of years have created rock layers that are on a much smaller scale compared to overall canyon walls. A particular kind of formation called a "tent rock" can be seen. A denser, more water-resistant layer of lava may have covered a layer of softer layer of lava below it. The darker lava could have been only a few feet thick while the lighter lava below was tens of feet thick.
Erosion could result in an upper layer of lava dividing into caps over softer lava. The softer lava directly below a cap was protected from eroding away compared to softer lava out in the open. A shape called a tent could be created, (see picture 2). Picture 3 shows three tent rocks that are similar for their caps and columns. Their closeness to one another suggests that they were once part of common upper and lower layers.
A cap rock can erode into a particular shape depending upon its physical composition. Picture 4 shows two pancake shaped cap rocks that are balanced above a softer rock. Each cap rock extends to one side in the open. Remarkably, each cap rock is balanced on the smaller rock that then sits on a larger pedestal column below. Picture 5 shows two cap rocks balanced at the top of columns. It also shows a white columnar rock that has lost its cap rock.
A tent rock means you do not need to dig down and around a boulder to determine what kinds of soil or rock it is sitting upon. Erosion instead has exposed the geology. Picture 6 shows an example. The dark cap rock is not sitting directly on a column of light tuff.
Instead, there is a section in between that is composed of orange colored soil and stones. Geology research shows that the Valles Caldera had lakes in it as one time. A stream or lake apparently laid the soil and rocks down on top of the white tuff. A flow of dark lava then occurred, covering the soil and rocks.
Other smaller eruptions occurred after the initial major eruption. The last smaller eruption occurred about 40,000 years ago. Tent rocks therefore provide a way of learning in more detail what occurred as part of volcanic eruptions. Different kinds of lava from the eruptions created different kinds of layers.
Picture 2: A dark denser lava cap protects the lighter lava below from eroding away. Photo by Robert Dryja
Picture 3: A layer of dark lava has broken apart and eroded into a set of three caps on top of lighter colored columns. Photo by Robert Dryja
Picture 4: The stone on top is eroded into the shape of a pancake but it still remains balanced in place. Photo by Robert Dryja
Picture 5: The white column on the right it has lost its cap. The two columns on the left had gas bubbles inside them. Bubble holes now appear on their sides. Photo by Robert Dryja