Ashley Dunne
Francis MacDonald
Earth 104A
2 December 2019
Pisgah Crater Research Paper
Abstract
A study of the relative ages of the flows around the Pisgah Crater, Mojave Desert, CA. Discussing previous studies done on Pisgah crater, and the regional setting helped piece together the geologic history of the lava flows. Then based on the way that the lava flowed over topography, we interpreted the relative ages of each of the three significant lava flows. Interpreting what we could see on the topographic maps combined with observations in the field, we drew conclusions that the flow on the top is the youngest.
In this study, the main goal was to piece together the history of the lava flows around Pisgah crater. We predicted the relative ages of each of the flows based on the way that the volcano erupted and the way that the lava is laid out around the crater. In the field, we had no background on the subject or area so based our hypotheses solely on observation and interpretation. So in this research paper, I will re-evaluate whether or not I still agree with the relative ages that I gave the lava in the field.
Several scientists have studied the region in past years. They studied the sequence of eruptions, elements, crystallization, relative ages, mineral analyses, and isotopes, among other things (Glazer). There has been extensive research done in the area in the past, with a lot still left up for interpretation. The relative ages have been disputed and debated in the past by countless scientists. Pisgah crater is located at the heart of the Mojave desert. The basalt found here are among some of the youngest in the world, representing a part of the Miocene volcanism which ranged from the Amboy Crater to Black Mountain. Pisgah lies below sea level and represents a strike-slip fault called the Pisgah fault.
There were three main flow types that we observed. The first is ropey Pahoehoe with large centimeter-scale plagioclase crystals (Fig. 1). It consists of vesicular basalt, pulled further down in the middle than on the sides because of more momentum and less friction. This caused ropey U-shapes in the lava. Our next contact was at that of the A’A that we observed. In this flow, the phenocrysts are smaller, more on a half centimeter scale. This flow is even more vesiculated than the last and contains a lot more olivine (Fig. 2). The next was fields of scoria for a little bit and then the next section was a similar flow to the first, of Pahoehoe but less ropey and with smaller plagioclase crystals (Fig. 3). This flow had the contact at the scoria, which we concluded came before the flow since the scoria is underneath and around the flow (Fig. 4).
Fig. 1: Ropey Pahoehoe at the first stop Fig 2: Vesiculated A’A
Fig 3: Scoria contact Fig 4: Less ropey Pahoehoe
After analyzing these flows and their characteristics, we concluded that the A’A is the oldest, then the smaller, less ropey Pahoehoe, and then the youngest is the ropey, larger crystalled Pahoehoe. The younger flow was filling in the topography of the older flow, and we could see that the A’A is younger than the scoria because it fills in on top of scoria and then the Pahoehoe fills in on top of the A’A. We can reveal relative ages by basic original horizontality rules, and we observed which flow filled in the others and figured out which one is older. In terms of the ropey and less ropey Pahoehoe, the more ropey Pahoehoe is found on top of the less ropey Pahoehoe in multiple places on the topographic map, (Fig. 5) indicating that the ropey Pahoehoe is younger.
Fig 5: Topographic Map of Pisgah Crater
After having analyzed lava flows and taken into account the regional setting of Pisgah crater, we came to the final conclusion on the relative ages of the lava flows. The A’A is the oldest and the ropey Pahoehoe is the youngest of all of them.
Comments