Earth and Planetary Sciences ETDs

Publication Date

10-10-1967

Abstract

A statistical method employing the Tukey Chi-square test and the vector method was used for determining the direction of primary flow lineation in volcanic rocks. Oriented samples were collected from the McCartys Basalt, the porphyritic andesite of Mount Taylor and the Bandelier and Battleship Rock Rhyolite Tuffs of the Jemez Mountains. All these localities are in New Mexico. Measurements were taken of the elongate particles in each horizontal thin section. The flow lineation direction in each slide was determined statistically by computing values of vector mean or chi-square orientation. The McCartys flow samples showed the lowest reliability. This was probably due to the complex flow mechanism and possible secondary crystallization of plagioclase laths. The following correlations and conclusions were drawn regarding the McCartys flow.

1. A random distribution of toothpicks and nails superficially resembles the felty texture of the basalt. The distribution of the plagioclase laths in the basalt is, however, not random.

2. A higher degree of preferred orientation exists toward the top and sides of the McCartys flow. This suggests a complex cooling history.

3. A higher degree of preferred orientation exists in a constriction than in any other part of the flow.

The following correlations and conclusions were drawn concerning the Bandelier and Battleship Rock Rhyolite Tuffs.

1. The rhyolite tuff samples not only show a good preferred orientation but substantiate the view that the location of the vent is in the Valles Caldera. This indicates that the movement of an ash flow is in part laminar. Laminar flow probably predominates in the late stages of movement of an ash flow.

2. At Battleship Rock there seems to be a direct correlation between the degree of welding and the degree of preferred orientation.

3. In the Battleship Rock welded tuff, pumice fragments are better indicators of flow direction than crystal fragments.

4. In the Bandelier Rhyolite Tuff, the pumice likewise was the best indicator of flow. Crystals and glass shards followed respectively.

The samples collected from the porphyritic andesite at Mount Taylor showed the highest degree of preferred orientation. A vent located within the Mount Taylor amphitheater was indicated. The following correlations and conclusions were drawn regarding the porphyritic andesite.

1. The groundmass plagioclase laths have a higher degree of preferred orientation than the plagioclase phenocrysts.

2. Differences in vector mean and chi-square orientation values exist within the same sample.

3. There may be great variation in values of flow lineation for two samples collected only a few feet apart.

4. The statistical treatment of orientation data may be used to differentiate an outcrop previously mapped as one flow into two or more flows.

The direction of flow azimuth indicates the absolute direction of movement at any point on a flow. Objective criteria must be used to indicate the direction of the flow azimuth. Two criteria were found to be reliable in ash-flow tuffs.

1. Fork-shaped glass shards with a long "handle” tend to have the long handle of the shard in the direction of flow lineation pointing away from the source. This criterion has a reliability of 62.9%.

2. Fork-shaped shards without a distinguishable long handle will have one of the three handles in the direction of flow lineation pointing away from the source. The reliability is 66.3%.

The blocking effect, the penetration effect, and the orientation of spindle-shaped objects either did not show high reliabilities or were not plentiful enough for reliability to be measured. Imbrication and blocking effects seem to be the best indicator of flow azimuth in vertical section.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Wolfgang Eugene Elston

Second Committee Member

Abraham Rosenzweig

Third Committee Member

J. Paul Fitzsimmons

Language

English

Document Type

Thesis

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