Tag: Geology

Strain In Butte, Montana

Presenter(s): Owen Smith − Geology

Faculty Mentor(s): Ray Weldon, Mark Reed

Poster 3

Research Area: Geology

From the uplift of the Rocky Mountains to the basin and range extension, Butte Montana has undergone dramatic tectonic deformation. This deformation does not just make for an interesting landscape but also affects the shape of mineral grains in the rock. The shape of grains can show us the amount of tectonic compression or extension the region has experienced, however the grains only record the amount of strain since it formed. Using quartz veins and the quartz grains that compose them, I measured the minor/major axis lengths of the grains. This shows us how the grain has been stretched or compressed relative to the veins orientation. The main method used for this analysis is the Fry method and it allows us to see the amount of compression or extension has occurred along the quartz veins. The results show us that if veins have not be cross cut then the grains are compressed along the vein orientation and extension occurs perpendicular to the vein orientation. When a vein does get crosscut, then the grains show less extension perpendicular to the vein and less compression along the vein orientation. This tells us that on the vein level, when a vein crosscuts another vein, there is strain accumulated parallel to the crosscutting veins orientation. This research will help complete the picture of the total amount of strain built up in the Butte, Montana region.

Decompression Experiments of the Mono Craters Eruptions of 1340 C.E.

Presenter(s): Eamonn Needham − Earth Sciences

Faculty Mentor(s): Jim Watkins, Thomas Giachetti

Poster 4

Research Area: Earth Sciences (Geology)

Funding: UROP mini grant

The Mono Craters, California eruptions of 1340 C.E. were a series of eruptions that produced relatively texturally homogeneous deposits, with the exception of the first bed. The initial eruptive deposits differ from later deposits in
the relative abundance of obsidian pyroclasts (quenched magma), volatiles (H2O and CO2), and microlites (minerals <100μm). These textural differences between Bed 1 and the other beds remain unexplained, but may be due to changes in decompression rates. To test the decompression rate hypothesis, a sample of synthetic Mono obsidian was run in a cold seal pressure vessel at eruptive conditions. The sample was kept at 850°C and 60 MPa for 2.5 days, and then was decompressed isothermally at a rate of 0.001 MPa/s until it reached 5 MPa. Following rapid quench, bubble number density and microlite number density were determined from scanning electron microscope images. In the future, more of these experiments will be run at different decompression rates, to see which decompression rate best match the textures of the Bed 1 samples. The calculated decompression rate will be compared to decompression rates of later deposits which were calculated in other studies using volatile concentrations. If Bed 1 has a different decompression rate than the later beds, this could explain the textural differences, and can be used to look at how the eruption initiated and progressed. This research could have implications for the ongoing debate of eruptive style transitions from explosive to effusive, which in turn will inform hazard mitigation for volcanoes exhibiting this behavior.