MECHANICAL (FREEZE-THAW & THERMAL EXPANSION-CONTRACTION) WEATHERING EXPERIMENTS ON JURASSIC DIABASE FROM THE GETTYSBURG BASIN AND THE ROLE OF ROCK TEXTURES IN FORMATION OF FRACTURES

Prominent features of the Gettysburg rift basin include ridges, knobs, crevices, boulder fields, etc. formed in Jurassic diabase (York Haven and Rossville). In the Gettysburg area these landforms had prominent roles in the Civil War and there is much interest in how these developed. Given that these igneous intrusions cooled under various crustal conditions, they exhibit a range of textures from fine aphanitic to moderately coarse phaneritic. This study examines the role of rock texture and mineralogy on the formation of fractures which are exploited by near surface weathering processes. Freeze-thaw and root-wedging processes are said to exploit regional flexure-formed joints as well as pre-existing microfracture networks; however, it is unclear as to how small-scale vertical and sub-horizontal fracture networks develop in these very hard diabase sill deposits. To explore these processes, replicate samples with different grain sizes were cut into cubes, hand-polished to remove irregularities in the surface of the samples, and thin sections were prepared for petrographic analyses. Samples were photographed and examined to ensure the absence of preexisting microfractures. Samples are currently being subjected to repeat freeze-thaw trials using liquid nitrogen to initiate fracture development through thermal contraction and expansion. Although experimentation is ongoing, initial trial results show a more complex process takes place. We have discovered that microfractures can develop quickly upon initial cooling. As samples return to room temperature, the surface of the samples experience condensation and develop rime ice. Fractures become noticeable and wick moisture into the sample where it appears to freeze, expand, and propagate microfractures further into the rock. It is not yet known how different textures will respond to these tests, but it is hypothesized that coarser grain sizes will be less susceptible to microfracture development due to higher yield strengths associated with interlocking fabrics of more slowly cooled intrusive rocks.