Non-linear magma ascent pathways.
Volcanologists understand that linear magma ascent is unlikely.
Instead, ascending magma is more likely to experience varying ascent rates, pressure fluctuations, and may even stall temporarily in a shallow storage region. Despite this, many experimental studies continue to model magma ascent as a linear process, starting from equilibration at pressures greater than 100 MPa and followed by decompression to final pressures of 20 MPa or higher. Given that volcanic activity is largely influenced by processes within the conduit and that linear ascent is improbable, I have started to explore non-linear ascent pathways in my research.
High crystal number densities from mechanical damage.
Crystal and vesicle textures in volcanic eruption products offer crucial insights into magmatic processes. Crystals play a significant role in the nucleation and growth of gas bubbles, influencing the overall degassing efficiency of magma and, consequently, the intensity of volcanic eruptions. Therefore, pyroclast textures are often analyzed and compared with experimental results obtained under controlled conditions to establish threshold conditions that can trigger shifts in eruptive style.
A longstanding puzzle in this field is the presence of numerous tiny crystals, or microlites, in dome and cryptodome samples. Despite extensive efforts, experiments have typically failed to replicate this texture, with observed number densities often an order of magnitude lower than those found in natural pyroclasts (see figure). In Lindoo and Cashman (2021)*, we introduced pressure fluctuations into decompression experiments to simulate the decompression of magma that stalls during ascent and the pressure cycling that occurs in non-erupted magma within the conduit during episodic explosive activity.
These pressure fluctuations caused fragile, dendritic microlites formed at lower pressures to break apart, leading to an apparent increase in number density (hatched [D2] symbols in figure), matching the higher densities observed in natural samples. This was a significant departure from the results of experiments that followed a more traditional decompression pathway (gray symbols in figure). We then compared our experimental results with samples from the May 18, 1980, Mount St. Helens blast dacite deposit. Our findings provide an explanation for the crystal textures observed in precursory ash and the blast deposit, offering valuable insights into the degassing history of the cryptodome.