Temporal variations in the constituents of volcanic ash and adherent water-soluble components in the Unzen Fugendake eruption during 1990–1991

Abstract

A change in the chemical compositions of volcanic gases is one of the noticeable phenomena that frequently occurs prior to an eruption. Analysis of the water-soluble components adhering to volcanic ash is available for remote monitoring of volcanic gases from inaccessible volcanoes. It is a secure method for monitoring volcanic activity without using particular devices. Prolonged volcanic eruption at the Unzen Fugendake volcano from 1990 to 1995 started with a phreatic eruption after 198 years of dormancy. Volcanic activity changed from a phreatic and phreatomagmatic eruption to a magmatic eruption with pyroclastic flows in May 1991. The relationship between the chemical composition of volcanic ash and the contents of the water-soluble components adhering to it are discussed in relation to the early stage of the long-term eruption. Volcanic ash ejected by phreatic and phreatomagmatic eruption before dome formation was the product of the alteration in the volcanoclastic materials beneath the surface. The ash had a high content of water-soluble components, which was caused by the absorption of hydrogen chloride and sulfur dioxide gases from magma into wet debris before dome formation. Volcanic ashes which were generated by pyroclastic flows after dome formation were fresh lava fragments. While the contents of water-soluble sulfate adhering to the ash noticeably decreased, those of water-soluble chloride adhering to the ash hardly decreased. The considerable decrease in the contents of water-soluble sulfate was caused by the reaction of volcanic gases with dry lava fragments. Contrary to this, the concentration of hydrogen chloride gas in ash clouds was extremely high, which obstructed the decrease in the water-soluble chloride content in the ash. Volatility of chlorine and sulfur from volcanic rock suggests that the inner temperature of pyroclastic flows was higher than 600∼700 • C at least.

Hot clasts and cold blasts: thermal heterogeneity in boiling-over pyroclastic density currents

“…Similar evidence for highly variable emplacement temperature of PDC deposits has been recognized in other studies. Previous studies of two different PDC deposits have estimated emplacement temperatures that range from about 48 8C, measured by thermocouple (Sparks et al 2002), to around 700 8C, estimated by the mobility of chlorine and sulphur between the gas and ash of a PDC (Nogami et al 2001). The highest emplacement temperature estimated from progressive thermal demagnetization data is approximately 680 8C .…”

“…Sawada et al 2000;Cioni et al 2004), have been used to estimate emplacement temperatures. Temperatures of emplacement for PDC deposits range from around 48 8C (Sparks et al 2002) to approximately 700 8C (Nogami et al 2001), and estimates of temperature can vary by 200 -300 8C within different parts of a single deposit (e.g. Zlotnicki et al 1984;Bardot 2000;Paterson et al 2010).…”

Development of a Blade‐type Crawler Mechanism for a Fast Deployment Task to Observe Eruptions on Mt. Mihara

“…Even in the case of mild eruptive activity, landslides and mudslides could arise due to the newly formed terrain. Therefore, observations are recorded and work procedures are conducted using unmanned aerial vehicles (UAVs) instead of humans (HEADQUARTERS, 2008; NASA, 2012; Nogami, ; Taddeucchi, ). For instance, a compact helicopter (Yamaha's RMAX‐G1) powered by an engine took aerial photographs and installed seismographs on Mt.…”