Breccias related to explosive volcanism

Abstract

Explosive breccia pipes were formed in the phreatomagmatic eruption of Taal volcano 60 km south of Manila, Philippines in September 1976. They were photographed in eruption which consisted of a series of small explosions occurring during the peak of activity at 10 sec intervals. These four breccia pipes formed on the collapse fault marking the margin of the much more intense eruption of 28 September 1965 when a diatreme 800 m in diameter formed. Renewed eruption in October 1977 on one of the breccia pipes blasted out the sealed plug and breccia fragments in all stages of alteration were included in the tephra. They ranged from barely agglomerated to strongly cemented through argillized and pyritized to totally replaced by silica and pyrite, fragments indistinguishable from those found in many mining districts. The violence of the explosion initiating a phreatomagmatic eruptive sequence results in strong crackling of the walls of the pipe and the crakle zone may become mineralized (cf. Braden, Chile). Usually the crater collapses on ring faults after eruption, forming a marginal tectonic breccia which can be mineralized (cf. Balatoc, Philippines.) The small craters with steep inner walls and tapering outer slopes which form during phreatomagmatic eruptions, sometimes containing lakes, are called maars and they are the surficial expression of an explosive breccia pipe or diatreme. Superficially similar craters are formed over kimberlite pipes. Hydrothermal explosion craters are somewhat similar. Collapse breccia pipes can form in the same sequence with explosive pipes. When one rising plug encounters ground water and explodes, fluostatic pressure on any other cupolas rising from the same magma drops rapidly, resulting in withdrawal of magma and collapse of walls and roof (Perry 1961). “Fluidization” is not thought to be of any significance in formation of breccia pipes (Wolfe 1980) contrary to Reynolds (1954) and many authors who have cited that work. An explosive breccia pipe is like an open window resulting in rapid depression of the isotherms of a pluton. A concentrated brine front can build up in minutes after an explosion. The very rapid cooling may explain why many explosive breccia pipes are altered by silica and pyrite only, the system being cooled before metallic solutions have time to replace breccia matrix or fragments.