Masaya, like Boqueron, is geochemically confusing. It has iron rich lavas, a type of magma evolution we call tholeiitic. It also has an active lava lake that is often visible from the rim of the active vent. There is no question that the lavas evolve at low pressure and therefore develop the tholeiitic trait.
The lava lake is at times sealed up. At other times, often for several years, Masaya has prolonged and voluminous gas eruptions. In the normal accounting of eruptions these gas plumes are often ignored or treated lightly. However, at Masaya enough sulfuric acid is produced to generate acid rain that causes significant destruction of property and agriculture (no doubt health problems as well). The is a deforested zone downwind to the WNW, where the low gas plume brushes the surface.
Masaya is a shield volcano, with a very gentle slope. The cones are minor bumps in the summit caldera that is about 15 km across. The volcano is a low caldera with a diameter of at least 60 km. It is not high, but it is huge.
Arc volcanoes are supposed to be calc-alkaline, composite cones but Masaya is a tholeiitic shield. Ironically, this unusual volcano is the global maximun in eruption of 10Be. This radioactive isotope is created near the earth's surface by cosmic rays. It can become concentrated in sediments on the sea floor. From there, it travels into the trench and, if not scraped off, can reach the roots of arc volcanoes in about a million years. It is the single most definitive tracer of subducted sediments that help trigger arc volcanism. Masaya, an oddball, is the global leader.
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