Welded tuffs and the Maelifell caldera, Alftafjordur volcano, south-eastern Iceland

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Abstract
SUMMARY: The ‘Maelifell caldera’ is the name given to a separate collapse area situated on the southern margin of the main caldera of the Tertiary Alftafjordur volcano. It is 2 km in diameter and is made up of inward dipping welded tuffs, agglomerate (including patches of welded agglomerate), tuffaceous sediments and rhyolite and andesite lavas. The welded tuffs occur as thin pitchstone sheets and as much thicker felsitic masses: the former contain inclusions of basalt glass and are good examples of the simultaneous eruption of acid and basic magmas. It is suggested that the caldera represents the core of a parasitic volcano, beneath which there was a high level acid magma chamber, the immediate source of the local acid rocks. The magma chamber was intersected by numerous intrusions of basic magma, and these caused explosive eruptions and the emission of acid and basic magma to form thin pitchstone sheets. Decreased pressure in the magma chamber after eruptions caused repeated collapse within the cal

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