ALBERT

Description: Sag-hegy is the remnant of a complex volcano consisting of several phreatomagmatic pyroclastic sequences preserved in immediate contact with a thick (c. 50 m) coherent lava body. Due to the intensive quarrying, the inner part of the lava has been removed, leaving behind a castle-like architecture of pyroclastic rocks. The outcrop walls thus demonstrate the irregular morphology of the lava, which was emplaced in a NW-SE-trending ellipsoidal vent zone in a phreatomagmatic volcano. Pyroclastic beds in the quarry wall are cross-cut by dykes and sills, inferred to have been fed from a central magma zone. Thin (〈10 cm) strongly chilled, black, angularly jointed aphanitic basaltic lava mantles the pyroclastic sequence, and has a corrugated margin as a consequence of sudden chilling against the cold and wet phreatomagmatic tephra in the inner wall of the tuff ring crater. These corrugated zones are inferred to be a characteristic textural feature, indicating extensive mixing of lava and host tephra which led to peperite formation along the outer rim lava lake. A spectrum of peperite formed along the lake margin, and fluid oscillation, due to fluidization of the wet tephra, disrupted a steam envelope formed around the lava, causing basaltic magma to invade and mix with the phreatomagmatic tephra. The presence of unconformities in the tephra ring facilitated the formation of sills fed from the central lava body.