Then on 30 July — the last day of eruptive activity, lava was erupted at the Duraznero fissure and vent. Only residual fumarolic activity and thermal emissions then occurred until 30 July when the Duraznero vent and fissure re-activated. This caused the southern arm — the Cumbre Vieja to be the sole source of volcanic activity. Such earthquakes are undetectable except by specialist equipment and therefore at best may be the precursory activity for an eruption at some time in the future. Cumbre Vieja Spanish : Old Summit is an active although dormant volcanic ridge on the volcanic ocean island of La Palma in the Canary Islands , Spain , that erupted twice in the 20th century — in , and again in He was startled to hear noises emanating from the Duraznero vent and became extremely alarmed when material started being expelled from the vent.