Excursion guide for field trip V2 : ‘Island of Terceira’

• Main Author: Self, Stephen
• Publication Date: 1982
• Format: Article
• Language: eng
• Source: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Download full: http://hdl.handle.net/10400.3/4884
• topic: Volcanism; Terceira Island (Azores); Azores Archipelago

The island of Terceira is 406 km2 in area and rises to 1021 metres above sea level. It consists of four strato-volcanoes grouped along a prominent fissure zone (Fig. 1). Two volcanoes, Pico Alto and Santa Barbara, are active and the other two, Guilherme Moniz and Cinquo Picos, are believed to be extinct. The fissure zone may be the sub-aerial expression of the Terceira Rift, regarded by Krause and Watkins (1970) as a secondary spreading centre. Terceira shows a great diversity of lavas and pyroclastics for an oceanic island and is noteworthy for voluminous production of peralkaline salic magma. Of the four volcanoes forming the island ; three are composed of both basic and salic rocks and one has only salic rocks exposed. Since the emergence of the island a compositionally bimodal population of rocks has been represented. The products of over 100 eruptions in the upper Terceira Group have been recognized. These include ignimbrites, pumice fall deposits, salic lava extrusions, strombolian scoria deposits, basaltic lava flows and littoral (surtseyan) basaltic tuffs. Basaltic activity is concentrated along the fissure zone which bisects the island diagonally from NW to SE. Volumetric studies give the rate of accumulation of new crust along this small spreading centre; 5.46 km3 of new material has been erupted on the island in the past 23,000 years, of which over 4 km3 is comendite pantellerite composition. The island's economy is dominated by agriculture and dairy farming. Much of the water for the maintown of Angra do Heroísmo (approximately 20,000 population) comes from underground springs or streams in the lava tubes of a 2000-year-ald basalt in Guilherme Moniz Caldera. The island has a good system of roads. Almost the entire papulation lives around a 5 km wide coastal strip (Fig. 2).