Volcanoes of Terceira Island

The Azores archipelago is located in a complex geological setting dominated by the American, African and Eurasian plates boundaries interplay. Terceira island lies in the central part of the so-called Terceira Rift, a north /west and south/east structural line that extends from the Mid-Atlantic Ridge into the east/west Azores-Gibraltar Fault. The active volcanic systems at Terceira Island include the central volcanoes of Santa Barbara and Pico Alto and a basaltic ridge that crosses the island along a general north /west and south/east trend. Since its settlement in the 15th century only one volcanic eruption took place on land in 1761. Submarine eruptive activity occurred in 1867 and 1998-2001 along the Serreta Submarine Ridge, a volcanotectonic structure that extends west of the island.

Furnas do Enxofre is the unique fumarolic field known at Terceira Island. It is located at Pico do Galhardo, a trachytic dome emplaced where the north /west and south/east basaltic ridge intersects Pico Alto Volcano.

Furnas do Enxofre (sulphur pits)

Caves

Where there are volcanoes, there are caves - and the Azores have spectacular caves in profusion. More than 200 have been discovered.

The islands owe their birth to being at the triple junction of the Eurasian, North American and African tectonic plates, where volcanic eruptions break through, raising the ocean floor and covering them with lava and volcanic ash.

Some caves are simply vast pits where the crust and new rock didn't quite fit. Most, however, are lava tubes, where the outer crust cooled and the inside flowed away. This makes for very long caves - the Gruta das Torres on Pico is more than 5km long.

The most spectacular, seen from both above and below ground, is Terceira's Algar do Carvão, a huge volcanic pit with caverns leading to an underground lake. There is also a fascinating visitors' centre.

Algar do Carvão

Serreta Volcanic Ridge

The submarine eruption that started on December 1998 from multiple vents along the Serreta Volcanic Ridge, about 10 km west of Terceira Island, Azores, continued through March 2000. Vents along the ridge were very active between December 1998 and September 1999. Activity then declined to very low levels with rare surface manifestations through December 1999. Activity increased again in late January 2000.

Several times during 1999 basaltic lava balloons were observed floating in the eruptive area. These "balloons" are very hot, gas-rich, lava fragments produced from small submarine lava lakes/fountains. During ascent to the surface, magmatic gas exsolves from the hot fragments, increasing the volume of the balloon while the crust is glassy and expansible. Once at the surface, interaction between the hot blocks and seawater produce white steam columns that can be seen from land when meteorological conditions are favorable. The blocks eventually sink after the gas escapes.

Lava causing white steam on the surface

Lava balloon from the Serreta Ridge off Terciera

An oceanographic mission supported by the national Foundation for Science and Technology was carried out in April 1999 to study the geological/geophysical characteristics of the eruption and its impact on local ecosystems. Scientists from the University of Azores, University of Lisbon, University of Algarve, Instituto do Mar, and Instituito Hidrográfico used a remotely operated vehicle that crossed an impressive submarine volcanic plume just above an active eruptive center at about 380 m depth. This plume was formed by volcanic particles of ash and lapilli size along with gas bubbles and lava balloons up to 2 m in diameter.

On 28 January 2000 a yellowish spot was observed at the sea surface above the eruptive area due to the dispersion of a volcanic plume that rose from a new vent located at about 250 m depth. The area of water discoloration caused by the plume was visible almost continuously for about a month, reaching a maximum diameter of 8 km on 24 February. The plume was generated by multiple eruptive pulses from different eruptive centers located within a few hundred meters of each other.

Seismicity along the ridge related to the eruption continued through the end of March, but at low levels. Since the beginning of this volcanic crisis the physical and chemical parameters of waters and fumarolic gases from Terceira Island have been monitored, with no changes detected. Another submarine eruption took place in this general location in June 1867. At that time five months of strong seismicity destroyed about 200 houses at Serreta.

Don Joao de Castro Bank

Don Joao de Castro Bank is a large submarine volcano that rises to within 14 m of the sea surface roughly halfway between Terceira and San Miguel Islands. A submarine eruption during December 1720 produced an ephemeral island that attained a length of 1.5 km and an altitude of about 250 m before it was eroded beneath the sea surface two years later. The volcano (also spelled Dom Joao de Castro) was named after the Portuguese hydrographic survey vessel that surveyed the bank in 1941. Two youthful parasitic craters, one tephra covered and the other sediment free, are located on the NW flank. The submarine volcano has an impressive fumarole field and remains seismically active.

Don Joao de Castro Bank

Caldeira de Santa Bárbara

An attractive trip can be made to the volcanic western part of Terceira and the Caldeira de Santa Bárbara (1,022m/3,353ft), the highest point on the island, a volcano with a number of explosion vents and collapsed craters. Northeast and east of the volcanic plateau, between Pico Alto, Pico da Bagacina and Furnas do Enxofre, are several crater lakes and fumaroles.

Serra de Santa barbara

Slowly climbing up the slopes of Serra de Santa Bárbara, this scenic road leads through magnificent landscapes, with Japanese cedars and colourful hydrangea bushes dominating the views on the lower levels. With about 8 kilometres in diameter, this volcanic massif, which once created the western part of the island, encircles the lush green crater of the same name on its southwest side. With 1,021 metres above sea level the island’s highest peak, the volcano of Santa Bárbara marks the southern rim of the Caldeira de Santa Bárbara, which has a diameter of around 2 km, and a depth of only 150 m, with several small volcanic cones protruding from its bottom. A well-indicated asphalted road leads up to the top of the dormant volcano - which was last active in 1867 when the lava streams from a lateral flank badly damaged the village of Serreta - where a beautiful belvedere provides superb panoramic views making it possible to see the outlines of the neighbouring islands Graciosa, São Jorge and Pico on the horizon on clear days.

The about 827 m high Pico Rachado marks the northern rim of the caldera and in the north-eastern part of the Serra lies the small and tranquil Lagoa Negra, much visited by migrating birds during the winter. Four small volcanic cones in the southeast of the volcanic massif form the area of the Mistérios Negros, part of it was only created during the eruption of 1761, which explains the rather scarce vegetation in this area.

Mistérios Negros

Photo credit Ge-Questa

Not suitable for bathing but ideal for picnics and meditation surrounded by beautiful lush green nature and divine tranquillity, the small, idyllic lakes of Lagoa do Negro, below the western flank of the 662 m high Pico Gordo, and Lagoa das Patas, also called Lagoa da Falca, at the bottom of the volcano Santa Barbara on the road near the Pico das Duas, are worthwhile a halt and spending some relaxing time there. Right next to the lake do Negro lies the entrance to the Gruta do Natal, an almost 700 m long, tunnel-like grotto where every year on Christmas day a mass is said, hence the name (‘Natal’ which means Christmas in Portuguese).

The whole region of Serra de Santa Bárbara inclusive the Caldeira de Santa Bárbara, the Mistérios Negros and its lakes form the 1,100 hectares large nature reserve of Florestal Natural da Serra de Santa Bárbara e Mistérios Negros, established for its botanical, scientific and scenic value.