Sirens of the Azores
Sete Cidades
The caldera at the west end of the island
Sete Cidades is a volcanic caldera in the municipality of Ponta Delgada, at the western end of São Miguel. It holds two lakes, a village of 793 people, and the most active volcanic system in the Azores.
What is it?
A caldera, near-circular, 5.3 kilometres across and up to about 630 metres deep, formed roughly 36,000 years ago.
More precisely, three successive collapses — about 35,700, 28,750 and 15,740 years ago. The walls run from 200 metres on the west side to 500 on the east. Inside are lakes, pumice cones, lava domes and maars.
Lagoa Azul and Lagoa Verde sit at 260 metres and are joined by a bridge. Smaller crater lakes inside the caldera include Lagoa de Santiago and Lagoa Rasa.
THE NUMBER TO REMEMBER
Seventeen eruptions inside this caldera in the last five thousand years.
It is the most active volcanic system in the entire archipelago.
What has erupted, and when?
1439–1444: Eruption at the Caldeira Seca pumice cone. The exact date was never recorded. Frutuoso: returning navigators found the west of the island "completely changed", with pumice and tree trunks floating offshore.
1444: Explosive event on the south-west side of the caldera — conventionally the last intra-caldera eruption.
3 July 1638: Submarine eruption off Candelária, throwing up a ring-shaped island. It lasted 29 days.
1811: Submarine eruption off Ferraria, creating the temporary Ilha Sabrina; houses damaged onshore.
1880: The most recent activity — a submarine vent west of the volcano.
★ The 1439–1444 eruption is the one that matters to the island's story. It falls inside the window traditionally given for the settlement of São Miguel — so on the conventional chronology, the first people to settle the west end arrived at a coast that had just been rebuilt. → Porta do Diabo
Did anything erupt here in 1713?
No. And because the date is attached to this place in several places, it is worth being exact about what 1713 actually was.
What is recorded for São Miguel in 1713 is floods, in December.
And in older volcanological compilations, an eruption at Pico das Camarinhas — which is on the western flank of the Sete Cidades edifice, at Ponta da Ferraria, twelve kilometres from these lakes.
Current geosite dating puts that lava flow at around 1140 AD instead.
Nothing erupted inside the Sete Cidades caldera in 1713.
Is it protected?
Área de Paisagem Protegida das Sete Cidades, code SMG17, about 2,173 hectares.
Also a Ramsar site, a UNESCO Global Geopark geosite of national significance, and a CORINE Biotope site.
Where does the name come from?
From a medieval story about an island that was never found.
Insula Septem Civitatum — the legend that in AD 734 a Visigothic archbishop and six bishops fled the Muslim invasion of Iberia with five thousand followers, and founded seven Christian cities on an undiscovered island in the Atlantic.
There is no evidence the island of the Seven Cities ever existed. European cartographers drew it anyway, for two hundred years, and when the Azores turned up in roughly the right ocean the name came with them.