Spectacular volcanoes form the Azores islands, which rise out of the Atlantic Ocean hundreds of miles from neighboring continents. The lush green vegetation enveloping these islands conceals their exceptional geologic setting: they are in a highly active tectonic environment, growing above the triple junction plate boundary between the North American, Eurasian, and African (Nubian) tectonic plates. Since the 15th century, when arriving Portuguese settlers first carved out a living, much has changed in the appearance of the islands—but starting deep below the surface, frequent earthquakes and volcanic eruptions continue. Recently, I had the good fortune to join a tour group and admire the impressive landscapes of two of the islands while hiking along many miles of trails. I’ve learned much to share about both the past and present of this unique and highly scenic region.
Location of the Portuguese Azores and Madeira islands, and Spanish Canary Islands, in the northern Atlantic Ocean (Wikimedia)
Islands in the Middle of the Atlantic Ocean
The nine Azores islands form a chain that spans about 370 mi (600 km) and is located around 870 mi (1,400 km) west of mainland Portugal and 2,100 mi (1,930 km) from the closest approach of North America, at Newfoundland, Canada. Surface areas of the islands range from the largest, San Miguel, at 293 sq mi (759 km2) to the smallest, Corvo, at 7 sq mi (17 km2). The archipelago is relatively young, with volcanic activity occurring on five of the islands in historic times (about 550 years) and evidence of faulting in rock formations with ages younger than 1 million years (Mid-Pleistocene to Holocene age).
Rockwalls viewed during flight into Lajes airport on Terceira Island
When Portuguese explorers arrived in the mid-1400s, they found no inhabitants on the islands, although there may have been earlier visits by Vikings or other societies. Settlers reaching the islands began clearing fields of vegetation and volcanic rock for crops and pasture. They constructed their buildings and an extensive system of stone walls with the volcanic rocks they cleared. These walls cover the landscape, and hundreds of years later, many still stand, marking property boundaries and protecting agricultural fields from harsh winds. Now, farmers continue to construct new ones for specific uses—rocks are readily available.
Rock walls protecting grapevines at the Materramenta Winery on Terceira
The mild, sub-tropical climate and fertile volcanic soils of the islands make the land suitable for a variety of grains, potatoes, fruits and vegetables, and pastures for livestock. Beginning in the late 1400s, favorable Atlantic Ocean currents and prevailing winds made the Azores an important staging outpost for sailing ships to take on board provisions and fresh water. Whaling ships visited the islands, and whaling became an important industry for Azoreans for hundreds of years. In the 1980s, whale hunting ended, followed by a pivot toward whale tourism. Today, dairy farming and other agricultural production are important economic pursuits, while tourism is welcomed and rapidly increasing.
Earthquakes and Volcanoes
The Azores islands are formed by the highest volcanoes on the Mid-Atlantic Ridge, a great zone of extension (spreading center) separating the ocean crust and stretching to Iceland. Researchers postulate the existence of the Azores Hotspot based on an area east of the ridge that is elevated and has thickened crust, attributed to increased heat flow and melt production. Some researchers have found evidence that the hotspot has been drifting northward for millions of years. As the North American, Eurasian, and African plates slowly shift positions, the Mid-Atlantic Ridge, the Azores Hotspot, and many faults and fractures control island topography. The magma supply to the Azores is large enough to keep island volcanism active; however, it is lower than the intense volcanism of Iceland or Hawaii.
The Azores Plateau underlying the islands probably formed by magma production from the Mid-Atlantic Ridge between about 10 and 4 million years ago. In contrast, most of the islands are younger than 1.5 million years old. All types of volcanic eruptions have occurred on the islands, ranging from Hawaiian-style spatter cones to explosive pumice cones. Most rock types range from basaltic to trachytic (a silicic, mildly alkaline rhyolite) to pumice. The geologic setting is complex, presenting many research opportunities for geologists!
Bathymetry of Azores triple junction with islands marked in yellow (Wikimedia)
Volcanic eruptions are a part of life in the Azores. Since the 1400s, when historical records began, at least two dozen terrestrial and nearby submarine volcanoes have been reported. The most recent surface eruption occurred in 1957, when a small island grew out of the sea and merged with the island of Faial. The last submarine eruption recorded was between 1998 and 2000 near Terceira Island. All the islands have active volcanism, except for Santa Maria Island. There is no temporal pattern for the volcanic activity on the nine Azores islands, in contrast to the Hawaiian Islands, which show age progression from northwest to southeast.
The Azores have also experienced many earthquakes, with maximum expected magnitudes varying from 6 to 7 (moment magnitude). There have been several historical earthquakes of approximately magnitude 7, including events in 1522, 1614, 1841, and 1980. Extensive damage and thousands of deaths were associated with some of these events. Focal mechanisms indicate strike-slip (dextral and sinistral), normal, oblique, and a few reverse events.
Mt Pico on Pico Island, the highest mountain in Portugal at 7,713 ft (2,351 m). The lava flow is from a 1718 eruption and reached the coast of the island. Eventually this volcano will probably collapse and a caldera will form. (Wikimedia)
A Few Observations
My visit to the Azores included visits to both Terceira and San Miguel islands, each with extraordinary evidence of the regional volcanism. Multiple cycles of volcano formation, collapse to form a caldera, and explosion are recorded in the rock record. San Miguel is the largest and most volcanically active island in the archipelago, with recent eruption landforms of a few thousand years to only hundreds of years old.
Satellite photo of San Miguel Island, showing Sete Cidades in the top left of the image and Furnas on the right (Wikimedia)
There are three active central volcanoes on San Miguel Island—Sete Cidades, Agua de Pau (Fogo) and Furnas. Each of the large calderas contains a lake, and each has had several eruptions within the past 5,000 years. When settlers first arrived in the mid-1400s, there is evidence that Furnas was erupting, and there were major eruptions of Furnas in 1630 and Fogo in 1563 (each about as large as Mt St Helens in 1980). The most recent submarine eruptions near the island were in 1907, 1911, and 1981.
Twin lakes in the Sete Cidades caldera; steep caldera walls in the background
The enormous caldera of Sete Cidades on the western end of the island has a diameter of 3.7 x 3.1 mi (6 x 5 km) and a multi-stage history. The present caldera formed during collapses around 36,000, 29,000, and 16,000 years before the present. At least nine eruptive centers that formed craters within the caldera have been active since the last major collapse. Several craters show characteristics of magmatic explosive eruptions, including some craters where eruptions resulted from magma interaction with groundwater or a lake (phreatomagmatic eruptions). The last eruption within the caldera occurred about 500 CE, with an eruption adjacent to the caldera in the 15th century and more recent eruptions from submarine vents along the coast of the island.
Sete Cidades caldera with post-collapse craters lined with trees and town next to the lake
On the eastern end of the island is the volcanic complex of Furnas. The oldest caldera has a maximum diameter of about 4.3 mi (7 km). Furnas includes several central (younger) calderas and many active fumarole fields, where hot gas and steam accompany hot springs and boiling pools of water. There have been several caldera collapses, with the most recent dating back about 100,000 years and a younger collapse at 35,000 years. Numerous explosive eruptions within the calderas have occurred during the past 5,000 years, with historical eruptions in 1439 and 1620.
Furnas lake
Several thousand people currently live in villages on the floors of both the Sete Cidades and Furnas calderas. The dangers from future eruptions are accepted, in part because of the value of the fertile agricultural land, and at Furnas, natural springs of sparkling, CO2-rich water. Nonetheless, both volcanic complexes are active, and any large explosive eruption would be catastrophic. My group spent three nights at a hotel in the Furnas caldera, and although I enjoyed relaxing in the adjacent hot springs, whenever I noticed the tinge of sulfur in the air, I felt uneasy contemplating the magma that was surely churning not far below.
Steam rising from geothermal pools in the town of Furnas
Life in the Azores
Human settlement on the islands brought major changes to the flora and fauna. Bats were the only native mammals; gradually people introduced sheep, cattle, goats, horses, and hogs, along with mice, rabbits, lizards, frogs, and many other creatures. Trees were felled for fuel and construction material, rapidly denuding the islands, and exotic trees, shrubs, and ornamental species from other continents were brought in for fuel, lumber, and windbreaks. Some invasive plants have shallow root structures, and in many areas where the exotics have replaced native plants, landslides have become more common.
Living in the middle of a vast ocean brings many challenges. The geographic isolation of the Azores islands requires independent electrical power generation systems on each island. While diesel fuel is currently the major source of power, the island utilities also harness renewable energy from wind, hydro, and geothermal sources; together, these provide roughly one third of the power generated on the islands. Geothermal resources are especially important, as plate boundary areas have high subsurface heat gradients, providing an essentially inexhaustible heat source. Power plants to harness this energy are on Terceira and San Miguel islands, and plans for expansions are in the works.
Geothermal plant for electrical power between Lake Fogo and Ribera Grande on the north side of San Miguel
The Azoreans who live on the nine islands are resourceful and resilient. The lush green landscapes and coastal areas of rugged rock and crashing waves are lovely. There is an extensive trail system that provides fabulous views, especially of the impressive calderas. It is a great place to visit; however, many tourists are probably unaware that their holiday is on islands with active volcanoes!
Lava flow and waves near Biscoitos on Terceira Island
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Thank you for this post. I was curious as to which company you traveled with. I have been on several geology trips with Geoworld Travel (out of Wales) this company travels to the Azores.
A minor correction:
“Recently, I had the good fortunate to join a tour group and admire the “. You may want to change fortunate to fortune.
Thanks, Deborah! I traveled with Road Scholar — the guides and guest speakers were all quite knowledgeable. Thanks also for spotting the typo — it is now fixed.
Thank you for a fascinating article.
Thank you, Larry! The Azores have so much interesting history!
Flew into Lajes 40 years ago on our way back from Turkey. Had time to drive halfway around the island for lunch. Preflight briefing warned to be careful of ramp strikes. That is a Navy aviation term that describes the sheer cliff face dropping into the water on the approach end of the runway. It mirrors what a naval aviator sees while trying to land on a carrier. Neat place, though. Cheers –
Sounds great – thanks!
Wonderful post! We almost made it last October but decided to spend more time on the Portuguese mainland. A trip there is in our future!
Thanks, Wayne! The Azores are impressive — especially the San Miguel Island calderas!
Excellent reading! These islands are truly dynamic as well as beautiful. I wonder if others also associated the sulfur smell with churning magma! Thank you for this comprehensive description of the Azores.
Thanks, Laurie!