The Azores have a depth of caves, but on a volcanic set of islands – caves are really lava tubes. These are natural caves formed by lava flowing beneath the hardened surface of a lava flow long ago. The tubes acted as lava drains lava during an eruption, so once the lava flow ended and the rock has cooled and left a long channel. When the volcanic eruption ended, plants on the lava fields particularly near the cave entrances slowly grew up over the centuries. There are two types of lava rock that can be viewed in the many caves of the Azores: Lajido – smooth lava, and the more rough and porous Bisciotos lava.
Lava tubes are likely to exist on geologically active planets or moons, like here on the Earth. To see them on earth, here are some caves that are open to the public!
Algar do Carvão is an ancient lava tube right in the center of Terceira. Terceira has numerous caves, but this is the one that is open for exploration. The site is open to the public daily Monday to Friday from May to September, between 3-5 p.m. Access to the site may be arranged by emailing the Montanheiros (http://www.montanheiros.com). The word Algar means a natural cavity in the earth that, unlike most caves or caverns, is vertical. The word, Carvão mean coal, as the cave is quite dark. After a descent about one hundred meters into the earth there is a pool of crystal-clear water. The cave itself is remarkable for its stalagmites and minerals. The caves, 40.5 hectares in length, have been designated a “regional natural monument.”
The Gruta das Torres (Grotto of Towers) is a highlight of any visit to Pico. This set of caves is located outside of Criação Velha, in the Town of Madalena, on the western end of Pico. The cave system formed from a series of lava flows about 500 to 15,00 years ago. The caves are a group of interconnected lava tubes between 2–72 ft. widths, created during different geological periods. It has a cave height between3.6–49 ft. and is located at an elevation of some 980 ft. With an estimated length of over 3.2 miles, it is the longest lava cave in the Azores. Access to the cave is managed by AZORINA, SA. The Visitors' Support Centre at the entrance to the cave has a circular sweep and a linear spatial design – that prepare visitors for the spectacle below. It was awarded the "European Union Prize for Contemporary Architecture Mies van der Rohe Award 2007."
Gruta do Carvão is set in the western part of Ponta Delgada on São Miguel, and is the longest lava cave on the island at 1912 meters divided in three separate sections. The one to the North (“Paim” Section) is 880 meters, the one to the South (“João do Rego” Section) runs 300 meters and and middle one (“Secadores de Tabaco” or ”Rua de Lisboa” Section) is 701 meters long. However some old field observations indicate a bigger extension that it might have been more than 2.5 miles long. The cave’s age is between 5000 to 12,000 years old, possibly dating to the Sete Cidades volcano and/or the Fogo volcano.
The Furna do Enxofre, an impressive volcanic cave, is located inside Graciosa’s Caldera. Connected to the outside through two large fissures, the Furna do Enxofre (Sulfur Cave) features a perfect dome inside with a height of approximately 40 meter in its central part. Access is through a tower built in 1939, featuring a spiral staircase with 183 steps cut into the rock. The "cathedral" of the volcanic caves of the Azores features a cold water lake and mud sulfur fumaroles , after which the cave is named. The cave is open between 11.00 a.m.and 2.00 p.m., when daylight reaches the interior through two entry vents.