Magnetic study of the Furnas caldera (Azores) | Annals of Geophysics Article Sidebar PDF Published: 1997-11-25 DOI: https://doi.org/10.4401/ag-3917 Keywords: Furnas caldera, magnetic anomalies, hydrothermal alteration Main Article Content I. Blanco Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain A. Garcìa Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain J. M. Torta Departamento de Volcanología, Museo Nacional de Ciencias Naturales, CSIC, Madrid, Spain Abstract A local ground magnetic study of the Furnas caldera (S. Miguel Island, Azores) has provided new insight into the magnetic structure of this volcano. Analysis of the data comprised removal of the IGRF, reduction to the pole, pseudogravity integration and upward continuation. Also, a spectral method was applied to estimate the depth to the magnetic sources, as well as a 2.5D forward modelling technique. Magnetic properties obtained at the laboratory for some representative sample rocks were considered in the modelling process. The most relevant features are the existence of an important negative anomaly inside the caldera and of an intense positive anomaly to the south of the coast. The former points out a decrease in the magnetization of the caldera filling materials with respect to the surrounding rocks, which could be explained as the result of post-eruption processes such as hydrothermal alteration. This is expected as Furnas has an active hydrothermal system probably related with a magmatic reservoir at high temperature. The positive anomaly suggests the existence of a strongly-magnetized body beneath the south coast. Article Details Issue Vol. 40 No. 2 (1997) Section OLD Open-Access License No Permission Required Istituto Nazionale di Geofisica e Vulcanologia applies the Creative Commons Attribution License (CCAL) to all works we publish. For any reuse or redistribution of a work, you must also make clear the license terms under which the work was published. This broad license was developed to facilitate open access to, and free use of, original works of all types. Applying this standard license to your own work will ensure your right to make your work freely and openly available. For queries about the license, please contact ann.geophys@ingv.it. How to Cite (1) Blanco, I.; Garcìa, A.; Torta, J. M. Magnetic Study of the Furnas Caldera (Azores). Ann. Geophys. 1997 , 40 (2). https://doi.org/10.4401/ag-3917 .

powerful earthquakes. The island of São Miguel in the Azores is the site of the Sete Cidades volcano and caldera, which are the byproducts of historical Atlantis (Ancient Greek: Ἀτλαντὶς νῆσος, romanized: Atlantìs nêsos, lit. 'island of Atlas') is a fictional island mentioned in Plato's works Timaeus and Critias as part of an allegory on the hubris of nations. By describing Atlantis as a naval empire from the west that had conquered most of Europe and Libya, Plato purposely created a literary contrast with the Achaemenid Empire, the great land-bas The… According to Critias, the Hellenic deities of old divided the land so that each deity might have their own lot; Poseidon was, appropriately, and to his liking, bequeathed the island of Atlantis. The island was larger than Ancient Libya and Asia Minor combined, but it was later sunk by an earthquake and became an impassable mud shoal, inhibiting travel to any part of the ocean. Plato asserted that the Egyptians described Atlantis as an island consisting mostly of mountains in the northern portions and along the shore and encompassing a great plain in an oblong shape in the south "extending in one direction three thousand stadia [about 555 km; 345 mi], but across the center inland it was two thousand stadia [about 370 km; 230 mi]." Fifty stadia [9 km; 6 mi] from the coast was a mountain that was low on all sides ... broke it off all round about ... the central island itself was five stades in diameter [about 0.92 km; 0.57 mi]. In Plato's metaphorical tale, Poseidon fell in love with Cleito, the daughter of Evenor and Leucippe, who Another passage from the commentary by Proclus on the Timaeus gives a description of the geography of Atlantis: That an island of such nature and size once existed is evident from what is said by certain authors who investigated the things around the outer sea. For according to them, there were seven islands in that sea in their time, sacred to Persephone, and also three others of enormous size, one of which was sacred to Hades, another to Ammon, and another one between them to Poseidon, the extent of which was a thousand stadia [200 km; 124 mi]; and the inhabitants of it—they add—preserved the remembrance from their ancestors of the immeasurably large island of Atlantis which had really existed there and which for many ages had reigned over all islands in the Atlantic sea and which itself had like-wise been sacred to Poseidon. Now these things Marcellus has written in his Aethiopica. Marcellus remains unidentified. Other ancient historians and philosophers who believed in the existence of Atlantis were Strabo and Posidonius. Some have theorized that, before the sixth century BC, the "Pillars of Hercules" may have applied to mountains on either side of the Gulf of Laconia, and also may have been part of the pillar cult of the Aegean. The mountains stood at either side of the southernmost gulf in Greece, the largest in the Peloponnese, and it opens onto the Mediterranean Sea. This would have placed Atlantis in the Mediterranean, lending credence to many details in Plato's discussion. Diodorus Siculus describes the Atlanteans as living in North Africa, and as having gone to war with the Amazons. The fourth-century historian Ammianus Marcellinus, relying on a lost work by Timagenes, a historian writing in the first century BC, writes that the Druids of Gaul said that part of the inhabitants of Gaul had migrated there from distant islands. Some have understood Ammianus's testimony as a claim that at the time of Atlantis's sinking into the sea, its inhabitants fled to western Europe; but Ammianus, in fact, says that "the Drasidae (Druids) recall that a part of the population is indigenous but others also migrated in from islands and lands beyond the Rhine" (Res Gestae 15.9), an indication that the immigrants came to Gaul from the north (Britain, the Netherlands, or Germany), not from a theorized location in the Atlantic Ocean to the south-west. Instead, the Celts who dwelled along the ocean were reported to venerate twin gods, (Dioscori), who appeared to them coming from that ocean. The location of Atlantis in the Atlantic Ocean has a certain appeal given the closely related names. Popular culture often places Atlantis there, perpetuating the original Platonic setting as they understand it. The Canary Islands and Madeira Islands have been identified as a possible location, west of the Straits of Gibraltar, but in relative proximity to the Mediterranean Sea. Detailed studies of their geomorphology and geology have demonstrated, however, that they have been steadily uplifted, without any significant periods of subsidence, over the last four million years, by geologic processes such as erosional unloading, gravitational unloading, lithospheric flexure induced by adjacent islands, and volcanic underplating. Various islands or island groups in the Atlantic were also identified as possible locations, notably the Azores. Similarly, cores of sediment covering the ocean bottom surrounding the Azores and other evidence demonstrate that it has been an undersea plateau for millions of years. The area is known for its volcanism however, which is associated with rifting along the Azores triple junction. The spread of the crust along the existing faults and fractures has produced many volcanic and seismic events. The area is supported by a buoyant upwelling in the deeper mantle, which some associate with an Azores hotspot. Most of the volcanic activity has occurred primarily along the Terceira Rift. From the beginning of the islands' settlement, around the 15th century, there have been about 30 volcanic eruptions (terrestrial and submarine) as well as numerous, powerful earthquakes. The island of São Miguel in the Azores is the site of the Sete Cidades volcano and caldera, which are the byproducts of historical volcanic activity in the Azores. The submerged island of Spartel near the Strait of Gibraltar has also been suggested.