The Nova Iguaçu Volcano, State of Rio de Janeiro, Brazil, is a working hypothesis for geological interpretation of the volcanic rocks taking place at the north-eastern border of the Mendanha massif. Klein & Vieira (1980a; b) proposed an extinct volcano of the dinosaurs age with volcanic cone, volcanic crater, and volcanic bomb at Nova Iguaçu. The mass-media diffused this hypothesis all over Brazil, causing a social phenomenon. There took place a movement for transformation of Nova Iguaçu Municipal Park into “Geopark of the Nova Iguaçu Volcano” of the UNESCO. However, since 2006, the articles of scientific journals have clarified the volcanological geology of the area, identifying the rocks with volcanic appearance as constituent of subvolcanic intrusive rock bodies (Motoki et al., 2007a; b; c). The hypothesis of the Nova Iguaçu was now extinct in the academic communities, however the volcano myth still continues in sightseeing promotion groups, being supported by strong popular desire. The recent researches revealed that in Nova Iguaçu region, explosive volcanic eruptions occurred with provable formation of volcanic edifices. After the eruptions, an intense uplifting and consequent regional denudation took place. Therefore, the cones, the craters, the lavas, the pyroclastic flow deposits, and the volcanic bombs have been completely eliminated by the strong tropical erosion of this region, exposing underground geologic structure on the present Earth’s surface. The Municipal Park is underlain by the geologic bodies corresponding to the magma chamber, composed of syenite and trachyte, and subvolcanic conduit, made up of pyroclastic rocks, of a depth of 3 km, such as pyroclastic dykes. That is, no extinct volcano is present at Nova Iguaçu. The outcrops are world rare geological examples that expose show underground geologic structures of a volcanic area.

The scientific documents about the hypothesis of the Nova Iguaçu Volcano are exposed at "Homepage do Vulcão de Nova Iguaçu", http://vulcaodenovaiguacu.tripod.com, however most of them are written in Portuguese.

Locality

The locality in question is present at the Municipality of Nova Iguaçu, about 35 km to the west-northwest of the Rio de Janeiro City, Brazil. At the north-east flank of the Mendanha Massif, about 2 km to the south-southeast of the Nova Iguaçu railway station, a horseshoe-formed valley open to the north takes place. Its upstream has semi-circular form elongated to north-east ward, 800 m long, 700 wide, and 180 m deep. The center of this morphology is present at S26°46’25”, W43°28’5”. This area is surrounded by the peeks with altitude more than 400 m, such as Contenda Rock, Austral Rock, Confeito Rock, and Contento Peak, which are underlain by pyroclastic rock. This rock is originated from volcanic explosion made up of rock fragments. However, the own valley is constituted by trachyte, another igneous rock with massive texture. The presence of the pyroclastic rock was the main justification of the Nova Iguaçu Volcano hypothesis.

Proposal of the volcano hypothesis

The hypothesis of the Nova Iguaçu Volcano was proposed by geologists Viktor Carvalho Klein and André Calixto Vieira (Klein & Vieira, 1980a; b), based on the occurrence of the pyroclastic rocks, including volcanic bombs. These authors considered the pyroclastic rock as of pyroclastic flow deposits that constitute a “extraordinary well-preserved complete volcanic edifice” (Klein et al., 1984). Based on these works, a doctor degree these is concluded (Klein, 1993). The bread-crust bomb was a highlight (Klein et al., 2001). A pamphlet of geological field excursion guide also was presented (Vieira & Klein, 2004). However, above-mentioned opinions were not published in scientific journals, therefore the Nova Iguaçu Volcano hypothesis could not transform into a scientific theory accepted in academic communities.

Later development

At the beginning of the 21st Century, another research group realised geological studies related to the Nova Iguaçu Volcano hypothesis. They are composed of geologists of multiple institutions such as: Mauro Geraldes, Ana Maria Netto, Diana Ragatky e Lílian Silveira of the Rio de Janeiro State University (UERJ), Sergio Valente of the Rio de Janeiro Federal Rural University (UFRRJ), Edson Mello of the Rio de Janeiro Federal University (UFRJ) e Felipe Medeiros e André Ghizi of the Rio de Janeiro State Department of Mineral Resources (DRM-RJ), etc. This research group proposed a volcanic crater at Nova Iguaçu (Loc. 1, Medeiros & Ragatky, 2004; Geraldes & Netto, 2004). The volcanic crater model was commented personally by geologist André Calixto Vieira, however it never had been presented in the scientific communities. Another proposal of this group was trachytic lava flows (Geraldes & Netto, 2004; Silveira et al., 2005; Valente et al., 2005) at Dona Eugênia Valley (Loc. 6) and the Vigné Quarry (Loc. 7). However, the latter two works denied existence of the volcanic crater because the supposed crater is underlain by massive trachyte, and not, volcanic breccia (Silveira et al., 2005; Valente et al., 2005). Based on these works, a master-degree thesis was concluded (Silveira, 2006). However, above-mentioned works were not published in scientific journals.

Scientific diffusion

In 2004, o DRM-RJ started a scientific diffusion campaign of the geology of Nova Iguaçu, with special attention of the Municipal Park, with the objective of the transformation of the scientific language into popular ones. As a part of the project “Caminhos Geológicos” (Mansur & Erthal, 2004), which means “Geological Tracks”, some panel-plates were installed at strategic points of the hypothesis of the Nova Iguaçu Volcano (Mansur et al., 2004). These materials were shown at the homepage of the DRM-RJ (Ghizi et al., 2004). This information was widely known in sightseeing promotion groups of Nova Iguaçu and Rio de Janeiro (GuiaCamp, 2006; Carioquinha 2006; Trilha a Pé, 2006; Tirus Baixada, 2007). In this way, the exitct volcano with a volcanic crater at Nova Iguaçu began to be believed by the publics.

Social phenomena

From 2004, Brazilian mass-media such as the Rede Globo (2004; 2005a) and Folha de São Paulo (2005) diffused the hypothesis of Nova Iguaçu Volcano, based on the interviews of a geologist. He affirmed that the geologic structure of Nova Iguaçu is the only one volcano in Brazil with clear morphology of volcanic crater and volcanic cone. In parallel, public speeches were realised at CREA-RJ (Rio de Janeiro Regional Counsel of Engineering and Agronomy) and DRM-RJ (Rio de Janeiro State Department of Mineral Resources). The contents were retransmitted by non-scientific sources, such as the homepage of Nova Iguaçu Municipal Government (2005), of Mesquita Municipal Government (2005), the neighbour city, and Wikipedia Nova Iguaçu (2005). Under this influence, the Nova Iguaçu Topshopping held the Volcano Exposition (Topshopping Nova Iguaçu, 2004). The Rio de Janeiro State Financial Tribnary declared the supposed volcanic crater as a natural sightseeing attraction point of Nova Iguaçu and Mesquita (TCR-RJ, 2004). These non-academic sources had a special interest of the volcanic crater, commenting an extremely long period volcanic eruption, from 72 million years to 40 millions years old, which is incompatible with the scientific study of the Earth’s volcanoes.

At the beginning of 2005, Folha de São Paulo (2005), a Brazilian major newspaper, diffuse that the mining activity of the Vigné Quarry (Loc. 7) were destroying the only intact volcano of Brazil, devastating the sharp volcanic crater morphology. This information was reproduced immediately by: CPRM (2005), Brazilian Geological Survey; MTC (2005), Brazilian Ministry of Technology and Science; SBPC (2005), Brazilian Society of Progress of Science; FUNDEP (2005), Brazilian Foundation of Research Development; FAPERJ (2005), Rio de Janeiro State Foundation of Research Support; Nova Iguaçu (2005) Municipal Government. A judicial process of the same contents was sent to the Public Ministry of the State of Rio de Janeiro, requiring immediate interdiction of the quarry. This process motivated the execution of the TAC project (Instrument of the Adjustment Terms of Behavior Applied to Mining Companies of the State of Rio de Janeiro; Erthal et al., 2004). The last report (Valente et al., 2005) affirmed presence of the volcanic edifice, however denied the volcanic crater, recommending reopening of the mining with reduced scale.

In this period, the hypothesis of the Nova Iguaçu Volcano expanded to become a social phenomenon, and many events of sightseeing, commercial, artistic, administrative, and educative activities were held. They were based on the unconditional belief of the extinct volcano with volcanic crater as if an established scientific theory. The environmental education was a big regional highlight (Guedes & Mansur, 2004; Geraldes, 2006a; b). There took place a movement for registration of the Municipal Park to the UNESCO as “Geopark of the Nova Iguaçu Volcano”. Some presentation in scientific events (Geraldes & Netto, 2004a; b; Geraldes, 2006a; Mansur et al., 2004) and the homepage of the DRM-RJ (Ghizi et al., 2004; Medeiros et al., 2004) called the Municipal Park as Geopark. Certain newspapers diffused, base on the interviews of another geologist, that the UERJ and Petrobrás (Brazilian National Petroleum Company) were researching the oilfield by means of the Nova Iguaçu Volcano (Clipping UERJ, 2005; UERJ em Dia, 2005; Jornal do Brasil, 2005; Jornal Saber Ciência, 2005; Gaspet SC, 2005), investing one million dollar in two years.

Although, the above-mentioned social phenomenon had poor scientific base, its diffusion was sensational. A multi-institutional research group (Motoki et al., 2007d) alerted this uncontrolled fever: Quo Vadis ?!

Scientific doubts

On the contrary to the ardent social phenomenon, the scientific communities maintained a prudent attitude. According to the former explanation, no article favor to the volcano hypothesis was published in qualified scientific journals. In addition, there appeared an oppositional opinion presented in scientific events by geologist Akihisa Motoki. Rodrigo Soares, Marcela Lobato and Giannis Petrakis of UERJ, Susanna Sichel of UFF, José Ribeiro Aires of Petrobrás, Ana Maria Netto of UERJ (Motoki et al., 2004; 2005), showing that the geology and morphology of Nova Iguaçu are incompatible with the volcano model. This opinion was published (Motoki & Sichel, 2006), marking the first scientific article of Nova Iguaçu geology.

Geological discussion

Since 2006, the hypothesis of the Nova Iguaçu Volcano has been discussed in scientific events and academic journals. The interested points are: 1) volcanic crater; 2) volcanic cone; 3) volcanic bomb; 4) lava flow; 5) pyroclastic flow; 6) regional denudation level.

Volcanic crater

The volcanic crater is defined geologically to be a circular negative morphology formed by an explosive eruption (MacDonald, 1972; Motoki et al., 2007a). The eruption forms breccia with large clasts, called explosion breccia. Homepage of DRM-RJ (Ghizi et al., 2004) indicates that the crater should be present at the horseshoe-formed valley situated about 2 km to the south of the urban zone. Vieira & Klein (2004) presented the N-S geologic cross-section of the supposed crater, showing a prominent volcanic cone. However, the topographic map of this area indicates inexistence of such morphologic characteristics. The inner walls of the valley are not sub-vertical, but of low angle of 15 to 20°, being comparable with a slope formed by erosion. Klein & Vieira (1980b) inferred that the supposed crater area might be underlain by volcanic agglomerate, that is, volcanic breccia with large clasts. However, the rock present there is massive trachyte. This geological observation demonstrates inexistence of the volcanic crater at Nova Iguaçu.

Volcanic cone

The volcanic cone corresponds to the cone-formed positive morphology originated from the accumulation of solid eruptive materials. Vieira & Klein (2004) presented an illustration of the Nova Iguçu volcanic cone, of 1.7 km of diameter, 250 m of relative heights, and 27º of flank angle. If so, the cone must appear prominently on the north-eastern flank of the Mendanha Massif, and easily recognised from Nova Iguaçu, and neighbour cities, however, such cone is not seen. The topographic map and satellite images do not confirm it (Motoki et al., 2007a).

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