volcanic bomb is one type of essential fragment originating from a magma drop, launched from the crater in viscous semi-solid state. Indications of vesiculation, volume expansion, and plastic deformation are observed. In case of high-viscosity magma, such as andesite and dacite, the consolidated clast surface is cracked by the volume expansion originated from vesiculation. Such a bomb is called bread-crust bomb. Silveira et al. (2005) and Valente et al. (2005) proved that more than 50% of the clast present in the volcanic agglomerate of the Contenda Rock are volcanic bombs. Klein & Vieira (1980b) and Klein et al. (2001) commented on the presence of bread-crust bomb. However, Motoki et al. (2007b) revealed that these clasts are not porous, but composed of massive trachyte. In addition, no signs of plastic deformation are observed. The altered clast surface with porous appearance is attributed to weathering of fabrics, namely pseudovesicular structure. Therefore, it is concluded that no volcanic bomb is present at Nova Iguaçu. Lava flow Geraldes & Netto (2004a; b), Silveira et al. (2005) and Valente et al. (2005) concluded that the trachyte of this area was made up of lava flows which covered the erosion surface of the syenitic rock. If so, the trachyte must be younger than the syenite. However, field observation of the contact outcrops demonstrated that the trachyte intrudes into the metamorphic syenite, forming an intrusive body older than the syenite (Motoki et al., 2007a). Geraldes & Netto (2004a; b), Silveira et al. (2005) and Valente et al. (2005) considered that the trachyte is distributed only along the valley of the crater in a strip 2 kilometers long and less than 100 meters thick. This body has a horizontal extension larger than 15 kilometers and stretches from the base to the top of the Mendanha massif, with more than an 800-meter drop in elevation (Motoki et al., 2007a). Valente et al. (2005) show isolated occurrences of hexagonal fractures as evidence of trachyte lava flow. The trachyte does not have the characteristics of lava flow and lava dome (Motoki et al., 2007a). The hexagonal fractures were identified to be horizontal columnar joints of trachyte dike. These geological observations indicate that the trachyte is an intrusive body, being the oldest body of the felsic alkaline magmatism of this area, and is clearly different from the lava flows overlying the eroded surface of the syenite. Pyroclastic flow Klein & Vieira (1980a;