Monumento Natural Regional da Gruta do Carvão
Countless stalactites can be seen on the ceiling of the cave: some are conical in shape with a smooth surface, resulting from the solidification of drops of lava; others are irregular in shape, with a whitish colour and very fragile, resulting from phenomena caused by the alteration and sedimentation of the water that infiltrated into the cave. This water also caused the oxidisation of the basalt rocks that form the Gruta do Carvão, giving them red and orange tones. As in other caves in the Azores and on the island of São Miguel, what we see here is clear evidence of the different levels of lava that flowed through the inside of the cave at various times in the past.
The presence of countless volcanic micro-structures prove the importance of the Gruta do Carvão for our knowledge and interpretation of basalt volcanism on the island of São Miguel.