Brilha J.B., Sequeira Braga M.A., Proust D., Dudoignon P.

Presented in the V Congresso Nacional de Geologia (Lisbon, 1998) and published in Comunicações do Instituto Geológico e Mineiro, tomo 84, fasc. 1, B164-B167

Key-words
Columnar jointing; volcanic neck; tephrite; fracturation.

Abstract:
The columnar jointing in the Penedo de Lexim volcanic neck is characterized by a Hexagonality Index of 1.03, indicating the mature nature of the fracturation pattern. The evolution of the cooling process in depth (2000 m) is responsible for the development of columns with some different aspects in relation to prisms of sub-aerial lava flows.