Durban, Afrique du Sud, 16 - 20 septembre 2002 - J. L. van Rooy and C. A. Jermy, éditeurs ISBN No. 0-620-28559-1 ESTIMATING THE INTACT ROCK STRENGTH OF A ROCK Geology - rocks and minerals Basalt Basalt is a mafic extrusive rock, is the most widespread of all igneous rocks, and comprises more than 90% of all volcanic rocks.