Popping Rocks and Lava Tubes from the Mid-Atlantic Rift Valley at 36° N

DURING the 1972 Midlante cruise of the RV Jean Charcot to survey a limited area of the rift valley south of the Azores, a dredge (CH-DR11) was taken at a depth of 1,360 fathoms in a depression near the foot of the eastern wall at latitude 36° 49.3′ N and longitude 33° 15′ W. Two types of rocks were recognized from their structure and their degree of weathering. One type is “slab-like” rock, 2 to 13 cm in diameter and 1 to 2 cm thick (Fig. 1a and b). The other type of rock consists of oval glassy pebbles (1 to 5 cm in diameter, Fig. 1c) which were observed to jump and explode, one or two at a time, for 3 d after the dredge haul was emptied on to the deck of the ship.