Final answer:
Basalt is the volcanic rock formed when effusive, mafic lava erupts underwater and cools very quickly.
Step-by-step explanation:
The type of volcanic rock that is formed when effusive, mafic lava erupts underwater and cools very quickly is basalt. Basalt is an extrusive igneous rock that is commonly found in the oceanic crust. It is formed when lava quickly cools on the seafloor, resulting in fine-grained rock with very tiny crystals.