Submarine volcanoes and volcanic vents are common features on certain zones of the ocean floor. Some are active at the present time and, in shallow water, disclose their presence by blasting steam and rock-debris high above the surface of the sea.
Many others lie at such great depths that the tremendous weight of the water above them results in high, confining pressure and prevents the formation and release of steam and gases. Even very large, deepwater eruptions may not disturb the ocean floor.
The famous black sand beaches of Hawaii were created virtually instantaneously by the violent interaction between hot lava and seawater.