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 explosive release of
steam and gases. Even very large, deep-water eruptions may not disturb
the ocean surface.
[Illustration: Schematic representation of a typical submarine eruption
in the open ocean.]
The unlimited supply of water surrounding submarine volcanoes can cause
them to behave differently from volcanoes on land. Violent, steam-blast
eruptions take place when sea water pours into active shallow submarine
vents. Lava, erupting onto a shallow sea floor or flowing into the sea
from land, may cool so rapidly that it shatters into sand and rubble.
The result is the production of huge amounts of fragmental volcanic
debris. The famous “black sand” beaches of Hawaii were created virtually
instantaneously by the violent interaction between hot lava and sea
water. On the other hand, recent observations made from deep-diving
submersibles have shown that some submarine eruptions produce flows and
other volcanic structures remarkably similar to those formed on land.
Recent studies have revealed the presence of spectacular,
high-temperature hydrothermal plumes and vents (called “smokers”) along
some parts of the mid-oceanic volcanic rift systems. However, to date,
no direct observation has been made of a deep submarine eruption in
progress.
During an explosive submarine eruption in the shallow open ocean,
enormous piles of debris are built up around the active volcanic vent.
Ocean currents rework the debris in shallow water, while other debris
slumps from the upper part of the cone and flows into deep water along
the sea floor. Fine debris and ash in the eruptive plume are scattered
over a wide area in airborne clouds. Coarse debris in the same eruptive
plume rains into the sea and settles on the flanks of the cone. Pumice
from the eruption floats on the water and drifts with the ocean currents
over a large area.
[Illustration: Submarine eruption of Myojin-sho Volcano, Izu Islands,
Japan on September 23, 1952.]