Active volcanoes at rest for long periods
How many active volcanoes are there in the world?
As you read these words, only a few dozen volcanoes on land are spewing out lava, ash and gas.
However, just as a person with a cough doesn't cough all the time active volcanoes also have
long pauses between eruptions. To understand these pauses, let's think about how a volcano
works. Melted rock (magma) rises up within the earth and accumulates beneath the surface,
gradually developing a reservoir. As more and more magma gathers, the pressure increases until
finally, lava explodes through the surface of the earth. Then the volcano is quiet again until many
years or centuries later, when enough magma has built up to cause another eruption.
It's difficult to say for sure whether a volcano has finished permanently or is just quietly building
up magma. By studying many volcanoes all over the world, scientists have come up with a rule of
thumb: If a volcano has erupted at least once during the past 10,000 years, it's called an active
volcano. It's likely to erupt again.
So to answer your question, there are about 1,500 active volcanoes on land. And there are many
more volcanoes on the ocean floor!
Most volcanoes on land happen around the "Ring of Fire"-along strings of islands and the edges
of continents that surround the Pacific Ocean. These are places where one surface plate dives
beneath another (called subduction, in plate tectonics).
You can make a map showing the Ring of Fire and other active volcanoes using our Web site,
atlas.geo.cornell.edu
T IS AN UNDERWATER VOLCANO?
An underwater volcano is just a magma chamber beneath the surface of the water. When the volcano
erupts, the cold water cools the lava quickly to rock. The cooled lava builds up to form a rock and
when it breaks the surface, it forms an island.
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, deepwater eruptions may not disturb the ocean surface.
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 seawater 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 seawater. Loihi Submarine Volcano - the most recent expression of the
hotspot that produces the Hawaiian Island chain - NOAA
On the other hand, recent observations made from deep-diving submersibles have shown that some
submarine eruptions produce flows and other volcanic structures remarkable similar to those formed on
land.
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.
What happens when an underwater volcano erupts?
In: Volcanoes [Edit categories]
When the hot Lava or Magma touches water, the water is boiled into steam.
This steam rises and can be seen as an eruption of air out of the ocean. As
the Magma cools, it hardens forming new \"ground\". If the flow of magma
continues and there is enough formation to break the surface of the ocean,
a new island is formed. (see Hawaiian Islands)
If the eruption is great enough, there could be a significant increase in global
temperature caused by the steam and raising water temperature in the area. All
volcanic eruptions have an effect on global temperature. Mt St Helens increased the
planet by 1.5 degrees, now imagine an eruption like that under water? Steam Baked!
If the volcanoe erupts with a large enough force it could also cause a
tsunami{ giant wave}