Abstract:  Calderas in the distant past have gone off with 
devastating results.  Everybody knows about the dreaded 
Calderas under the 'dry' land from TV specials.  Our own 
Yellowstone is a standard dreaded caldera and is a prime 
present day candidate for yet another release.  The magma 
underneath is getting grumpy with hundreds of small 
earthquakes occurring indicating current substantial 
underground shifting.  There are others in the world as 
well.

But... what about the theoretical prospects of a reality of 
a caldera or worse, calderas, being located under the ocean 
floor, now silted in with age and currently invisible to us 
going off underocean somewhere?

The Hawaiian Volcanic islands have in the past 'exploded' 
with spectacular results, devastating everything around.

There have been others.

Now what, in Joseph_Sixpackian theory, has an underwater 
caldera got to do with anything important on any of the 
local planets within our own solar system?

The current standard idea is that the ocean floor would open 
up and belch out a bunch of magma and immediately be cooled 
and contained by the the weight of the cold ocean waters 
above.

Glop would build up underocean, cooled and hardened by the 
cold ocean Waters and voilá, an island or islands would 
be formed after a time.

Warm steam clouds (distilled water - fresh) would pour down, 
a coconut would land and minutes later, bugs, spiders, 
birds, grass, trees, and shopping centers arrive followed 
immediately by the island being surrounded by trash and 
rubbish in the waters around the islands.  

But... there is another scenario we might consider...

The possible generation of the dreaded dastardly devastating 
H-y-p-e-r-c-a-n-e !

The immense mass of VERY HOT water from an undersea caldera 
might recompose itself above at first as a hurricane and 
then as a hypercane in trying to reach thermodynamic 
equilibrium.  Hypercane winds are theoretically reputed to 
be in the area of 580 miles per hour.  That will take you 
hat off...

The theoretical hypercane might be able to open up a 
somewhat large ultraviolet (uv) hole in the upper, upper 
atmosphere above it that would play havoc with flora & fauna 
upon the earth below.  The duration of the UV hole may exist 
for months, perhaps, even years.  The damage would be 
incalculable.

The caldera's released magma from below would probably 
result finally in a somewhat large island of volcanic stuff 
if joe's intuitive guess is any good. as well as sending 
possibly cubic miles of distributed ash over thousands of 
miles beneath perhaps for years as well.  Things could get 
real cold.

Core samples dug deep down into and to the base of many 
islands bottoms now might reveal them to be just pure 
volcanicly sourced stuff.

Undersea remnants of old exploded undersea calderas might 
exist as well.  Hot spots upon the ocean floor might reveal 
a lot.  It is a job for the undersea scientists, however.

Strange sea and ocean areas between adjacent land areas 
might be a good place to look for current hot spots, 
currently silted over the ancient once active calderas.

A very hot caldera generated ocean, temporarily devoid of 
sufficient sustaining oxygen and biochain food elements 
might be responsible in having put to rest many of the 
ancient dinosauric life form chains both in and out of the 
ocean.

Anyhow, if you throw hot rocks into a pot of cold water, the 
water warms up, and sometimes it gets hot enough that you 
can make a nice cup of tea, or perhaps even, some soft 
boiled eggs. The physics is simple, the predicting the exact 
results is not.

Consult a real scientist who has studied such matters for 
deeper insights into such speculative theoretical realities.

But if the rocks underneath you are getting hot and wiggly, 
go somewhere else so you won't end up being the soft-boiled 
egg.