Re: Do underwater volcanoes produce obsidian?

Yes, in fact glass is commonly produced in underwater eruptions, but it 
would be pretty uncommon to find the hand-sized chunks of glass that most 
people think of when they think of obsidian.  

Volcanic glass is formed whenever cooling is rapid enough to forestall the 
formation of crystals in a cooling melt (some crystals may have formed 
while the liquid was sitting in a magma chamber, but the bulk of the rock 
solidifies without crystalizing).  This glass can be massive, like the 
textbook obsidian, or it can be solidified glass foam (known as pumice), 
or fragmented glass foam (known as ash).  

When magma erupts underwater, one of several things can happen.  If the 
water is deep, the pressure prevents the gasses in the magma from bubbling 
out of solution and the result is rapid cooling of the surface without 
fragmentation.  Pillow lava, which has a glassy surface layer, is the most 
common form.  The water depth also prevents the water in contact with the 
hot magma from boiling.

At shallow depths, gas disolved in the magma can come out of solution and 
form bubbles, which dramatically increases the volume, sometimes 
explosively.  This increases the surface area exposed to the water, which 
can drive additional explosions as described below.

Whenever hot magmas come in contact with water at low pressure, the water 
can be heated enough to boil.  Frequently, that boiling happens 
explosively, with enough force to fragment the erupting magma.  Surtsey in 
Iceland is a perfect example of this.

David Smith
Discovery Center of Science & Technology
Bethlehem, PA