[colorshapetexture (OP)]

I thought ocean magma was more dense than continental?

Floating rocks? Please explain how this works with subduction?

Anyway.. Thought ya'll would enjoy this cruise. Cheers!

http://yacht-maiken.blogspot.com/2006/08/stone-sea-and-volcano.html [nofollow]

[Bass]

As you differentiate magma, it becomes more silicic.  Island arcs are great examples of subduction zones creating more silicic volcanic rocks (including pumice).

 

[evan_au]

Oceanic volcanoes like Hawaii can happen in the middle of a seafloor plate, far from a subduction zone.

In the case of Hawaii, there is a row of volcanoes, with the most volcanic activity on the one at the end. This is usually attributed to a hotspot in the mantle, which is moving relative to the seafloor, leaving a chain of volcanoes. The older ones are now beneath sea level, so perhaps they are sinking...

[Mazurka]

The photos show pumice which is a result of rapid cooling of blobs of lava ejected from a volcano.  In the magam chamber it is under pressure and when it is ejected it depressurises, whilst cooling leaving lots of bubbles frozen in the rock matrix.  It can only form in high viscosity lavas (felsic or silicic). 

This frozen rock foam floats until capiliary action fills more than a ctricial number of vesicles (rock bubbles) and it sinks.

The romans used it as a low denisty aggregate for concrete domes (such as the Pantheon)

 Technically it is a description of a texture rather than of compostion so is not limited to oceanic settings.  I do not understand the referrence to subduction except as Bass says that island arc volcanism tends towards highly differentiated lavas... 

[colorshapetexture (OP)]

Thanks for the info. Very informative.
 Sorry for the subduction dig. Out of place there.
 Cheers.