Lava balloon
Apparently, they are generated when gases trapped within magma form large bubbles that eventually rise to the sea surface.
The outer part of the crust is highly vesicular and striated and has delicate flow structures that can be seen using a scanning electron microscope.
[4] Lava balloons have been described from Terceira Island in the Azores,[3] at Teishi Knoll of Izu-Tobu (Japan) in 1989,[2] El Hierro, offshore Pantelleria (Foerstner volcano, Italy) in 1891 and Kealakekua Bay (Mauna Loa,[5] Hawaii) in 1877.
[3] They surfaced in batches over a span of several months, clustering in particular areas that appear to reflect the position of active volcanic vents on the seafloor but also wind and ocean current driven transport.
[12] Balloons usually floated for less than 15 minutes before sinking again as water penetrated them through cracks in the crust and gases escaped.
[3] Remotely operated underwater vehicle (ROV) observations of the putative vent area found debris that may have come from lava balloons.
[16] As reported by fishers, black balloons of lava floated on the sea, sometimes propelled by steam jets and sometimes exploding with up to 20 metres (66 ft) high debris fountains.
[20] At El Hierro, lava balloons were erupted from 27 November 2011 until 23 February 2012[1] and often exploded upon reaching the sea surface.
On the seafloor close to the vent were balloons with various shapes including amphora-like and sizes reaching over 4 metres (13 ft).
[22] Towards the end of the eruption, some lava balloons had a thin layer of solidified magma around a glassy core and appeared to float for longer times, allowing them to reach the coast.
[26] Large floating pumice blocks such as these observed in Kikai, Japan, in 1934–1935 may be comparable to lava balloons,[28] but they are produced by eruptions of felsic magma, which are rich in silicates and lighter elements.
[6] Lava balloons are probably limited to a depth range of 30–1,000 metres (98–3,281 ft): too deep, and gas bubbles do not form; too shallow, and degassing fragments the rocks.
Only a few sufficiently large balloons can rise all the way to the sea surface; smaller ones fill quickly with water and sink.
[30] Water that penetrates the lava can boil and the resulting vapours can inflate the balloons and make them float,[15] although for Terceira a non-water gas composition has been inferred.
[3][d] The high gas content and low viscosity of the magma during the Terceira eruption allowed balloons to form despite the vents being located at considerable depth.
