Amygdules, or a tale of two solutions
While the name comes from the Greek for almond via anatomy and refers to the tonsil shape of the bubbles, the geological reality involves a tale of erupting lava releasing its dissolved gases as it rises through the layers of crustal rocks and the ambient pressure releases. Like a spurting champagne bottle these gases drive pretty much all eruptive activity from the most gentle to the most intense. When the lava cools however they remain as gas bubbles within, known as vesicles. Somewhat later in geological history the cooling lava's residual heat feeds hydrothermal convection cells of percolating rainwater, that cycle up and down through the porous rock picking up dissolved minerals on the downward cycle and precipitating the elements in new forms as it rises and cools. These minerals often precipitate in hollows within the rock, giving us beautiful geodes in larger bubbles, veins in fractures and smaller mineral filled amygdales (as they now become) in the smaller bubbles.
Nature created the beauty in the photo through this two stage process, whereby gas first exsolved out of magma to create the spaces, and then the oxygenated waters of the Earth filled them with beautiful minerals, in this case calcite and zeolites in a fine grained basalt. Earth, fire, air and water all in one, creating a wonderful distilled elemental quintessence, how's that for Nature's poetry?
Loz
Image credit: Bernardo Cesare http://microckscopica.altervista.org/en/
