The type of rock most likely to contain holes left by trapped gases is volcanic rock, a category of extrusive igneous rock. These holes are known as vesicles, and the resulting texture is described by geologists as vesicular. This texture is the preserved structure of gas bubbles that were unable to escape the molten rock as it rapidly solidified near or at the Earth’s surface.
Igneous Rocks and Rapid Cooling
Vesicular texture is a defining characteristic of extrusive igneous rocks, which are formed from lava cooling quickly outside of the Earth’s crust. Magma that cools very slowly deep underground forms intrusive igneous rocks, like granite, which typically have large, interlocking mineral crystals and no vesicles. The rapid cooling of extrusive rock prevents the gas bubbles from fully escaping the liquid, trapping them within the solid structure. This quick solidification increases the viscosity of the lava, making it thick and sticky, which physically impedes the movement of the bubbles. If the lava were to cool slowly, the gases would have ample time to migrate out and vent into the atmosphere.
The Mechanism of Vesicle Formation
The gas bubbles that create vesicles originate from volatile components dissolved within the magma deep beneath the surface. These components are primarily water vapor, but also include carbon dioxide and sulfur dioxide. Deep within the Earth, the immense pressure keeps these volatile substances dissolved within the molten rock, similar to how carbon dioxide is dissolved in an unopened bottle of soda.
As the magma rises toward the surface, the lithostatic pressure exerted by the overlying rock decreases. This reduction in pressure causes the dissolved volatile components to exsolve, meaning they come out of solution and form discrete gas bubbles. This process is known as vesiculation and is the driving force behind many volcanic eruptions. The bubbles rapidly expand as the pressure continues to drop, increasing the overall volume of the magma. If the eruption is particularly explosive, the magma can be fragmented into a frothy material full of expanding bubbles. When this frothy lava lands and cools almost instantly, the bubble walls solidify around the trapped gas.
Identifying Specific Vesicular Rocks
The two most recognizable examples of vesicular rocks are pumice and scoria, which are differentiated by their composition, color, and density. Pumice is a light-colored, highly porous volcanic glass that forms from felsic magmas, which are rich in silica. This rock often has such a high concentration of tiny, interconnected vesicles that its overall density is less than water, allowing it to float. The abundance of gas bubbles gives pumice a foamy, glassy texture.
Scoria, by contrast, typically forms from mafic magmas, which are darker, rich in iron and magnesium, and have a composition similar to basalt. It is usually dark in color, ranging from black to reddish-brown, and is denser than pumice due to having thicker walls between its vesicles. Scoria’s vesicles are generally larger and fewer in number compared to pumice, and the rock almost always sinks in water.
A third common example is vesicular basalt, which is simply a dark, fine-grained basalt lava flow containing noticeable vesicles. While both pumice and scoria can be ejected as fragmented material, vesicular basalt often forms in the upper crust of a lava flow. The distinctions between these three rocks highlight how the original magma’s chemistry and the specific cooling conditions influence the final vesicular texture.