The name for a rock with holes depends entirely on how those voids were created. Geologists classify these rocks based on the mechanism of void formation, resulting in several distinct categories. The appearance of these voids—whether tiny bubbles, large channels, or clean perforations—is a direct clue to the natural process that shaped the stone, such as volcanic activity, chemical weathering, or biological action. There is no single scientific term that accurately covers every type of “holey” rock.
Vesicular Rocks Formed by Trapped Gas
Many rocks with holes result from rapid cooling during a volcanic eruption. These are scientifically known as vesicular rocks, a texture characterized by numerous small cavities called vesicles. Vesicles form when gases dissolved within molten magma, primarily water vapor and carbon dioxide, come out of solution as the magma rises and pressure decreases. This process is similar to bubbles forming in an opened soda bottle.
If the magma cools quickly before these gas bubbles can escape, the bubbles become frozen in place within the solidified rock. The resulting texture is highly porous, and the size and distribution of these voids depend on the magma’s composition and viscosity. Pumice and scoria are two prominent examples, both classified as extrusive igneous rocks.
Pumice is a light-colored, low-density volcanic glass formed from felsic (silica-rich) magma. This highly viscous magma traps gas bubbles into a frothy, foam-like structure with thin walls, often allowing the rock to float on water. Conversely, scoria forms from less-viscous mafic (iron and magnesium-rich) magma, resulting in a darker, denser rock. Scoria features larger, more irregular, and thicker-walled vesicles, giving it a cinder-like appearance that causes most samples to sink.
Porous and Cavernous Rocks Formed by Dissolution
Another category of rocks with holes is created by external chemical processes, particularly the dissolution of minerals by water. This process is most common in sedimentary rocks, such as limestone and dolostone, which are composed primarily of carbonate minerals. These rocks are described as porous when they contain small, interconnected voids, or cavernous when they feature much larger openings.
The formation of these voids begins when rainwater absorbs carbon dioxide, turning it into a weak carbonic acid. This acidic water percolates through the rock, reacting with and dissolving the calcium carbonate within the stone. Water preferentially flows along existing fractures and bedding planes, slowly enlarging them through this chemical reaction.
When this dissolution process continues on a massive scale, it forms large subterranean voids known as caves, which are hallmarks of karst topography. Smaller, non-interconnected voids, such as geodes or vugs, can also form by similar dissolution processes where minerals are removed from pockets. This chemically driven creation of holes differs fundamentally from the mechanical trapping of gas bubbles in volcanic rocks.
Bioerosion and Perforated Rocks
A third mechanism for creating holes in rocks is bioerosion, the breakdown of hard substrates by living organisms. This process is noticeable in marine environments, such as coastlines and coral reefs, where various organisms physically or chemically bore into the rock. Organisms like boring clams, sponges, and marine worms create distinct perforations as they excavate tunnels to seek shelter or access food.
These bioeroders use mechanical drilling and chemical secretions to create holes in solid rock, resulting in a perforated appearance different from the bubbly texture of vesicular rocks. The holes are often clean, circular, and follow the path of the organism. Rocks with holes bored completely through them are often given common names like “hagstones” or “holey stones,” regardless of the precise geological or biological cause.