There are a few main ways that rocks develop holes, such as weathering, organisms, or even dissolved mineral molds.
In the article that follows, we’ll explain.
What Causes Holes In Rocks? (EXPLAINED)
Whether it’s a druse, geode, lithophysa, miarolitic cavity, boring from pholads or mollusks, pit, pocket or pore, vesicle or vug, rocks gain holes via weather, organisms living their autonomous lives, or even dissolved mineral molds.
Rocks with holes don’t get that way via only one mechanism. There are many factors at play, and sometimes even vast lengths of geological time passage.
Druses, geoges, lithophysae, pits, pockets and vugs are all closer to cavities, caves, caverns or crevasses, rather than weather-caused sedimentary holes like miarolitics and molds from disintegrated minerals, or from bored openings by organisms.
Primary holes are generally caused by formation via volcanic gasses trapped in bubbles that eventually escape, while secondary holes are typically created by water erosion, wind blasting, wave crashing, glacier abrasions, boring by creatures, or mold left behind from dissolving minerals or the decay of dead organisms.
Holes in rocks are most frequently made because softer minerals and rock strata dissolve faster than harder ones.
Echinoderms like Strongylocentrotus Purpuratus will also bore into rocks.
Holes in rocks don’t last forever, though, due to the six steps in Earth’s natural rock cycle: weathering and erosion, transportation, deposition, compaction and cementation, metamorphism and finally, rock melting.
Weathering or erosion is when rain and wind carve into rocks, changing their shape.
Transportation is when wind or living organisms carry rock particles to other areas or lands, changing the rock compositions and environments, adding to or subtracting from land recipes and mineral or sedimentary ecosystems.
Deposition is when rock particles raise river beds in island-like sections, causing rivers to change their form as well, which then cycles the sediment and rock dance in whole new ways.
Either above or below water, sediments can stack up to where the weight and pressure fuses the bottom layers, creating compaction.
When dissolved minerals fill gaps and solidify, this is called cementing, and after a long time of compaction and cementation, sedimentary rock is created.
Tectonic plate activity causes sedimentary or igneous rocks to sink down underground over long periods of geologic time, where high heat and pressure catalyze them into “metamorphic rock,” the fifth stage in the rock cycle.
Finally, they are pushed down far enough into the heat and pressure to become magma, which can then be blown out of an erupting volcano.
Because of this ancient geologic routine, you can find sedimentary, igneous, and metamorphic rocks even where you live!
Perhaps the most popular holey stone is often called a hag stone or adder stone, witch stone, fairy stone or other various forms of “magical” stone names.
The earliest known discoveries of this rock were in the Northern German and Baltic seas.
However, they are also found in Egypt, called aggry or aggri, or Britain, where they might be called serpent’s eggs or snake eggs.
Fishermen often fastened them to the bows of their boats to keep witches or evil spirits at bay.
They are most often found in riverbeds or on beaches.
Hag stones are usually made from flint with a glassy appearance, but it is possible to find some made from limestone or sandstone as well.
Hag stones are formed typically by water erosion, grinding of other stones, and the boring of bivalve mollusks called “piddocks.”
These organisms have a “foot” muscle that grips the edge of the stone, then uses its toothed shell as a scoring drill of sorts by rotating the rock until it can crawl into the hole it bored, then extend a siphon out to catch its phytoplankton food.
If made by a mollusk, there’s a chance your stone hole could be misshapen because the piddock’s shell kind of “wings out.”
If a perfectly round hole, most likely it was formed via weathering, erosion and/or dissolved mineral methods.
Holey rocks are most often found near the oceans or tropics near volcanoes, but can also be found in mineral-rich mountains or any aggressive-weather terrain, and even your local stores.
There are plenty of famous landmarks where people can bear witness to the awesome power of corrosion and weathering.
Papago park in Arizona is believed to be twenty million years old, and it contains large, angular blocks of granite as well as landslide deposits, indicating these rocks formed at the base of a steep mountain front that has since been eroded away.
Clasts formed small, then water pooling in them eroded the rest away until the rock earned its holey nature.
In Utah, aside from the amazing Arches of Moab, the Canyonlands Country along highway 191 earned a spot on the tourist map for its “Hole ‘N The Rock” formation.
This rock holds a home!
Its hollowing was not done by nature, however, but rather by the Christensen family about a century ago.
Pleistocene glacial geologists and hydrogeologists sometimes drilled holes in rocks or bedrock to try and gather rock data.
Environmental geologists and oil field workers also drill holes in rock, but these are the kinds of voids we holey stone seekers are much less likely to find; and would we want to?
To have a home carved out of rock, though? Would you roll with that?