Rock Discoveries

Uncovering the Fascinating World of Rock Holes: Types Causes and Unique Features

Rock Holes in Geology: What You Need to Know

When we think about rock formations, some examples that often come to mind are towering mountains, majestic canyons, and intricate rock arches. However, one wonder that is not as talked about but just as fascinating are rock holes.

Rock holes are the result of various natural processes, whether it be erosion, soft mineral breakdown, or even boring organisms. In this article, we will explore the causes and types of rock holes and how these fascinating formations contribute to our geological wonders.

Types of Rock Holes Formation

Rock holes can be categorized according to how they came to be. The two primary forms of rock hole formation are simultaneous and epigenetic/secondary.

Simultaneous Holes occurs during lava solidification and caused by vesicles. When a lava flow cools, gases trapped within the rock start to expand and escape.

This causes vesicles or tiny pockets to form. Vesicles are usually small but can also be large enough to form holes that are visible to the naked eye.

Sometimes, lava flows that have coalesced create tubes and tunnels, which enable them to expand over long distances. In some instances, the interior of these tubes or tunnels can form vaulted collapse chambers, which give way to rock hole formation.

Such holes have a distinctive circular shape, and depending on the size of the vesicles, they can range between a centimeter to over a meter in diameter. Epigenetic/Secondary Holes, on the other hand, are holes that form due to weathering, erosion, boring organisms, or soft mineral breakdown.

For instance, when soft minerals, such as gypsum or salt, dissolve due to contact with moisture, they can create different-shaped holes ranging from rounded to box-like shapes. On the other hand, weathering and erosion due to water, wind, glaciers, or waves, can wear away at rocks resulting in fascinating features that range from simple potholes to highly complex networks of winding underground passages.

Such holes are common in caves, underground river systems, and karst landscapes. Furthermore, boring organisms, such as mollusks, worms, sponges, and piddocks, are often responsible for creating holes in marine environments.

Causes of Rock Holes Formation

As previously mentioned, there are three primary causes of rock hole formation in geology. These causes include weathering and erosion, boring organisms, and trapped gases.

Weathering and Erosion – This cause of rock hole formation is often due to various elements. Water and waves continuously batter coastlines and cliffs, gradually wearing away at the rock.

Windblown sand, over time, can carve holes or even unique-looking arches and spires out of rock surfaces. Glaciers, massive sheets of ice, moving across the earth’s landscape have carved out valleys, lakes, and other landforms.

Boring Organisms – When it comes to Marine environments, we see rock holes in the form of burrows, which are created by different types of boring sea creatures. Mollusks, worms, and even sponges can tirelessly burrow into rocks, weakening them over time and creating holes in the process.

Trapped Gases – This cause of rock hole formation is a result of gases that become trapped within igneous rocks during solidification. Basalt, scoria, and pumice are typical rocks that have trapped gases.

These gases serve as nuclei around which crystallization occurs, leaving hollow spaces that become observable holes or cracks in the rocks.


Rock holes may not be the most talked-about geological formations, but they are still fascinating all the same. The way they form through different natural processes leads to unique shapes, and they can be found all over the world.

Hopefully, this article has given you a better idea about rock hole formation, from how they come to be to the causes behind them. Even if you’re not much of a geology enthusiast, the next time you spot a rock hole, you might be pleased to know how it was formed.

Types of Rock Holes and Their Presence in Different Rocks

Rock formations can sometimes leave holes in them that provide interesting insights into how they were formed and what processes acted upon them. There are different types of rock holes, each formed through unique geological processes.

This article explores the different types of rock holes and their presence in different rocks.

Types of Rock Holes

In Sedimentary Rocks

Sedimentary rocks are formed when accumulated sediments are cemented together. Usually porous in nature, these rocks can form different types of holes.

Pores are the most common form of hole in sedimentary rocks. Pores can vary in size and shape and can be present in rocks like sandstone and limestone.

Limestone is particularly notable in the presence of solution cavities, which are holes caused by the dissolving action of weakly acidic groundwater.

In Igneous (Volcanic) Rocks

Igneous rocks form when molten magma or lava cools and solidifies. The various types of holes that appear in these rocks are lithophysa, miarolitic cavity, vesicles, druses, vugs, and amygdules.

Lithophysa is a type of vesicle in volcanic rocks that has a star-like shape. Meanwhile, miarolitic cavities are irregular, elongated, or rounded spaces ranging from a few millimeters to several centimeters in diameter.

Vesicles, on the other hand, are formed by gas bubbles becoming trapped within the lava. Druses are small cavities shaped like grape-like clusters of crystals, while vugs are pockets or holes within rocks.

Finally, an amygdule is a void space within the volcanic rock that is filled with minerals.

In Intrusive Rocks

Intrusive rocks, also known as plutonic rocks, are formed from magma that cools and solidifies beneath the earth’s surface. These rocks also have miarolitic cavities that are formed because of the volatile gases being trapped during the cooling process.

At the Beach

Beaches also have their share of rocks with holes. Hag stones have been a part of Celtic folklore for centuries and are considered talismans that protect against evil spirits.

These stones have one or more naturally occurring holes in them. Odin stones, Adder stones and Holey stones all have similar characteristics, and despite their different names, share similar significance.

These rocks with holes are often associated with tafoni, a term for small, honeycomb-like weathering patterns that occur on rocks that are composed of sandstone, granite, or other materials.

In Ocean (Sea) Rocks

Ocean rocks also exhibit holes that result from erosion and weathering. Salt is particularly harsh in sound, especially when it is heated by the sun.

Salt crystals are known to expand up to three times their size as sea water dries from around them. In time the expanding crystals break down surrounding rock, leaving behind a network of holes in the rock.

These holes are known as tafoni and are common in rocks near the ocean.

Presence of Holes in Different Rocks

Different rocks exhibit varying degrees of porosity and susceptibility to weathering and other natural forces leading to the formation of holes. Igneous rocks, such as basalt, pumice, and scoria, have the highest concentration of holes amongst rocks.

Pumice, in particular, often has large vesicles. Sedimentary rocks like sandstone and limestone are highly porous and often exhibit cavities and solution cavities, respectively.

Intrusive rocks, like granites, gabbros, and pegmatites, also have miarolitic cavities due to the volatile gases trapped within during the cooling process.


Rock holes are a fascinating sight that provides insights into geological processes. Different types of rock holes exist due to various processes, whether it be volcanic or sedimentary rock formation, weathering, erosion, or exposure to saltwater.

The presence of these holes in different rocks contributes to their unique beauty and the stunning geological formations that continue to amaze us.

Unique Forms of Rock Holes

Rock holes are fascinating geological formations that enlighten us about the natural processes that act upon rocks. Apart from the common forms of rock holes such as vesicles, miarolitic cavities, and solution cavities, certain types of rock holes exhibit unique features.

In this article, we will explore two forms of rock holes: potholes and geodes.


Potholes are round-shaped holes that can form on bedrock or streambeds. These holes are usually formed by frictional erosion which occurs when circulating water with rock fragments gets trapped in the depression of a larger rock.

The rocks then rotate and grind away at the bedrock, sometimes enlarging the depression into a pothole.

Potholes can vary in size from small and shallow to deep and large enough to be dangerous for swimmers.

Potholes form in a variety of rock surfaces. They are common in granite, gneiss, and weathered metamorphic rocks.

One fascinating example of potholes is the Devil’s Kettle Falls located in Minnesota. It is a waterfall where one side of the river cascades 50 feet into a giant hole that seems to lead to nowhere.

Scientists and locals alike have been trying to figure out where the water that goes into the hole goes for decades, with no answer yet.


Geodes are another unique form of rock holes that are lined with crystals. These hollow cavities can be found in volcanic rocks, such as pumice and scoria, but can also be found in sedimentary rocks.

Geodes begin as bubbles in volcanic rock that are filled with gas. Over time, the gas escapes and leaves void spaces within the rock.

These spaces are then filled with minerals over many years, or even millions of years, creating beautiful crystal linings.

Geodes are formed in various sizes, ranging from a few centimeters to over four feet. They can be spherical or irregularly shaped and sometimes have several different layers of crystal.

Geodes are usually found in large groups within the same area, making their discovery all the more rewarding. The most common minerals found inside geodes are quartz, calcite, and amethyst, although other types of crystals and minerals can also be found.

Geodes can be found in several locations globally, but some of the most significant geode deposits are found in the United States, specifically in the states of Kentucky, Illinois, and Iowa. The Keokuk Geode is a famous geode found in the American Midwest and is highly regarded for its beautiful crystal formations.


Rock holes are relatively common geological features, and there are a plethora of different types of formations that can be found. Some forms, such as potholes, are created by the mechanical action of wind or water, while others, like geodes, are formed through the slow accumulation of minerals.

The unique characteristics of these rock holes provide us with a better understanding of the natural processes that occur around us. From the beauty of rare crystals within geodes to the intrigue of potholes that seem to lead down into the depths of the earth, we can appreciate the fascinating qualities and discoveries that can be made with these unique forms of rock holes.

In conclusion, rock holes are fascinating geological features formed by a variety of natural processes, from weathering and erosion to volcanic activity and the action of boring organisms. Different types of rocks exhibit different types of holes, from potholes and solution cavities in sedimentary rocks to miarolitic cavities and vesicles in igneous rocks.

Exploring these unique forms of rock holes helps us better understand the forces of nature that shape our planet and highlights the beauty of geological formations. Here are some frequently asked questions about rock holes:


What causes potholes?

Potholes are formed by frictional erosion, occurring when circulating water with rock fragments gets trapped in the depression of a larger rock. 2.

What is a geode? A geode is a hollow cavity, usually spherical in shape and lined with crystals, found in volcanic and sedimentary rocks.

3. What type of rock has miarolitic cavities?

Miarolitic cavities are common in igneous rocks, particularly in granites, gabbros, and pegmatites. 4.

How are rock holes formed in ocean rocks? Rock holes can form in ocean rocks due to the action of saltwater and weathering, known as tafoni, which creates honeycomb-like weathering patterns.

5. What are some common minerals found inside a geode?

Quartz, calcite, and amethyst are some of the most common minerals found inside geodes, although other types of crystals and minerals can also be found.

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