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Holes in Rocks Explained: How Are Formed & What Causes Them

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Holes in rocks can be formed in strikingly different ways. Sometimes holes are a result of water erosion – the simplest and the most obvious way of hole formation. But you will be surprised to know that holes can be formed at the very first stages of rock solidification. Moreover, some holes in hard and almost indestructible rocks can be produced by small mollusks. Surprised? Get on board!

Holes can be found in all three types of rocks. Holes can be secondary and created by weathering. Additionally, holes in beach pebbles are created by boring mollusks. Primary holes are created simultaneously with the volcanic rock formation because of the gas bubbles trapped during lava solidification.

Swiss cheese-resembling cliffs on the seaside, boring mollusks and burrowing creatures, champagne corks… Do you see any connection? After reading the following article, you will see the connection and will be able to explain a lot of intriguing things around.

What Causes Holes in Rocks and Why Do Rocks Have Holes in Them?
What Causes Holes in Rocks and Why Do Rocks Have Holes in Them?

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Why Do Rocks Have Holes in Them

Holes in rocks are a sign of the next stage of the rock cycle. Holes are a result of different natural processes like water erosion, wind erosion, waves and glacier abrasion, fluctuation of temperature and humidity, living organisms’ activity, and so on. 

Holes in rocks are usually a sign of rock weaknesses. Softer minerals and rock strata can be destroyed much faster than the harder ones. In most cases, holes are the result of weathering and erosion. Such volcanic rocks like pumice and scoria receive their holes during the solidification of lava.

Different processes are responsible for hole formation. Holes are mostly indicators of rock weaknesses. However, sometimes holes are formed at the same time as a rock.

Such holes can be considered an essential part of the rock and form a specific vesicular texture, as in the case of basalt

What Causes Holes in Rocks

It’s not correct to put just one mechanism of hole formation on all types of rocks. Holes can be formed simultaneously with rock or millions of years later. Different types of rock undergo different mechanisms of hole formation.

Weathering, which is represented by water and wind erosion, the abrasive activity of waves, and the boring and burrowing activity of living organisms, creates holes in rocks. Holes in rocks can be also created simultaneously with rock solidification, as in the case of basalt, pumice, and scoria rocks.

Simultaneously formed holes are typical for igneous volcanic rocks like basalt, scoria, pumice, and sedimentary rocks like sandstone and limestone.

The tiny holes between the individual grains of sand in sandstone rock are called pores. The pores in a rock collectively make up its porosity, which is an important property to know in groundwater, geotechnical studies, and petroleum geology.

Holes can be created millions of years after the rock formation. These secondary holes are mostly caused by exogenic processes, mainly weathering and erosion.

Why Are There Holes in Rocks at the Beach

Almost perfectly round holes in rocks at the beach puzzle all rock lovers. At first glance, it’s hard to find a rational explanation for this surrealistic formation. However, inventive nature has an answer.

Holes in rocks at the beach can be created in two ways. The most common one is when holes are caused by boring sponges, burrowing worms, or mollusks, such as piddocks. These creatures produce almost perfectly round holes. The second cause of holes is water erosion, which wears weaker parts of the rock away.

Stones, with a naturally occurring hole running through them, are known as hag stones. Holes can be caused in nature by boring sponges, burrowing worms, or mollusks, such as piddocks.

Piddocks – are the marine bivalve mollusks of the family Pholadidae. These rock-boring bivalves effectively drill into the rock, which breaks off and erodes into cylinder-shaped holes a few centimeters across.

Piddocks live their lives inside that shelter and stick their siphuncles out to filter the seawater. Sometimes shells of bivalvia can be observed inside the holes.

But in most cases, these are dead organisms. Otherwise, mollusks try to escape if their shelter is about to be destroyed.

TIP: Do you know what rocks you can find on the beach most often? Check out the list of the most common beach rocks in the article below:
25 Most Common Rocks on the Beach: How & Where to Find Them?

What Makes Holes in Ocean (Sea) Rocks

Some other bigger holes can be observed at the seaside. It can be attractive Swiss cheese-like landforms. Also, there can be huge fascinating gothic-like arcs, which small boats can go through.

The mechanism is a bit different than that explained before. For sure, there are no such big mollusks to bore such holes. Luckily! 

Holes in ocean rocks or cliffs are created by the erosion activity of wind, waves, and saltwater. Softer layers of rocks and softer minerals are worn away faster, creating intriguing textures and surfaces. Tafoni – is a separate name for a series of holes in a rock created by a honeycomb weathering.

Salt expansion, which works similarly to frost wedging, occurs in areas of high evaporation or near-marine environments.

Evaporation causes salts to precipitate out of solution and grow and expand into cracks in the rock. Salt expansion and wind activity are the main causes of tafoni – a series of holes in the rock. 

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What Kind of Rocks Have Holes in Them?

What Kind of Rocks Have Holes in Them?
What Kind of Rocks Have Holes in Them?

All types of rocks (sedimentary, igneous, metamorphic) can be subjected to hole formation. Here we will give you some examples of rocks and shed light on the individual processes that take place.

Igneous volcanic rocks such as basalt, pumice, and scoria have holes called vesicles. Sedimentary rocks like limestones and sandstones can have natural small holes called pores. Pebbles on the beach have holes created by boring mollusks. All types of rocks can have holes created by the weathering of soft minerals.

Presence of Holes in Intrusive Rocks

Intrusive rocks can have both simultaneous holes, which form during magma or lava solidification, and secondary holes if rock massive are subjected to saltwater, wave abrasion, or living organisms activity.

Holes in intrusive rocks are not so common, as they usually solidify slowly and deep beneath the earth under high-pressure conditions. However, sometimes there are some kind of holes in pegmatites. These holes are called miarolitic cavities and are usually the source of fine crystals and gemstones.

Usually, intrusive rocks like granite or gabbro have no holes, as they are formed under high-pressure conditions that don’t allow any gas bubbles and holes.

However, there is a special type of small irregular form cavity found in coarse-grained igneous rocks pegmatites. 

Miarolitic cavities are formed as a result of a dramatic change in pressure. When the pressure decreases, dissolved gas from magma can create big bubbles of irregular shape. These cavities are called miarolitic.

TIP: Oceans and seas are full of interesting rocks. Do you know which rocks are most often found in the oceans? Find out more in the article below:
Which Gemstones Come From The Ocean? Corals, Pearls & more!

Presence of Holes in Igneous (Volcanic) Rocks

Igneous volcanic rocks have holes that were formed simultaneously with the solidification of lava. These holes are a part of the rock texture. Sometimes holes are big enough to create geodes – beautiful geological formations lined with crystals of amethyst.

Holes in igneous volcanic rocks are explained by the presence of dissolved gases. These gasses are trying to escape while the lava is still liquid and become trapped as soon as lava solidifies. Examples of such rocks are basalt, pumice, and scoria. Holes are usually called vesicles.

Pumice and scoria are igneous extrusive (volcanic) rocks. They form outside of the volcano, usually on top of lava flows. The top of these lava flows become very frothy and, when they cool, the gasses in the lava expand and escape forming air holes or vesicles in rock.

The physics of the described process is similar when you open a bottle of champagne. The difference in pressure releases gas bubbles in champagne after the opening.

Sometimes volcanic rocks look a bit like Swiss cheese – they have dozens of holes in them. Molten rock often contains dissolved gases.

These gases bubble out of the lava when it is still a liquid, but as soon as the lava solidifies, the gas becomes trapped.

The specific texture of the volcanic rock, which contains a lot of holes, is called vesicular. Vesicles that are filled with minerals are called amygdules.

How Are Holes in Rocks Formed

The classical process of hole formation is weathering and erosion. Different exogenic agents like water, wind, waves, glaciers, and living organisms are in charge of hole formation.

The general process of hole formation is the weathering of softer minerals from the harder matrix. Sometimes holes can be produced by boring sponges or mollusks activity. Holes in igneous volcanic rocks occur because of lava degassing and quick cooling. Gas bubbles are trapped in lava and perceived as holes.

Sometimes enigmatic big holes in the size of several meters are caused by an interesting natural process. Holes are produced by circulating water, holding a smaller rock or pebble up against a large rock. The resulting friction erodes a “pothole” into the larger rock.

TIP: Have you ever wondered why rocks have different colors? And how do these colors come about? Find out more about rock colors in the article below:
Rock Colors: What Determines Color & Why Different Colors

FAQ About Rocks with Holes

Still did not find the answer to your answers about rocks with holes? Find frequently asked questions in the section below:

What Are the Holes in Igneous Rocks Called

Holes in igneous rocks, created as a result of gas bubbles solidification in lava, are called lithophysa in high-silica lavas like rhyolite, a miarolitic cavity in pegmatites, vesicles in basalt, pumice, and scoria; druses and vugs in the case when holes are lined with crystals.

What are Rocks with Holes in Them Called

The first group of rocks with very obvious holes found on the beach are called hag stones, Adder stones, Odin stones, holey stones, witching stones, chicken gods, and so on. Holes are produced with boring mollusk. The second group of rocks is igneous volcanic rocks like pumice and scoria.

What Causes Small Holes in Rocks

There are three main causes of small holes in rocks. The first one is weathering – soft minerals are worn away from the harder mass. The second one –  is trapped gas bubbles in igneous volcanic rocks. The third one – is holes made by boring sponges, burrowing worms, or mollusks, such as piddocks.

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Holes formation in rocks can be simultaneous and secondary. Simultaneously holes are created during the rock formation. Holes in basalt, pumice, and scoria are a result of trapped gases during lava solidification and are called vesicles.

Epigenetic or secondary holes in rocks are the next stage of rocks’ lifecycle. Every rock is subjected to weathering. Weathering doesn’t work similarly on all types of rocks and minerals.

Some softer rocks and minerals decay faster than others. This makes different parts of rocks disintegrate at different speeds. Soft minerals, which are worn away first, leave holes.

The main weathering agents which are in charge of hole formation are water, wind, waves, and glaciers.

Another mechanism of rock formation takes place with the help of living organisms like mollusks, worms, and sponges. These creatures can bore holes in rocks. The most common boring creature, which creates holes, is piddocks – small marine bivalve mollusks of the family Pholadidae.

To sum up, here are the main causes of holes:

  1. Trapped gas bubbles during the rock solidification.
  2. Weathered and worn away soft minerals in the hard matrix.
  3. Wind and saltwater commonly work on the seaside rocks.
  4. Waves abrasion on the near-the-sea cliffs. 
  5. Boring and burrowing living organisms like mollusks, worms, and sponges.

Pick up any rock with a hole and train your imagination on the history of this hole formation by applying one of the five factors above. Nature has a lot of hidden secrets to surprise you!

TIP: Some rocks have holes on the surface, and geodes and thundereggs have a hole inside. Do you know the difference between these two rocks? Find out more in the article below:
The 7 Main Differences Between Geode and ThunderEgg