Why do sandstone rocks sometimes have holes in them?

We are diving into the intriguing realm of sandstone rocks, one of our favorite rocks at Mini Me Geology. One of our readers, Matthew, wrote in and asked: Why do sandstone rocks sometimes have holes in them?

What is Sandstone Rock?

Banded Sandstone Sedimentary Rock

Sandstone is a sedimentary rock made of small (typically 0.25 millimeters to 2 millimeters) grains of minerals, rock, and/or organic material. The majority of grains in sandstones are the mineral quartz. Sandstone can have different colors and textures because of the composition of sediment and minerals during its formation. Some great examples of the different appearances of sandstones are white sandstone, yellow sandstone, and banded sandstone which is a variety having bands of different colors with varying thicknesses.

The entire process of sandstone formation is a slow geological cycle that spans millions of years. The rock forms in oceans, rivers, lakes, and deserts when wind, water, or ice deposit sand grains in a “quiet” area. These grains accumulate in layers over long periods of time.

How do Sandstone Rocks Form with Holes Inside?

Yellow Sandstone Sedimentary Rock

Once the loose sand grains settle on the land surface in a desert or on the floor of an ocean, lake, or river, they are “glued” together. This process is called lithification, and it has two primary processes that are compaction and cementation. While there can be other processes at play, these are the more common methods of sandstone formation. Compaction occurs when the heavy weight of the top grains puts pressure on the bottom layers. Over time, the weight squeezes the sand grains together, pushes water from between the grains, and “compacts” the grains so they are closer together.

Minerals present in groundwater and ocean water will “cement” the compacted sand grains and bond the grains together. Typically dissolved calcite or quartz in the water will precipitate as they move through the holes between the grains acting like a super glue that then holds them together. The mineral “glue” may completely fill the holes between the sandstone grains or it may leave the spaces open. Sandstone rocks with holes are known as porous sandstones. A “pore” is the open space between the sand grains in a rock. The porosity of a sandstone is a measure of the amount of pore space (holes) inside the rock. The higher the porosity, the larger the open space between the grains.

Sandstone Rocks with Holes Inside are Great Underground Storage Areas

White Sandstone Sedimentary Rock

The holes that are left inside a sandstone rock are great places to store water, oil, or natural gas underground. Yes, you heard it right! These porous sandstones play a crucial role in storing valuable resources beneath the Earth’s surface. Just like people, not all sandstones are the same! Some sandstones have a few tiny holes, while others are riddled with larger cavities.

And there you have it! We unraveled the mystery of why some sandstone rocks have holes inside. So, the next time you stumble upon one of these holey wonders, remember the secret world they hold beneath the surface ”“ a world of storage spaces and geological marvels!

If you have questions, Ask a Geologist!

If you have questions about rock, minerals, or any other aspect of geology, send it to us and we are happy to answer either on the website or on video! You can submit your questions on our website or email them to us at rockinfo@minimgeology.com.

Keep exploring, keep wondering, and who knows, you might just uncover the next great mystery of the Earth’s fascinating rocks! Happy rock hunting!

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