Characteristics of Basement Rocks (Properties and Examples)

When we talk about basement rocks, we’re not talking about the cinder blocks that make your home’s foundation surrounding the basement.

Basement rocks are the oldest kind of rock, so they make an important foundation for any budding geologist or rockhound to study.

In this article, you’ll get to know some commonly known basement rocks, as well as information about basement rocks to better understand them.

Characteristics of Basement Rocks (Properties and Examples)

In geology, the terms “basement” or “crystalline basement” refer to rocks that sit underneath a sedimentary platform or some sort of cover.

They can also be found underneath sedimentary rocks in general or sedimentary basins.

These sedimentary rocks or sedimentary basins that cover basement rocks can fall under the igneous category or metamorphic rock classification.

Because sediments, sedimentary rocks, or some combination of the two settle over basement rocks, geologists sometimes call them “sedimentary cover” or just “cover” for short.

Facts about Basement Rocks

As we mentioned previously, basement rocks are the oldest kind of metamorphic and igneous rocks.

They create the crust of masses of land throughout the globe.

Basement rocks frequently take on the form of granite, but they can have other compositions, as well.

For those who are trying to identify basement rocks, they are pretty obvious to spot with the naked eye.

For starters, basement rocks are large–they make up the entire crust of continents, after all. Basement rocks make up massive landmarks such as the depths of the Grand Canyon.

This kind of rock is usually free from many fossils or other ancient impressions, since it predates many of the creatures that were fossilized in many younger kinds of rock.

The Vishnu Schist (also known as the Vishnu Basement Rock) is located in the depths of the Grand Canyon and it is a popular example of basement rock that modern-day geologists can visit easily.

Basement rock tends to have an uneven appearance on a large scale.

If you see the basement rock from a birds’ eye view, you will notice that it frequently has contrast between sedimentary rocks such as limestone or sandstone that are laid over the basement rocks.

Depending on where you try to find basement rocks, you may not be able to view them since they are frequently covered in other types of rock and sediment or they’re hidden deep in canyons.

While the sedimentary rocks that form over basement rocks can be thin, they can be as thick as three miles tall.

Unfortunately, if the sedimentary rocks deposited over basement rocks are several miles thick, you may only get small slivers of basement rock peeking through.

This may not be visible on the ground level.

Basement rock, on the other hand, is usually fairly thick. It can be up to ten times as thick as the layers of sedimentary rock that form a crust over it–between 20-30 miles at times.

Because of the variations in thickness of sediment, basement rock is sometimes easy to see.

At other times, it is impossible to view without some serious deconstruction of nature’s beauty.

Outside of the Vishnu Basement Rock that sits at the deepest point of the Grand Canyon, you can also spot exposed basement rock at the Shenandoah National Park in Virginia in the United States and across the globe in New Zealand, the Korean Peninsula, and the Congo.

For rockhounds, basement rock is more of a sight to remember in pictures than a potential find to add to the rock collection.

How Do Basement Rocks Compare to Similar Types of Rock?

Compared to sedimentary rock, basement rocks are pretty difficult to observe outside of certain areas across the world.

Basement rock is also significantly older than some comparatively younger types of rock such as volcanic rock and sedimentary rock.

As a result, these rocks are obscured from view and relatively difficult to reach.

Basement rocks are also usually smoother than rocks in the upper layers because they formed well before creatures could make fossils in them or leave traces of themselves behind on their surface.

Basement rocks tend to be older than Permian evaporites and limestones that formed on top of them.

In the geological formation timeline, evaporites created a weak zone where tough limestone cover could shift over the evaporites.

As a result, basement rocks were visually distinguished from the cover and exposed only small veins of the basement rocks.

A wide range of rock types create basement rocks. Some of the more common ones include feldspar, quartz, and garnet.

You may recognize these names as beautiful gems that you would buy in the jewelry store or celebrate finding on a hike.

However, with basement rocks, these usually present as veins of gorgeous metamorphic and igneous deposits that shimmer subtly underneath swaths of limestone or other sedimentary rock.

You also won’t want to attempt to mine basement rocks that look just like your favorite stone.

Because basement rocks are so old, they frequently exist in places that are federally protected and preserved.

Therefore, it’s illegal to try to take them home. Most of them would be too hard to access or even see to attempt to mine from them, anyway.

Basement rocks are usually studied in various sects of geology including sedimentology, basin geology, and petroleum geology.

Crystalline basement rocks don’t capture the attention of researchers of basement geology as much.

The reason for this is that crystalline basement rocks usually don’t offer valuable resources such as petroleum or natural gas.

Regardless of what type of basement rock you are studying, you will probably notice that basement rocks often incorporate a variety of types of rock.

They can have several different types of rocks combined, including volcanic rock, metamorphic rock, and igneous rock.

Sometimes, basement rocks also have fragments of oceanic crust in them. These fragments, also known as ophiolites, got stuck between tectonic plates.

This substance then folded and metamorphosed to become the material we see in these rocks today.

Some theories suggest that a large percentage of the basement rock we see today used to be ocean crust many millions of years ago.

The theory goes that the oceanic crust then metamorphosed a great deal and eventually converted to continental crust.

We’ll never know how basement rocks actually formed–they are hundreds of millions of years old–but we can appreciate the beauty that they possess today and the stable foundation they give our continents.

Conclusion

In short, basement rocks are the oldest type of rocks on earth.

They are called basement rocks because they form the foundation or “basement” under layers of sediment and sedimentary rock, also known as “cover.”

Basement rocks can be difficult to find because of their covered nature, but in the United States, the deepest part of the Grand Canyon is a good spot to find examples of basement rocks in the Vishnu Schist.