The Al Naslaa rock formation is Earth’s most bizarre geological feature

The Al Naslaa rock formation, found in Saudi Arabia, is one of Earth’s greatest mysteries.

The Al Naslaa rock formation located in Saudi Arabia is made of high-density sedimentary rock, and shows significant evidence of weathering and erosion. However, the pedestal beneath it has eroded more quickly, the petroglyphs upon it are thousands of years old, and the extremely smooth fissure down its center is not yet fully explained.

Two boulders, six meters (20 feet) high, stand atop their own pedestals, separated by a perfectly smooth crack.

This view through the center of the fissure that separates the two sides of the Al Naslaa rock formation showcases how smoothly and cleanly the crack appears separating the two pieces of the boulder. As such, it’s a common tourist attraction, but also a common font for conspiracy theories concerning its origin.

Possessing petroglyphs up to 4000 years old, Al Naslaa’s central fissure is iconic.

Al Naslaa is interesting not only from a geological perspective, but also from an archaeological one, as the numerous glyphs that it contains are thousands of years old.

Although its cause remains debated, other terrestrial features hint at solutions.

These limestone formations in the Torcal de Antequera, Spain, show the erosion of rock by wind, sand, rain, and other sources of water, which can cause different layers of rock to erode at different rates, as well as fissures to form in both the horizontal and vertical directions.

In the United States, Balanced Rock displays similar “unstable equilibrium” features.

Balanced Rock, shown at left in this photo, is one of the most iconic sites in Arches National Park. Although the center of mass of the boulder at the top of the pedestal is, in fact, over the pillar, it is only a matter of time before continued erosion brings this structure down.

In sedimentary rock, different layers erode at different rates, commonly creating pedestal-like structures.

In Arches National Park in Utah, United States, this feature is colloquially known as “Marching Men,” although in reality it’s composed of higher-density sandstone sitting atop a layer of a mudrock/sandstone base that’s interbedded with limestone and gypsum. This lower layer erodes more easily, giving rise to the sight of heavier objects sitting atop scrawnier bases and pedestals.

As the supporting material erodes, these ancient structures risk collapse.

Balanced Rock, shown here in photos from before 1975 (black and white) and after 1976 (color), showcases how weathering and erosion can lead to the collapse of structures over time. At right, the structure once known as “Chip off the Old Block” fell during the winter of 1975-1976, and someday, Balanced Rock will fall as well.

Egypt’s White Desert contains similarly unstable features as well.

This natural feature of eroded limestone in Egypt’s White Desert is one of many such structures where a massive boulder sits atop a natural, thinner pedestal. Erosion of different geological layers at different rates will often lead to unstable equilibrium configurations such as this.

Combinations of wind, rain, sand, and flowing water all carve these structures over geological timescales.

This structure from Al Farafrah in the White Desert in Egypt shows a common sight: a large, weathered boulder atop a weathered spire/pedestal, where rocks deposited at different geological epochs weather and erode at different rates.

Meanwhile, New Zealand’s “Split Apple Rock” shows a similar, almost-perfect split.

New Zealand’s Split Apple Rock is composed of granite, and dates back to the Cretaceous period more than 65 million years ago. The cause of the split down the rock’s center was a naturally occurring joint, and was likely broken apart by a combination of rains and ocean waves.

Planes of weakness in granite structures, known as joints, enable water-based weathering to cleave boulders apart.

This rock, located in Abisko, Sweden, shows signs of not only weathering, but fracturing along a glacially-scoured outcrop. These features are common in the region, and their examination can teach us about postglacial weathering rates.

The backside of Al Naslaa contains another cracked joint parallel to the main fissure: a potential clue.

The Al Naslaa rock formation has a spectacular crack that’s perfectly smooth, with both edges parallel, running down its center. However, a parallel crack can be seen on one side, running the full height of the structure, providing a potential clue to the fissure’s origin.

All throughout the Arabian Peninsula, eroding sandstone and limestone lead to magnificent features.

The Al-`Ula city was part of the Kingdom of Lihyan located in northwest Saudi Arabia, and has many of the same geological features to its rocks as those found elsewhere in the vicinity. Many of the structures found in Saudi Arabia and across the Arabian Peninsula date back more than 2000 years, with some going all the way back to the Bronze Age.

Earthquakes and/or fault lines may have also played a role in creating Al Naslaa’s fissure.

The fact that there are many small horizontal cracks across both sides of the boulder, but that those cracks don’t necessarily line up with one another, strongly suggests that many of these cracks formed from weathering and erosion processes that took place after the boulder had already been separated into two.

Although its collapse is inevitable, Al Naslaa remains an enduring geological wonder.

This view of the “back side” of Al Naslaa shows a number of features that can be more easily seen not to align precisely from the left side to the right, suggesting that faulting may have been at play in dividing this boulder into two.

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