The Geology of AlUla

The landscape, the colors, the valleys, the mountains, all tell us about the geology story of AlUla.

The story of AlUla

is written in the rocks.

The landscape, the colors, the valleys, the mountains, all tell us about the geology story of AlUla.A story that begins long before the Nabataeans built the spectacular tombs at Hegra. A story that is written in the rocks.

Wherever you are in AlUla, you will see rocks. The rock faces surrounding Old Town. Amazing natural formations as you drive towards the Sharaan Nature Reserve. Rugged, sharp hills near the airport. Ancient volcanic landscapes. All around AlUla you will see an incredible variety of striking landscapes. Just walking down the street and you will see rocks. Red rocks that seem to change their features as the sun moves throughout the day. And dark rocks that look like an enormous table jutting high above the ground.

AlUla is a very special place for geology. We can find metamorphic, igneous and sedimentary rocks all in this region. This history can be seen as we move through AlUla from the south to the north, travelling almost a billion years through time.

Oldest AlUla

In the south of AlUla, you can find the regions oldest rocks, dating to around 900 million years old. With age the rocks have their wrinkles, on an enormous scale. They have been buried, crushed, slightly melted, and thrust up, changing their original composition. These are metamorphic rocks: rocks that have been changed from their original form.

Older rocks are often at the mercy of our planet’s never ending cycle of continents, the plates on the Earth’s crust are floating on hot, molten rock, being forced to crash into each other, spread apart from one another, or pushed down under one another (known as plate tectonics). When rocks are forced down into the Earth, the pressure and heat melt the rocks and new crystals form. Sometimes the rock is only melted slightly, and there are places in the south where we can find some conglomerates, which have been slightly melted. These are evidence of underwater avalanches in AlUla around 900 million years ago. This ancient environment has almost been obliterated by time.

Here, in this more jagged landscape, we can also find very old rocks that were formed from molten rock (igneous rocks). Fractured, folded, and distorted through 900 million years of movement, they still help us understand the history of the planet. Ancient volcanoes covered the landscape. Hot molten rocks (called magma) flowed and forced it’s way through cracks in the crust, some never reaching the surface. It was a violent period, one which we are just beginning to understand.

A story through deep time

The time of the seas

The most familiar, and the most iconic, rocks in AlUla are the beautiful light-red sandstones. These formed around 510 – 480 million years ago. The planet was a very different place. There was no life on land at this time, no animals, no forests. Without plants to help keep the soils and sediment together, there was enormous amounts of erosion from the land, and massive amounts of sediment were deposited.

During this time, AlUla was close to the coast, with shallow beaches, sometimes huge rivers and deltas over time. The clues are in the rocks, we can see marks left by the soft movement of the tide 500 million years ago, or large round pebbles in fine sand moved by fast flowing rivers. And we can even find traces of life. Tracks in the sediment made by extinct marine creatures called trilobites. We haven’t found any fossils in AlUla yet, but we are looking.

These rocks were a lot lower than they are today, they formed by the sea at the sea level. As more and more sediment was deposited, the sediment from the ancient beaches and deltas were pushed down into the Earth. The weight of the sediment above, and the heat and the pressure, squeezed out any water, and baked the sediment to form the sandstones we see today. The powerful movement of plate tectonics thrust the rocks high up, where they have been slowly eroding for over 15 million years. Some of the most famous landforms in AlUla, such as Elephant Rock, have been sculpted by the greatest artist of all; nature.

From within the Earth

There are some parts of AlUla that have dark rocks exposed, not the typical sandstones that are iconic to the region. Enormous areas covering hundreds of square kilometers, of flat black rocks. These are the remains of relatively recent lava flows. Between 10 million and just half a million years ago, AlUla would have been even hotter than it is today, as volcanoes erupted across the landscape. They were not the typical explosive type you might imagine when you think of volcanoes, like Mount Fuji or Mount St Helens, which form from the melting of the Earth’s crust as it is forced down into the Earth (called subduction).

The volcanoes in AlUla formed in a very different way. An enormous bubble of magma rose from deep inside the Earth. Sometimes these giant bubbles of molten rock do not reach the surface, and cool forming huge granite formations. When they do reach the surface, volcanoes form. And on Harrat Uwayrid, the largest nature reserve in AlUla, there are dozens of extinct volcanoes scattering the landscape. It shows that it must have been an incredibly active place, with many volcanoes active at once.

The black rocks that can be found on these volcanic fields (which are called harrats) are basalt. This is a very pure rock that has small crystals, and it was formed from hot magma from inside the Earth, not from other melted rock. Basalt is a lot harder than sandstone, and where it has erupted, it has covered the older sandstone and protected it from erosion

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