The Prophet Muhammad‘s ﷺ migration from Makkah to Madinah passed through dozens of mountains. Yet only one became inseparably linked to one of the defining moments in Islamic history.
Mount Thawr is home to the cave where the Prophet ﷺ and his companion, Abu Bakr al Siddiq, remained for three days while evading their pursuers during the Hijrah. Today, the mountain stands as one of Islam’s most significant historical landmarks.
Beyond its religious importance, however, Mount Thawr also preserves a geological history stretching back hundreds of millions of years.
According to American geologist Dr Thomas Winslow Sisson of the United States Geological Survey’s Volcano Science Centre, the mountain’s granite rocks may have begun forming deep beneath the Earth’s surface around 800 million years ago.
Granite Forged Deep Underground
Dr Sisson explains that Mount Thawr consists primarily of hard granite, an igneous rock that did not form at the Earth’s surface.
Instead, it originated as molten magma deep within the Earth’s crust. Over time, this magma intruded between underground rock layers before cooling and solidifying extremely slowly beneath the surface. Millions of years later, tectonic activity uplifted these rocks while erosion gradually exposed them, ultimately forming the mountain visible today.
Historical geological studies of the Makkah region indicate that this magma did not cool during a single event.
Instead, it crystallised during two major geological phases separated by a long interval.
The older phase dates to between 824 and 802 million years ago, while the younger geological episode occurred approximately 664 million years ago.
Scientists determine these ages using radiometric dating, a natural geological clock contained within rocks.
Granite contains tiny amounts of naturally occurring radioactive elements, such as uranium, which decay at a constant rate into stable elements like lead. By measuring the products of this decay, geologists can calculate a rock’s age with remarkable precision.
According to Dr Sisson, the uranium-lead dating method performed on zircon crystals offers the highest level of accuracy and is regarded as the international standard for dating granite.
Zircon crystals are exceptionally durable, preserving their radioactive records over immense geological timescales with little interference from external conditions.
Although scientists already have reliable estimates for the age of rocks across the wider Makkah region, Dr Sisson says Mount Thawr’s granite itself could still be analysed using this technique to establish its precise age.
Ancient Rocks, Younger Mountains
Dr Sisson distinguishes between the age of the mountain’s rocks and the age of the mountain itself.
While the granite that forms Mount Thawr and much of the Arabian Shield solidified beneath the Earth’s surface more than 800 million years ago, these rocks remained buried for an extended period.
The mountain itself only emerged much later, during the Miocene Epoch.
According to Dr Sisson, Mount Thawr’s formation is closely linked to one of the region’s most significant tectonic events: the opening of the Red Sea.
Around 25 million years ago, the Arabian Peninsula began separating from the African continent, creating the Red Sea.
This tectonic movement initiated the gradual uplift of western Saudi Arabia.
Between 14 and 10 million years ago, the region experienced a particularly intense period of uplift, raising the mountains of Makkah and the Hijaz to the elevations seen today.
Unlike famous mountain ranges such as the Himalayas, which formed through the collision and compression of tectonic plates, the mountains around Makkah developed through an almost opposite process.
During the formation of the Red Sea, the Earth lost a substantial portion of the dense, heavy rocks beneath the crust, known as the mantle lithosphere.
At the same time, intense heat rising from deep within the Earth caused the crust beneath the Hijaz region to expand and become less dense.
As its density decreased, the crust became more buoyant, gradually rising above the partially molten mantle beneath it and lifting the mountains with it.
Dr Sisson compares this process to the uplift occurring today around the Afar Depression within Ethiopia’s Great Rift Valley, where the Earth’s crust is being pulled apart and internal heat is raising the surrounding highlands through the same geological mechanism.
How the Cave of Thawr Was Formed
Dr Sisson also offers a geological explanation for the formation of the Cave of Thawr itself.
Based on photographs of the mountain and its summit, he says the granite rocks have gradually lost their sharp edges through a long process known as weathering.
Over many centuries, wind, along with repeated temperature changes between day and night, slowly eroded the edges of the rocks.
This gradual process smoothed them into enormous rounded granite boulders, known scientifically as spheroidal boulders.
The next stage occurred when gravity and natural ground movement caused some of these massive, rounded boulders to roll down the mountain slopes.
One particularly large boulder eventually came to rest across a wide natural fracture between two rock masses, forming what effectively became a natural roof.
The protected space left beneath this enormous rock created the geological formation now recognised as the Cave of Thawr.
Three Geological Records Hidden Within the Mountain
According to Dr Sisson, Mount Thawr is far more than a landmark of Islamic history.
He describes it as a geological archive preserving three distinct chapters of the Earth’s ancient past.
The first record reveals the precise moment when molten magma cooled and solidified underground, marking the birth of the mountain’s granite.
This information can be obtained using uranium-lead dating on microscopic zircon crystals found within the granite.
The second record identifies when those rocks were finally uplifted to the Earth’s surface.
Deep underground, rocks remain extremely hot. As tectonic forces and erosion gradually expose them, they cool over time.
Scientists can determine this uplift history using methods such as apatite fission track dating, which examines microscopic damage left by radioactive particles inside apatite crystals, or uranium-thorium-helium dating performed on zircon.
The third record reveals how long the mountain has remained exposed at the surface.
Once rocks are fully uncovered, they begin interacting with cosmic rays, high energy particles arriving continuously from outer space.
These particles penetrate only the upper few centimetres of exposed rock, creating unique radioactive isotopes.
By measuring these isotopes in laboratory samples, scientists can calculate how long the rocks have been exposed to the atmosphere.
Taken together, these three geological records transform Mount Thawr from a site of immense religious and historical significance into a remarkable natural archive preserving extraordinary chapters of Earth’s geological history.
