Why the Strait of Hormuz is a geological wonder

About a quarter of the world’s seaborne oil trade passes through the Strait of Hormuz, the roughly 30-mile-wide waterway between the Persian Gulf and the Gulf of Oman. This narrow passage is one of the world’s most prominent maritime “chokepoints,” high-volume shipping routes with the power to wreak havoc on the global economy if disrupted—which has been the case since late February due to conflict in the Middle East.

It’s also a geological marvel as one of the places on Earth where you can see the collision of two continents, says Mike Searle, professor of Earth Sciences at Oxford University’s Worcester College.

That evidence is visible in the geology across the region—from the Zagros Mountains of southern Iran to the skinniest point of the Strait of Hormuz, where the Musandam Peninsula of Oman juts north like a dagger toward Iran.

Remarkable for its steep, black rock cliffs and jagged coastline of drowned valleys, a type of estuary that forms when rising sea levels flood river valleys, the Musandam Peninsula is also one of the rare places on Earth where ophiolite—rocks that are typically buried deep under the ocean crust—are “absolutely spectacularly well exposed,” Searle says. "It's by far the biggest, best ophiolite complex anywhere in the world."

Yet the same geologic process that made the Strait of Hormuz so unique is also what leaves it so vulnerable.

How the Strait of Hormuz formed

Many of the world’s shipping chokepoints are straits, defined as a narrow body of water that connects two larger bodies of water. These have been formed naturally over millions of years through the shifting of tectonic plates and rising sea levels with the melting of glacial ice sheets. Seafarers have taken advantage of these geographical shortcuts for centuries as they typically are faster than navigating around continents on the open ocean.

(The Bab el Mandeb is another strait with an outsized role in history.)

For the Strait of Hormuz, this process began roughly 35 million years ago with the collision of two continental plates: the Arabian plate to the south and the Eurasian plate to the north.

At the time, these continents were separated by the ancient ocean Tethys, named for the mythological Greek Titan of the sea. The Arabian plate began to push northward under the Eurasian plate, a process called subduction, ultimately consuming Tethys as the two continental plates and the landmasses above them knitted together, says Mark Allen, head of the Earth Sciences department at Durham University in the U.K. who has studied the Arabia-Eurasia collision.

“And the neat thing about continental collisions is they are not over in an instant,” Allen says. “The deep forces that make the plates move can still operate after you’ve brought these continents together for tens of millions of years.”

As the Arabian plate continued to bulldoze its way under Eurasia, Allen says, both plates eventually began to shorten and thicken “just like if you crash two cars together.” As these two plates crumpled together, they created the present-day Zagros Mountains in Iran.

(The lost continent of Zealandia has been mapped for the first time.)

This tectonic movement also created the conditions for the Strait of Hormuz. Think of the Arabian plate like a bendy ruler, Allen says. When you place something heavy on one end of it—like a mountain range—the ruler begins to dip down and create a depression. In the case of the Arabian plate, this depression formed the Persian Gulf and the Strait of Hormuz.

That’s where rising sea levels come in. Around 20,000 years ago during the Last Glacial Maximum, the water in the Persian Gulf was so shallow you could have walked across it in certain points, Allen says. But as ice sheets began to melt, global sea levels rose substantially—about a hundred meters in 15,000 years. (“Pretty rapid to a geologist,” Allen says.) Over time, this brought water to the eastern coast of what is now Iraq and flooded the Persian Gulf. At one point water from the Tigris and Euphrates rivers also filled in the Strait of Hormuz.

A valuable—and vulnerable—landscape

If you look around the Strait of Hormuz, or wander through the landscapes directly on either side of it, you’ll see the traces of the continental collision in stunning ways.

To the north, the tectonic forces that have been shortening and thickening the crusts of the colliding plates have “created the most spectacular landscapes in the Zagros Mountains of southern Iran,” Allen says. The mountain range is layered with sedimentary rock including sandstone, shale, and limestone. The latter is particularly strong and resistant to erosion, Allen says, meaning you can walk on a single bed of limestone for many kilometers.

(Rare chunks of Earth's mantle found exposed in Maryland.)

“Zagros has long been seen as a paradise for structural geologists, the kind of geologists who are interested in how and why rocks form, because you can walk over these huge structures and study them from satellite images,” he says. “It's about as striking a geological landscape as you can find on Earth.”

The region is also known for its salt glaciers and salt domes created as the salt from deep within the Earth was pushed up through the folds created by the continental collision. “In places it literally flows down the hillsides like a rock glacier,” Allen says.

To the south, the Musandam Peninsula is part of the Al Hajar Mountains in Oman that run all the way down the northeast coast of Arabia. Those mountains are mostly composed of ophiolite, Searle says, a slab of the Tethyan oceanic crust and mantle that was thrust onto Arabia during a continental collision between 95 and 60 million years ago during the late Cretaceous. The same geologic forces that created the Strait of Hormuz also tilted the Musandam Peninsula eastward to the point where it pinches the Strait.

Most notably, this continental collision also gave the region its staggering oil reserves.

For hundreds of millions of years before the Arabian plate collided with Eurasia, it was sitting at just below sea level, Allen says, accumulating all the kinds of rocks needed to form oil and gas reserves. Over time, the collision of the plates trapped these pockets of oil and gas under the rocks in the northern end of the Arabian plate—which now sits underneath Iran, Iraq, and parts of Syria.

“What is distinctive about the Middle East is the sheer scale of it,” Allen says. “All of this happened over a vast area, over a long period of time, with traps that are huge so that economically you're not spending all your money drilling into a reservoir that's going to drain dry in a few years, they've got fields that last for many decades.”

Transporting this oil and gas to the rest of the world, however, requires first passing through the Strait of Hormuz—and its pinch point carved out by the Musandam Peninsula.

The Musandam Peninsula is still actively on the move, albeit at a geological pace. Searle points to a paper he and colleagues published in 2014 showing that the Musandam Peninsula is still thrusting northward toward the Zagros Mountains.

“And the Strait of Hormuz will gradually close,” Searle says—although that’s not likely to happen for at least another 10 million years.