287-million-year-old granite containing microscopic crystals that act as atomic clocks. Helping scientists date when Asia's mountains rose.
The mountain's granite heart
The rock that forms Doi Suthep, a mountain overlooking Chiang Mai, is a uniform granite that first crystallized deep underground. While the hook specifies an age of 287 million years, geological studies often present a range. Research indicates the granite intrusion is part of the Central Granite Belt of Thailand, with U-Pb dating of related granites in the region pointing to formation during the Triassic period, between 221-210 million years ago. Some older studies place the granite in the Carboniferous period, around 330 million years ago. This magma cooled slowly beneath the surface, allowing for the formation of microscopic crystals.
The entire mountain mass was later pushed upwards as a result of the Himalayan Orogeny, the tectonic collision between the Indian and Eurasian plates. This large-scale geological event, which began around 50 million years ago, was responsible for lifting the granite from deep within the crust to its current elevation of 1,676 meters.
Zircon timekeepers
Embedded within the Doi Suthep granite are microscopic crystals of the mineral zircon (ZrSiO4). These crystals are exceptionally useful for geologists. When zircon crystallizes from magma, its structure readily incorporates uranium atoms but strongly rejects lead. This allows for a precise dating method.
The uranium isotopes trapped in the zircon begin to decay at a constant, known rate. Specifically, Uranium-238 (²³⁸U) decays into Lead-206 (²⁰⁶Pb) with a half-life of about 4.47 billion years. Scientists can measure the ratio of parent uranium atoms to daughter lead atoms within a single zircon crystal. This ratio is a highly accurate atomic clock, recording the time that has passed since the crystal solidified from magma. This process, known as Uranium-Lead (U-Pb) dating, is a reliable method for determining the age of ancient rocks.
By analyzing these zircon crystals, geologists can determine the crystallization age of the granite pluton itself. Further analysis using different techniques on the same crystals can reveal subsequent thermal events. For example, studies on zircon and monazite from Doi Suthep have identified tectonothermal events in the Triassic (221-210 Ma) but also in the Late Cretaceous (around 83-67 Ma) and the late Eocene-Oligocene (34-24 Ma), recording the complex history of the mountain's formation and uplift.
💡Fun Facts
Zircon crystals are so tough they can survive erosion and be recycled into younger sedimentary rocks, carrying their original age information with them.
Some of the oldest mineral fragments ever found on Earth are zircon crystals from the Jack Hills of Australia, dated to about 4.4 billion years old.
The Doi Suthep-Pui mountain range is part of the Thanon Thong Chai Range, which also includes Thailand's highest peak, Doi Inthanon.
The radioactive decay of elements like uranium within the Earth's crust and mantle is a significant source of the heat that helps drive plate tectonics.
The national park is generally accessible daily from 8:00 AM to 4:00 PM for services like campsites and visitor centers. The main temple on the mountain, Wat Phra That Doi Suthep, is typically open from 6:00 AM to 8:00 PM.
Admission
Entry to the national park to visit major waterfalls or use campsites costs 100 Baht for adult foreign visitors and 50 Baht for children. There is a separate entrance fee of 50 THB for foreigners to enter the Wat Phra That Doi Suthep temple area.
Accessibility
The terrain is mountainous. The main temple is reached by a steep, 306-step staircase, though a funicular elevator is also available for a small fee. Park trails can be steep and are best suited for those with good fitness.