Multi-Tiered Magmatic Plumbing System of the Tatun Volcano Group, Taiwan: Insights from Mineral Thermobarometry and Reservoir Evolution

Beneath the bustling streets of Taipei, a geological question has long lingered in the minds of scientists and the public alike: is the Tatun Volcano Group, a cluster of mountains just north of the city, truly dormant, or is it merely sleeping? These volcanoes are not the towering, snow-capped giants of the Pacific Rim that erupt with terrifying frequency; they are older, quieter, and their last known activity occurred thousands of years ago. Yet, their proximity to a metropolis of millions makes understanding their inner workings a matter of urgent safety. To answer whether these mountains could wake up, geologists must look inside the Earth's crust, where magma—the molten rock that fuels eruptions—waits in hidden chambers. This process is governed by a "plumbing system," a network of underground reservoirs and pathways where molten rock cools, mixes, and sometimes stalls before it can reach the surface. By studying the crystals trapped within ancient lava flows, scientists can reconstruct the temperature, pressure, and depth of these hidden chambers, effectively reading the history of the volcano's last breaths to predict its future behavior.

A team of researchers recently drilled deep into the heart of Mount Tatun to retrieve a core sample, a long cylinder of rock that preserves a vertical history of the volcano's activity. From this core, they extracted thirty-three samples of solidified lava, representing four distinct layers of ancient eruptions. Their goal was to examine the microscopic crystals within these rocks, specifically focusing on a mineral called amphibole. Amphibole is a key indicator because it only forms under specific conditions of heat and pressure deep underground. When magma rises too quickly or changes its chemical environment, these crystals begin to break down, forming distinct rims or changing their internal structure. By analyzing the size, shape, and chemical composition of these crystals and their rims, the researchers could determine exactly how deep the magma had sat, how fast it was moving, and what happened just before it erupted.

The study revealed that the Tatun Volcano Group operates through a complex, multi-level system rather than a single, simple chamber. The magma journey begins deep in the Earth's crust, where it cools slowly and forms crystals like orthopyroxene. As this molten rock rises, it stalls at various depths, creating a series of underground "waiting rooms." The researchers found evidence that magma stagnated at depths ranging from 4 kilometers to as deep as 22 kilometers. At these intermediate levels, the magma cooled further, allowing amphibole crystals to grow. However, the story does not end with a simple ascent. The data suggests that fresh, hot magma from deeper below frequently intruded into these stagnant, cooler pockets. This mixing of hot and cold magma acted like a wake-up call, reactivating the dormant magma and triggering the final journey to the surface.

Crucially, the speed of this final ascent was measured by looking at the edges of the amphibole crystals. When magma rises rapidly, the pressure drops, causing the amphibole to decompose and form a thin rim of new minerals. The thickness of these rims allowed the scientists to calculate that the magma moved upward at a very slow pace, traveling between 0.004 and 0.011 meters per second. To put this in perspective, this is a slow, steady crawl rather than a violent rush. Such a slow ascent rate explains why the Tatun volcanoes have historically produced gentle, flowing lava rather than explosive, catastrophic blasts. The magma had time to cool and release its gases gradually, resulting in effusive eruptions that spread across the landscape rather than blasting it apart.

The research also uncovered a significant change in the volcano's plumbing system over time. By comparing the depths where magma stalled in the past with modern seismic data, the team found that the main magma reservoir has retreated deeper underground. In the late Pleistocene epoch, the top of the magma reservoir sat at a depth of about 4 kilometers. Today, that ceiling has dropped to 8 kilometers, suggesting the volcano's magma supply has diminished and the reservoir has shrunk. This retreat is linked to a shift in the region's tectonic forces; the Earth's crust in northern Taiwan is no longer stretching and thinning as it did millions of years ago, which has likely reduced the upwelling of hot mantle material that feeds the volcano.

Despite this apparent decline in activity, the study highlights a lingering concern. While the main magma reservoir has moved deeper, a known hydrothermal reservoir—a pocket of hot water and steam—remains at a shallow depth of 2 to 4 kilometers. The researchers suggest that if fresh magma were to intrude into this shallow zone today, it could prime the system for a new eruption. The crystals in the core samples showed that amphibole decomposition often begins in these shallow, hot environments. Therefore, while the volcano may be less active than it was in the distant past, the potential for future activity remains. The most prudent path forward involves closely monitoring this shallow hydrothermal zone, watching for any signs that new magma is stirring the waters beneath the surface, ensuring that the city above remains safe from the slow, steady heartbeat of the Earth below.