A volcano beside Taiwan’s capital may look like one old mound. This study finds it is a layered record of separate eruptions, separated by roughly 170,000 years. In northern Taiwan, the Tatun Volcanic Group is a volcanic field that poses a direct threat to the Taipei metropolitan area.

Understanding when eruptions happened, how they behaved, and how long they lasted matters for judging future hazards. But the eruptive history remains difficult to understand because detailed geological maps are lacking, well-exposed volcanic sites are rare, and dating these relatively young rocks is challenging.

Its position beside Taipei, among several volcanic peaks and above a deep underground zone of partly melted rock, makes the hazard’s local setting easy to grasp. So the researchers treated Shamao as more than an isolated case. They aimed to understand how small volcanic domes live, erupt, and change over time.

The core idea was to connect the shape of the land with what the rocks preserve. In everyday life, it is like reconstructing a building’s history from both its floor plan and the layers inside its walls. That connection can help identify when these small, highly crystal-filled volcanic systems are prone to collapse and produce fast-moving currents of ash and broken rock, even where exposure is limited.

To map Shamao in detail, the researchers used an airborne laser survey that produced a bare-earth surface map. It revealed the volcanic landform beneath vegetation and other cover. They used shaded relief, contour lines, and slope views to reveal the volcanic edifice, landslides, craters, and flows.

A long underground core taken from the side of one depression became important evidence about its depositional and explosive history. The core contains no lava-flow intervals. Instead, it consists mainly of broken volcanic material extending well below the base of the nearby West Flow.

Differences in rock character, grain size, color, and weathering divide the core into two main layers. The age results support the sequence suggested by the land’s shape. The oldest feature, West Flow, erupted about 196,000 years ago. The West and East Domes formed much more recently, beginning about 24,000 years ago.

Shamao was therefore built during separate periods, not in one continuous event. The researchers favor an explanation in which West Flow, East Dome, and West Dome are separate batches of magma extracted and erupted from a partly solidified underground magma system at different times.

That model explains why the rocks differ slightly: each extraction came from a different part of the underground system, with different minerals and amounts of melt. It also fits the different eruption ages, which make a single shared batch of magma extremely unlikely.

The complex rock textures point to repeated mixing between magma and crystals as the system was replenished and tapped again. The overall reconstruction identifies two distinct eruptive episodes. The earlier episode produced the laterally spreading West Flow, while the later episode built a composite dome.

That later phase was punctuated by several major dome-collapse events, showing that the story includes not only construction, but repeated breakdown. The practical goal is to understand the hazards these volcanic complexes may pose, which nearby populations are exposed to them, and the risks to their social, economic, and political well-being.

This Shamao study is a first step toward understanding potential volcanic hazards, nearby populations’ exposure to them, and the risks those hazards pose to their socioeconomic and political well-being. Shamao’s shape records repeated magma arrivals and collapses, not one isolated event.

That kind of reconstruction can help show how volcanic hazards may affect people living near the Tatun volcanic field.