Under Taipei’s skyline sits a volcanic complex that looked, at first glance, like one slump. The evidence instead reveals multiple dome-building episodes, repeated collapses, and eruptions separated by roughly 170,000 years. The Tatun Volcanic Group looms over Taiwan’s metropolitan capital region, but the threats it poses are only vaguely known.

Shamao is investigated as one of the youngest and smallest features of that group. A high-resolution, LiDAR-based Digital Terrain Model, supplemented by sparse outcrops and a drill core, reveals an episodic, multi-event pyroclastic density current succession, particularly block-and-ash flows, rather than a single slump event.

That evidence identifies overlapping West and East Domes and an older West Flow extending about 650 meters southwestward from the domes’ flank. Volcanic dome growth is highly variable, and structural instability during extrusion can trigger dangerous density pyroclastic currents, especially block-and-ash flows that can be deadly and difficult to predict.

Observations from dome-forming eruptions over the past several decades have significantly advanced understanding of the dome-forming process. The Tatun Volcanic Group is a Quaternary volcanic field in northern Taiwan that poses a direct threat to the Taipei metropolitan area. Understanding eruption durations, styles, and the timing of recent activity is crucial for evaluating future hazards.

That broader history remains limited by a lack of high-resolution geological maps, rare well-exposed volcanological outcrops, and challenges in dating young, intermediate-compositional volcanic rocks. In the main map, the yellow Shamao complex sits on the southwestern edge of the pink Qixing subgroup, beside the Taipei City area.

The blue dashed anomaly forms a broad arc around the volcanic field, while the numbered crater symbols show the other major edifices distributed across the surrounding terrain. The Shamao reconstruction is intended as a foundational step toward a systematic petrological and hazard framework for the Tatun Volcanic Group, not merely as a localized case report.

The study addresses the lifecycles, eruption periodicities, and rheological controls of small-volume volcanic domes. Its integrated approach combines high-resolution LiDAR topography, lithofacies from surface outcrops and subsurface core, precise geochronology, and petrological analyses.

The approach links crystallinity and viscosity with geomorphological outcomes to reconstruct the physical and magmatic evolution of Shamao. A detailed geomorphological map used a bare-earth Digital Terrain Model produced from a two-by-two-meter airborne LiDAR grid by Taiwan’s Geological Survey and Mining Management Agency.

Hillshade, contour, and slope functions in ESRI ArcGIS version 10.9 revealed details of the volcanic edifice, landslides, craters, and flows. The investigation examined outcrops, one deep drill core from the Geological Survey and Mining Management Agency, and several unpublished drill cores.

Eleven samples were collected during three field excursions between April 2022 and January 2024. Field observations and drill-core interpretation examined relationships with surrounding rocks, flow or bed form, and thickness. For lava-flow outcrops, special attention went to cooling and emplacement structures such as joints, vesicles, chilling, and chilled margins.

Volcanic clastic material was evaluated through composition, grain size, roundness, sphericity, and sorting. Samples were selected to be fresh, dense, and vesicle-free to minimize alteration and weathering effects, and argon-argon analyses were performed at the WiscAr Laboratory at the University of Wisconsin-Madison.

The samples were crushed, sieved, ultrasonically leached in three molar hydrochloric acid for fifteen to thirty minutes, and repeatedly rinsed with deionized water. Phenocrysts were removed from groundmass separates by magnetic sorting and hand-picking before the purified separates were packaged and irradiated for two hours.

The one-point-one-eight-six-four million-year Alder Creek sanidine was used as a neutron fluence monitor. Shamao lies in a humid, subtropical environment with intense chemical weathering, dense vegetation, and extensive urbanization. Naturally exposed outcrops are exceptionally rare, making conventional field-based stratigraphic correlation difficult.

The sequence analysis therefore relies primarily on high-resolution LiDAR geomorphology, supplemented by lithological data from limited outcrops and subsurface drill cores. Shamao is a young andesitic volcano within the twenty-five-kilometer-wide Tatun Volcanic Group.

The complex comprises three principal landforms: East Dome, West Dome, and West Flow. East and West Domes form the edifice of Mount Shamao, while West Flow extends about 500 meters westward from the mountain’s southwestern base.

A 120-meter-long core called BH15, retrieved from the side of one depression, was critical for interpreting the depositional and explosive history of those depressions. BH15 contains no intervals of lava flows; instead, it consists mainly of brecciated volcanic clastic material extending below the basal elevation of the adjacent West Flow.

Lithological characteristics, grain-size distributions, matrix coloration, and prominent weathering horizons divide the core into Unit 1 from zero to 100 meters and Unit 2 from 100 to 120 meters. In the left-hand column, the two wavy weathering markers occur near the base of the long upper interval, separating otherwise similar breccia packages.

The core photographs show a visible contrast: the upper rows contain larger, more intact grayish blocks, whereas the lowest rows are darker and dominated by smaller, heterogeneous fragments. Shallow slopes around East Dome’s summits suggest rubbly surfaces where breccia thinly covers lavas that formed in place.

The outer flanks of the coarse clastic slope are stabilized near the angle of repose. East Dome is therefore interpreted as a compound structure of two domes capped and flanked by block-and-ash flows shed continuously during dome emergence and collapse.

The cross-section shows West Dome as a steep, blunt-peaked body perched on the collapsed northern flank of the broader East Dome. Beneath both domes, dashed conduits descend to the basement, while the West Flow forms a shallow, roughly seven-degree sloping apron toward the southwest and the phreatic crater occupies the intervening depression.

The pervasive rubble cover and smooth, intermediate-angle slopes of thirty to forty-five degrees imply that West Dome’s flanks are not merely rock-fall accumulations. Instead, the surface is interpreted as an extensive composite apron formed by frequent, small-scale block-and-ash flows generated continuously during exogenous ascent.

Shallow summit slopes and massive, polygonally jointed boulders indicate that the uppermost blocks were emplaced while hot and moved little after cooling. West Dome is consequently interpreted as a central lava core mantled near its summit by hot-emplaced auto-breccia and flanked by dome-collapse pyroclastic-density-current deposits.

The studied samples share three common petrographic features, beginning with a general lack of vesicles, implying that the source magmas were either fluid-undersaturated or mostly degassed. The samples also have high crystallinity, and the study uses crystals for crystalline phases much coarser than the volcanic groundmass, excluding groundmass microlites.

The crystals likely include a mixture of phenocrysts and antecrysts, which are recycled crystals in a magmatic system. Plagioclase is the most abundant mineral, while clinopyroxene, orthopyroxene, hornblende, and iron-titanium oxides are less abundant. All studied samples are porphyritic, crystal-rich volcanic rocks with plagioclase as the dominant mineral.

Crystal content is nearly fifty percent in West Flow and sixty to seventy percent in West and East Domes. These highly crystallized magmas are highly viscous and suitable for forming the West Flow and the East and West Domes. The three principal features nevertheless show notable textural differences: West Flow contains two pyroxenes and no hornblende, East Dome contains two hornblende generations, and West Dome contains little clinopyroxene and one fine-grained hornblende generation.

The study summarizes argon-argon dating results in Table 3, with complete data provided in an online resource. Sample SHM25-A-1 comes from a roadcut into West Flow. Argon-argon dating of two of the three principal features supports the temporal relationships derived from topographic observations.

West Flow is the oldest feature, erupting around 196 thousand years ago, whereas the West and East Domes formed much more recently, beginning around 24 thousand years ago. Figure ten shows laser incremental-heating results for two Shamao samples: the West Flow sample, SHM24-A-1, and the East Dome sample, SHM24-B-1.

The age spectra define plateaus of one hundred ninety-six point four plus or minus two point three ka and twenty-four point two plus or minus one point eight ka, respectively, while the inverse isochrons provide corresponding age estimates. These results support the authors’ interpretation that the West Flow is much older, whereas the domes formed more recently.

Geochemical data from Shamao are shown on total alkali-silica and zircon-to-titanium-oxide over niobium-to-yttrium diagrams. On the total alkali-silica diagram, the studied samples are classified as basaltic andesite to andesite. The zircon-to-titanium-oxide over niobium-to-yttrium diagram further confirms their sub-alkaline affinity and uses immobile trace elements to minimize the influence of diverse crystal populations.

Two West Flow samples remain interesting because they plot close to the boundary between sub-alkaline and alkali basalts on that diagram despite their sub-alkaline character on the total alkali-silica diagram. Figure twelve uses Harker diagrams to plot magnesium oxide, aluminum oxide, total iron, calcium oxide, titanium oxide, and potassium oxide against silicon dioxide, comparing samples from the Shamao and Qixing areas and different sampling studies.

Most panels show an overall decrease as silicon dioxide increases, but the points are scattered rather than smoothly aligned; calcium oxide in the West Flow samples is specifically noted to rise locally. The potassium oxide pattern is important because it differs from the trend expected from simple crystal fractionation, arguing against a single straightforward fractionation series.

The overall Shamao features are not fundamentally different from those formed during modern dome-building episodes, where dome construction involves transitions between endogenous and exogenous growth. Endogenous growth involves internal magma intrusion and lateral inflation that forms a massive basal core, but continuous swelling can oversteepen the outer carapace and trigger gravitational collapses and pyroclastic-density currents.

Exogenous growth involves highly viscous magma breaching the surface as steep, blocky spines or lobes that frequently undergo brittle failure and shed auto-brecciated talus and block-and-ash flows. At Shamao, intact lavas near the dome base likely preserve early endogenous inflation, while steep summits and extensive pyroclastic-density-current successions suggest later exogenous spine extrusion drove vertical growth and catastrophic collapses.

The simplicity and size of West and East Domes imply that they formed in less than one to several years, although the similarly wide but twice-as-tall Mount St. Helens dome formed over several years. The volume of Mount Shamao is approximately zero point sixteen cubic kilometers.

Cross-sections suggest East Dome accounts for roughly seventy percent, or about zero point one one two cubic kilometers, and West Dome for thirty percent, or about zero point zero four eight cubic kilometers, of that total. One proposed cause of East Dome’s collapse is that upwelling West-Dome magma destabilized its flank, after which West Dome emerged through the collapsed portions of East Dome.

A second proposed cause is continuous exogenous growth in southern East Dome, which produced gravitational instabilities and spontaneous collapse of its upper parts. Both possibilities represent common internal volcanic processes frequently observed during modern dome extrusion.

Figure seven presents a southwest–northeast cross-section through the Shamao complex, separating the present East Dome, the collapsed East Dome, and the West Dome above the pre-Shamao surface. It proposes two phreatic craters: a visible, post-East Dome crater, and a hidden crater intersected by borehole BH15 that predates the East Dome’s collapse.

This matters because it provides a relative sequence linking explosive crater formation to dome growth and collapse. The favored explanation is that West Flow, East Dome, and West Dome each represent a batch of magma extracted and erupted from a mush system at different times.

Different extraction events could produce geochemical differences because they occurred in parts of the mush with different mineralogical combinations and melt fractions. The model also better explains the geochronological data: different eruptive ages between West Flow and the Domes make a relation to one common magma extremely unlikely.

Textural complexity points to magma-magma and magma-crystal mixing, processes favored when a mush system undergoes multiple events of magma recharge and extraction. In the central reservoir, the color scale shows melt fraction changing from near zero at the blue base to near one in the yellow upper layers, with alternating bands and lens-shaped zones rather than a uniform body.

Three separate feeder dykes rise from this mid-crustal region toward the surface, while a deeper, dashed-outlined source is shown below roughly thirty kilometers. The practical goal of this study and ongoing work on the thirty volcanic complexes of the Tatun Volcanic Group is to understand potential hazards, nearby population exposure, and risks to socioeconomic and political well-being.

The Shamao study is the first attempt toward that goal. Shamao was built in stages: an older West Flow erupted around 196 thousand years ago, while the West and East Domes formed much later, around 24 thousand years ago. That history shows why small volcanic systems still matter for hazard assessment.