Geology of Taiwan

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Geology of Taiwan (short, easy version)

How Taiwan formed
- Taiwan is a young and active landmass created where the Philippine Sea Plate collides with the Eurasian Plate.
- The island sits at a complex boundary where subduction (one plate moving under another) has changed direction over time.
- Much of Taiwan’s crust is a mosaic of old pieces called terranes that were pushed together and uplifted by ongoing plate collision.

Terranes and major regions
- Terranes are chunks of crust that came from different places and combined to form Taiwan.
- The island has several key zones:
- Eastern stratigraphic region: the Coastal Range on the east, made of newer rocks (Neogene age) including melange (mixed rock) and volcaniclastics, plus various formations from volcanic material and sediments.
- Central Mountains: a tall belt of rocks with very old, metamorphosed rocks and complex ancient formations.
- Western stratigraphic region: western part has deformed and buried older rocks covered by newer sediment; not all older rocks are visible at the surface.
- Western piedmont region: flat, near-coastal areas with limestone, dolomite, and laterite (a rusty soil) forming in Holocene to Pleistocene times.
- Volcanic rocks region: northern Taiwan has a string of volcanoes and volcanic rocks; rocks also occur on Green Island, Orchid Island, and Penghu.

Central Mountains and older rocks
- The oldest exposed rocks are metamorphic and found mainly in the Central Mountains.
- Permian-age rocks may be the origin of some of these, now exposed as gneiss, schist, marble, slate, and related rocks.
- Notable units include the Danana’ao (Tananao) Group with distinctive schists and gneisses; other formations include Tailuge Marble, Kainanagang Gneiss, and various schists.
- The Yuli Schist (Cretaceous) and other belts show different metamorphic grades (greenschist to blueschist) and are important for understanding Taiwan’s deep past.
- In the west, some of these old rocks are buried under younger layers but influence the mountain geology and mineral deposits.

Eastern stratigraphic region and coastal rocks
- The Coastal Range on Taiwan’s east is the newer, active belt tied to the Luzon arc.
- Rocks here include melange, tuff, and other volcanic materials from Neogene time, with later formations like the Lichi Formation (melange) and other volcaniclastics.
- The Pinanshan and Milun Conglomerates (Pleistocene to older) are found in eastern fault-bounded areas and record past river and shore processes.

Western stratigraphic region and lava-centered rocks
- The west exposes older Cenozoic sediments that have been strongly folded and heated, with younger Quaternary plains on top.
- Deeper surface rocks are known from boreholes and include a mix of siltstone, sandstone, shale, and occasional coal seams.
- The region includes various formations such as Yunlin (Cretaceous) and Wangong (Paleocene volcaniclastics), with other units (Shuangji, Wuzhishan, Yeliu) showing a mix of volcanic and sedimentary rocks.
- In the south, ophiolite mélanges at Kending (Kent) record pieces of oceanic crust pushed onto land.

Western piedmont and coastal features
- The Hengchun Limestone and related near-coast sediments formed after major late-Pleistocene movements.
- Laterite soils and river terrace gravels develop on the western side, with Holocene mud, sand, and coral deposits forming a broad coastal belt.

Volcanic rocks and igneous rocks
- The Tatun Volcanic Group sits at Taiwan’s northern tip; Qixing Mountain is the highest peak for Taiwan’s volcanoes.
- Other volcanic rocks occur on Chilungshan, Green Island, Penghu, and Pengjia Island, typically as andesite, tuff, and breccia.
- Magmas largely come from subduction of the Philippine Sea Plate beneath the Ryukyu arc, with mantle melting producing the rock types found on Taiwan.
- NW Taiwan also hosts alkaline rocks (picrite, alkaline basalt, and Trachyandesite) created under tectonic tension on the continental margin.
- The volcanic rocks are rich in certain elements (potassium, strontium, rubidium, rare earth elements) and show complex mixing with crustal material.

Structures and tectonics
- Taiwan features several structural features like synclines (valleys framed by folds) and major faults (Hengchun Fault, Chaochow fault, and others) that outline the island’s deformation.
- The landscape shows the result of arc-continent collision, thrusting, andContinental crust being pushed up to form mountains.
- The region experiences frequent earthquakes; notable events include the 921 Chi-Chi earthquake (Chukou fault-related) and other major quakes in the past.
- The whole island sits in a high seismic hazard zone due to ongoing plate convergence and crustal deformation.

Offshore geology
- Offshore, features include the Kaoping Slope and Hengchun Ridge, the southern Longitudinal Trough, and the Huatung Ridge.
- The Luzon Arc partially appears at the surface around Lüdao and Orchid Island.
- The Ryukyu Trench to the east is a major plate boundary; the Taitung Canyon and nearby basins connect to larger oceanic structures.
- Undersea structures and canyons influence earthquake activity and submarine geology.

Geophysics and crust
- Taiwan shows strong gravity and magnetic signals tied to crustal structure and tectonic processes.
- The crust is about 30 km thick overall, thickening under the Central Range and thinning toward the coasts.
- Heat flow is particularly high west of the Longitudinal Valley, indicating active crustal processes.

Hydrology and rivers
- Many rocks in Taiwan have low porosity, so groundwater is limited in some areas.
- The country has hot springs, especially near the northern volcanic region (Beitou and Chingshui Geothermal area).
- Rivers carry large amounts of sediment to the sea; some rivers move tens of millions of tons per year, with the Zhuoshui and Peinan rivers noted for high sediment loads.

How scientists study it
- The first geological map of Taiwan appeared in the late 19th century, with subsequent maps over the 20th century showing more rocks and regions.
- Modern work combines field studies, boreholes, and geophysical data to map formations, ages, and tectonic history.

Key takeaways
- Taiwan is a dynamic collage of old and new rocks formed by complex plate interactions.
- The island’s mountains, coasts, volcanoes, and faults all reflect ongoing subduction, collision, and crustal deformation.
- Earthquakes are a natural part of Taiwan’s geology, and offshore processes continue to shape its landscape.