Geological Aspects of Taiwan
Joost Kuckartz, National Taiwan University, November 2007
Taiwan is one of the best locations for
geological research, as it’s located near the edge
of the continental Eurasia plate and the oceanic
Philippine plate. As can be seen on the image on
the right, the Philippine plate is pushing the
Eurasia plate. However, south of Taiwan this
plate is moving on top of the Eurasian plate, and
in the north it’s moving under the Eurasian plate.
To understand more about what this means for
Taiwan, we first have to introduce the plate
tectonics.
Before the 20th century people thought the Earth consisted of fixed features. However, in
the beginning of the 20th century several scientific breakthroughs pointed in the direction
of the Earth consisting of moving parts. The evidence for the current plate tectonic theory
was formed in the 1960s. If you were to look at a map of the world and you would see the
western coastal line of Africa and the eastern coastal line of America, they look like they
would fit together. In fact, 250 million years ago, the earth consisted of only one
continent named Pangaea. The Earth’s tectonic plates have been moving since they’ve
been there.
The Earth basically consists of a core, a mantle
and a rigid crust. Interesting for plate tectonics
are only the top parts, called lithosphere and
asthenosphere. The lithosphere consists of the
crust and the uppermost part of the mantle, and
the asthenosphere is located in the mantle only,
positioned below the lithosphere. The image on
the left clarifies the composition of Earth’s
internal layers.
In the theory of plate tectonics, the asthenosphere is a layer with low viscosity, while the
lithosphere is a more rigid layer floating atop the asthenosphere. Because of heat transfer
from the internal earth to the outer layers, the lithosphere was able to start moving.
However, the lithosphere is not passively floating on top of the asthenosphere, because of
the different densities of the plates it’s also believed to have an influence in the tectonic
plate motion.
Because this movement continued for the past 250 million years (and also before),
collisions between plates took place (convergent boundaries). Also, plates move away
from each other (divergent boundaries) or move next to each other in different directions
(transform boundaries). These boundaries can be found everywhere in the world, and
they’re the main source for earthquakes and volcanoes. The image below contains each of
the mentioned boundaries.
The image shows a divergent boundary in the middle of the ocean, an example of this
boundary can be found in the middle of the Atlantic Ocean. A divergent boundary on
land is hardly found, only Iceland is known to be a
country with a divergent boundary. Also uncommon
are transform boundaries, however the San Andreas
Fault in California, USA is known to be a transform
boundary. The most common boundary is the
convergent boundary, and Taiwan is on such a
boundary as can be seen on the first image of this
document.
So to be able to explain Taiwan’s geological features, we need to understand what
happens at a convergent boundary. There are three possibilities of convergent boundaries:
a collision between two continental plates, two oceanic plates or a continental and an
oceanic plate. In all cases one of the plates will move below the other plate (the
subducting plate) and all will create mountain ranges, above or below sea. The following
images explain the formation of these ranges.
The mountain ranges are created because one plate pushes the other, crumbling the
lithosphere to form mountains. Often volcanic ranges (volcanic arcs) are also formed
because the lithosphere will enter deep into the asthenosphere where the rising
temperature causes water in the rocks to evaporate. This in case lowers the melting
temperature of the surrounding mantle so magma can rise to the surface.
All these properties of subducting plates can be found in
Taiwan as well. The mountain ranges are formed because the
Philippine plate is pushing the Eurasia plate. Taiwan
however is a special case, because the Philippine plate is
moving on top of the Eurasia plate but also moving below
the Eurasian plate in the north of Taiwan. Therefore some
volcanic activity is present throughout the island, but most of
it in the northern part, north of Taipei. You won’t be able to
find active volcanoes here, but hot springs are commonly
found.
Being on the edge of a tectonic plate means that earthquakes
are common. The results of the movement of the plates on
top and below each other can be found in the mountains
where layers of rock can be seen to have experienced sudden
shifts. The movement can also be experienced in everyday
life through the many earthquakes Taiwan has. As
destructive and unpredictable as they might be, they are the
main source for plate boundary research.
As can be seen on the map above, there are
many active fault lines in Taiwan. A fault
line is a location where evidence of the
relative movement of the earth can be found.
These fault lines are caused by earthquakes
in this region. An example of how
earthquakes help scientists in mapping fault
lines can be seen in the image on the right.
This information is taken from the seismic
information of the Chihshang earthquake at
the Coastal Range (east of Taiwan) in May
1995.
A famous example of fault lines is the Chi-Chi earthquake of September 21, 1999, where
a new fault line was created in the west of Taiwan. A magnitude 7.1 earthquake caused a
surface rupture over a range of 100 kilometer with a maximal horizontal and vertical
displacement of 10 meters. Geological survey in the mountains show that this happened
many more times, however this recent earthquake gave scientists much more data for
research. When such an event will occur again will remain a question. Earthquakes still
can’t be predicted.
Sources:
- Wikipedia: http://en.wikipedia.org
- Lecture of course “Exploring Taiwan, Geographic Environment and Resources” at the
National Taiwan University
- Joshua Barnes, Institute for Astronomy, University of Hawaii:
http://www.ifa.hawaii.edu/~barnes/ast110_06/tprai.html
- Judson L. Ahern, Department of Geophysics, University of Oklahoma:
http://geophysics.ou.edu/solid_earth/notes/seismology/seismo_interior/seismo_interior.ht
ml
- United States Geological Survey, “This Dynamic Earth, the Story of Plate Tectonics”:
http://pubs.usgs.gov/gip/dynamic/dynamic.html
- Department of Geological Sciences Research, Central Washington University:
http://www.geology.cwu.edu/facstaff/charlier/currentprojects/taiwan.html