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Report Field trip of Special Topics on Earth Science
Name : Esilestari Hutahaean
Title: Rocks Observation in Tian Xiang and Chishingtan Beach
Background of this field trip is to improve students understanding of geological
materials (in theory), which is about rocks and minerals. Thus, students are expected to know
how the physical forms of an outcrop, how characteristics of rocks and how rocks and minerals
process itself in nature. The basic theory given in lectures are generally easier to understand and
imagine. But the reality on the ground, is not as easy as the author imagined. Thus, we need a
further observation and directly concerns the appearance of the objects of geological rocks and
minerals in order to obtain an understanding of the expected. This observation can be done
directly through field trip.
The objective of field trip is to direct observation of the rocks and minerals at some
places in Tian Xiang and Chihsingtan beach. Students can conduct observation directly about the
appearance of rocks and minerals. Students are expected to understand the real situation on the
ground.
Location I:
Name: Tian Xiang
Time : 10.30 a.m
Melange zone is bodies of mixed rocks including blocks of different ages and origin,
commonly embedded in an argillitic, sandy, or ophiolitic matrix, or more rarely in a carbonatic,
evaporitic, or volcanic matrix. Reflecting the lack of internal continuity of strata and/or contacts
because of high stratal disruption. Melanges represent a significant component of many
collisional and accretionary type orogenic belts. Their internal fabric is a result of different
processes, such as tectonic disruption, mass transport, diapirism and fluid expulsion, glacial
shear and loading, and gas hydrate generation, and their interplay. These processes commonly
occur at shallow crustal depths in the presence of excessive interstitial fluid pressure, which
facilitates brittle and mesoscopically ductile deformation (Festa, A et al 2010). In our fieldtrip we
can see melange zone in Tian Xiang, like in the picture below.
Melange zone in Tien Xiang, Taiwan
In Tien Xiang we can see melange zone and commonly existing rocks are schist, marble,
and granite.
1. Schist
Schist in Tien Xiang
https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/schist.html
Texture
: foliated (foliation on mm to cm scale).
Grain size
: fine to medium grained; can often see crystals with the naked eye.
Hardness
: generally hard.
Colour
: variable (often alternating lighter and darker bands, often shiny).
Mineralogy
: mica minerals (biotite, chlorite, muscovite), quartz and plagioclase often present
as monomineralic bands, garnet porphyroblasts common.
Schist is a metamorphic rock, formed by the metamorphosis of mudstone / shale, or some
types of igneous rock, it has been subjected to higher temperatures and pressures.
Porphyroblasts are common in schist, and they provide information on the temperature and
pressure conditions under which the rock formed. Due to the more extreme formation conditions,
schist often shows complex folding patterns. There are many varieties of schist and they are
named for the dominant mineral comprising the rock, e.g. mica schist, green schist (green
because of high chlorite content), garnet schist etc (Auckland.ac.nz. 2005).
2. Marble
Marble in Tian Xiang
https://s3.amazonaws.com/usermedia.venngage.com/0a6195668d06c6829ff5730d619d67a5.jpg
Texture
: granular.
Grain size
: medium grained; can see interlocking calcite crystals with the naked eye.
Hardness
: hard, although component mineral is soft (calcite is 3 on Moh's scale of
hardness).
Colour
: variable - pure marble is white but marble exists in a wide variety of colours all
the way through to black .
Mineralogy
: calcite.
Marble is a metamorphic rock formed when limestone is exposed to high temperatures
and pressures. Marble forms under such conditions because the calcite forming the limestone
recrystallises forming a denser rock consisting of roughly equigranular calcite crystals. The
variety of colours exhibited by marble are a consequence of minor amounts of impurities being
incorporated with the calcite during metamorphism (Auckland.ac.nz. 2005).
3. Granite
Granite in Tien Xiang
https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/granite.html
Colour
: variable but typically light-coloured.
Texture
: phaneritic (medium to coarse grained).
Mineral
: orthoclase, plagioclase and quartz (generally more orthoclase than
plagioclase), often with smaller amounts of biotite, muscovite or amphibole
(hornblende).
Silica (SiO2) : 69%-77%.
Granite is a felsic, generally equigranular, relatively light coloured intrusive rock. It
comprises some of the oldest known rocks on Earth, and is the most abundant basement rock
underlying the relatively thin sedimentary rock cover of the continents. Granite is produced in
volcanic arcs, and more commonly in mountain building resulting from the collision of two
continental masses. The earliest continental masses were products of the accumulation of
volcanic arcs, and this is why granite lies in the cores of all of the continents. Granite is the
plutonic equivalent of rhyolite (Auckland.ac.nz. 2005).
Location II:
Name: Chishingtan beach
Time : 13.30 p.m
Kind of rock that we found at Chishingtan beach:
1. Amphibolite
https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewrecord.php?cID=4913
Amphibolite is a weakly foliated metamorphic rock dominated by amphibole and plagioclase
feldspar. Amphibole is often hornblende, but may be grunerite, actinolite or tremolite, rocks
dominated by glaucophane, crossite and reibeckite are usually blueschists rather than
amphibolites. Plagioclase is usually albite. Accessory minerals may include biotite, quartz,
garnet, calcite, wollastonite, andalusite, staurolite, kyanite, and/or sillimanite
(Imperial.ac.uk.2013).
2. Conglomerate
https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/conglomerate.html
Texture
: clastic (coarse-grained).
Grain size
: > 2mm; clasts easily visible to the naked eye, should be identifiable.
Hardness
: variable, soft to hard, dependent on clast composition and strength of cement.
Colour
: variable, dependent on clast and matrix composition.
Clasts
: variable, but generally harder rock types and / or minerals dominate.
Conglomerate is a sedimentary rock formed from rounded gravel and boulder sized clasts
cemented together in a matrix. The rounding of the clasts indicates that they have been
transported some distance from their original source (e.g. by a river or glacier), or that they have
resided in a high energy environment for some time (e.g. on a beach subject to wave action). The
cement that binds the clasts is generally one of either calcite, silica or iron oxide. The matrix can
consist solely of the cementing material, but may also contain sand and / or silt sized clasts
cemented together among the coarser clasts (Auckland.ac.nz. 2005).
Discussion:
Alluvial river is river in which the bed and banks are made up of mobile sediment
and/or soil. Alluvial rivers are self-formed, meaning that their channels are shaped by the
magnitude and frequency of the floods that they experience, and the ability of these floods to
erode, deposit, and transport sediment, so I think river in Tien Xiang classified into alluvial
channel, because consist from sediment, and the shape of the river change because frequently of
the water.
References:
Auckland.ac.nz. 2005. Auckland.ac.nz homepage <https://flexiblelearning.auckland.ac.nz/
rocks_minerals/rocks/index.html>. Accessed 7 June 2016.
Festa, A et al. 2010. Mélanges and mélange-forming processes: a historical overview
and new concepts. International Geology Review. 52: 1040–1105.
Imperial.ac.uk. 2013. Imperial.ac.uk homepage. <https://wwwf.imperial.ac.uk/earthscienceand
engineering/rocklibrary/viewglossrecord.php?gID=00000000122>. Accessed 7 June
2016.