Back to School – AW#38

This month’s Accretionary Wedge topic is “Back to School”, hosted by Anne Jefferson.

Anne has asked us to share “about the things that we don’t often learn or teach in school but that may turn out to be quite important in the real world” as geology students / professors / professionals / enthusiasts.

I have got deep reflections on the topic, not only as a geologist practicing outside the academia, but also as a student who is still attending a post-grad school. There are so many things that we should have learnt, or qualities or personality to be developed. My boss always says that he wish his subordinates to “be passionate about the subject (geology)” and “be creative” or “love to do fieldwork” etc.. These are all true, but I would like to share here two things that, I believe, are important.

The first quality is daring to challenge to old school of thoughts. Students or scholars from the western world may find it quite common to challenge their bosses or supervisors. However, in the Chinese culture, we are not accustomed to dispute with someone who is more senior or more experienced, or to question ideas which were established by some old, emeritus  professors. Daring to challenge others, and be able to defend your own idea, to me, is a paramount important thing here.

I have learnt my lesson when I was doing my MPhil back in 2003. My research topic was a controversial issue over the origin of a rock unit. After three years of investigation, I came up with an interpretation which basically disagree with what a well-respected and experienced geologist has believed. So, I put my findings and interpretation in my thesis. But I was not brave enough at that time to publicly defend my own idea. Looking back, I regret this a lot. Because some unhappy events subsequently occurred, which could have been prevented.

The second important thing is “always be prepared to learn new thing“. To be honest, I always think that I have not learnt enough about geology. Sometimes, I just feel I know so little, that makes me very anxious. The reason is partly because geology is such a broad subject, and new ideas come up everyday. The truth is, we always have new things to learn in geology. This is why this subject is so exciting and fun. Passion is the driving force, but getting ready to be a life-long learner is also needed. This is also to do with getting ready to accept new challenges.  So, I have not stopped learning yet, not until the end of my day.

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Kinky Columns


I was inspired by Callan’s interesting post last week on the patterns of some cooling columns.  Callan has explained, using some nice illustrations, the concept that the cooling joints are formed perpendicular to the cooling front (or thermal gradient), and that some curved columns can be described as a result of corrugated cooling front.

But then, I asked myself, how can I explain the kinky columns found here in Hong Kong? Can they be explained by the presence of wavy cooling front as well?

The kinky columns are well exposed on an old quarry face near a reservoir in southeast Hong Kong. The entire rock face is over 30 m in height. The diameter of individual columns here is about 1.2 m. The entire unit has been tilted at 20-30 degrees to the east (as shown by the consistently inclined long-axis of the columns).

I focused on the kinky columns exposed on the middle part of the rock face, and tried to draw the possible cooling fronts. The pattern of the cooling fronts will be somewhat like this.

It seems that some cooling fronts are converging while others are diverging. Does it make sense? I cannot easily think of a reason for the waviness of the cooling fronts.

Another good exposure is found not far away. Similarly, I tried to mark the cooling front on the picture.

Again, the cooling fronts seems to be wavy. In the middle of the photos, the kinky columns are intruded by a mafic dyke. However, the mafic dyke does not seem to have affected the cooling pattern and the formation of the cooling joints. It is apparent that the intrusion post-dated the formation of cooling columns, although the time gap between these two events is uncertain.

So far so good, right?

But there is a major issue. If you look at the first picture again, you will see that the kinking is actually not restricted to a single kink-band. There are multiple kink-bands, occurring repeatedly and are sub-parallel to each other. They extends for several hundreds of metres, although no conjugate sets are found. This kind of kinky pattern is also seen at other parts of the same tuff unit.

So, if we are trying to explain the kink bands as the products of wavy cooling fronts, then this would suggest that the same wavy pattern of the cooling front should have repeated again and again at different position in the cooling unit. I doubt how this could have happened. Alternatively, the kink-bands could be the result of tectonic movement. However, as I have said, no conjugate sets are found. So, this explanation is questionable.

The one explanation we have used to use: the kink-bands could have formed by slump folding when the cooling unit was still “semi-solidified” and able to deform in a “plastic manner”.  The columns might have bent towards the same orientation during the tilting of cooling tuff unit, probably under its own weight, to form the kink-bands (as a kind of slump folding?). This seems to be a better explanation. However, one issue remains: how can the cooling columns (and the tuff unit) on one hand “deform in a plastic manner”, but on the other hand formed the kink-bands, which is essentially a brittle deformation??

Anyone have any clues?


Posted in Geology, Hong Kong | 1 Comment

My Favourite Geology Word AW#35

This is my first time joining the Accretionary Wedge Geoscience Blog Carnival. The current topic for AW#35 is “What is your favourite geology word?” (hosted by Georneys).

My favourite geology word is this one — MYLONITE

Definition: A fault rock which is cohesive and characterised by a well developed schistosity resulting from tectonic reduction of grain size, and commonly containing rounded porphyroclasts and lithic fragments of similar compositions to minerals in the matrix. Fine scale layering and an associated mineral of stretching lineation are commonly present. Brittle deformation of some minerals may be present, but deformation is commonly by crystal plasticity. (source)

Back in 2000, when I was just starting my career as a geologist, I was assigned to carry out a petrographic study of dynamically metamorphosed rocks in northwest Hong Kong. This was when I first got to understand this word – MYLONITE.  In Chinese, the word is translated to 糜稜岩 (Míléngyáng). I actually think that this is the most ‘beautifully’ translated geology word in Chinese.

I have some pictures of mylonite from Hong Kong.

This is a mylonite outcrop. The protolith (parent rock before deformation) is a pebbly quartz sandstone. Several years ago, I wrote a short ‘poem’ describing this outcrop.



(Even strong rock can be twisted and deformed.)

The dynamic metamorphic zone in northwest Hong Kong is quite wide. Therefore, the metamorphism has affected not only the sedimentary rocks, but also some granites and volcanic rocks in the region.

This is a hand specimen of mylonitic granite. The porphyroclasts of quartz set in very fine-grained, dynamically recrystallised matrix, showing clear foliation.

This is a mylonitic volcanic rocks under microscope.

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Finding Mt Doom (Mt Ngauruhoe)

The best part of studying in New Zealand is that the geology here is fascinating. And I have got a chance to do a field trip to Tongariro National Park this year!

There are three active volcanoes in the Tongariro National Park: Mt Tongariro (1967m), Mt Ngauruhoe (2287m) and Mt Ruapehu (Tahurangi, 2797m). I can’t tell you how much I love these mountains (volcanoes), they are just beautiful.

Mt Ngauruhoe (2287m)

Mt Ngauruhoe is a classic stratovolcano (or cone volcano). It has been built up by many phases of pre-historic and historic eruptions, spilling out lavas, volcanic bombs as well as pyroclastic flows. The latest eruption occurred in 1975, during which block-and-ash pyroclastic flows came down on the northern flank of the volcano. The dark grey, rubble-like rocks (in the front part of the picture) are one of these lava flows. You can probably see and identify some younger lava flows on the slopes as well.  These lavas and pyroclastic deposits are mostly andesitic in composition.

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Where exactly is Hong Kong – geologically speaking?

You may have heard that Hong Kong is a tiny but busy city somewhere in southeast China (well, some people do mistakenly think that Hong Kong is a city in Japan…).  Some of you may have visited Hong Kong as a tourist, or have briefly transited via the international airport.

So, where exactly is Hong Kong? Geologically speaking?

Hong Kong is located at the month of the Pearl River, which is the third longest river system in China and crosses much area of Guangdong and Guangxi provinces.  The Pearl River drains into South China Sea, which is to our south and southeast. We are now located at the passive continental margin in the southeast side of the Eurasian landmass. Pretty stable and free of active volcanoes or strong earthquakes. However, in the geological past, we did underwent a period of time when things were much more…violent.

Geologically speaking, the southeast China consists of a large crustal block, known as the South China Block (some people call it South China Sub-plate). The South China Block has been present for a long period of time. Some of the basement rocks are Archean in age, i.e. over 2.5 billion years old! Before the start of Palaeozoic era (some 550 million years ago), the South China Block was part of the Gondwana supercontinent. It broke up from the great landmass and started drifting northward in the early Palaeozoic, together with many other crustal blocks. Finally, the South China Block collided with its brother, the North China Block in the late Palaeozoic, and joined the family of the Eurasian Plate.

We do not have any early Palaeozoic rocks exposed in Hong Kong. The oldest rocks that are now present here is Devonian in age (about 410 million years old). The tectonic environment here was relatively stable in southeast China for most of the late Palaeozoic.

At the start of Jurassic Period (or perhaps earlier), an active plate boundary was formed in southeastern margin of the South China Block, where the palaeo-Pacific Plate converged and dived under the Eurasian Plate. Many active volcanoes were present at that time. Probably because of some unusual configurations of this subduction zone, a 1200 km wide igneous province (mostly granitoids and associated volcanic rocks) was formed in southeast China. The age of these rocks span from some 200 to 90 million years old.

About 80% of the exposed rocks in Hong Kong are igneous rocks that formed during Middle Jurassic to Early Cretaceous (between 180 to 140 million years ago). For examples, the prominent landmark of Hong Kong — the Lion Rock, is made of the 140 million year old granite, which have exhumed from the deep ground.  The highest Peak, Tai Mo Shan (957 m), consists volcanic rocks of 164 million years old.

After the cessation of volcanic activities, the land began to subside, and basins were formed. It has been said that there are more than 100 pull apart basins identified in Guangdong Province. These basins were then filled up by Cretaceous or younger sediments, some have preserved numerous dinosaur eggs and some are oil-bearing.

I have given here a very brief description of the geological history of Hong Kong. Stay tuned, more is coming.

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My Blog

Well, blogging is new for me, but I am ready to give it a try. I presume most of my blog posts to come will be around geological topics, or my life as a geologist. Some posts may be about … Continue reading

Gallery | 4 Comments