VR 360 Geology Field Trip - Lai Chi Chong
Good morning everyone! Today we're going to Lai Chi Chong Geological Park. The Lai Chi Chong Geological Park is on the south east of the Tolo Channel, so this is in the north part of Sai Kung. So this location has a unique rock formation during the volcanic activities about 146 million years ago. So this is so-called Hoi Ha Road, so we are going to start from this location and hike 2.8 kilometres to the site. So in order to get here, we can start from HKUST and take the bus or take the taxi, and ride to here, and then we go on the hiking to the site location for the visit.
So 2.8 kilometres to go. Are you ready? (Ready.) Let's go! This is the youth hotel, very good for the travellers to stay here. After 50 minutes of hiking, we arrived the Lai Chi Chong. So the site is in front of us, only several hundred meters to go. Okay, the last few meters. Let's go!
Finally we arrived. This is Lai Chi Chong Pier, so people can take a boat to visit this location. Today we chose to hike through the Hoi Ha Road.
So there's also a ferry actually come on and go for the Lai Chi Chong. All right. We arrived to the Lai Chi Chong Geological Park. The park actually is in the south east of the Tolo Channel. We can see the Lai Chi Chong Pier. The rock formation is mainly starting from this location all the way to the west for about 150 meters. There's a variety of different sedimentary rocks originated from volcanic eruptions.
Siltstone, tuffaceous siltstone, sandstone, mudstone and also coarse ash, fine ash volcanic tuff. So we can just start our trip here. At this location you can see weathered coarse ash volcanic tuff. You can see the very variable grain size of different size within the fine matrix. So this particle is relatively large, the mineral composition are mainly feldspar and quartz as we expected before, because this location is quite close to the volcanic source, therefore relatively coarse ash volcanic tuff is formed here. So because volcanic ashes are blown by the wind, large particles will be accumulated close to the source, and that's why we have coarse ash volcanic tuff here.
During the time it was weathered, you can clearly see the weathering started from the joint and some are weathered into relatively weaker materials already, not strong enough. The whole Sai Kung area if you look around, we have a lot of volcanic tuff around us, which was formed during the volcanic period. Let's go along the site and to have a more exposure. You can see here on the ground, the weathering started from the joints already. Very nice to see the watermark here.
Here we can see the site map. We are standing here close to the pier, we go all the way to the west part, there's a 150 meters of the rock formation which is related to the volcanic activities. Here is a very nice illustration about the formation of this type of rock. In this site, we can see tuff, tuffite, mudstone and siltstone. They are kind of formed during the volcanic eruption period. And here is a nice illustration to
see the formation. So the volcanic activities, they were intermittent during 146 million years ago. So volcanic eruption blow up to a lot of volcanic ashes, rock fragments. They erupted and deposited in the lake environment.
So the river water kept bringing the volcanic ashes and also silt into the lake, where there was pressure around it, actually compressed them into the rock. And also this location, we have a lot of folding structures, tilted or faulted by the earth's movement. Later on I can show many examples here. So finally the whole area was exposed to the surface, so here is what we are here. In this site, we have a variety of different type of rocks,
and we just saw that coarse ash volcanic tuff. We also have fine ash volcanic tuff. We have tuffaceous siltstone, sandstone, mudstone, and also we have rhyolite, which is directly related to the volcanic activities. There's a lot to see at this site, so let's go forward to the west. Here we have a very nice exposure of completely weathered (decomposed) volcanic tuff. You can see that this is a completely weathered (decomposed) volcanic tuff. The weathering is throughout.
The rock actually is very weak, so we can easily scrape it without any problem. We can see that the completely weathered (decomposed) volcanic tuff preserved the original rock texture. You can clearly see the joint, original joint, weathering started from the joint and come inside, so the whole rock turn into the clay-like material inside and out, but still they preserve the original rock texture. That's how we define it as completely weathered (decomposed) volcanic tuff. On the top of this, you can clearly see they are already became the residual soil. Residual soil actually lost original rock texture, so the rock texture is completely destroyed. So this is completely, clayey and sandy materials as a residual soil. So clearly you can see this division
of residual soil and a completely decomposed volcanic tuff, a very nice location to see this exposure. Here we can see a layered strip of rock which is called tuffaceous siltstone. It's also weathered. The rock is in between volcanic rock and a sedimentary rock because they were sedimented in the lake environment due to the volcanic origin.
So when the volcanic ashes settled down in the lake, it formed layers of lighter coloured, and also rainfall can also bring silt material from higher ground and also deposited into some kind of darker coloured material. So this kind of interbedding was originally horizontal as {is} normally the sedimentary rock, but also this is because of volcanic origin so we give this a name called volcaniclastic rock. This feature is quite unique in the Lai Chi Chong formation. You can clearly see a magnificent rock formation over all this area. They are all tuffaceous siltstone, very beautiful pattern. You look on the ground. You can see very colourful rock layers. There's a red, there's white, there is also black.
So the red is due to the weathering, and this is also the striped, interbedded layer. They are all tuffaceous siltstone, some are tuffaceous sandstone and even finer mudstone. Here is also the bedded tuffaceous siltstone but this layer is quite special. You can come close and take a look. This layer is particularly strong,
this is called chertified layers. It's called the chertification process. So original rocks are still tuffaceous siltstone, but after deposition in the water, the water is filled with silica, silica is SiO2. The silica infiltrated into this layer and take all the pore space between the grain particles, so they virtually fill in the pore space. If you look at this chertified layer, you can still see some original layering, but it is very dense without pore space because the pore space was filled with silica. We can pick up a rock here because you can see the rock is highly fractured with a very sharp corner, can easily cut your finger and also it's very strong and hard, so I can strike with my hammer, while this layer. So this layer is quite strong, the hardness is more than 7. If I strike harder with my hammer,
you can smell some kind of crude oil smell because of the sparkling of the iron from my hammer. So the harder it is, the stronger the sparkle and the smell. This is quite magnificent formation because of the chertification. Once the layer was chertified, it is much stronger than the rest. We can see the differential weathering, this layer against the bottom layer. The bottom layer is weaker, it was eroded more than a stronger layer.
So here you can clearly see a very beautiful formation, it was tilted in that direction and the bottom was eroded more, and this thick layer itself is a chertified tuffaceous siltstone layer. From this angle, you can see a lot of interbedding, the whole formation was tilted to that direction to the south direction by almost like a 20 degree. Come close to here, you can clearly see all this banding, they're kind of very lightly twisted because as I explained, they were settled in the water environment, before it was completely solidified into rock, they can be overturned or moved by the earth's movement. So you can see the kind of a flow patterns which is formed during the deposition process.
This is a very nice part, you can take a closer look here. Very nice. Also this layer seems to be a bit chertified so it's stronger than the rest. This something remains over here. This is another chertified layer, very strong. Let's go on.
You can see this layer is quite special at this location. If you follow me, you can see the layer actually leads all the way and turn over and come back, so it's highly distorted. This feature locally you see like here, we call folding, so the folding actually started to overturn over here. So this is because, before the rock layer is fully solidified into rock, the earth movement could actually deform the rock layers and overturn into a very complex formation during a small scale like this. So you can clearly see that this location,
the dipping location along this direction. If I walk over here the dipping changes to that location. So this small scale folding is a very nicely exposed in this location. Very nice and you can see the chertified tuffaceous siltstone layer dipping down in that direction, while over there the dipping towards the ocean, so they are kind of changing the dipping direction within a short distance. There are a lot of good rocks on the ground around this location. You can come close to this layer, actually this layer appears to be finer, much finer, they are actually mudstone. The mudstone has very fine grains so you can see these scratches,
see, leave some marks like the skin of babies, a very tender and very smooth, very nice mudstone layers. Come closer to this site, if you close zoom in, you can see it's like a cheesecake. This is actually the quartz vein, actually intruded into the cracks, you can clearly see this hard part, the whiter part, they are quartz, and the remaining part, they are siltstone. So the quartz actually intruded into it as very small cracks, to fill in very small cracks. Because the quartz is much harder and stronger, it's more resistant to weathering, so if you zoom in to take a look, you can clearly see all these areas are kind of protruded, it's three-dimensional sculpture. So I like this rock the most, this is the intruded quartz veins.
Let's take a look around and walk around this rock. You clearly see the quartz vein, quite a lot of holes on it, it is like a three-dimensional natural sculpture, the magnificent sculpture by the nature. So this part, the hard part, is made by quartz. Okay, this is the beautiful Lai Chi Chong. We actually started from this east to the west part.
We walked around 150 meters until to this end, so this is almost the end of this site. Along there, there's more to see about beaches, also big boulders etc.. People are very happy hiking over there.
It's a beautiful day, I hope you enjoy the site visit. Okay, it's nice and beautiful day, but we have to go back, so let's go back to the site and to the pier. This will be a nice walking back because we can see all this rock formation again.
Again, coarse ash volcanic tuff, weathered. Very fine, very smooth mudstone layer. Here is a small scale folding around the coast.
Look at here again, to see how the rock is actually turned. Beautiful chertified tuffaceous siltstone. Differential weathering erosion show up here.
Very nice multi-colored rock layers, striped, layered. Tuffaceous siltstone layer. Now let's go around the beach.
The ground actually has been greened due to the growth of some biomaterials. A lot of people here today to actually enjoy the site visit, enjoy the site view. They may not be geologists but i can see they do enjoy the unique environment in Lai Chi Chong. Here's a site for completely decomposed tuff and the residual soils. You can look at the weathering here.
The weathering actually turned the rock into a reddish orange colour. Okay, now actually we finish the site visit. Thank you all to come with us and also thank my students for coming all the way with me. See you next time. Let's say thank you, all the students actually to take effort to come to Lai Chi Chong. Okay, see you next time!
2021-05-04 10:36
