It is unusual to get so excited about geology in the middle of a city but this EarthCache, in the heart of Singapore, is worth more than a passing glance. It is next to Exit 4 at the Outram Park MRT station; the exit you’d use going to or from Chinatown. At Outram Park ascend the Exit 4 stairs and leave the MRT. Immediately on the right, examine the natural collage of metamorphic minerals making up the stone cladding around the entrance to the Dorsett Hotel in the corner of the precinct, next to the station. This can be viewed from the pedestrian precinct.
The Geological Background
The rock itself is a garnet-staurolite schist which has been sourced from the Chapada Diamantina region of Bahia state, in the Northeast of Brazil. It is a regionally metamorphosed rock which probably started life as a fine-grained mud on the bottom of the sea-floor. This was compacted by further sediment deposited on top to form a sedimentary rock called shale. The ocean floor was later compressed by tectonic forces and the shale subjected to high temperatures and pressures as it was squeezed between two converging lithospheric (tectonic) plates. The temperatures and pressures generated were sufficient to allow the original clay minerals in the shale to recrystalise (but not melt) to form minerals such as mica (both muscovite and biotite), garnet and staurolite. Figure 1 shows the range of minerals you might expect to see in a rock of this type.
Figure 1
This mineral composition is typical of a group of metamorphic rocks (a metamorphic facies) called the amphibolite facies that forms under medium pressure and average to high temperature. Both garnet and staurolite are index minerals; the presence of which gives an estimate of the temperatures and pressures of metamorphism at the time of their formation. (Figure 2).
Figure 2
Whilst the minerals that make up the finely-crystalline groundmass of the rock and are difficult to see without a hand lens, the ruby coloured garnets and darker, blade-like staurolite form large, well-shaped (euhedral) crystals called porphyroblasts, which gives the schist a porphyroblastic texture. (Figure 3). A porphyroblastic texture is one in which larger minerals are set in a finer crystalline groundmass. This may show a 'preferred mineral orientation' in which minerals are aligned/orientated as a result of pressure from one direction as they grew.
Figure 3
To log this EarthCache please send the answers to the questions below through the Message Centre. But more importantly just take a few moments to stand back and admire Nature’s Artwork.
Logging details
The co-ordinates will take you to view the facing stone from the pedestrian precinct next to Exit 4 of the Outram Park MRT station. Here you will find the answers to the following questions.
- What other minerals (in addition to micas, garnet and staurolite) does Figure 1 indicate you would expect to find in the groundmass of this garnet-staurolite schist ?
- Refer to Figure 2. State the highest temperature and highest pressure at which a rock of the Amphibolite facies is likely to have formed.
- Estimate the size of the largest garnet and the longest staurolite crystals you can see.
- Describe the orientation of the staurolite crystals and suggest if you think they show a 'preferred mineral orientation" (i.e: a particular direction of growth of the crystals).
- Look closely at the garnets and staurolite crystals and try to work out which of the two might have formed first. Explain your reasoning (See hint if you need help)
- Using a scale, take a photo of any part of the stone to show the beauty of Nature’s Artwork. Use an appropriate object as the scale (this could be whatever is available – GPS unit, coin, pen, ticket, finger; use your imagination).