1. Some Microscopic features of
Volcanogenic Rocks in Jurong Formation,
Singapore
KYI KHIN (2015)

2. Location: Jln Kilan Barat (Rail Corridor)
Depth: 28.00-29.00m
Welded Tuff (Ignimbrite)

3. Rheomorphic structures (flow structures) in ignimbrite
PPL XN

4. Rheomorphic structures (flow structures) in a ignimbrite
PPL XN

5. Location: Jln Kilan Barat (Rail Corridor)
Depth: 29.00-30.00m
Spherulite: Typically rounded, radiating arrays of crystal fibers produced by the high temperature
devitrification of volcanic glass. In felsic rocks, the crystal fibers are generally composed of alkali feldspar and
a silica polymorph (either quartz or cristobalite), whereas in mafic rocks the fibers commonly consist of
plagioclase and/or pyroxene. Spherulites typically have diameters of 0.1-2.0 cm, but can be muchlarger
(commonly up to 20 cm). Isolated spherulites are generally spherical, but adjacent spherulites may impinge
upon one another to produce long chains that are often aligned with flow foliation (Mc Phie et al. 1993: pp.
24–25).
Welded Spherulitic Tuff

6. Rounded lithic fragment of older welded tuff, displaying marked compaction and distortion of shards
PPL PPL

7. Rounded lithic fragment of older welded tuff, displaying marked compaction and distortion of shards
PPL XN

8. Location: Jln Kilan Barat (Rail Corridor)
Depth: 31.00-32.00m
Green Lapilli Tuff

9. Glass shards and clasts in fine-grained ground mass
PPL XN

10. Glass shards and clasts in fine-grained ground mass
PPL XN

11. PPL XN

12. Location: AYE_LP.320
Depth: 54.00-55.00m
Non-welded Ignimbrite

13. Pyroclasts, lithics and crystals in fine-grained ground mass
PPL XN

14. Pyroclasts, lithics and crystals in fine-grained ground mass
PPL XN

15. Pyroclasts, lithics and crystals in fine-grained ground mass
PPL XN

16. The term "ignimbrite" (from the Latin igni- "fire" and imbri- "rain") was coined by the New Zealand geologist
Peter Marshall in 1935. This term was originally used only to refer to welded tuffs. These are pyroclastic
rocks that were so hot right after the deposition from the pyroclastic cloud that individual clasts adhered to
each other. The term "ignimbrite" includes all pyroclastic flow deposits, no matter whether they are welded
or not.
An ignimbrite is a welded pyroclastic rock that contains abundant flattened juvenile clasts often originally
pumice. The flattened clasts within ignimbrites are termed fiamme and range from lapilli-sized (>2 mm) to
block-sized (>64 mm). The layered texture produced by fiamme is termed a eutaxitic texture. The
groundmass of ignimbrites is usually dominated flattened vitric shards, but can contain lithic and crystal
fragments. The fine-grained groundmass of many ignimbrites has a reddish colour due to high temperature
oxidation of iron, in particular in the upper parts of a pyroclastic flow deposit. Less welded flows tend to be
white or grey, whilst intensely welded flows are often dark grey to black. Recrystallisation and alteration of
glass within ignimbrite is common, in particular in ancient examples.
PPL XN