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Geology of Indonesian Pumice

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Geology of Indonesian Pumice

Pumice or pumice is a type of rock that is light in color, contains foam made of glass-walled bubbles, and is usually referred to as silicate volcanic glass.

These rocks are formed by acidic magma by the action of volcanic eruptions that eject material into the air; then undergo horizontal transport and accumulate as pyroclastic rock.

Pumice has high versicular properties, contains a large number of cells (cellular structure) due to the expansion of the natural gas foam contained therein, and is generally found as loose material or fragments in volcanic breccia. While the minerals contained in pumice are feldspar, quartz, obsidian, cristobalite, and tridymite.

Pumice occurs when acidic magma rises to the surface and suddenly comes into contact with outside air. Natural glass foam with / gas contained in it has the opportunity to escape and the magma freezes suddenly, pumice generally exists as fragments that are ejected during volcanic eruptions ranging in size from gravel to boulders.

Pumice commonly occurs as melt or runoff, loose material or fragments in volcanic breccias.

Pumice can also be made by heating obsidian, so that the gas escapes. Heating performed on obsidian from Krakatoa, the temperature required to convert obsidian into pumice averaged 880oC. The specific gravity of obsidian which was originally 2.36 dropped to 0.416 after the treatment, therefore it floats in the water. This pumice stone has hydraulic properties.

Pumice is a white to gray, yellowish to red, vesicular texture with orifice size, which varies in relation to each other or not to a scorched structure with oriented orifices.

Sometimes the hole is filled with zeolite/calcite. This stone is resistant to freezing dew (frost), not so hygroscopic (sucking water). Has low heat transfer properties. Pressure strength between 30 – 20 kg/cm2. The main composition of amorphous silicate minerals.

Based on the manner of formation (desposition), distribution of particle size (fragment) and the material of origin, pumice deposits are classified as follows:

  1. Sub-area
  2. Sub-aqueous
  3. New ardante; i.e. deposits formed by the horizontal outflow of gases in lava, resulting in a mixture of fragments of various sizes in a matrix form.
  4. Result of re-deposit (redeposit)

From the metamorphosis, only areas that are relatively volcanic will have economical pumice deposits. The geological age of these deposits is between Tertiary and present. Volcanoes that were active during this geological age included the Pacific Ocean fringe and the path leading from the Mediterranean Sea to the Himalayas and then to East India.

Rocks similar to other pumice are pumicite and volcanic cinder. Pumicite has the same chemical composition, origin of formation and glass structure as pumice. The difference is only in particle size, which is smaller than 16 inches in diameter. Pumice is found relatively close to its place of origin, while pumicite has been transported by the wind for a considerable distance, and was deposited in the form of fine-sized ash accumulation or as tuff sediment.

The volcanic cinder has reddish to black vesicular fragments, which were deposited during the eruption of basaltic rock from volcanic eruptions. Most of the cinder deposits are found as conical bedding fragments ranging from 1 inch to several inches in diameter.

Potential of Indonesian Pumice

In Indonesia, the presence of pumice is always associated with a series of Quaternary to Tertiary volcanoes. Its distribution covers the areas of Serang and Sukabumi (West Java), the island of Lombok (NTB) and the island of Ternate (Maluku).

The potential for pumice deposits that have economic significance and very large reserves are on the island of Lombok, West Nusa Tenggara, Ternate island, Maluku. The amount of measured reserves in the area is estimated at more than 10 million tonnes. In the Lombok area, the exploitation of pumice has been carried out since five years ago, while in Ternate the exploitation was only carried out in 1991.