Theories on Origin of Nogas Island

One American who had visited the island described the island as mostly made up of old coral, shells and volcanic pumice. This came as a surprise for seldom have I encountered materials which discuss geology of the island and it really sparked my interest on probing the origin of the island.

Pumice is solidified frothy lava. It can be formed when lava mixes with water. Its porous nature is due to the simultaneous actions of rapid cooling and rapid depressurization.

According to experts from the Philippine Institute of Volcanology and Seismology (Philvolcs), the southern tip of Panay Island is composed of marine and pyroclastic (volcanic) rocks that are older than two million years. The age of the “volcanic pumice” in Nogas Island could not be ascertained at this point due to lack of information. It is not easy to conclude for no reference mentioned whether the island formed together with the Panay mainland or later.

The southern part of Panay is located in a known active “collision zone” of tectonic plates. An evidence of this collision is the presence of the Negros Trench running along the southern tip of Panay. This area is where the Sulu Sea basin subducts or the plate moves beneath another which at this case is the Negors Trench. (Source: GeoScience Journal Vol. 10 No. 3, p. 305-320, Sept 2006).

The subducted plate (read: the Sulu Sea basin plate) is dragged downwards into the earths mantle until it reaches a depth where high temperatures melt the rocks. The resulting magma then rising along vertical fissures and reaches through the surface through a volcanic vent. When fissures open up in the sea floor, volcanic islands formed as a result. There is no known volcano in Anini-y, Antique but the existence of Sira-an Hot Spring and the Negros Arc in Negros Island are the evidence of this tectonic activity. Earthquakes usually occur at these zones and is said to be able to shift the plates by up to 5m or about 15ft at once.

It could possibly be that that along this area magma might have risen along a fissure. However, the percent composition of pumice in Nogas Island is unknown that its presence in the sland might have been caused by other processes. Pumice is said to be a common product of explosive eruptions. Could it possibly be that in the past, Kanlaon, the nearest volcano is the area had erupted explosively and volcanic materials have reached this point? (Picture from: http://www.platetectonics.com/)

Another view is that there might be pumice in tahe surface but the base composition of the island might not be igneous in nature. Some studies have found that pumice being buoyant have the capacity to float across great distances of water. This phenomenon is called pumice rafting. There is also a possibility that the rock was misidentified as pumice. I am not certain about this as I am no geologist and during my last visit on the island the rocks caught least of my interest.

Nogas Island may have formed through organosedimentary buildup. Notice that mapping of the island revealed reef(s) surrounding it.

The reef flat may have formed through combination of abiotic process such as deposition of sand, wave erosion other natural process and biotic process through formation and build-up of coral and other calcerous algae. Sand may have collected then compressed, and maybe turned to rock through lithificatition and on this compressed sand corals and coralline algae thrived. It could be that corals and calcerous algae have grown on top of one another and formed a three dimensional framework that is modified in various ways by other organisms and inorganic processes.


The events described above may have led to the formation of the island. It could be that either of the processes explains the birth of the island or it could be interaction of events. It is important to probe the origin and to understand the island formation for geologic history is an important influence on an island’s natural resources especially its flora and fauna.