Formation of the Philippine Islands

The Earth’s Lithospheric Plates

The nature of the Earth is unique and has no close likeness. It may be compared to a very hot ball of molten materials which gradually become cold. In time, the outer portion of this ball cooled and hardened like a shell. It is called the crust. The crust, however, is not an entirely a solid covering. It is cracked into several big blocks called plates. A single plate can be as large as a continent and can move independently from other surrounding plates. Around the Earth, there are seven (7) major plates and over a dozen intermediate-sized and smaller plates.[1] For the study of the formation of the Philippine islands, the most important of the major plates are: the Eurasian plate and the Indo-Australian plate. While the most important of the intermediate-sized plates is the Philippine Sea plate.

The Eurasian plate is the bedrock of what encompasses the continents of Asia and Europe. It is very stable plate which includes the submerged margins of the continents of Asia and Europe. These are called continental shelves. The extreme southeastern portion of the Eurasian plate, which is a part of Southeast Asia, is a continental shelf. The region is called the Sunda Shelf. The highland sections of this shelf emerged as islands. These islands, which include the Philippine islands of Palawan, Mindoro and Romblon, geologically belong to the Eurasian plate. The Sunda Shelf and its islands is known as the Sundaland block of the Eurasian plate.

The Indo-Australian plate is found south of the Eurasian and the Philippine Sea plates. It is generally oceanic, being submerged by Indian and Pacific Oceans, but it holds two gigantic land masses – the island continent of Australia and Indian subcontinent. Recent researches, however, show that these two land masses are moving independently of each other, thus, may actually be parts of separate plates.[2]

The Philippine Sea plate is found east of the Eurasian plate. It is the bedrock of the major islands of the Philippines, Indonesia, Taiwan, and the Marianas. 

Formation of Island Arcs

Below these plates is the mantle, a very hot section of the Earth where rocks are at the point of or is actually melting. These hot and molten rocks stream steadily making the plates above it move about. The plates move or rotate almost unnoticeably and at times collide against each other.[3] Upon collision, the heavier plate slide beneath the lighter plate and is dipped against the hot mantle of the earth. This event is called subduction.

The subducting plate carries with it some crust sediments down into the subduction zones. The heat of the mantle melts the edges of the down-turned plate and the sediments it carried. The molten plate is called magma.

The magma, being light, is pushed up against the crust. As it tries to escape to the surface, it sometimes creates violent explosions. As it progressed, belt of volcanoes were formed on above the dominant plate. When the subduction occurred underwater, like when an oceanic plate descends beneath another oceanic plate, the belt of volcanoes formed results in island arcs. The Philippines is an assembly of several island arcs.

The Philippine Island Arc System

The territory of the Philippines is composed of many island arcs formed by several incidents of subduction. The island arcs are collectively called Philippines island arc system. Each major Philippine island had a complex natural history.

With the exception of Palawan, Mindoro and Romblon, most of the Philippine islands are considered to have been parts of island arcs formed at the southern edge of the Philippine Sea plate millions of years ago.[4] As part of the Philippine Sea plate, the islands moved northward as the plate rotated clockwise. These roving islands, known as the Philippine Mobile Belt, eventually collided with the Sundaland. The collision resulted, among others, in a series of subductions around Philippine archipelago.

On the western border, are the subductions along trenches of Manila, Negros, Sulu and Celebes where the plates of the South China Sea, Sulu Sea and Celebes Sea are subducting beneath the Philippine Sea plate. These eastward subductions resulted in the emergence of the island arcs of Luzon, Negros, Sulu-Zamboanga and Cotabato.[5]

On the eastern frontier, are the subductions along East Luzon trough and Philippine trench. These westward subductions resulted in the formations of the eastern island arcs of Northern Sierra Madre, Southern Sierra Madre-Polillo-Catanduanes and the East Philippine arc.[6] In time, some of these arcs merged together forming big islands like Luzon and Mindanao.

The Luzon arc is a complex belt of volcanoes extending from the Coastal Range of southeastern Taiwan through the volcanic islands north of Luzon, the Luzon Central Cordillera, and the Western Luzon arc, ending at Marinduque Island. The arc has been active since the Oligocene period to the present.

The Negros arc consists of two overlapping arcs of different ages. The combined arc system extends 400 km and includes eastern Panay. The arc system is terminated against the Philippine fault in the north and abuts the Sulu-Zamboanga arc to the south. Cretaceous basement includes marine sedimentary rocks and pillow basalts, exposed in southeast Negros and serpentinized ultramafic rocks in northeastern Masbate. In the older western arc, Eocene to Oligocene andesitic to dacitic volcanic and clastic rocks host a Miocene dacitic diatreme complex at Bulawan in southwest Negros and middle Miocene dioritic intrusions in northeast Masbate. In the younger eastern arc, middle Miocene to Pliocene andesite flow breccias, volcaniclastic rocks and conglomerates are overlain by late Pliocene andesitic volcanic rocks and Quaternary andesite to basalt stratovolcanoes. The two arcs are the product of subduction beneath Negros but the polarity of the older arc is not clear; it has been interpreted to be situated above a west-or east-dipping subduction zone.

The younger arc is probably the product of east-dipping subduction at the Negros trench, which currently appears to be inactive or almost so and associated with a slab that extends to a depth of about 100 km.

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[1]Tarbuck, Edward J. and Frederick K. Lutgens, Concepts and Principles in Physical Geology of the Earth, 8^th ed., (Singapore: Pearson Education South Asia Pte Ltd, 2005), 56.

[2]Strahler, Alan and Arthur Strahler, Introducing Physical Geography, 4^th ed., (USA: John Wiley & Sons, Inc., 2006), 440.

[3]Christopherson, Robert W. Geosystems: An Introduction to Physical Geography, 5th ed., (New Jersey: Pearson Education, Inc., 2003), 328.

[4] Robert Hall, Reconstructing Cenozoic SE Asia

[5]Yumul, Jr., Graciano P., Carla B. Dimalanta, Victor B. Maglambayan and Edanjarlo J. Marquez, Tectonic Setting of a Composite Terrane: A Review of the Philippine Island Arc System, 12 Geosciences Journal 1, (March 2008), 7 − 17.

[6]Yumul, Jr., Graciano P., Carla B. Dimalanta, Victor B. Maglambayan and Edanjarlo J. Marquez, Tectonic Setting of a Composite Terrane: A Review of the Philippine Island Arc System, 12 Geosciences Journal 1, (March 2008), 7 − 17.