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.
[1]Tarbuck,
Edward J. and Frederick K. Lutgens, Concepts
and Principles in Physical Geology of the Earth, 8th ed., (Singapore :
Pearson Education South Asia Pte Ltd, 2005), 56.
[2]Strahler,
Alan and Arthur Strahler, Introducing
Physical Geography, 4th 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.
what theory does this respond to?
ReplyDeleteIs it Plate tectonics or the sunda land theory
sorry late reply, its plate tectonics.
ReplyDeleteHow visayas was formed?
ReplyDeletesir, was Philippines formed by an oceanic to continental plate?
ReplyDeleteNo, it is formed by Oceanic to Oceanic Plates
Delete