Plate Tectonic Theory
From WikiPilipinas: The Hip 'n Free Philippine Encyclopedia
Plate Tectonic Theory states that the Earth's outer layer is fragmented into plates that are in constant motion. The movement's rate has been determined to be approximately 5 - 10 cm per year (2 - 6 inches per year), depending on location of the plates.
These plates ride atop a part of the Earth's mantle called the asthenosphere. It is a hot, dense and partially molten (but not liquid)part of the Earth. Moreover, it flows with a type of movement called convection.
Convection currents beneath the plates move the plates in different directions. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. As plate movement occurs, the plates sometimes stick together and then slip. This sudden slippage causes vibrations known as earthquakes.
Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the Earth's surface and in their change in position relative to each other. On the other hand, Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges.
Plate tectonics theory helps us understand the underlying causes of the major topographic features of the Earth, as well as the reasons why some areas of the world are frequently devastated by earthquakes and volcanic eruptions.
 What Is A Tectonic Plate
A tectonic plate (also called as lithospheric plate) is a massive, irregularly shaped slab of solid rock, composed of both continental and oceanic lithosphere. The lithosphere consists of the Earth's crust and part of the uppermost mantle. The Earth's surface or lithosphere is divided into about 7 large plates and 20 smaller ones.
Two types of crust are present in the lithospheric plates:
- Thin (5-7 km), dense (3.0g/cm3), basaltic oceanic crust, dark, fined-grained igneous rock.
- Thick (35 - 40 km, ranging to 60-70 km in some mountain ranges), low density (2.7 g/cm3), granitic continental crust, light-colored, coarse-grained igneous rock
 Types of plate boundaries
There are three major types of[ [tectonic plate boundaries]]. These include:
- Divergent plate boundaries where plates move apart from one another.
- Convergent plate boundaries where plates move toward one another.
- Transform plate boundaries where plates slide past one another.
 The Formation of the Philippines
The Philippine is estimated to have formed during the Pleistocene Period 2 million B.C. It has distinct geographical features and has an array of land and water forms. These can all be explained by the Plate Tectonic Theory.
The Philippine is the product of the continental drift of the Philippine Sea Plate. The collision of the Eurasian plate (where the Asian lands once lay) and the Philippine Sea platewith the Pacific plate and Indian-Australian plate created the current low and high lands, islands and seas in the Philippines. Evidences of these were the Sierra Madre range and Zambales range in the eastern upward movement of the Philippine Sea plate while it was being squeezed between the movement of the Eurasian plate and the Pacific plate.
The Philippines was considered as Proto-Philippinewith the islands of Bicol, Leyte, and Eastern Mindanao that were formed during the Cretaceous Period100-65 million B.C. It was concentrated in the Equatorial Zone in between the Philippine Sea Plate and the Eurasian plate which served as the first islands that shaped the country.
The Eurasian plate moved eastern-southward while the Pacific plate went norther-westward during the Eocene Period (55-35 million B.C.). Then, the Proto-Philippine moved away from the Equatorial Zone and advanced near the western part of the Pacific plate. This pushed upwards the Sierra Madre range and the Zambales range again.
Back then Palawan and Zamboanga were connected to the Eurasian plate. They stirred towards the east and joined together the islands of the Philippines. During this movement, three major island groups emerged: Luzon which covered Luzon up to Samar, Eastern and Central Mindanao and Zamboanga and Western Mindanao.
The continental drift caused some volcanic activities and earthquakes. It also created trenches of which volcanoes arise. Examples of these can be seen at the eastern part of the country, like Bicol and Leyte. They were located at the Philippine Sea trench and the East Luzon trough which resulted form the collision of the Philippine Sea plate and the Pacific plate. It can also be seen in the western part of the country like the volcanoes in Batanes, Tagalog region, Panay-Negros, Zamboanga-Sulu. There are also trenches in Manila-Negros-Sulu-Cotabato due to the continent drift of the Eurasian plate and the Philippine Sea plate.
In addition, faults were created. There were earthquakes because of the movement of the faults. There were constant earthquakes that started in Mindanao that ran through Leyte, Masbate, and Quezon until it halted at Cordillera. The formation of rivers and valleys in Marikina and Central Luzon were assumed to be the result of the convergent plate boundaries.
South China Sea widen because of the aperture in between the Eurasian plate and the Microcontinental Block during the early Miocene period. A major event during the middle Miocene period was the collision of the Microcontinental Block of Palawaninto the original islands of the Philippines which caused volcanic activities in Sulu.
In the late Miocene period, the volcanic eruptions ceased in the islands of Sulu but the Pacific and Philippine plates continued to move. The northern part of the Manila trench crashed into Taiwan that caused the low lands of Luzon to appear from the ocean. The Manila Trench reached the Western Mindanao in the last part of the Miocene period. It was cut-off and became Negros and Cotabato trenches when Mindoro-Palawan and the Peninsula of Zamboanga were pinned to the original islands of the Philippines. It was also during this time that the eastern part of the Philippines sank in the Philippine trench area. This was the start of the formation of the Philippines in its current location, form and shape.
 Contributions of The Plate Tectonic Theory
The[ [theory of Plate Tectonics]] revolutionized the understanding of geology. Some geologic facts were found out to be interrelated as a result of the Plate
- The distribution of volcanoes along the edges of continents and around the rim of the Pacific Ocean (Ring of Fire)
- The association of deep ocean trenches with volcanic mountain chains,
- The presence of a huge undersea mountain range with a central valley, which encircles the globe (see image above),
- The geographic distribution of mountain ranges (and their various ages and types of deformation),
- The geographic distribution of earthquakes, which occur in lines, and deep earthquakes which occur along inclined planes,
- The geographic distribution of certain types of fossils
- The distribution of certain types of sedimentary rocks which can be used as paleoclimate (ancient climate) indicators,
- The age of the oceanic crust
- Sediment thickness distribution patterns in the ocean basins.
- Gore, Dr. Pamela. 2005. Plate Tectonics. http://facstaff.gpc.edu/~pgore/Earth&Space/GPS/platetect.html (accessed April 30, 2008).
- Klous, W. Jackquelyne and Tilling, Robert I. The Dynamic Earth: The Story of Plate Tectonics. US: Diane Publishing, 1996. http://books.google.com.ph/books?hl=en&id=oCzogRW7d9UC&dq=plate+tectonic+theory&printsec=frontcover&source=web&ots=pq0-_DjSUD&sig=kHYVstJVdFvftde_l92CZ3hkWok#PPA2,M1 (accessed April 30, 2008).
- Punongbayan, Raymundo S. 1998. Atlas of the Philippines. Quezon City: Environmental Center of the Philippines Foundation.