Lesson 4: Formation of Landforms

• The formation of several landforms in the country lies in the concept of plate tectonics. The plate's constant motion and interaction with each other produce mountains, volcanoes, and earthquakes.
• According to plate tectonics theory, the lithosphere is a strong rigid layer the uppermost mantle and crust that were broken into several major and minor segments called tectonic plates.
• The lithospheric crust is thinnest in the oceans an thicker in continents. It lies on top of the asthenosphere, a weak and ductile region in the mantle, which allows the lithosphere to move as a separate component.
• There are seven major tectonic plates which include North America, South America, Pacific, African, Eurasian, Australian-Indian, and Antartic Plates.
• In addition, there are also intermediate-sized plates such as the Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia, and Juan de Fuca plates.

  • These plates are further divided into regions then into microplates such as the Sundaland Plate.
  • The edges of these plates are marked by volcanic and seismic activities which lead to the formation of spectacular landforms.
• There are three main types of plate boundaries, namely: divergent, convergent, and transform fault. These plate boundaries also mark the occurrence of earthquakes and the formation of volcanoes and mountains.
• A divergent boundary is formed when two plates move away from each other resulting to an upward movement of molten material and formation of new seafloor. This type of boundary is also called constructive margin.
• The ocean floor is a landform produced by a divergent boundary. An example would be the Atlantic Ocean which was formed by the separation of African, Eurasian, and American continents.
• A convergent boundary is formed when two plates move toward each other. It is also known as a destructive margin. This type of boundary also marks subduction zones where the denser lithospheric plate, which is the oceanic lithosphere, descends into the mantle. It is where the oceanic crust is destroyed and recycled.
• Transform plate occurs when two plates simply slide past each other. it is also known as the transform fault boundary. In this type of boundary, there is no formation or destruction of the new lithosphere.

  • That is why it is also called a conservative margin.
  • Most of the transform fault boundaries can be found on the ocean floor, but there are instances where they also cut across the continental crust.
• Molten material rises in a divergent boundary, where melting occurs due to decompression of hot asthenosphere as the plates move apart.

  • As the cracking and expansion continue, more magma is extruded which later on becomes the young oceanic crust.
  • As the spreading continues, the oceanic crust becomes wider.
• Older and denser oceanic crust moves away from the ridges and is recycled in subduction zones.
• In a convergent boundary, volcanoes and mountain ranges can be formed depending on the type of crust colliding with each other.
• Earthquakes occur in convergent boundaries as the descending plate sinks below the other plate.
• Earthquake locations form a planar zone called the Wadati-Benioff zone, and it usually starts in the oceanic trench and dips at a steep angle away from the ocean basins.

  • These earthquakes occur at depths shallower than 70 kilometers caused by the grinding and pushing of the cold, brittle descending plate against the other plate.
  • This is one of the reasons why active seismicity is observed in subduction zones.
• When the sinking plate starts to melt, the molten and hotter magma slowly rises. Some of it will reach the surface, erupt through a vent or fissure, and eventually produce a volcano.
• When continental lithosphere converges with the oceanic lithosphere, the continental block remains buoyant while the oceanic crust starts to sink into the mantle.
• The oceanic-continental convergence forms continental volcanic arcs. A classic example is the Andes Mountains near the Peru-Chile Trench.
• When two oceanic crusts converge, the denser oceanic block sinks beneath the other. This oceanic-oceanic convergence produces a volcanic island arc or island arc. Some examples are Indonesia, Japan, and the Philippines.
• When two continental plates converge, it produces spectacular mountain ranges, also known as orogens, such as the Himalayas. This occurs when rocks in continental plates are folded into linear belts, metamorphosed, and pushed upward.
• Subduction takes place in continental boundaries wherein one plate moves under another and sink to the mantle due to gravity.

  • The region where this process takes place is called the subduction zone. This area has a high rate of earthquake and volcanism.
• The Philippines is composed of many island arcs and orogens produced by subduction.
• Majority of the islands are considered as part of the Philippine Mobile Belt (PMB).
• Several million years ago, the Philippine sea Plate collided with the Sundaland Plate, a part of the larger Eurasian plate.

  • The collision resulted in a complex system of tectonic interactions of the Philippine Sea Plate and the Sundaland Plate, featuring several subduction zones and orogenies.
• In the Western border, the South China Sea, Sulu Sea, and Celebes Sea Basins are subducting beneath the PMB. These different subductions, along with many others, resulted in the emergence of island arcs of Luzon, Negros, Sulu, Zamboanga, and Cotabato.
• On the Eastern border, the subduction along the East Luzon Trough and Philippine Trench resulted in the formation of island arcs of Northern Sierra Madre, Southern Sierra Madre-Polillo-Catanduanes, and the East Philippine Arc.
• The Palawan blocks (North and South Palawan, Mindoro, Western Panay, Romblon Islands) were said to come from the southeast edge of the continental crust of China as part of the Eurasian plate.

  • This is also one of the reasons why Palawan is not seismically active unlike the rest of the Philippines.