3 LESSON_Q1

• When two tectonic plates collide, the oceanic crust usually subducts beneath the continental crust because it is denser than the continental crust
• If you visit a place in the Pacific known to be along converging plates, you should NOT expect to see rift valleys
• A deeper part of the ocean floor is most likely a trench
• At a mid-ocean ridge, you can expect to find relatively young rocks
• When crustal plate A is moving away from crustal plate B, the expected average rate of change in position between A and B is a few centimeters per year
• The plate boundary formed between the Philippine plate and the Eurasian plate is a convergent boundary
• Most of the Philippine islands are products of the subduction process and originated geologically in an oceanic-oceanic convergence
• Most of Earth's earthquakes, volcanoes and mountains occur at plate boundaries
• Complex mountain systems such as the Himalayas are the result of continental-continental convergence
• At a divergent plate boundary, plates move apart, resulting in the upwelling of material from the mantle to create new seafloor
• Alfred Wegener is known for the continental drift hypothesis
• Focus


  The point of origin of an earthquake
• New crust is created by magma from the mantle
• When two continental plates collide, folded mountains can form
• Earthquakes can happen when two plates slip past one another
• A converging plate boundary is where two plates move toward each other, causing one of the slabs of the lithosphere to subduct beneath an overriding plate
• A divergent plate boundary is a region where the crustal plates are moving apart
• A transform fault plate boundary is produced when two plates slide past each other
• Converging continental and oceanic plates

  1. Oceanic crust subducts beneath continental crust

  2. Subducted crust melts to form magma

  3. Magma rises to form volcanoes on continental plate

  4. Earthquakes occur due to grinding of plates
• Trench


  • Deepest part of the ocean formed by subduction of oceanic crust
• Converging two oceanic plates

  1. Subduction forms trench

  2. Earthquakes can generate tsunamis

  3. Melting of subducted crust forms volcanic island arc
• Many parts of the Philippines originated from oceanic-oceanic convergence
• Majority of the Philippine islands are part of the Philippine Mobile Belt, except for Palawan, Mindoro, and Zamboanga
• Trenches like the Manila-Negros-Cotabato Trench system and the Sulu Trench are products of the collision of the Philippine Mobile Belt and the Sundaland block of the Eurasian Plate
• Trenches like the Philippine Trench and East Luzon Trough are products of the subducting Philippine Sea Plate beneath the Philippine archipelago
• The subduction of oceanic lithospheres underneath the Philippine archipelago creates active volcano chains
• The constant dipping movement of slabs induces frequent moderate to strong earthquakes at various depths, and gives rise to mountain ranges
• Converging two continental plates

  1. No subduction occurs

  2. Collision leads to formation of mountain ranges

  3. Shallow earthquakes occur
• The collision of India and Eurasia about 40-50 million years ago began the formation of the Himalayas
• Activity on Converging Two Continental Plates

  1. Place a block of wood at each end of the clay strips

  2. Slowly push the two blocks together towards the center

  3. Observe what happens to the clay
• Collision zone


  Formed when two continental plates converge
• Since they are both less dense, NO SUBDUCTION occurs
• Collision of continental plates

  • Leads to the formation of mountain ranges
  • Occurrence of shallow earthquakes
• Two large land masses, India and Eurasia, collided to begin the formation of the Himalayas


  About 40 to 50 million years ago
• Since subduction is impossible with two colliding continental plates, pressure is released by pushing the crusts upward and forming the Himalayan peaks
• Divergent plate boundaries


  1. Plates tend to move apart

  2. New materials from the mantle ooze and slowly become new ocean floor
• Formation of rift valleys and oceanic ridges are indications that the crust is spreading or splitting apart
• Most divergent boundaries are situated along underwater mountain ranges called oceanic ridges
• The spreading rate at these ridges may vary from 2 to 20 cm per year
• When a spreading center develops within a continent, the crust may break into several segments