Plate Tectonics
DIFFERENT TYPES OF PLATE BOUNDARIES
1. Divergent Boundary – Wherein plate moves apart, creating a zone of tension. Formation of rift valleys and oceanic ridges are indicators that the crust is spreading or splitting apart. Most divergent boundaries are situated along underwater mountain ranges called oceanic ridges.
2. Convergent Plate Boundary – In which two plates move forward each other, causing one of the slabs of the lithosphere to subduct beneath an overriding plate. Converging oceanic plates will cause formation of trenches, and these trenches will become sources of earthquakes. The leading edge of the subducted plate will eventually reach the mantle causing it to melt and turn into magma.
3. Transform Fault Boundary – Where plates slide or grind past each other without diverging or converging. The best example of this plate boundary is San Andreas fault which is bounded by North American Plate and the Pacific plate. The immediate concern about transform fault boundaries are earthquake activities triggered by movements along the fault system.
The Convergent Plate Boundaries has three types:
Oceanic-Continental – One plate is oceanic there are volcanoes found in lines that outline the subduction zone. Earthquakes also happen in these zone. For example, the Alestian islands that border Southern Alaska are in island arc.
Oceanic-Oceanic – One of the plate (oceanic crust and lithospheric mantle) is pushed, or subducted,under the other that mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust’s melting point that leads to the formation of magma.
Continental-Continental - Are when two plates collide, they have a density lower than the mantle which prevents subduction (submersion of plates under each other) so they create mountains, such as the world famous Himalaya Mountain Range.
CONVERGING OCEANIC CRUST LEADING PLATE AND CONTINENTAL CRUST LEADING PLATE
During collision of two plates; one has continental edge while the other has an oceanic edge. The event gives rise to the formation of a volcanic arc near the edge of a continental leading plate. The reason for this is because the denser oceanic crust undergoes what we call a subduction process or the bending of the crust towards the mantle. Since the mantle is hotter than the crust, the tendency is the subducted crust melt forming magma. Addition of volatile materials such as water will cause the magma to become less dense, hence allowing it to rise and reach the crust once again and causing volcanic activities on the continental leading plate. For the oceanic crust, one important geologic feature is formed, and that is the trench. Also called submarine valleys, ocean trenches are the deepest part of the ocean. One of the deepest is Philippine trench with a depth of 10,540 meters.
Another subsequent effect of the continuos grinding of plates against each other is the occurrence of earthquakes. The subduction of plate can cause earthquakes at varying depths. Most parts of the occational shallow earthquakes – where the focus is within 60 km of the Earth’s surface. Of the total energy released of earthquakes, 85% comes from shallow earthquakes. Meanwhile, about 12% of energy originates from intermediate earthquakes or those quakes with a focal depth range of 60 to 300 km. Lastly, are the deep earthquakes whose origin is more than 300 km to 700 km below the Earth’s surface.
Converging Oceanic Plates
Like the first type of convergent boundaries discussed earlier, converging oceanic plates will cause formation of trenches, and these trenches will become sources of earthquakes. Underwater earthquake, especially the stronger one, can generate tsunamis. The Japanese term for “harbor wave,” tsunami is a series of ocean waves with very long wavelengths (typically hundreds of kilometers) cause by large-scale disturbances of the ocean. The Leading edge of the subducted plate will eventually reach the mantle causing it to melt and turn into magma. The molten material will rise to the surface creating a volcanic island arc parallel to the trench. Volcanic island arc is a chain of volcanoes position in an arc shape.
Converging of Two Continental Plates
When two continental plates converge, a collision zone is formed. Unlike the other two types of convergent types of boundaries, subduction ceases for this particular type of convergence. No trench, no volcano, and definitely no island arc are created during the process. Instead, what is created is a large group of tall mountains called mountain ranges.