Module 1 Week 4

Question 1

1. Explain the concept of Seafloor Spreading.

Answer 1

• The mid-oceanic ridges, which was discovered through mapping the ocean floor, were sites for new oceanic crustal materials to be created. The oceanic crust spreads apart as new magma introduces new crustal material.

Question 2

2. What drives seafloor spreading?

Answer 2

• The new magma, which is the heat source, drives seafloor spreading as oceanic lithosphere moves away from it.

Question 3

3. Explain how linear magnetic anomalies are formed and why it used as a proof for seafloor spreading.

Answer 3

• The mid-oceanic ridges record the prevailing magnetic polarity since it flips very often. However, because of seafloor spreading, allowing creation of new crustal materials. And then a reversal of magnetic polarity happens which will then be preserved in the rocks. Symmetries in each side of the oceanic crust record reversals in the magnetic anomaly, and you can only explain this pattern if seafloor spreading is invoked.

Question 4

4. Why are oceanic basins underlain by old oceanic lithosphere tend to be very deep?

Answer 4

As seafloor spreads, the oceanic lithosphere becomes progressively colder, older, and much denser. Since it is denser, the tendency of the lithosphere is to subside, making it very deep.

Question 5

5. Explain the Plate Tectonics Theory.

Answer 5

• The Plate Tectonics Theory was discovered by plotting out the epicenters of earthquakes. The geophysicists from Colombia University then found a pattern wherein the epicenters clustered along distinct zones, specifically along the plate boundaries. Thus, according to the theory, the earth’s outer brittle layer is subdivided into lithospheric plates that are in constant motion with respect to each other.
• The theory unifies the Continental Drift Theory and the Seafloor Spreading Theory. It explains various geologic phenomena such as the occurrence of earthquakes, volcanoes, mountain building, and many more.

Question 6

6. What are the three types of plate boundaries?

Answer 6

• Divergent: These are sites where new oceanic crustal materials are being generated and they spread apart. This can be found in oceanic ridges and rifts.
• Convergent: Occurs when two lithospheric plates meet. The convergent plate boundary also has three types, ocean-ocean, ocean-continent, and continent-continent.
• Conservative: Transform faults

Question 7

7. What type of plate boundary do the Red Sea and the African Rift Valley represent?

Answer 7

• Divergent plate boundary

Question 8

8. How are island arcs formed? Give examples.

Answer 8

• When the two oceanic plates meet, one of them is thrusted down wherein a subduction zone is formed. Because of the convergence of the two plates, partial melting of the lithosphere is initiated because of the introduction of water that lowers the melting temperature of rocks. Rising above the ocean floor are volcanic islands known as island arcs.

Question 9

9. What caused the uplift of the Mountain Range?

Answer 9

• When two continental lithospheres converge, neither will be subducted because both are buoyant. Instead, this will result to up-thrusting of the lithosphere, forming mountain ranges.

Question 10

When an oceanic lithosphere converges with a continental lithosphere, it is the oceanic lithosphere which is down-thrusted through a process known as subduction. Why not the other way around (i.e., the continental lithosphere is subducted)?

Answer 10

• The oceanic lithosphere is down-thrusted because it is much denser than the continental lithosphere.

Question 11

11. Why are transform faults also referred to as “Neutral Plate Boundaries”?

Answer 11

• In a transform fault type of plate boundary, lithospheric plates are neither created nor destroyed. They simply move past each other.

Question 12

12. What is the origin of “Hot Spots” such as the volcanic island chain of Hawaii? How are they useful in determining absolute plate motion?

Answer 12

• Hot Spots are another proof of plate motion. These occur when a mantle plume rooted deep within the mantle punches upwards to the moving lithosphere. As volcanoes move away from the hot spots, the volcanoes become extinct and eroded because there is no more heat source. By tracing the path of the old volcanic chain, absolute plate motion will be determined.

Question 13

13. In what type of tectonic plate boundary does the Philippine Archipelago lie on? What tectonic features are associated with this type of plate boundary?

Answer 13

• The Philippine Archipelago lies on a Convergent plate boundary, specifically the oceanic-oceanic type. As a consequence of the convergence of the two oceanic lithospheres, subduction is formed. The outward manifestation of that subduction is a trench, which is the Philippine trench. However, not only does it have subduction on the right side, but there is also subduction on the other, which is the Manila trench, and another subduction on the lower part of the archipelago. And because we are in an active zone of collision, we have a lot of volcanoes, earthquakes, and landslides.