Plate Tectonics

1. Plate Tectonics The Driving Force of Earthquakes

2. Before we can understand earthquakes and their effects, we need to have a basic understanding of the driving force behind them, plate tectonics. Bent rails from 1976 Guatemalan earthquake.

3. What is Plate Tectonics ? • Plate tectonics is a theory whereby the earth’s lithosphere is broken up into numerous fragments, or plates, each of which independently rides or drifts on the plastic asthenosphere.

4. The vast majority of earthquakes occur along the boundaries of these tectonic plates. The above diagram shows the epicenters for recent large earthquakes throughout the world. Notice how the epicenters tend to fall in long belts, with few outside these regions.

5. If we look at a map of the Earth’s surface, one of the features which is immediately apparent is the apparent jigsaw puzzle fit of the continents of South America and Africa.

6. In fact, based on this and other lines of evidence, the German meteorologist Alfred Wegener theorized that approximately 200 million years ago all of the present day continents were joined together into a single landmass or “supercontinent” he termed Pangea. Alfred Wegener

7. According to his theory, over the past 200 million years, the continents had drifted over the surface to their present positions. But how was this possible? To understand this we must examine the Earth’s interior.

8. What Does the Interior Look Like? • First, we must examine what we mean by the terms lithosphere and asthenosphere. If we look at a cross-section through the Earth’s interior, we see that it is layered, with a crust, mantle and a 2-part core (an outer liquid core, and a solid inner core). How we know this we will examine at a later time.

9. The Crust • As you can see from the previous diagram, Earth has 2 distinct forms of crust, oceanic crust which underlies all of our ocean basins and continental crust which makes up our continents.

10. Oceanic Crust • Oceanic crust is made entirely of the rock basalt, and is between 5 and 10 km thick. Basalt is very rich in both of the elements iron (Fe) and magnesium (Mg). As a result of its’ composition, oceanic crust tends to be quite dense.

11. Continental Crust • Continental crust by contrast is largely granitic in composition and is approximately 35 to 70 km thick. It is compositionally rich in elements such as silicon (Si), sodium (Na), and potassium (K). As a result, it tends to be much less dense than the oceanic crust.

12. The Mantle • Contrary to popular belief, the Earth’s mantle is not a liquid, but is instead a solid composed largely of the rock peridotite. As pressure and temperature increase as you go further towards the core, the peridotite metamorphoses,or changes, into new rock types.

13. The Asthenosphere • Within the upper mantle, pressures and temperatures are high enough to cause the solid rock to become a plastic. The zone where this occurs is called the Asthenosphere. Silly Putty would be a perfect analogy to what the mantle is like here.

14. The Lithosphere • The solid crust, plus the solid upper mantle above the plastic asthenosphere act as a single package, which we call the Lithosphere.

15. The Core • As mentioned earlier, Earth has a 2-part iron core, an outer core that is liquid, and an inner core which is solid. At first that may seem backwards, but near the center of the Earth, pressures are so high that temperatures cannot rise high enough to cause the inner core to melt.

16. Harry Hess and Seafloor Spreading • In the period following World War II, considerable research was done on the ocean floor and some remarkable discoveries were made by Harry Hess, a naval commander and geology professor at Princeton University. Harry Hess

17. Hess had determined several important facts about the seafloor: • The existence of a mid-ocean ridge system (an approximately 10,000+ foot high volcanic mountain chain that bisects the oceans). • The ocean floor is young (less than 200 million years old, compared to the 4 billion age of the continents). • Age progression on sea floor (oldest adjacent to the continents and youngest at mid-ocean ridges).

18. Based on his findings, he formulated his theory of “seafloor spreading”. According to his theory, because it it less dense, hot asthenosphere rises upwards in certain areas of the Earth’s mantle. This is similar to the idea of a hot air balloon rising. As the air is heated it becomes less dense and begins to rise. The rising asthenosphere then causes the overlying lithosphere to bend upwards. The rising asthenosphere also as it rises, begins to partially melt.

19. As the solid lithosphere bends upwards, it begins to crack and break. The rising molten asthenosphere now can reach the surface, where it can cool and solidify forming new seafloor. As the magma solidifies, its’ density increases and as a result, starts to sink. However, since hot asthenosphere continues to rise upwards, it cannot. Instead of sinking, the cooled magma spreads outwards to the sides. The cycle then repeats and older cooling magma is replaced by younger. These areas are called “divergent boundaries”. In this way, continents will “drift” over the Earth’s surface.

20. If the Earth’s lithosphere is separating at “divergent boundaries”, it must be pushed together in other regions. These are termed “convergent boundaries”. Since there are 2 distinct forms of crust, 3 possible types of convergent boundaries are possible: • Ocean-Continent • Ocean-Ocean • Continent-Continent

21. Ocean-Continent Convergence • Convergence between oceanic and continental lithospheric plates generally results in the denser oceanic lithosphere being pushed under, or subducted beneath the less dense continental lithosphere. This is the case in South America where the Nazca Plate is being subducted beneath the South American Plate, and also in the Pacific Northwest where the Juan DeFuca Plate is subducted beneath the North American Plate

22. Ocean-Ocean Convergence • In the case of convergence between 2 oceanic plates, the older, colder and therefore more dense oceanic plate will preferentially be subducted beneath the younger oceanic plate. This is the case in the region around Japan. Notice in the lower figure how seismicity in Japan is tied to the tectonic boundaries.

23. Continent-Continent Convergence • The final possibility involves the convergence between two continental plates. In this case, continental lithosphere, which is low in density, cannot be subducted. Instead, the lithosphere is pushed upwards, forming tremendous mountains, such as the Himalayas and the Alps.

24. Hot Spot Volcanism • At both divergent and convergent boundaries, we have volcanic activity. But what about areas such as Hawaii, and Yellowstone National Park, where volcanic activity occurs in the middle of a tectonic plate, not associated with either of these boundaries? These areas are called “Hot Spots”.

25. In these situations, hot material rising from the lower mantle rises upwards. As it rises, it partially melts forming a magma. It then “punches through” the lithosphere and at the surface forms a volcano. As the lithosphere moves over this mantle “plume”, a chain of volcanoes will form.

26. Transform Plate Boundaries • The final situation that needs to be addressed is that where two tectonic plates simply slide past one another. As one can observe, the mid-ocean ridge is not a continuous straight line, but instead consists of short segments offset from one another. Connecting these offset segments are transform faults.

27. Pacific Plate • Quite possibly, the most famous example of a transform fault is the San Andreas Fault in California. People often think that with a large earthquake on this fault, California will fall into the ocean. However, the two sides are simply sliding past one another. North American Plate

28. In summary, we have seen how that due to motions within the Earth’s mantle, several different types of tectonic boundaries will develop: divergent, convergent and transform. In addition to these, areas where material is rising from the lower mantle will form hot spot volcanism. In the next section, we will look at faulting and see how it is related to tectonism.

29. Activities • Activity #1: How Tectonic Forces Affect Faults Determine a fault's slip by knowing the tectonic environment around it -- and vice versa.