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(10) Solid Earth. The student knows that plate tectonics is the global mechanism for major geologic processes and that heat transfer, governed by the principles of thermodynamics, is the driving force. The student is expected to: (a) investigate how new conceptual interpretations of data and innovative geophysical technologies led to the current theory of plate tectonics; (b) describe how heat and rock composition affect density within Earth's interior and how density influences the development and motion of Earth's tectonic plates; There are many forms of energy in our Universe, only some of which we have learned about so far. The principles of thermodynamics govern how and why these energy forms are transferred. Law 1: Conservation of Energy. The total amount of energy in the universe is constant. This means that all of the energy has to end up somewhere, either in the original form or in a different form. Tell one example that illustrates the first law that we’ve learned about so far. Law 2: Entropy Disorder in the universe always increases. As the disorder in the universe increases, the energy is transformed into less usable forms. Thus, the efficiency of any process will always be less than 100%. Tell one example that illustrates the second law that we’ve learned about so far. Law 3: Law of Absolute Zero All molecular movement stops at a temperature we call absolute zero, or 0 Kelvin (-273oC). Since temperature is a measure of molecular movement, there can be no temperature lower than absolute zero. Since we don’t really focus on this 3rd Law, you really won’t find an example illustrating this that we’ve learned…however, you should still be familiar with it! Zeroth Law: This law states that if object A is in thermal equilibrium with object B, and object B is in thermal equilibrium with object C, then object C is also in thermal equilibrium with object A. This law allows us to build thermometers. For example, the length of a mercury column (object B) may be used as a measure to compare the temperatures of the two other objects. 1. Describe the first Law of Thermodynamics and tell how it applies to something we have already learned in Earth Science. 2. Describe the second Law of Thermodynamics and tell how it applies to something we have already learned in Earth Science. What we have learned before about convection currents within the mantle, should help you to make a connection with the science of how the Earth’s tectonic plates move about the surface. You can see the hot plumes here, formed at the mantlecore boundary. These plumes not only form the Earth’s “hot spots”, but they are responsible for helping to form the convection currents within the asthenosphere which drive the plates. • The current scientific theory that explains the large-scale movements of the lithospheric plates. developed during the early 1900s, and later accepted by the majority of the scientific community when the concepts of seafloor spreading were developed in the late 1950s and early 1960s Let’s start at the beginning…with Alfred Wegener It’s not easy being a scientific genius, as Wegener found out early on! He was met with: “Utter, damned rot!” “If we are to believe the continental drift hypothesis, we must forget everything we have learned in the last 70 years and start all over again,” Anyone who “values his reputation for scientific sanity” would never dare support such a theory” Wegener began as a meteorologist, and studied climate patterns on Earth, largely in Greenland and polar regions. It is here he observed the movement of icebergs and glaciers. As a Meteorologist, • he pioneered the use of balloons to track air circulation • became the first to use kites and tethered balloons to study the polar atmosphere as well. • Early 20th century geologists viewed continents as fixed features that could rise and fall, but not move around. • Slow shrinking of the Earth was considered to be the cause of mountain building. • Connections of former land bridges and seaways could explain all stratigraphy and distributions of fossils. This was essentially the Atlantis myth of appearing and disappearing continents. Wegener noted several inconsistencies. 3. How do thermodynamics influence plate tectonics? 4. What is plate tectonics theory, and who “piloted” the notion? 5. What did we think was happening before we knew about plate tectonics: • How did continents move or change? • Mountain building? • Fossil and other geologic consistencies from one continent to another? • Distribution of ancient climatic indicators does not make sense. • Close geographic fit of South America and Africa--like a torn newspaper. • Narrow mountain belts restricted to continental margins. • Isostasy of crust at two levels-oceanic and continental. • Distribution of certain fossils, such as Mesosaurus. • In 1912 the meteorologist Alfred Wegener described what he called continental drift, which spurred a debate that would end up fifty years later in the theory of plate tectonics. • Wegener thought that the present continents once formed a single land mass (which we now call Pangea) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to "icebergs" of low density granite floating on a sea of denser basalt. The biggest problem for Wegener was that he had no convincing mechanism for how the continents might move. Wegener thought that the continents were moving through the Earth's crust, like icebreakers plowing through ice sheets, and that centrifugal (caused by Earth’s rotation) and tidal forces were responsible for moving the continents. 6. Describe five inconsistencies that Wegener found with current theory: • 1. Climate: • 2. Fit: • 3. Mountains: • 4. Density and isostasy: • 5. Distribution: 7. Wegener couldn’t come up with a “mechanism” for his continental drift hypothesis. How did he think the continents moved around? During the 1940s, scientists began proposing that convection currents might have driven the plate movements, and that spreading of the continents may have occurred below the sea within the oceanic crust. • One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field direction. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time. Initially, this phenomenon was explained by what was called "polar wander" . It was assumed that the north pole location had been shifting through time, and that the plates and continents were “fixed”. An alternative explanation, though, was that the continents had moved relative to the north pole. This, of course, turned out to be true, and the “Theory of Plate Tectonics” was born. The second line of evidence for plate tectonics was our knowledge of Earth’s interior, and how it could influence the Pillow Basalt surface. We already know that the Earth’s interior is made up of progressively more and more dense material, as you travel towards the core. It also gets hotter, and hotter. Both of these facts influence the motion of Earth’s tectonic plates. You are also aware that the continental crust (largely made up of granite) is less dense than the oceanic crust (largely When the new crust forms at mid-ocean ridges, it is higher than made up of basalt). the surrounding oceanic plate. It forms a mountain. This “height” In places crusttomeet (subduction above thewhere rest ofoceanic the seaand floorcontinental allows gravity “push” on the zones), the that greater of oldthe lithosphere lithosphere liesdensity farther from ridge. relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. (Ridge push, Slab pull) 8. Explain one piece of evidence that helped prove Plate Tectonics Theory. 9. Explain another piece of evidence that helped prove Plate Tectonics Theory. 10. How does gravity influence plate movements? • At the oceanic ridges? • At the subduction zones? 11. Which is the greater influence on tectonic movement, slab push, or slab pull? Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. Three Thoughts: Mantle Dynamics Large scale convection currents in the upper mantle which are transmitted through the asthenosphere as the main driving force of the tectonic plates This theory was launched by Arthur Holmes Gravity Dynamics Ridge push, Slab pull. Remember, gravity dictates these two geological forces. At the mid-ocean ridge, gravity helps to “push” the plates apart. At convergent zones, gravity “pulls” one plate beneath the other. Slab pull is stronger than ridge push, by far. Rotation Dynamics Was Wegener right? Does the Earth’s rotation influence tectonic plate motion at all? There is new scientific evidence that centrifugal force, which causes Coriolis Effect, influences plate direction. 12. How do mantle dynamics influence plate movement? 13. How do gravity dynamics influence plate movement? 14. How do rotation dynamics influence plate movement? In 1971, geophysicist W. Morgan proposed the hypothesis of mantle plumes. In this hypothesis, convection in the mantle transports heat from the core to the Earth's surface in thermal columns, known as “plumes”. Mantle plumes carry heat upward in narrow, rising columns, driven by heat exchange (thermodynamics) across the core-mantle boundary. (known as the Gutenberg Discontinuity) Plume Lithosphere Mantle Core Heat These “hot spots” occur along plate boundaries, as well as midplate, such as with the Hawaiian Islands. This cycle is nothing more than an “updated” version of the rock cycle, that includes plate tectonic theory. It was developed by J. Tuzo Wilson in the late 1950s and 60s. The problem with the traditional rock cycle is that it implies that rocks just cycle endlessly from one to the other, as you can see from this simplistic diagram. James Hutton in his Law of Uniformitarianism envisioned earth processes cyclically but never getting anywhere, never evolving. The Wilson cycle, however, contains an evolutionary component: that is, it is not just cyclical, but it is cyclical with direction. In the Wilson Cycle, there are 9 stages (or A-I). If you were to compare the Wilson Cycle Stage A continent with the Wilson Cycle Stage I continent, you would find stage I is a far more complex continent containing more felsic igneous rock because of all the rock evolution taking place in the Wilson cycle. Stage A The Wilson Cycle simply depicts the rocks on planet Earth as having gone through a series of changes, and evolutions, and not a simple cycle, and describes the movement and formation of these rocks in terms of plate tectonics. Stage I 15. What are hot plumes, and how do they form? 16. What name is given to the boundary where hot plumes form? 17. Why is Hawaii (the big Island) the only Island within the Hawaiian chain to maintain an active volcano? 18. How is the rock cycle different from the Wilson Cycle? 19. How might a stage A continent differ from a stage I continent in the Wilson Cycle? 20. What is a “felsic” rock?