Lecture 4 Notes (Building the Earth: How it Works) Dr. Spotila GEOS 1004
Lecture 4 Notes (Building the Earth: How it Works) Dr. Spotila GEOS 1004 GEOS 1004
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This 7 page Class Notes was uploaded by Dylan Notetaker on Wednesday August 31, 2016. The Class Notes belongs to GEOS 1004 at Virginia Polytechnic Institute and State University taught by Dr. Spotila in Fall 2016. Since its upload, it has received 19 views. For similar materials see Physical Geology in Science at Virginia Polytechnic Institute and State University.
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Date Created: 08/31/16
GS1004 Spotila 31 August, 2016 Lecture 4: Building the Earth: How Earth Works Motivating questions: 4 Billion years ago, the Earth had a core and mantle, but no crust, and no tectonics like we see today. Why not? How was crust made? What is plate tectonics and when did it begin? How does the Earth's internal heat drive it like and engine, causing the outer layers to move? Crust was made through a much slower process (slower than differentiation of core and mantle) of distillation, from repeated partial melting of the mantle, which continues today key process = magmatism, volcanism distillation: concentration of a by-product of some process (in this case, melting) due to difference in density. LOOK-UP ITEMS. Understand what is meant by "partial melting" and “decompression melting” of the mantle. CRUST: the outer layer of the solid Earth, 10-40 km thick. There are two types; oceanic and continental crust. Oceanic crust: basalt composition; Si, O, minor Fe/Mg (but less than the mantle; oceanic crust is less dense than mantle). Where does oceanic crust get made? Happens at a mid ocean ridge (a type of active plate boundary) How and why? Solid (hot/ductile) mantle rises due to convection; depressurizes, melting temperature decreases, it partially melts This forms magma chambers beneath the ridges. Lighter magma rises to the surface. GS1004 Spotila 31 August, 2016 The magma is extruded to make the sea floor, which is the oceanic crust. Basalt: mafic rock (meaning dark). It is the most common rock in Earth's crust. LOOK-UP ITEMS. define "basalt", understand the difference between "mafic", "felsic", and "ultramafic" compositions. The process of magmatism at mid-ocean ridges results in new crust. This pushes the old crust apart. What is this process called? Sea floor spreading As a result young oceanic crust is found at the ridges; it gets older away from ridge in either direction. Mid-ocean ridges are a type of active plate boundary in plate tectonics. They are considered extensional. Oceanic crust makes up the sea-floor, thus explaining about 70% of the Earth's surface (or outer layer). What about the continents? Why are they higher? continental crust = less dense (2.7g/cm^3) oceanic crust = more dense (3.0g/cm^3) mantle = most dense (3.3g/cm^3) isostasy = height of surface depends on density underneath; maintain gravitational equilibrium at any point in the ductile mantle. Continents stick up higher because continental crust is less dense. LOOK-UP ITEMS. Understand what "isostasy" means. GS1004 Spotila 31 August, 2016 How was this less dense crust made? To explain continental crust, we need to back track and explain how oceanic crust is destroyed. What would it mean if there was no way of destroying the sea-floor made at mid-ocean ridges? The Earth would have to expand. It’s not. The oldest oceanic crust is also <200 million years old, meaning that it is getting destroyed somewhere. Where? Oceanic crust sinks back into the mantle at subduction zones after it cools, contracts, and densifies. There is a balanced cycle between new oceanic crust formed at mid- ocean ridges and that consumed at subduction zones; there is no net increase in volume of oceanic crust (i.e. no net expansion of the sea floor). So, what about making that continental crust? Subduction causes melting of oceanic crust at island arc volcanoes; separate the lighter magma (distillation) = continental crust Continental crust is rich in Si, 2 , but little Fe/Mg. Representative rock is granite/diorite; more variable than oceanic crust. The first continental crust forms by about 4.0-3.5 billion years ago. This is still locally preserved in some places known as Archean "cratons". Continental crust then grows gradually for 1.5 billion years. Continents are assembled from the inside-out by piecemeal accretion. By about 2.5 billion years ago, we have continental and oceanic crust, and normal plate tectonics. GS1004 Spotila 31 August, 2016 LOOK-UP ITEMS. define "craton". What time period does "Archean" refer to on the geologic timescale? Question of the Day: Where would you find the youngest oceanic crust? a) Offshore western Africa b) Any abyssal plain c) East Pacific Rise d) Offshore Western US GS1004 Spotila 31 August, 2016 GS1004 Spotila 31 August, 2016 GS1004 Spotila 31 August, 2016
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