GEOL 106 Complete Notes with Diagrams (winter 2014)

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Queen's University
Geological Sciences and Geological Engineering
GEOL 106
John Hanes

1 Earth System Engineering: an introduction 1 History 1969: humans 1 landed on the moon. “A new powerful idea is born” because of the photograph of the earth. Planet is a closed system with no renewable resources. Number of US laws on environmental protection dramatically increased since 69: 1972: “only one earth”view of earth changed. Raised public concerns (pop culture Scientific American and TIME). President of US commissioned a study of the Earth System. What do we need to know and do to live in concert with this fragile earth? Report of this commission (1989-1993)  goal:  to understand past, present and future of the whole earth system, from lives to atmosphere, outer core to inner core.  use this understanding to maintain the environment  Objectives: A. Understand processes and interactions of subsystems of the global earth system (hydrosphere, atmosphere, etc) on different scales. (The science component) B. Sustaining supplies of natural resources. C. Mitigate geological hazards and minimize them D. adjustment to effects of global environmental change. (E. improving the standard of living)  B C D (and E) = Earth Systems Engineering/Management component “Earth Systems Engineering , The world as human artifact” by Brad Allenby ESE is the “next great challenge for the engineering profession” Allenby’s hypothesis: humans have been engineering the earth since the start of the human race. To test the hypothesis, we need to study the past by 1 *Earth System engineering (ESE) =Earth System Management (ESM). They are both for the use and convenience of people. 2 1) Study tree rings: thickness depends on local climate. Rainfall/drought? forest fire scorches? 2) Ice cores from ice sheets Annual snow layers trap dust—some transformed into ice. Winter layer has darker color because of more dust. Can examine earth processes up to 600,000 years ago. e.g. a study of Pb contamination from Greenland Ice Sheet: Pb in Greenland Glacier Even 2000 years ago (Roman Empire_ humans have been using lead (unwittingly) managing the environment/earth system. other examples:  Fire: agriculture (burn to grow corps) generates CO2.  Rice paddies: produces methane (greenhouse gas). Scale of ESE/ESM  Has increased over time. We can see the change by simple observation (qualitative) or measuring (quantitative). e.g. how much material do humans move compared to natural processes? Metal Production  Earth moved by humans: 60 x 10^4 tonnes/year  Earth moved by natural processes: 16 x 10^4 tonnes/year. (We know by measuring sediment carried by river to the sea) *humans move 4X natural processes! 3 Post 1955 data: CO2 measured day to day shows dramatic increase th pre-1955 data: CO2 in ice sheets also shows human impact since 20 century. How does past ESE compare to future? Two main differences: 1) Scale: in the past, impact was local. 2) Intent: in past, effects were unintended and unanticipated. e.g. Thomas Midgely who invented leaded gasoline in 1921, Freon (CFC) in 1930-31 to do good. But these entailed enormous environmental costs. Species Extinction since 1800 Why has there been such a dramatic increase in the impact of humans on the earth system? 1) huge increase in population 2) increase in technology (e.g. Midgely) 3) demand for increased human level of affluence—production of wealth (recall Adam Smith sociology yeah!). “Big Picture” issues  anything we do is ESE: the question is the scale. Think globally, act locallyeverything has an impact. Allenby: ESC should be conscious, controlled.  To what end are humans engineering or should engineer the earth is a moral issue, not a scientific one.  ethical dimension: you need to define your goals.  sustainability? (def of sustainable development: meets the need of the present without compromising the future generation)  Is sustainability enough?  The wealth of world’s 300 wealthiest individuals is equal to the combined annual income of 41% of the human population. Sustainability cannot happen without equity. 4 Finally, Allenby: “Minimizing the Risk and scale of unplanned/undesirable perturbations in such systems in an obvious ESE objective.”  Risk management! 5 Risk Analysis and Risk Management 2 Scenario: government of Ontario proposes to build a Nuclear Reactor on the shore of lake Ontario in Kingston. Task: carry out risk analysis and management, focused on seismic risk. General approach:  hazards=something that might cause harm to people  e.g. landslides on Mars are not hazards unless the damage human properties (rovers)  a) “Natural” hazards AND b) anthropogenic hazards AND c) both often intertwined. (e.g. car accidents in snowy weather)  difference between a hazard and a resource:  resource=something useful  anything can be a resource or a hazard.(e.g. water) WATER e.g. “sewer gas” (HS2) can help regulate blood pressure but excessive amount can be lethal. e.g. DDT: insecticide or poison? How do you decide where the damage threshold is?  partly by empirical observations and scientific studies.  ULTIMATELY by society “preferences” (e.g. smoking) Over time, there may be changes in human tolerance/sensitivity to the hazard. Sensitivity to hazard over time: 6 1:1 Natural disasters and catastrophes are high-energy events caused by natural Earth Processes: Synergy: combination of two factors or more. e.g. snowstorm=snow + wind. The amount of both has to reach a certain level for the combination to be considered hazardous. If only one of them crosses the threshold and the other does not, then it’s not hazardous. Conversely, even though neither cross threshold on their own, the combination can be hazardous. Once past the damage threshold, there are the three main factors that control how severe the hazard event is:  Absolute amount/intensity: too much or too little  Duration of event: too long  Rate of change: how rapidly you move into the hazard zone (cross damage threshold)—may not have enough time prepare. Important note: after a hazard event, there are both losses and gains: e.g. generators, lumber, steel (reconstruction), businesses get more money. Haiti earthquake led to discovery of petroleum sources. Whether a hazardous event becomes 1) a disaster 2) catastrophe depends on several factors:  extent of human losses of life  extent of human injury  property loss  area affected (geographic scale)  group vs. individual (?) 7  reconstruction time (during which people’s everyday lives are jeopardized)  societal reaction/perception (e.g. Japanese people are used to earthquakes) The rich lose their money (75%) and the poor lose their lives (90%). Why is the distinction between disaster and catastrophe important? a) Amount of government/world aid affected by identification b) Historical perspective: how often do really big events happen? (gives you an idea how big the event is) Finally, don’t forget: How to measure P and S ? H H  empirical observations and scientific studies (science)  Social/economic impact studies (humanities)—not just a matter of dollar costs! Should we believe the “experts”?  Experts usually add a margin of safety in risk analysis, the safety factor, to be on the safe side. They won’t tell you the exact “limit”. Ultimately, whether we want to incur a risk is a personal/societal choice. Advantage of boldness vs. advantage of caution?  There are always opportunities foregone when we take precautions.  People’s perception of risk varies with the nature of the risk. For instance, people see airplane crash as more risky than car crashes, even though the probability of them dying in a car crash is far greater. The general approach to Risk Analysis 1) Understand the hazard (what’s an earthquake?) 2) determine the risks from that hazard for the region of interest (P X S ) H H 3) Determine ways to reduce H or H . e.g. avalanche: reducH S by telling people not to ski. RHduce P (how likely the avalanche is going to happen) if at all possible. 4) do a cost-benefit analysis: economic + environmental + social + “personal” choice. what risks are we willing to take for what benefits? 5) implement mitigation techniques if warranted (worth it…the cost, the panic, everything…) Earthquake: Step 1 – Understanding the Hazard Understanding the hazard: Earthquakes 3 a) What causes earthquakes? Movements on faults! 2 Fault: a break/crack in rocks along which there has been appreciable displacement Elastic Rebound Theory: The rigid part of the earth can store elastic energy.  earthquake happens when either a) a *fault forms or b) there is an episode of movement of *pre-existing fault. In both cases, stored elastic energy is released. For a big earthquake, fault motion needs to be only a metre. How do we recognize a fault?  look for rock/soil layers that have been shifted. 3 kinds of faults: “strik slip”: think about striking matches! ”dip slip” means one side going under! Many rivers occur along faults because the broken up rocks can be easily washed away. And we often build dams on such rivers because the rivers make canyons. 2 A fault is simply a plane along which two masses of rock move relative to one another. A plate boundary is a fault in which the opposite sides are different plates. All plate boundaries involve faults, but not all faults are at plate boundaries (for example, the New Madrid fault zone in the middle of the North American plate). Earthquake: Step 1 – Understanding the Hazard b) Where do earthquakes occur? focus: “point” of energy release on a fault. It is a whole line of energy, but considered a pin-point scale wise. Epicentre: directly above the focus on the surface of the earth. It is important because it’s the closest place to the focus at Earth’s system. c) How do we find the focus, the epicenter and the energy released? By simple observation of where the damage is greatest, or use the seismometer: For horizontal motion: Inertial mass: a mass at rest rends to stay at rest. It wants to stay at rest even though the spring’s shaking like crazy—so is the ground. So paper squiggles while mass does not, resulting in squiggly lines drawn on paper. The diagram drawn by a seismometer is called a seismogram. Seismometers are placed deep underground to prevent disturbances from trucks and stuff. Using 3 seismometers make sure that you pick up all directions (E/W, N/S, up/down) To find the focus, or how far away the earthquake is, we need to first know what seismic waves are and how they behave. 1) Seismic energy waves radiate from the focus as body waves: 2) When body wave motions reach surface, some get transformed into surface wave motions, which is most damaging to human properties. Earthquake: Step 1 – Understanding the Hazard Surface waves Body Waves P waves: push-pull (compression-rerefaction). Kind of like how a caterpillar moves, except it doesn’t bend but uses flexibility of a spring to “spring” forward. springs forward a lot, then compresses a little to gain momentum, then springs forward again. S waves: also called shear wave. Travels like when you wave a jumping rope on the ground. 1) The more elastic a material is, the higher the seismic velocity. (e.g. rubber ball and dough have similar density, but rubber ball bounces back way more) 2) The more dense a material is, the lower the seismic velocity. (e.g. Iron ball and foam ball, though both are not very elastic, iron ball bounces less because it’s more dense). Where e=density, k and ʮ=elastic moduli. Why are there two different elastic moduli? Because there are 2 different ways to measure velocity: 1) How well something compresses (K)—for S waves, there is no need for compressibility (K) because it does not go in back and forth spring motion. Earthquake: Step 1 – Understanding the Hazard 2) How easily it is to change shape (ʮ)—the easier it is to change shape, the less elastic something is. P will always travel faster than S because ʮ has a 4/3 (>1) coefficient and an added K term. Because of this, P usually stands for primary and S for secondary. Seismic velocities of P and S waves for rocks at Earth’s surface: P-waves≈5.6 km/s S-waves≈3.3 km/s P goes 2x faster than S! e.g. Earthquake in Toronto ≈ 250 km away, takes ≈ 1 minute for P to arrive in Kingston. For a UNIFORM EARTH, it takes 35 minutes for P to get to the other side of the globe and 60 minutes for S. BUT earth is not uniform. So to measure the distance to an earthquake focus, we need to use empirically recorded (from historical data) travel times, which equals the time between P and S wave arrivals on your seismograph (shorter the time gap, closer the focus). Since we don’t know seismic velocities at depth (only surface rocks), and since seismic waves travel from depths to the surface, we need to first create a travel time graph: How to locate a focus though? We need a minimum of 3 seismic stations to locate a focus: Earthquake: Step 1 – Understanding the Hazard Each of the respective distances (red) forms the radius of a circle. The intersection of these three circles is where the focus is. What depths do earthquakes occur at? Less than 700 km deep (outer 10% of earth—radius of earth is 67600 km) because outer shell is rigid enough and elastic enough for things to snap and break, whereas inner part is more squishy. 90% of earthquakes happen at depths less than 100 km. How to measure the energy released from an earthquake? 1) Qualitative measurement: The Mercalli Scale from 1-12 (subjective observation, varies with distance from epicentre) 2) Quantitative measurement: The Richter Scale. Measures the maximum amplitude of S-wave on the seismogram, which determines the energy released (corrected for distance). *one does not use surface waves because it depends on the material. Each integer on the scale represents an amplitude on the seismograph 10 times higher (exponential increase). It is an open-ended scale, meaning the maximum could theoretically go beyond level 10. The largest magnitude ever recorded is 9. Earthquakes smaller than 2.5 are not felt by humans, but could be dangerous for nuclear reactors. Each magnitude integer corresponds to 30x energy release. So a magnitude 5 earthquake releases 900 times more energy than a magnitude 3 earthquake. We found that out (and calibrate for distance) by comparing the waves on seismograms made by known quantity of explosives (e.g. dynamite/nuclear blasts) to the amplitude recorded for earthquakes. So, the largest recorded earthquake (Chile, 9.5) releases the energy equivalent of annual USA energy release! How frequent do earthquakes occur? Earthquake: Step 1 – Understanding the Hazard Big earthquakes happen rarely: Power-law relationship: on a log-log plot, you get a straight line (which means linear relationship between exponents). Magnitude Frequency <3 Over 100,000 per year >3 Over 30,000 per year >6 100 per year >7 20 per year In general, the closer to the centre, the greater the damage; The greater the magnitude, the greater the damage; But this is modified by natural and anthropogenic conditions. Natural: nature of soil and rocks; Anthropogenic: nature of the structure that we build. List of earthquake hazards: 1) Surface Faulting: Structures on the fault will be destroyed by the tearing actions. So don’t build right on active faults (e.g. San Francisco on San Andreas fault). 2) Ground Shakes: This is the greatest threat to buildings and people, even though far away from centre. In a BIG earthquake, shaking can be severe even hundreds of km away. Most damage is a direct consequence of surface wave motion. Nearer the centre, S and P waves also contribute to the damage. Aftershocks: small earthquakes that occur soon after the main shock with epicentre in the same area as main shock. It can happen minutes after, or YEARS later! Aftershocks can collapse already damaged/fragile buildings. Earthquake: Step 1 – Understanding the Hazard 3 Material Amplification Effect: when seismic waves slow down as they go into another material, some of the extra energy is transferred into greater shaking (less velocity, more shaking): -on solid rock: less damage to buildings -on sand soil: buildings shake more; The amount of shaking also depends on the nature of the soil and rocks in the area. Soft soils (like clay) shake more than stiff soils (sand) which shakes more than rock. 3) Ground Failure a) “Landslides” rocks and/or soils. Triggers sides of mountains etc to break free and fall down, even if epicentre is far away. b) Liquefaction (which only happens to soils, not rocks!!) Specific kinds of sand/clay soil can change strength when shaken and can flow like a liquid. e.g. Leda clay: e.g. Liquefaction of sand can cause sand volcanoes!!! Sand volcano A conical body of sand, resembling the form of a small volcano. Sand volcanoes are formed by the extrusion of liquefied sand through a local vent at the surface. The extrusion usually results from a highly liquefied sand below a confining surface layer. 4) Tsunamis: Japanese for harbour waves, also called (scientifically) seismic sea waves. Can be triggered by 3 Earthquake: Step 1 – Understanding the Hazard earthquakes undersea: A tsunami: The dip slip can dip up or down. In the reverse dip slip scenario, water is first pulled away from shore then comes back. Either way, water will roll back to fill in the empty spots. *Strike-slip generally doesn’t cause tsunami Pebble-water effect: many waves hit land!! Tsunami Waves: Amplitude 1 metre or more Wavelength hundreds of km long Speed 500 to 800 km/hour (as fast as passenger jets) As water approaches shore, the waves slow down due to friction on the upward slope (shallowing sea floor) and the water piles up to as much as 60 metres high. The shallower the water depth, the lower the maximum wave height (no big risk in Lake Ontario!) because: a) Only a small amount of water is displaced b) Low speed of wave—less energy for amplification due to friction. However, a big tsunami can happen if triggered by a big landslide: Earthquake: Step 1 – Understanding the Hazard 5) Fires (caused by Earthquakes): San Francisco 1906. 6) Disruption of water supplies and resulting diseases: e.g. cholera because of dirty water in Haiti. 7) Human induced seismic hazards: a) Dam construction: 1 scenario: Loading of earth by water changes the stress regimeearthquake: nd 2 scenario: water gets underneath, dam failure, lubricate fault on the other sideearthquake plus flood: However this also raises possibility of earthquake control: get rid of fault stress little by little with water. b) Mining: Earthquake: Step 2 – Assess the Risks Assess the Seismic Risk 4 a) locate and determine nature of faults in the area 1. look on the ground, and from the air and space but this method cannot detect hidden faults 2. set up seismometers to locate fault only effective if fault has already moved fault zones are VERY complex: e.g. Haiti has four big faults; each has its own little faults. Not all faults produce earthquakes Inactive: has not moved in the last 2 million years. Potentially active: has moved in last 2 million years. Active: has moved in last 10,000 years This number is subject to change especially when you want to build a nuclear plant. These are ultimately human decisions. Kingston area has both small and very big faults. b) Study history of earthquakes in that area. Which faults are active? What energy do they release? How long do big quakes occur? 1. Set up seismometers, which gives us idea which faults are most active, as well as frequency and magnitude of earthquakes. Collect as long a seismic record as possible. - The arctic region, eastern and western Canada are most prone to earthquakes. - However, there is no big risk in arctic because of low population. 2. Determine the recurrence interval of BIG (>7) earthquakes in that area. RECALL: power law relationship between frequency and magnitude. Earthquake: Step 2 – Assess the Risks Relative Time: 1) Law of superposition: for sediment layers, oldest are on the bottom, youngest on the top. 2) Law of cross-cutting relationships: if faults cut across layers, then fault is younger than the layers - look at human historical records (goes back (happens after layers are deposited. before seismometers) - dig trenches and pits on the active faults to identity BIG ancient fault movements Recurrence Intervals (cross section of the ground) e.g. 9 major events in 1400 years in LA: Step one: determine the relative age of the fault. 1400/9=160 years recurrence intervals. However, events don’t happen like clockwork—aren’t evenly spaced. Step two: determine the absolute age: Peat layers contain dead organic matter. Carbon 14 (C 14) dating to determine when the faults last moved. Measure the ratio of C14:C12, C 14 undergoes radioactive decay to become N14. 1 half life for C-14 is 5730 years. HOWEVER: whether a fault movement is “big” is subjective here. For less subjective evidence… c) Evidence of ancient tsunamis and ground elevations/subsidence. 1. Tsunami: produced by big earthquakes, dumps sand on swamp—use C14 dating. Earthquake: Step 2 – Assess the Risks 2. ground subsidence/elevation:  These events can instantly bury swamps with sand. You can date the dead organic matter Construct Probability and Earthquake-Hazard Maps Helps future planningshow where the earthquake probability areas are. a) Determine the geologic/geographic factors in the area: 1. Map out locations of rock, sandy soils, clay soils, etc; 2. Map out zones of sand and clay that are prone to liquefaction (risk zones) 3. Map out cliffs and hills that are prone to landslides; 4. Map out tsunami hazard zones (shorelines and oceans) and lakes with steep mountains. (inlets also have potential for major tsunami due to landslides) b) Determine human interactions with the potential hazard in the area: 1. What is the distribution of population? Particular attention to proximity of people to high-hazard zones. 2. What is the nature of the human infrastructure? Where are the buildings and roads WRT high-hazard zones? What are the buildings made of and how are they designed? Vancouver has biggest risk>Montreal>Ottawa>Victoria>Toronto>Quebec GeneralApproach Step 3 – Ways to Reduce P and S H H Ways to Reduce Probability and Severity of Hazards 5 To Try to reduce Severity (H ) 1. Apply land use planning and zoning - Use high-hazard areas for low population use (parks, golf courses) - “set-back” locations: build on either side of the fault, not on it (so houses won’t be ripped apart). - Don’t build on areas with soft soil or soils that might liquefy. 2. Apply stringent building codes a) Choose appropriate building materials Good Materials Bad Materials Wood Heavy masonry Steel Adobe Reinforced (with steel) concrete Stucco Un-reinforced concrete Why? flexibility Why? inflexible e.g. 2 earthquakes in 88/89 of similar magnitude: Loma Prieta, Calif. 65 dead vs. Armenia 50.000 dead. b) Choose resistant building design: Buildings have a natural period of vibration of: ~ one vibration per N/10 seconds, where N=number of stories. Taller building means longer period of vibration. * The worst damage occurs when the periods of vibration of ground and the building are the same, because they will amplify each other. Ground Periods Building period Soft soil: several seconds Tall building: several seconds Worst combinations Solid Rock: <1 second Short building: <1 second So, this is how you design buildings: Tall Buildings: Top moves more than bottom—make Buttress it: on either end. top narrow!! Bracket it: at corner Spring with dampers: Bolt it: between bottom panel and foundation Block it: vertical supply Panel it: plywood Isolate it: wheels on ground. Horizontal Joints: e.g. snake-like oil pipes to prevent strike-slip fault damage. Brace it: cross-piece. GeneralApproach Step 3 – Ways to Reduce P and S H H c) legislate and enforce regulations about construction (government involvement)—this ensures a) and b). All of the above aim at minimizing damage but they cost a lot of $$! 3) Set up warning systems and emergency-response plans a) earthquake warming systems and tsunami early warning systems: Earthquakes: Radio waves travel faster than seismic waves, so set up seismometers near faults and send signals to distant placeswarning 15 sec to 1 min in advanceenough to shut of gas, valves, stop trains, etc. Tsunamis: 1. Sensors on ocean floor that detect sudden change in depths 2. maintain natural shorelines mangrove trees=less impact of waves 3. limit population centres along shoreline 4. provide tsunami education. b) Provide emergency equipment and personnel - army involved, etc and red cross - provide emergency response plans c) Educate the public - booklets: before, during and after. - earthquake drills Self-organized criticality: critical state that’s 4) Try to predict earthquake—is it possible? unpredictable. Earthquake behavior a) long term: recurrence intervals (accurate to decades) is chaotic in the mathematical - look for seismic gaps which are high risk areas on faults that sense. got stuck—“locked”—and has lots of stored energy. b) short term: impossible… - scientists are less sure because of the chaos theory HOWEVER, changes preceding an earthquake might provide some short-term warning: In a fault, the main change as stress builds up is the formation of microcracks in rocks in a process called dilatancy or expansion in volumes. This can lead to: 1. Ground bulge (not always reliable) 2. Microseismicity and foreshocks - small seismometer in the mine shaft to predict rock e.g. How big a landslide will the next bursts and warn miners (“listen” to cracks) grain of sand cause? We don’t 3. Increase in radon in water wells—cracks in rocks release know!! radon gas. GeneralApproach Step 3 – Ways to Reduce P and S H H Try to reduce Probability (H ) 5) Try “controlling” earthquakes: Inspiration: areas with talc tend to be creeping segments that move smoothly without causing major quakes because talc lubricates. We can use water to lubricate faults, but you can accidentally overdo it. Final 2 steps!! Step 4 – do a cost-benefit analysis: economic + environmental + social + “personal” choice. what risks are we willing to take for what benefits? Step 5 – implement mitigation techniques if warranted (worth it…the cost, the panic, everything…) Volcanic Risk Background: Rock Cycle and Plate Tectonic Cycle Volcanic Risk 6 Introduction to the Rock Cycle and the Plate Tectonic Cycle What causes stress in the earth? release of Earth’s internal heat plate tectonic cycle!!! 6:1 Energy Acting on Earth External energy Internal energy Dominates lithosphere Over time, has a profound effect Wears down the Earth’s surface Building up the earth’s surface (erosion etc) 6.6 x 10^7 calories/day a) Sun: 37000 x 10^17 calories/day - The earth is a heat engine - drives hydrological cycle - causes volcanoes, mountain building, earthquakes, - drives oceanic conveyor belt (energy etc. balance, more heat up north) - drives air currents Daily, Earth receives 6000 times from the sun as from inside!! b) Gravity: 0.6 x 10^17 calories/day - Earth/moon interaction up to 17 m tides tidal energy dams!! - moons gravity stabilizes earth’s tilt c) meteorites: small amount 6:2 Rock Cycle Summarizes the interaction between internal and external energy. Igneous: formed where liquid rock (magma) cools and crystallizes Volcanic Risk Background: Rock Cycle and Plate Tectonic Cycle Sedimentary: formed when weathered “particles” get deposited and lithified. Metamorphic: Gets pushed down during subduction. changed by high temperature and pressure in the solid state. (If pushed down far enough it can melt and become igneous again.) 6:3 Internal Energy: Plate Tectonics Cycle Sources of internal heat: 1) Formation of solar system: 4567 million years ago a) the earth grew like a growing snowball from comets and stuff, and heats up through “impact of planetesmals” b) Sinking of iron to form core of the Earth—iron sinks towards centre during differentiation, which, like comet impact, gives heat. c) comet impact resulting in formation of the moon—more heat . 2) Decay of radioactive elements like radium Earth gets rid of internal heat by: 1) Conduction—lumping together of atoms (when you get burned, it’s the atoms bumping against your hand) - The deeper you go, the hotter it gets (e.g. down the mine) o - Average geothermal gradient ~ 25 C/km near surface. 2) Convection—large-scale movement of hot material Internal heat is NOT released uniformly across earth’s surface. Earthquake zones tend to be the same as volcanic zones and mountain ranges!! These are like cracks in egg-shells. - These zones reveal patterns of large-scale convection in the mantle - the plate tectonic theory summarizes convection: But the mantle is solid (we know it’s solid by seismic waves—both P and S go through mantle, but S doesn’t go through liquid outer core). So how does solid convect?? Volcanic Risk Background: Rock Cycle and Plate Tectonic Cycle Mantle is elastic-plastic, behaves like silly putty—both solid and liquid. it convects plastically: - plastic: during slow stress application - elastic: during rapid stress application Rocks are just like silly putty!! 6:3:2 Theory of Plate Tectonics 1) Outer part of earth consists of a number of rigid lithospheric plates.  plate boundaries are marked by earthquakes and volcanoes Plates are not the same as crust!! plate includes upper mantle, which is brittle. These two make up the lithosphere. Oceanic crust: 5 km thick Continental crust: 30 km thick The lithosphere (100 km thick) is the plate that slides around on the asthenosphere, which is plastic. - slides at rate of several cm per year (fingernail growth) - almost all igneous activity and mountain building and earthquakes is located along the plate margins. Type 1: Constructive plate margins (divergent): Upswelling part of the convective cycle - Some make new lithosphere plates, some make new ocean crust (if melts at surface due to friction. e.g. red sea, mid-Atlantic ridge. If it happens under a continent, continent will split, making room for new crust and oceans to form. e.g. east Africa. Type 2: Destructive plate margins (convergent): down-going part. takes plate down and destroys it. - The only kind of lithosphere that is dense enough to subduct is oceanic lithosphere (mafic). Trenches=where ocean lithosphere plates are subducted. a) Oceanic plate (lithosphere) under oceanic lithosphere: friction causes melting,
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