美國-工程地質學

1、GEOL 206Earthquakes, Volcanoes and Plate TectonicsInstructor: Chris DeVriesSummer 2007About your Instructor MSc. Candidate, McGill University, Earth and Planetary Sciences Volcanologist-in-trainingAbout the Course Twice-a-week meeting, no lab Tuesday/Thursday 18h00 21h00, H-937 Assumes very little s

2、cientific training high school science is plentyExaminations There will be two examinations in this course May 17 June 7 The exam you do best on will be worth 45%, the other exam worth 25% Exams will consist of Multiple choice True or False Fill-in-the-Blank Short paragraph answerEarthquakes, Volcan

3、oes and Plate Tectonics The process of plate tectonics, and interactions that it creates are the foundation of our dynamic world Crust is created and destroyed Grinding of plates at contact zones produce devastating earthquakes Magma is formed deep under the crust and sometimes makes its way to the

4、Earths surface All of the consequences of plate tectonics have their own consequences Its a large and complex system that by the end of this class, I hope you have a better understanding ofCourse ObjectivesIntroduce you to basic concepts in geologyPlate Tectonics The history of the theory of plate t

5、ectonics An overview of how the plates move and what happens when they interact Magma generation and faultingVolcanism What landforms can be attributed to volcanism Local and global hazards Mitigating hazards the study of volcanoes Volcanoes and global changeEarthquakes Where do earthquakes occur an

6、d why? When the earth shakes, what is happening? Can we predict earthquakes? Mitigating earthquakesThe Big Bang Still the accepted theory of creation in the scientific community Occurred 13-18 billion years ago A single point of concentrated energy exploded The immense heat of the explosion caused F

7、usion of quarks (fundamental particles) Formed protons (1H) and neutrons Fusion of protons and neutrons Formed 2H, 3He, 4He and 7LiFirst Billion Years The fundamental force of gravity caused particles to be attracted to each other Hydrogen and helium (by far the most abundant elements in the univers

8、e) formed large, gaseous bodies Heat from interactions between particles increased, as mass of these bodies grew First stars initiated nuclear fusion 1 billion years after the Big Bang Fusion of 2H to 4HeThe next few billion yearsFirst-generation stars started to run out of 2H fuelAs the energy outp

9、ut decreased, and their density increased (4He has twice the mass of 2H, but only slightly more volume), gravity caused stars to collapse into themselvesCollapse caused heat output to increase againFriction between particles increased as volume decreasedIncreased heat allowed for the fusion of heliu

10、m into carbon4He + 4He 8Be + 4He 12CSmall amount of 16O is also produced by adding 4He to 12CNew phase of star collapse sees12C + 12C 24Mg 20Ne + 4He16O + 16O 32S 28Si + 4HeProduction of even-numbered elements up to iron (Fe)When a stars core is Fe-rich, fusion becomes thermodynamically impossibleSt

11、ars explode catastrophically (supernova)Elements heavier than Fe are formed by neutron-capture of Fe during supernova eventsOdd-numbered elements are formed by various forms of fission of the even-numbered onesOur Solar SystemOur sun was formed in a nebula consisting of remnants of supernova Second-

12、generation starInner planets (Mercury, Venus, Earth, Mars) formed by the coalescing of planetesimals In an area of nebula that was very hot (1200 K)Temperature remained higher than melting point of iron, nickel and many silicate minerals due to Impact energy of planetesimals hitting planets Radioact

13、ive decay of elementsDense elements sank to the center of the Earth Core of nickel and ironLight elements floated to the surface Crust of aluminum, oxygen and siliconDifferentiationEarth Thought to be fully formed 4.6 billion years ago Began to cool Solid crust (mostly Al, O, Si) Mostly solid mantle

14、 (Fe, Mg, Si) Liquid outer core (Fe, Ni) Solid inner core (Fe, Ni)We will look at this in more detail next weekThe Earths inventory of light elements were primarily delivered during the accretion of our planetplanetesimals, meteors and cometsSilicate minerals (which are the primary component of all

15、pre-Earth planetesimals) always have oxygen in their structure (as part of the SiO44- tetrahedron), and often contain hydrogenCarbon and nitrogen came from carbonaceous chondrites, a less common type of meteor that accreted alongside the more common siliceous bodiesElemental (e.g. He, H2, N2) and mo

16、lecular (e.g. H2O, CO2) volatiles entered and formed Earths early atmosphere due to:Degassing of the mantleHeat from accretion (and from gravitational compression as the Earths mass and therefore gravitational force increased) caused minerals to melt and outgas some of their light element componentP

17、ost-accretion meteor impactsBrought additional gaseous components to EarthEarly degassed volatiles during accretion were lost to spaceOnce Earths radius was 40% of present-day, gravity was great enough to retain themEarly atmosphere probably was of similar composition to present-day volcanic emissio

18、ns H2O: 90%CO2+H2S+HCl+HF+N2+NH3+H2+CH4+Ar+He : 10%The History of EarthAtmosphere and OceansAtmosphere initially composed of gases escaping from molten rock Pre-solidification of mantle and crust Post-solidification (volcanic activity)Gravity kept most of these gases from escaping into outer spaceWh

19、en surface temperature of Earth cooled below 100C, water condensed Global rainstorm created oceans Water has existed in liquid form on the surface of the earth for the past 3.8 billion years Oceans achieved their present volume in a very short timeNote that life evolved in the absence of O2 gas O2 s

20、cavenged by reduced gases (e.g. H2, H2S, NH3) and by dissolved Fe2+ in the oceans Oxygen only began to accumulate in the atmosphere 2 billion years ago Due to photosynthesisMinerals Formal definition Minerals must be crystalline solids occur naturally be of definite chemical composition be inorganic

21、Crystalline Solids?Eg. Halite, NaCl (table salt)Cubic arrangement of Na+ (red) and Cl- (yellow) ionsHalite crystals are cubic in shape.Definite Chemical Composition? An analysis of the mineral will always produce the same ratio of elements. E.g. if you analysed NaCl, you would find a 1:1 ratio of so

22、dium to chlorine, always. E.g. if you analysed 2 different crystals of (Mg,Fe)2SiO4 (the mineral olivine), where Mg and Fe can substitute for each other in the structure, the molar ratio of Mg + Fe to Si would be 2:1 in both crystals.Inorganic? Organic compounds are those that contain carbon and hyd

23、rogen and are made by living organisms. Some organisms can make minerals E.g. shell-building organisms, shells of calcite or aragonite (CaCO3). Calcite is inorganic. Amber (petrified tree sap) is NOT technically a mineral, as it is organic.Silicate Minerals Most common igneous minerals Form four maj

24、or structures based on the silicate tetrahedron (SiO44-) To form stable minerals, silicate tetrahedra Charge-balance with positively-charged ions (ionic) Share oxygen atoms with other tetrahedra (covalent)Form 1: Isolated silicate structure Single tetrahedra joined by positively charged ions E.g. ol

25、ivine (Mg,Fe)2SiO4)Individual SiO44- tetrahedra joined by Mg2+ and Fe2+ ions (green balls)Form 2: Chain structures Single or double chains of tetrahedra joined by positively-charged ionsSingle and double chains of silicate tetrahedraAugite (a pyroxene) is a single-chain silicate mineralForm 3: Sheet

26、 structures Sheets of silicate tetrahedra joined together by positively-charged ionsA sheet silicateMica is a sheet silicateForm 4: Framework structure A framework structure contains silicate tetrahedra joined in a 3-dimensional array Quartz (below), SiO2 is the best example of a framework silicateR

27、ocks Made up of minerals Igneous Rocks Formed from solidifying magma (molten rock) Sedimentary Rocks Formed from the lithification (compaction and dehydration) of sediments in oceans and lakes Metamorphic Rocks High pressure and/or high temperature re-crystallise igneous, sedimentary or existing met

28、amorphic rocks in a process called metamorphism Contain same chemical elements in same ratios as original rock Water and other fluids can be lost or added however Contain different mineral assemblages than the parent rockThe Rock CycleShell-formingmarine organismsClassifying Igneous Rocks Common to

29、classify igneous rocks based on their silica content Felsic rocks have a high (65%) SiO2 content (fel feldspar, a common mineral, si silica) Mafic rocks have a low (50%) SiO2 content (ma magnesium, f ferric, referring to Fe iron) Intermediate rocks have a SiO2 content between felsic and mafic Ultram

30、afic rocks have very low (45%) SiO2 content Rule of thumb The darker an igneous rock is, the less silica it has There are exceptions!Intrusive vs. Extrusive Igneous classification based on whether the rock solidified below ground, or erupted from a volcano Intrusive rocks tend to be coarser-grained

31、as they cool very slowly underground, giving crystals time to form Plutons vs. dikes vs. sills (see diagram) Extrusive rocks are usually fine-grained, often so that you cannot even see individual crystals, as they solidify in minutes to hours.Any crystals that you do find in extrusive rocks will usu

32、ally have formed before eruptionThe major igneous rock typesUSGS, 2000(Granite)(Granodiorite)(Diorite)(Gabbro)*These terms will be revisited in greater detail*IntrusiveExtrusiveFelsicMaficHow Old is the Earth? Ussher (Irish archbishop) Creation occurred on Oct 23, 4004 BC. Counted back Biblical gene

33、rations Lord Kelvin (physicist) Used the rate of cooling of the Earth to calculate at what time it would have been completely molten Came to 100,000,000 years Current knowledge based on radiometric dating of oldest minerals (found on earth) and of meteorites to determine age of solar system Age of e

34、arth as we understand it: 4.6 billion years Why was Kelvin wrong?Geologic Time Geologists think of time differentlyHow do we measure time? Absolute time We can tell how old a rock is via radiometric dating techniques Work best on igneous rocks When an igneous rock crystallises, minerals within it so

35、metimes contain unstable, radioactive isotopes These decay at fixed rates, forming different elements (aka half-lives) By measuring the ratio of the “daughter-product” to the radioactive isotope, and determining how much radioactive isotope was present at the time of formation (via the half-life of

36、that isotope), you can calculate how old the rock is.Concept of Half-life Half-life the amount of time it takes a radioactive element to reach half its original amountHalf-lives, daughter products, and effective dating range of commonly-used isotopes for radiometric datingK-40Half-life of 1.25 billi

37、on yearsDecays to Ar-40 (argon)Effective in dating rocks that are 100,000 4,600,000,000 years oldU-238Half-life of 4.5 billion yearsDecays (eventually) to Pb-206 (lead)Effective in dating rocks that are 10,000,000 4,600,000,000 years oldRb-87Half-life of 49 billion yearsDecays to Sr-87 (strontium)Ef

38、fective in dating rocks that are 10,000,000 4,600,000,000 years oldC-14Half life of 5,730 yearsDecays to N-14 (nitrogen)Effective in dating dead, preserved organic matter that died 100 40,000 years agoCarbon dating Useful in determining age of organic material Vegetation, while alive, converts CO2 t

39、o living tissue (photosynthesis)12CO2 and 14CO2 maintain constant ratio in the atmosphere, and maintain constant ratio in vegetation, while it lives Upon death, the C-14 (radioactive) in a plant decays By measuring the radioactivity of dead organic matter, we can indirectly determine the current rat

40、io of 12C to 14C, and determine when the plant died Only useful for material younger than 40,000 years After 40,000 years, there is not enough 14C to measureRelative Time Principles of Original Horizontality Superposition Lateral Continuity Cross-cutting relationships Unconformities Disconformities

41、Angular unconformities NonconformitiesOriginal Horizontality and SuperpositionLateral ContinuityCross-Cutting RelationshipsUnconformities Disconformity Where layers of sediment are apparently “missing” Missing rock strata separates beds that are parallel to each other Usually identified by a gap in

42、the fossil record If in one layer you find fossils that are indicative of 320 million years old, and in the layer above it, you find fossils circa 200 million years old, you have a disconformity Erosion of the missing layers following their depositionAngular Unconformity Where a younger horizontal s

43、trata overlies older strata that are tilted or foldedExplained again by erosionNonconformity A contact between strata where an erosional surface on a pluton is covered by younger sedimentary or volcanic rock implies millions of years of erosion Often plutons are many km below the surfaceReview of Un

44、conformitiesDisconformities1) deposition of horizontal sedimentary layers2) erosion of one or more sedimentary layers3) deposition of new layers atop much older layers, out of sequenceAngular Unconformities1) deposition and lithification of sedimentary rock2) folding or tilting of layers3) erosion4)

45、 renewed deposition on the erosional surfaceNonconformities1) formation of pluton deep underground2) erosion of several km of overlying rock3) deposition and lithification (eventually) of new sediment on old igneous surfaceQuestionABCDEFGHIJ1) Put the letters in order from oldest to youngest2) Betwe

46、en what letters did the fault occur?3) Where did erosion take place?4) Are there any unconformities? If so, where, and what type are they?The Structure of the EarthDifferentiation of the Earth When Earth was molten, denser material and lighter material separated gravitationally Differentiation is re

47、sponsible for the structure of the Earth as we understand itStructure of the EarthInner CoreOuter CoreMantleCrustNot to scale!How do we know what is inside Earth? Seismic data Waves from earthquakes travel at different velocities through different media Network of seismometers detects delays as the

48、waves go through the Earth Infer different layers of different density and composition We will talk about this in great detail when we discuss earthquakes Density measurements Average density of the Earth is 5.5 g/cm3 Measure speed of earths rotation, revolving around the Sun, calculate density Average density of the crust is 2.7-3.0 g/cm3 Density must be much greater inside the earth to account for thisHow do we know what is inside Eart