The LithosphereThe LithosphereChapter 9Chapter 9
Geologic Time vs. Human TimeGeologic Time vs. Human Time
3How do we date rocks?How do we date rocks?–Relative time• Superpostion–Oldest rocks on the bottom, youngest rocks on the top–Absolute time• Radiometric dating–Atoms with unstable nuclei (isotopes) decay intostable forms–Each isotope has a specific decay rate–Half-life--the length of time it takes one half the isotope todecay–Measuring the ratio of unstable to stable material givesa date (1:1 means one half-life has passed) 3
4The oldest rocksThe oldest rocks Earth first condensed and formed into aEarth first condensed and formed into aplanet about 4.6 billion years ago.planet about 4.6 billion years ago. Oldest rocks are from the Acasta Gneiss inOldest rocks are from the Acasta Gneiss innorthwestern Canada: 3.96 billion years oldnorthwestern Canada: 3.96 billion years old OldestOldest grainsgrains of rock, from Western Australia:of rock, from Western Australia:bet. 4.2 and 4.4 billion years oldbet. 4.2 and 4.4 billion years old Will we ever find large pieces of the oldestWill we ever find large pieces of the oldestrocks or rocks that are even older?rocks or rocks that are even older?4
77Earth’s magnetic fieldEarth’s magnetic field• Circulations within Earth’s inner and outercores are thought to be the mechanism forthe magnetic field, called themagnetosphere, that protects Earth fromsolar wind and cosmic radiation.• The magnetic north and south poles migrate• Geomagnetic reversal also happens inirregular intervals77
88Which elements make upWhich elements make upEarth’s crust?Earth’s crust?• Core–Fe, Ni, Si (and/or) O• Mantle–O, Si, Mg, Fe, Ca, Al• 99% of the crust is made up of 8 elements:–O, Si (make up 74.3%)–Al, Fe, Ca, Na, K, Mg88These elements, and trace others, combine tomake up Earth’s minerals and rocks...
What is a Mineral?A mineral is…a naturally-occurring,homogeneous solidwith a definite(but generally not fixed)chemical compositionand a highly-orderedatomic arrangement.It is usually formed throughinorganic processes.
Okay…quick quiz…Okay…quick quiz…Does ice fit the definition of a mineral?
...homogeneous solid...• It can’t start out as one mineral and becomeanother mineral halfway through (but it canchange color and still be the same mineral)Tourmaline (Elbaite variety):Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4
• Quartz–SiO2 (silicon dioxide)• Olivine–(Mg, Fe)2SiO4...definite (but generally not fixed)chemical composition...
...highly-ordered atomicarrangement.• Carbon dioxide gas is made up of atoms thatare not all bonded together into a structure.• When frozen, CO2 is bonded together...forming...what?
The shape of the mineral crystal is the result ofits internal atomic structure.chainsflat sheetscubesrhombohedronsasbestos (crocidolite)muscovite micagalenarhodochrosite
What if a substance doesn’t fit thewhole definition?• It’s a mineraloid.Some substances look like minerals--but they LIE.• Glass is a mineraloid. It has an “amorphous” atomicstructure.- O- Si
As light passes through the glass panels, it is distorted bythe ripples in the glass. Which panel is the new one?
Biogenic minerals: Exceptions tothe “inorganic processes” ruleFormed by living things– The pearl and shells of oysters• Aragonite– The main mineral found in humanbones and teeth• Apatite– Diatoms and radiolarians in theocean• Silicate minerals• Their skeletons are used as polishingagents......(in your toothpaste!)
Is coal a mineral?Is coal a mineral?No.Coal, petroleum, and peat are NOT minerals.–They have no definite chemical composition–They have no ordered atomic arrangementPetroleum, coal and peat are mineraloidsBut…They can form minerals under certain conditions–If coal beds are heated to high temperatures andthe carbon in them is crystallized, they can formthe mineral, graphite
Mineral Formation:Mineral Formation:Non-organic formationNon-organic formation• Minerals can form from:–Magma–Steam (these minerals are called vaporites)–Mineral components left behind when waterevaporates (these are called evaporites)–Mineral components dissolved in water that solidifyagain (these are called precipitates)
From steam: vaporitesFrom steam: vaporites• Vaporite–Water near a magma source heats up, dissolvesmineral components–Mineral compounds “condense” and solidify at thesurfaceCommonly found aroundvolcanic vents, these sulfurcrystals are forming aroundKilauea Crater, Hawaii. Alsofound near oceanic volcanicvents.Harvesting sulfur crystals from a volcano in Indonesia
From steam: vaporitesFrom steam: vaporitesHarvesting sulfur crystals from a volcano in Indonesia
From evaporating water: evaporitesFrom evaporating water: evaporites• Evaporite–Water evaporates, leaving behind mineralcompounds dissolved in the waterSalt crystals like these form insalt beds along the edge ofHighway 84, just before theDumbarton Bridge.
• Precipitate–When in too high a concentration to remaindissolved in a liquid, the mineral componentscondense and may actually “rain” out of thesolution–Often occurs when warmer water, which canhold more dissolved solids, cools slightlyMineral Formation:Mineral Formation:Non-organic formationNon-organic formation
Experiment• Make a precipitate mineral at home...
From dissolved solids: precipitatesOolitic beach sand from the Caribbean, the Bahamas,and Great Salt Lake in Utah form from calciumcarbonate precipitating out of the water
From dissolved solids: precipitatesPyrite “sand dollars” form in sea water and collect on theocean floor
What is a Rock?What is a Rock?• A solid, cohesive aggregate of one or moreminerals or mineral materials–A few rock varieties are made up almost entirelyof one mineral• Example: massive accumulations of halite
The Rock CycleThe Rock Cycle• Rocks have been continuously forming andreforming over millions and millions of years• The rock cycle is a closed flow system
What drives the rock cycle?What drives the rock cycle?• Earth’s internal heat–Radioactive decay from its initial formation• The main power source for this flow system• The sun–Drives weather systems that erode material• Secondary power source
The 3 Classes of RockThe 3 Classes of Rock• Classified based on the processes whichform them–Igneous rocks–Sedimentary rocks–Metamorphic rocks
Igneous RocksIgneous Rocksfrom cooling magmasfrom cooling magmasfrom cooling magmasfrom cooling magmas• The kinds of rocks you end up with (andultimately the kinds of structures built by theformation of those rocks) are determined bythe chemistry of the magmas you start with
Felsic MagmasFelsic Magmas• The “fel” is for feldspar (made from Si, Al, O), “si” is forsilicates (made from Si and O)• Lighter in color, lower in density than mafic minerals• Continental crust is predominantly felsic material• Cooler magmas, containing lots of silica (SiO2)• Highly viscous (resistant to flow)• High concentration of gases under high pressure• Gases can’t rise easily, so they stay trapped until nearthe surface• As pressure is released at the surface, thesepressurized gases tend to explode– (Example: Mount St. Helens)
Felsic MagmasFelsic MagmasExplosive Mount St. HelensExplosive Mount St. Helens(Pre-eruption)
Felsic MagmasFelsic MagmasExplosive Mount St. HelensExplosive Mount St. Helens(May 18, 1980)(May 18, 1980)
Felsic MagmasFelsic MagmasExplosive Mount St. HelensExplosive Mount St. HelensMt. St. Helens Today
• The “ma” is for magnesium, the “f” for iron(Fe--ferris); contains more metallic, heavyelements (Al, Mg, Fe, K, Ca)• Darker in color, higher in density• Oceanic crust is predominantly mafic• Low viscosity (flow easily for long distances)• Very little gas content, relatively little SiO2• Hotter magmas, so gasses stay dissolved• Rarely explosive– (Example: Hawaiian volcanoes)Mafic Minerals and RocksMafic Minerals and Rocks
Mafic MagmasMafic MagmasHawaiiHawaii2002-10-11 view northwest across coastal plain of Kilauea from WestHighcastle lava delta to Pu`u `O`o in upper right skyline
Intrusive vs. ExtrusiveIntrusive vs. ExtrusiveIgneous RocksIgneous Rocks• Intrusive igneous rocks—magma coolsbeneath the crust; crystals have more timeto form; harder, more erosion-resistant rocks• Extrusive igneous rocks—magma cools onthe surface (lava); crystals don’t have time toform good crystal faces, if it cools fastenough, no crystals will form(obsidian--“volcanic glass”)
Extrusive Igneous RocksExtrusive Igneous RocksBasalt and ObsidianBasalt and Obsidian
Sedimentary RocksSedimentary Rocks• The result of erosion, transportation, deposition, andlithification• Sediment—Weathered rock fragments that havebeen transported and deposited, usually by water(or by air or by glacial ice movements)
Erosion:Erosion:Sediment is made throughSediment is made throughMechanical or Chemical WeatheringMechanical or Chemical WeatheringMechanical or Chemical WeatheringMechanical or Chemical Weathering• Mechanical weathering—The physical force of aparticular process acting on rocks, such as waterpounding rocks in a riverbed- OR -• Chemical weathering—Chemical reactions, usually inthe presence of water, which operate to change thestructure of the minerals, and break them apart–(Example: iron in the presence of water and oxygen)
TransportationTransportation• Distance of transport and speed of the fluidmedium determine size, roundness andsorting of transported material
DepositionDeposition• Deposition generally occurs in flat layers calledstrata (or “beds”)• Principle of Original Horizontalitystates that material is originallydeposited in horizontal layers andlater is shifted as it is affected bycrustal movements
Sedimentary RocksSedimentary RocksBadlands National Park, SDBadlands National Park, SD
Horizontal layersHorizontal layersafter tectonic movementafter tectonic movement
LithificationLithification• Lithification is the process of turningsediment into rock• Compaction and cementation–As sediment is deposited, the addition of morelayers causes compaction (like a trash compactor)–Dissolved minerals such as silica recrystallize inpore spaces–Sediment grains are cemented together
Metamorphic RocksMetamorphic RocksAny kind of rock subjected to heat and/or pressure
Two types of metamorphismTwo types of metamorphism• Regional metamorphism–Occurs over 100s or 1000s of sq. miles–Common in subduction zones (where crustalplates collide)• Contact metamorphism–Localized–Heat and pressure of rising magmas “bakes” thesurrounding rocks