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exam2 notes - EXAM 2 2.12.2007 Chapter 12 Volcanism Magma...

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Unformatted text preview: EXAM 2 2.12.2007 Chapter 12 Volcanism Magma after is erupts at the Earth’s surface is called Lava. VolcanoMountain formed from erupted lavas and other material. Volcanism 4. is a fundamental process in building the Earth’s crust 5. a major natural hazard in places 6. Analysis of lavas allows geologist to infer properties of the Earth’s interior. 7. gases escape into the atmosphere Types of Lavas Mafic-Basalt Intermediate-Andesite Felsic-Rhyolite Basaltic lavas- dark in color, hot 1200 degrees Celsius, low viscosity can flow downhill at 10 km/hr 8. flood basalts- erupt on flat terrain spreads out in sheets. Successive flows build up plateaus. Figure 12.16 9. other lavas on land called Pahoehoe and Aa figure 12.3 10.Pillow lavas- formed by underwater eruption. Ellipsoidal, pillow like blocks – 1m wide figure 4.12 e.g. formed at/near mid ocean ridgesspreading centers. Can later Rhyolitic lavas- felsic, light in color, erupted at <900 degrees Celsius. High viscosity, lavas tend to pill up into thick, bulbous deposits figure 12.11 Andesitic lavas: properties in between basaltic and Rhyolite. Textures of Lavas: Generally fine grained or glassy. Vesicles- holes in top of lava formed by gases escaping before solidification, figure 12.6. Rhyolitic lava with lots of holes is so light it floats in water- pumice. Pyroclastic deposits- water and gases in magma can be released explosively shattering lava and overlying rocks forming pyroclasts. If very fine, 2 mm in sizes, called volcanic ash. Other pieces may be much larges figure 12.8. When pyroclasts fall to the Earth and cool they form tuffs/volcanic breccias. Pyroclastic flows- hot ash, dust, gases ejected from volcano as glowing cloud. Rolls downhill at 200 km/hr. Little warning, can lead to great loss of life/property figure 12.10. These did most of the damage at Pompeii in 79 AD. Eruptive stylesCentral Eruptions figure 12.11 A. shield volcano- broad gently slopping made up of many thin basaltic layers e.g. Hawaii. B. Volcanic dome- bulbous mass of felsic lavas piled up at/near vent. E.g. top of Mt. St. Helens C. Cinder cone- ejected material (mainly pyroclasts) dip away from the summit, E.g. Cerro Negro. D. Composite (strato) volcano- made up of “alternating” layers of pyroclasts and lavas (generally andesites) build up classical volcano shape (quite steep), e.g. Mts. Hood/St. Helens/Vesuvius/Etna/Fujiyama. E. Crater- pit at the summit of many volcanoes. Maybe much wider than “vent” from which eruption occur. Can be >100 m wide and deep. F. Caldera- after violent eruption, empty “magma chamber” no longer able to support roof. Volcanic structure collapses, forming basin like depression, often miles wide. Lake forms figure 12.12/11. G. Phreatic explosions- hot magma meets sea or ground water, forming super heated steam and causing violent explosions figure 12.5 E.g. Krakatoa (Indonesia) – 1883 explosion heard in Australia thousands of km away. Killed 30,000 people (tsunami) Fissure Eruptions 11.On land- form flood basalts (figure 12.15/16). Many cover – 1 million sp. Km, apparently “erupted quickly”. 12.At spreading centers- form entire sea floor, topped by pillow lavas. 13.Ash flow deposits- huge eruptions of pyroclasts cover large areas and 1km thick. Never seen by man, but in past occurred at Yellowstone. Other phenomena: Lahar- torrential mudflow of wet volcanic debris- very dangerous. Mix of water/rain/ice with pyroclasts or lava. Forms “slurry” – rolls down fast, wiping out towns/villages. Unpredictable Nevado del Ruiz (1985)- killed 10,000. Volcanic gasesgenerally made of water vapor, carbon dioxide, sulfur dioxide, oxides of nitrogen. If mainly gases heavier than air can form thick carpet in country side. Silent killerasphyxiation. Hydrothermal activity- interaction between circulating ground- water and buried magma. Geysers formed, Old Faithful. Can use for geothermal energy- clean system. Global Pattern of Volcanism (figure 12.19/20) “Ring of fire” around Pacific Ocean caused by subducted plates leading to (mainly) Andesitic volcanism. In areas, volcanic islands formed- Japan, Philippines (two oceanic plates convergence.) In other areas, mountain chain on continent formed e.g. Andes (continental plate oceanic plate convergence). Some volcanoes located at/near divergent plate boundaries (Iceland). 2.14.07 Many intraplate volcanoes and flood basalt areas thought to be linked to hot spots (plumes), thin jet of molten material rising rapidly from core-mantle boundary (figure 6.22). As Pacific plate has moved over the (Hawaiian) hotspot, it has formed a chain of volcanic islands and seamounts figure 12.21. Other island chains in pacific also formed in the same way. Can calculate plate velocities from such “hot spot tracks”- controversial. Volcanism and human affairs: Generally can not predict eruptions accurately (short time frame) (box 12.1). Pyroclastic flows and lahars most dangerous figure 12.24. Many major cities in danger- Tacoma (WA) Naples (Italy). Collapse of Minoan civilization caused by volcanic eruption in Aegean? Explosions on volcanic islands can trigger tsunamis- waves that travel at >500 km/hr across open sea and break 20m high at coastline. Little time for warning costal areas. (Violent) eruptions throw lots of dust/ash and gases into the atmosphere. Can cause plane engine to “seize up”. Fine dust leads to spectacular sunsets for many years. Can also cause minor global climatic changes- cooling. Examples seen in 1810s following big eruptions in Indonesia. Others have scaled up from here. If flood basalt eruptions release lots of gas/dust (??), can lead to dramatic climate changes. A school of thought suggests faunal extinction events – 250 and 65 million years ago caused by flood basalt eruptions in Siberia and India, respectively. Positive aspects of volcanism 14. Soils derived from volcanic materials are very fertile. Examples in Mediterranean Sea area and Colombia (Juan Valdez). 15. (Early) atmosphere and ocean formed by volcanic “eruptions”. 16. Geothermal energy possibilities- almost limitless, clean source of energy. Figured 12.27 Weathering and Erosion (Chapter 16): Plate tectonics and volcanism build mountains. These are then worn away to yield source sediments. Weathering- chemical decay and physical breakup caused by rain ice wind snow. Erosion- processes that loosen and transport soil and rock downhill (slope). Rocks (minerals) weather at different rates (figure 16.1) Factors controlling weathering rate table 16.1 17.Parent rock (e.g. granite) Different minerals weather at different rates figured 7.3- feldspar alters readily to clay minerals 18.Climate- weathering speeds up as temperature and rainfall increase. 19.soil- formed by breakdown of rocks –holds on to moisture. Also houses plants/animals that cause weathering. 20.length of time- the longer the more complete the weathering. WEATHERING OF TWO TYPES : CHEMICAL AND PHYSICAL Chemical weathering. Minerals in rocks react with air and water- chemical reactions (new minerals are formed). Feldspar (especially Plag) alter readily to form clay minerals. Since feldspars are dominant clay minerals important component of sediments- clays contain water. Alteration occurs with pure water slowly, speed up by dissolved carbon dioxidewater is weakly acidic . Acidic water will dissolve carbonates totally- leaving no acidic. residue. (Calcium bicarbonate is soluble). Different minerals show different types residue. of stability to chemical weathering table 16.3 2.16.07 Divide into 3 parts: 1. Least stable –non-silicates-halite and carbonates 2. intermediate -all silicates- generally Bowen’s reaction series in reverse. intermediate-all Minerals formed at highest temperatures, alter easily, those formed at lowest temperature (quartz and clays) - least readily. 3. Most stable- non silicates formed by weathering. Oxides and hydroxidesstable“rust layer” formed from mafic minerals only gets thicker with time. Thus different rocks (with different mineral components) chemically weather at different rates. Physical weathering breaks up rock into smaller pieces- no chemical reactions involved figured 16.3. Agents are water, (frost), alternating heat and cold, organismand helped by “cracks” (joints) in rocks figured 16.7-10. Special cases are exfoliation - large sheets of rock peel off from the outcrop and exfoliationspheriodal weathering- curved layers peel off boundaries (smaller scale exfoliation. Physical and chemical weathering go on simultaneously- and help each other figure 16.3,7-10) Soil - the residue of weathering. Soil is formed as a result of weathering of Soiloutcrops/bedrock. An important component is Humus- decaying animal/vegetable products – is mildly acidic and gives blackish color to soil. Dig ditch down in soil to reveal soil profile (layers –“horizons”) Top most A horizon rich in humus, but has lost all “soluble” material. Contains clay and quartz. Next layer- B horizon – little or no humus but rich in iron oxides and soluble material from A horizon. Lowest- C horizon grading into decaying/broken bedrock. Different types of soil are formed from different starting rocks (igneous, sedimentary) under different climatic conditions. Where we have very hot, humid weather acting on rocks for long time (e.g. the tropics), resultant soil tends to be poor for agriculture. Support lush vegetation initially but no lasting power. People practice slash and burn tactics. Paleosols- ancient soils preserved in rock record (e.g. northern Arizona) study of Paleosolsthese unravels details of ancient climates. Thus Paleosols indicate that oxygen was present in the atmosphere billion s of years ago. Human activity – building houses malls highways and deforestation and causing acid rain has sped up weathering; have also loaded soil in areas with salt, pesticides, toxic chemicals that slowly leak into ground water. Raw material of sediments : In addition to soil, you get sand, silt, clay, iron oxides sediments: (detritus- “broken grains”) as major components going into detrital sediments formed elsewhere. Also have dissolved calcium bicarbonate and salt carried by running water, that on arrival as destination (lakes/oceans) can form biochemical sediments. CHAPTER 5 Sediments and Sedimentary rocks Much of the Earth’s surface (including the seafloor) is covered by sediments. Most derived by weathering of rocks on continents. All processes occur at or close to the Earth’s surface (low temperature and pressure). Studying sediments and sedimentary rocks, helps us to understand Earth’s climate in the past changes in weathering processes. General processes involved in order : order: 21. Weathering of rocks 22. Erosion- removal of material formed by weathering of rocks 23. Transport of material- by rivers, glaciers, and wind 24. Deposition- particles settle or precipitate out. Sediment formed. 25. Burial- layers of sediment accumulate, compacting older layers 26. Diagenesis- pressure, heat, chemical reactions, “lithify” (harden) sediments to form sedimentary rocks. (1 and 2 both yield broken grains (detrital or clastic material) as well as dissolved material) Products in Clastic part depends on Starting rock type and intensity of weathering. Table 5.1. Feldspars alter relatively easily to clay minerals; if sediment contains feldspar, it denotes low amount of weathering. At highest weathering amounts, only quartz and clay minerals survive. Give rise to the two main types of clastic sedimentary rocks - - sandstones (quartz dominant) and shales (clays dominant). -sandstones Bio chemical sediments form from seawater- undissolved mineral remains of organisms, as well as minerals precipitated form sea water. Transportation- most sediments are transported by water or currents. Rivers play an important major role. Strength of currents important. Strong ones carry both coarse and fine detritus. Moderately strong currents carry fine detritus (mainly clay) and some coarser material (silt), but not coarse sand, which is deposited. Weak currents carry only the finest detritus (fine clay), and when these are checked, deposit fine muds (deep oceans). Distance of transportation affects both size and angularity of clastic particles figure 5.3. When transportation stops, sedimentation beings. As current carrying particles of all sizes slows, biggest particles settle out first, smallest ones last, gravity. Tendency of variations in current velocity to segregate sediments according to size is called Sorting . Well sorted sediment has particles all about the same size. Poorly sorted Sorting. sediment has particles of many different sizes. Figure 5.3 2.23.2007 Oceans and lakes may be regarded as “mixing bowls”. Input of material form rivers, rain, glaciers. Lose water by evaporation over a short period of time(<100 years) amount of water in oceans remains constant. But over long periods, sea level may alter dramatically (100million or more) e.g. glacial epochs. Ocean water shows “salinity” caused by dissolved material. Amount of salinity is balanced by addition of “salts” by rivers, and removal by precipitation. Of main interest to us: Calcium (bi) carbonate, Sodium chloride. Sedimentary basins - cover >10000 km2, and contain thick accumulations of basinssedimentary rocks. They are prime sources of oil and gases. Basins (figure 5.4) 27. Following, seafloor spreading breaking up a continent, get long narrow rifts filled with sediments and igneous rocks. 28.Later, cooling of material near continents causes subsidence (below sea level), creating space to collect plentiful land derived sediments. Called thermal subsidence basins e.g. East coast of North America. Load of collected sediments causes further depression subsidence creating more space to collect sediments. Near continent can get thick 10km thick sediments/sedimentary rocks. (Figure 5.4) 29. Flexural basins occur when plates converge. Weight of overriding plate causes underlying plate to bend down- creating a flexural basin. Oil reserves in Iraq occur in such a basin. Sedimentary environment. Defined by a geographic location, characterized by combination of geological processes and environmental conditions (figure 5.5) Location : on continent, near shoreline deep ocean. Location: Geological processes: nature and strength of currents, plate tectonic setting processes: (burial of sediment?> volcanic activity) Environmental conditions; kinds and amount of water (fresh saline), topography (mountainous, coastal plain, shallow or deep ocean). Nature and amount of biological activity. Clastic sedimentary environments: On continents- alluvial (stream), desert, lake, glacial. continentsAt shoreline- deltas, beaches, tidal flats. shorelineOceans- on continental margin (relatively shallow water), or deep ocean, sands and mud’s are deposited. These contain terrigenous sediments- derived from land (continent). Biochemical sedimentary environments show chemical and biochemical precipitation from (sea) water. Most important – carbonate environments- marine settling with calcium carbonate, the main sediment. Many organisms extract CaCo3 from sea water to form shells. When they die, their shells accumulate to form sediment. Commonly found in warm, subtropical waters. Evaporate environment formed where warm partially isolated sea water evaporates more quickly than it can mix with marine sea water. Main product= halite . halite. Sedimentary structures structuresBedding- parallel layers of different grain sizes or compositional – reflect successive Beddingdepositional surfaces. Bedding may be thin (few mm) or quite thick (meters) figure 5.1. Most bedding is nearly horizontal at the time of deposition. Cross bedding- sets of beds can be deposited by wind or by water. Inclined at large bedding- angles (<35 degrees) from the horizontal. Are formed when grains are deposited on steeper down current slope of sand dune or sandbar. Figure 5.7. Graded bedding - each layer (few cm thick) progresses from coarse grains at the beddingbottom to fine grains at the top. Many such beds may lie on top of each other , with a total thickness several hundred meters are called Turbidites, formed by special variety of ocean bottom current called turbity current. Ripples- small sand dunes with the long dimensions at right angles to the current. RipplesCan distinguish between symmetrical ripples formed at beach waves in both directions, or asymmetrical ones formed by wave in single direction (sand bar/sand dune) figure 5.7. Found also in ancient rocks figure 5.8 and can infer conditions of deposition by using Principle of Uniformitarianism. Bioturbation - cylindrical tubes few cm in diameter, extending vertically through Bioturbationbeds figure 5.10. Are remnants of burrows and tunnels made by marine organisms. In ancient rocks can deduce behavior of organisms and thus reconstruct sedimentary environment. Diagenesis- figure 5.12 after deposition and burial, sediments undergo physical /chemical changes and lithify- turn into (hard rock). By burial temperature may rise to -150 degrees Celsius. And A. compaction occurs water squeezed out. Most important for clayey sediments B. cementation - occurs – minerals are precipitated in the cementationopen pores of the sediments. Helps bind grains together. Sediment Sedimentary rock Mud Shale Sand Conglomerate 2.26.07 Clastic - look to particle size table 5.3 ClasticLarge pieces- boulder/cobble/pebble; called conglomerate. Medium pieces- sand called sandstone . sandstone. Fine pieces- silt, called siltstone Very Fine pieces- clay called mudstone, shale. Figure 5.15. shale. Subdivisions of sandstonesContains 25% feldspar- arkose Contains fair amount of clay- greywacke These components tell us what was weathered and nature/intensity of weathering figure 5.16 Fine grained sediments (siltstones/shale) are deposited by the gentlest currents. Shales often contain 10% carbonate, and/or organic material. -Relative to abundance of major sedimentary rocks- 75%, sand dominated rocks 10% carbonate rocks 15% Chemical sediments- table 5.4 figure 5.17 Classification based on chemical composition. Most important are carbonates- often made up of shells and skeletons of foraminifera - tiny creatures that live in surface foraminiferawaters figure 5.18 If the rock contains Calcium carbonate- limestone/chalk some magnesium included is dolostone . dolostone. Important occurrences figure 5.18 30. in reefs- large ridge like structures made by corals 31. carbonate platform- shallow extensive flat areas e.g. Bahama banks Evaporites- precipitate inorganically from evaporating sea water in arid regions. EvaporitesMay contain carbonates, sulfates, and chlorides. As evaporation precedes with little or no water input), Calcium carbonate precipitates first next is calcium sulfate (gypsum) and finally sodium chloride (halite). Many evaporite sequences very thick- hundreds of meters. Cannot be by total evaporation of deep ocean (say), Rather need: 32. enclosed body of saline water with 33. constricted access to open sea 34. limited freshwater added by rivers 35. arid climate Water steadily evaporates, level falls, drawing in ocean water through access over long period of time, evaporation of large amounts of saline water forming thick evaporate deposits e.g. Mediterranean sea figure 5.20. Chert - bio-chemically precipitated silica ChertPhosphates- from nutrient rich waters PhosphatesIron formations - generally in old rocks, which is less oxygen present in the formationsatmosphere. Coal made up of diagenetically altered swamp vegetation. Oil and gas (not rocks) formed from digenetic alteration of organic material in pores of sedimentary rocks. Found mainly in sandstones and lime stones. Only sedimentary rocks can contain fossils. Chapter 6 Metamorphic rocksProduced when igneous or sedimentary rocks are subjected to high pressure and temperature for long periods of time inside of the Earth. Can also metamorphose a metamorphic rock (subtle changes only). A good place is near convergent plate boundaries, and especially deep inside mountain belts (e.g. continent-continent collision). Going on today, deep inside Alps, Himalayas. Will only see rocks perhaps hundreds of millions years later when brought to (and exposed) at the surface figure 6.3. The new rock develops new minerals and texture with all “changes” taking place in the solid state (no melting). However, fluids (water) often play a major role. Regional metamorphism - large volumes of rocks subject to high pressure and high metamorphismtemperature for long periods of time (many millions of years). Deformation (bending or breaking) of rocks is not uncommon figure 6.3) Contact metamorphism- smaller volumes of rock, subjected to high temperature metamorphismfor shorter periods of time (e.g. around cooling plutons). Generally minerals in rocks formed at low temperatures change (more) readily during metamorphism e.g. clay – mica. Regional metamorphism takes place close to convergent plate boundaries. Plates pushing past each other, cause directed pressure; force (pressure) in one of 3 dimensions more than in two figure 6.3 This directed pressure gives rise to new texture call foliation/lineation in many metamorphic rocks. Temperature increases steadily with depth inside of the Earth , as does the pressure but not the same everywhere. Figure 6.1 Rate at which temperature increase with depth inside Earth is called the geothermal gradient . Average value is 30 degrees Celsius/ km for metamorphic gradient. conditions. Pressure more difficult concept. At depth of 15km rocks squeezed together 400 times more than at the surface of the Earth. 2.28.07 In P-T space can define areas where we get low, intermediate, high grade, metamorphic rocks. Fig 6.2 (Most) regional metamorphic conditions are represented through central (diagonal) part of figure 6.2 Regional metamorphism- e.g. cooking large turkey in oven. Can have high/low grade) temperature of cooking for long periods of time- cannot stimulate increase in pressure. Contact metamorphism- e.g. frying a steak on skillet. Relatively high temperature for short period of time. Rocks subject to regional metamorphism when P and T are rising (or holding steady) we get prograde metamorphism. Can go on for many millions of years during this stage get many mineralogical and textural changes in the rock. Water comes out of rock and moves around facilitating changes. Figure 6.9. Much later when the rock is moving towards the earths surface (plate tectonic/weathering) P and T often quickly, we get retrograde metamorphism . metamorphism. Few mineralogical/textural change as (a) low T inhibits changes (b) by rocks are “dry”. Metamorphic textures: result form directed pressure. Platy minerals (mica) line pressure. up parallel t one another giving rise to “streaky” appearance- foliation , figure 6.4. foliation, As grade of metamorphism increases, mica grains grow larger. Thus shale first turns to slate then to schist then to gneiss. Figure 6.4 gneiss. Amphibole grains (needle shape) can also line up under directed pressure giving rise to lineation (texture). New minerals begin to grow slowly, garnet being an important example figure 6.6. We need to be more specific than saying rocks are high/intermediate/low grade metamorphic rocks. Metamorphic facies- grouping rocks under different grades of metamorphism from faciesdifferent parent rocks. Thus: different kinds of metamorphic rocks are formed from the same parent rocks under different grades of metamorphism. Different kinds of metamorphic rock come from different parent rocks under the same grade of metamorphism. Occurrence of certain (index) minerals in the rock, help us in estimating the (index) highest P and T that the rock was subjected to. Better still to use groups of index minerals to define P and T conditions (or facies of metamorphism). Index minerals are formed (are stable) under only limited range of P and T conditions figure 6.7 6.8. Garnet is an important example. Other minerals (plagioclase feldspar/quartz) are stable under very wide range of P T conditions. These cannot serve as index minerals. -The occurrence of certain newly formed green colored minerals (chlorite/epodite) in low grade rocks helps us to define green-schist facies of Mm (rock is foliated ). Table 6.2 foliated). -At intermediate grade, get amphibole developing in the rock (mica may or may not be resent). This is the amphibole facies (rock is foliated/lineated). -At the highest grade of metamorphism, mica and amphibole breakdown to form pyroxene (see Bowen’s reaction series- temperature rising). Rock becomes dry. This is granulite facies - non foliated/lineated. faciesT and P conditions of facies (figure 6.7) Greenschist- 450 degrees Celsius low-ish pressure Amphibolites- 650 degrees Celsius intermediate pressure Granulite- 800 degrees Celsius high pressure. Another important facies is called blueschist facies- defined by low temperature -300 degrees Celsius and high pressure. Rock is foliated and contains an unusual blue colored amphibole. These unseal P-T conditions are only met close to subduction zones. All four belong to regional subduction zones. Finally on facies diagram figure 6.7 get hornfels facies- shows intermediate-high temperatures, very low pressure. These are contact metamorphic rocks, formed by heat transferred from plutons (e.g. dykes/sills) to surrounding rocks. Rocks are “sugary textured” show no foliation/lineation , since there is no directed pressure foliation/lineation, involved and there is only a short heating time. Two unusual regional metamorphic rocks containing one mineral each . Quartzite - formed by metamorphism of sandstone (only quartz present) QuartziteMarble- formed by metamorphism of limestone/chalk (only calcite present) Marblefigure 6.5 Note: regional metamorphic rocks are non foliated/lineated only if necessary minerals (mica/amphibole) re not present. Examples: granulite, marble, quartzite,. All contact metamorphic rocks (hornfels facies) are non foliated lineated. 3.2.2007 Chapter 8 “geological time” Geologists study phenomena on the scale of sec/minutes (e.g. earthquakes) to 100 of millions of years (formation and erosion of mountain belts= regional erosion) Two types of measurement relative and “absolute”. Relative datingRelative dating. Important sedimentary rocks dating. 36. principle of original horizontality- sediments generally laid down in horizontal layers figure 8.4 37. principle of superposition in a set of undisturbed beds, the one at the bottom is the oldest and the one at the top is the youngest. 38.if beds are broken (faults) or intruded by igneous rocks, these events occurred after deposition of sediments. Figure 8.4 No single location has continuous sedimentation, to get a full record must co relate rocks from different places. Difficult over large distances. Fossils as timepieces Rock record shows many different life forms, most of which came into existence at some point in time in the past and then became extinct later on. For each life form see evolution over time. Thus given life form (trilobite figure p. 169) shows change of head shape number of segments and legs over millions of years. Law of faunal correlation . Rock that contain the same group of fossils showing correlation. the same features were formed at the same time. Form good Correlation you need good fossils 39. common life form 40. widespread geographic occurrence 41. lived over relatively short period of time Such fossils are called index fossils. fossils. Using above laws especially fossils, can co relate rocks from all around the world figure 8.5. We find out that there are places in which sets of bed are missing. This could be due to: Missing beds were never deposited; or Beds were laid down but have subsequently been eroded away. Boundaries marking these “missing beds” are called unconformities shown by wavy lines in figures. Such marks of missing time may represent immense stretches of time and can be of three types Angular unconformity - tilted sedimentary beds below and horizontally tilted unconformitybeds on top. Disconformity- horizontal sedimentary rock beds both below and above the unconformity. (Generally recognized by sharp change in fossil content across boundary.) Nonconformity- igneous/metamorphic rocks overlain by sedimentary rocks. All three are seen in wall of Grand Canyon. 8.7 Can have very complex series of geological events 8.8/9 that have to be reconstructed form the rock record we see today. Using all of these techniques on sedimentary rocks we build up the geologic column (time scale) with the oldest rocks at bottom and youngest at top. 8.11. From basically four main units (eras) based on sharp fossil content differences. Each of these then subdivided based on finer changes in fossil content. Initially separated into two EONS . The older one with very few good fossils EONS. Precambrian , the younger one with good fossil content- Phanerozoic “life bearing” Precambrian, . Latter subdivided into three eras based on type fossils present. Oldest- Paleozoic “old life”, dominated by fish and amphibians Next- Mesozoic “middle life” dominated by reptiles. Youngest- Cenozoic “recent life” dominated by mammals 8.11 Each era of these is then subdivided into periods based on their refinement of exact fossils present. Paleozoic era is divided into: Permian (youngest)/Carboniferous, Devonian/Silurian/Ordovician/Cambrian (oldest) Mesozoic era divided into: Cretaceous (youngest)/Jurassic/Triassic (oldest) Cenozoic era divided into: Quaternary (younger)/ Tertiary (older). 8.11 3.5.07 Second Exam is chapters 12, 16, 5, 6 ,8. look at test 2 and test 3 Boundaries between Eons/Eras represent “sharp” changes in fossil content. Precambrian- Cambrian boundary “exploration of life forms” Paleozoic- Mesozoic boundary – 90% of life forms died out suddenly Mesozoic- Cenozoic boundary -60% of life forms died out suddenly. What caused these quick changes in life forms at the Earth’s surface? Were they catastrophic in nature? When did they occur? Note: boundaries between various Periods also show difference in fossil contents. Absolute (radiometric) dating. Need some sort of geological clock. Use radioactive materials. General form: Parent ( P- unstable) changes at a fixed rate to daughter (P- unstable) (D- stable ) figure 8.12. If we can measure the amount of P and D in a rock/ stable) mineral and rate of change is known we can figure out its age What does age mean? For igneous rocks represents the time when it changes form a liquid to solid (i.e. when the rock solidifies) Examples of P-D pairs used. 42. Radiocarbon (14C) changes quickly to nitrogen. 43. an isotope of potassium (K) changes (slowly) to argon (Ar) 44. Isotopes of uranium (U) changes very slowly to isotopes of lead (Pb) More about rate of change. Concept of half-life figure 8.13. When the clock starts, have only (N) at atoms of parents, no daughters. Time (e.g. years) it takes for half of the P to change into D called half life. After one half life (measured in say years), have N/2 atoms of parent only. Missing N/2 (N- N/2) atoms have changed to daughter (D/P=1). After two half lives, have only N/(2x2) (i.e. N/4) atoms of parent and 3N/4 atoms of daughter (D/P=3). After three half lives have only N/(2x2x2) (i.e. N/8) atoms of parent and 7N/8 atoms of daughter (D/P= 7). And so on for more half lives. Note: at any given time the number of parents plus the number of daughter parents can neither be created nor destroyed but can change form. Example: A mineral contains 10 million atoms of P and 150 million atoms of D. If the half life of this pair is 50 million years old, what is the age of the rocks? Note D/P=15. After 4 half lives have N/2x2x2x2 (i.e. N/16) of parent and N-N/16 (i.e. 15N/16 atoms of daughter D/P=15. So we are dealing with four half lives each of 50 million years old. Rock is 50x4=200 million years old. More P-D pairs table 8.1 ~For U-Pb, half-life= 4.5 billion years used method to date fairly old U-Pb, rocks/minerals, - 10 million years to 4.5 billion years. ~For K-Ar , half-life= 13 billion years use method to date rocks/minerals – K-Ar, few k. years to 4.5 billion years. For radiocarbon , half-life -6000 years; Rocks do not contain much carbon. Use this radiocarbon, method to date once living material in the range – 0-100000 years. In this case the clock starts when the living material (tree/person) dies. Now we use methods to “calibrate” geologic time scale. Problem; Geological time scale based on sedimentary rocks only, and radiometric ages only possible for igneous rocks. Sedimentary rocks cannot be radiometrically dated. For sedimentary rock, “age” is when sediment laid down (fossil enclosed). Detrital sedimentary rocks contain broken (solid) grains that have radioactive clocks ticking in them for a long time. Best Method: Date ash beds found mixed with sedimentary rocks. These follow the Principle of Superposition. Rocks above the ash bed are younger, Rocks below the ash bed are older. Figure to be shown in class WMWMWMWMWMWMWMWMWMWMW H 100 Ma G F E MWMWMWMWMWMWMWMWMWMWMW D C B ^300 Ma A Angular unconformity… ash bed Ages: A+B >300 Ma CDEF <300 Ma, >100 Ma GH <100 Ma Important examples of dating. U-Pb method. a. Age of the earth- 4500 million years – based on dating of meteorites formed at the same time as the Earth. b. explosion of life forms at base of Paleozoic occurred 550 million years ago, c. faunal extinction event at Paleozoic- Mesozoic boundary occurred 250 million years ago. 3.7.2007 K-Ar method. d. faunal extinction event at Mesozoic- Cenozoic boundary occurred 65 million years ago, e. man-like creatures first appeared in East Africa 5 million years ago. f. correct age (1925 years) obtained on ash material from Vesuvius eruption of 79AD (death knoll for creation-alists) Radiocarbon method g. Native American entire “new world” 30 thousand years ago. h. ice man froze to death in the Alps 5500 years ago. i. cloth on which Shroud of Turin is seen, was made in 1300 AD Other important geological events “dated” 1. oldest rocks dated on earth 4200 million years older. Records wiped out by (big) accretion events. 2. oldest rocks showing signs of life (algae) on earth 4000 million years old. 3. Appalachian mountains formed in complex series of vents 400 300 million years ago when Africa and Europe “crashed into” north America. 4. current cycle of plate tectonics began 200 million a ago. Pangea breaks up. North America from Africa and Europe= about 180 million years ago. South America form Africa 130 million years ago. Whole Atlantic Ocean created by divergent plate boundaries since 200 million years ago 5. India comes into hard collision with Eurasia 50 million years ago. Himalayas rise rapidly in the last 15 million years; big effect on climate. Radiometric methods “work” 45. ages obtained by these methods agree with those obtained by tree ring counting. 46. correct radiometric age obtained for 79 AD Mt. Vesuvius eruption. 47. radiometric age always in right order, i.e. agree with relative dating. 48. sea-floor spreading plate velocities obtained from radiometric results, in excellent agreement with those obtained by modern “physics” methods e.g. measuring “distances” using layers. ...
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