Lecture 11 - Biography of Earth PDF

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Biography of earth, history of geology...

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Class 11: Biography of Earth -

Clear record of earth history in continental crust rocks does not begin until 3.85 Ga - Time between birth of earth and this is Hadean Eon because earth was like an inferno - Internal differentiation: gravity pulled molten iron down to earth’s center, where it formed the core, leaving a mantle composed of ultramafic rock - Soon after that, a protoplanet collided with earth and blasted away a lot of the mantle, resulting debris coalesced to form the moon, which was only 20,000 km away when first formed - Earth was so hot most of its surface was magma, that eventually sank, at 4.4 earth might have become cool enough for solid rocks to form at surface - During Hadean Eon, volatiles originally in mantle were released from volcanoes, becoming toxic atmosphere of water - First oceans maybe accumulated soon after when water condensed as rain - Inner solar system underwent intense meteor bombardment between 4.0 and 3.85 Ga, so probably pulverized and melted all of earth’s crust, atmosphere, and ocean

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At 3.85 Ga, boundary between Archean and Hadean Eons, crust was cool and stable enough for rocks to survive - Significant volumes of new continental crust were generated - Crust formed from mafic igneous rocks that extruded/intruded at convergent plate boundaries or hot spot volcanoes, rocks were too buoyant to subduct so sutured together to form larger blocks at earth’s surface, led to blood basalts, as collisions continued blocks coalesced into proto-continents - By end of Archean Eon, 80% of earth’s crust had formed - CO2 went from being major component of atmosphere to trace component, atmosphere changed from foggy mixture of H2o and CO2 into being a transparent gas dominantly composed of N2 (once oceans formed)

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Life has probably existed since 3.5 Ga and maybe 3.8 Ga, rocks of this age contain chemical signatures of organisms, oldest undisputed forms of bacteria and archaea occur in 3.2 Ga rocks (Archean) - Stromatolites: distinctive layered mounds of sediment - Cyanobacteria after 3.2 Ga secret mucus and sediment settles to it, new cyanobacteria colonize top of the sediment, build mound upward - Submarine hot-water vents, black smokers, hosts of first organisms - Earliest life was heat loving bacteria or archaea that dined on pyrite at dark depths in oceans, later organisms could do photosynthesis and moved into

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shallower water As Archean ended, first continents formed and life colonized sea and shallow water, plate tectonics formed, continental drift occurred, mountain belts and erosion, atmosphere accumulated oxygen though still small, Archean air was unbreathable Proterozoic Eon spans 2 billion years, from 2.5 Ga to beginning of Cambrian Period at 542 Ma, encompassing half of earth’s history - Surface changed from fast-moving plates and small continents to more familiar world of slow plates, large continents, oxygen rich atmosphere - New continental crust grew at slower rates, by middle of eon 90% of earth’s continental crust had formed, proto-continents collided with each other so larger continents assembled, interiors became isolated from heating by igneous activity along its margins, so they cooled and became rigid (craton is a region of cold, stable continental crust), all of today’s crust existed by 1 Ga - Throughout shields today, outcrops expose PreCambrian basement of igneous and metamorphic rocks older than 1 Ga, small hills and valleys but no mountain ranges, most of North America’s is in Canada - Canadian shield consisted of Archean crust blocks sutured together by Proterozoic orogens - Successive collisions brought about supercontinent called Rodinia by 1 Ga, last major collision brought about Grenville orogen - Between 800 and 600 Ma, Rodinia turned inside out when Antarctica, India, and Australia broke away from North America and collided with South America, creating supercontinent Pannotia - Important steps in evolution of life, before most life was prokaryotic, in Proterozoic proliferation of eukaryotic life took place - Ciliate protozoans (single-celled organisms with fibers) at 750 Ma - In next 150 million years, several types of multicellular organisms existed and together constituted the Ediacaran fauna, survived into beginning of Cambrian before going extinct - With appearance of photosynthetic organisms, oxygen entered the atmosphere and at 2.4 Ga concentration of oxygen in atmosphere increased dramatically (great oxygenation event), other environments could not absorb oxygen produced by organisms so it went in the air as gas - Banded iron formation: colorful sedimentary beds formed by iron settling out of the ocean between 2.4 and 1.8 Ga, alternate layers of iron and jasper - Radical climate shifts happened at the end, accumulates of glacial sediment, found in regions that were located at the equator - In one model glaciers covered the land and the entire ocean surface froze, resulting in snowball earth, oxygen levels dropped in the sea so

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many life forms died off bc ice cut off oceans from atmosphere - Ice also disallowed CO2 from dissolving but did not prevent volcanic activity from adding to Co2, which trapped heat so earth warmed up Closed by earth warming up and continents drifting apart, shells appeared, fossil record became more complete, end of Proterozoic and Precambrian

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Phanerozoic Eon: Paleozoic, Mesozoic, Cenozoic

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Early Paleozoic (Cambrian-Ordovician) - Pannotia broke up, yielding smaller continents like Laurentia, Gondwana, and Siberia, new passive-margin basins formed along these edges - Sea level rose so continental interiors were flooded with epicontinental seas, now cratonic platforms, regressions marked by unconformities, and transgressions, marked by accumulations of sediment, took place (layer of strata in Grand Canyon is marked by these) - In Middle Ordovician, Laurentia’s eastern margin rammed into a volcanic island arc and the resulting collision, Taconic orogeny, deformed and metamorphosed strata of continent’s margin produced the Appalachians After Cambrian began, Cambrian explosion took place, occurred with supercontinent breaking up so might have to do with production of new ecological niches that resulted - First animals to appear had simple tube or cone shaped shells, soon the shells became more complex - By end of Cambrian, trilobites grazed sea floor with mollusks, brachiopods, etc. so food chain arose with plankton at the bottom - Organisms crawled or swam over reefs composed of mounds of sponges - Ordovician saw first crinoids and vertebrate animals, jawless fish - At end of Ordovician, mass extinction took place because of brief glaciation and sea level lowering

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Middle Paleozoic Era (Silurian-Devonian, 444-359 Ma) - Climate warmed, sea level rose, continents flooded again, huge reefs grew, layer of fossiliferous limestone on continents, distinct orogenies took place, western margin of North America continued to be a passive-margin basin but in Late Dvonian, it ceased being a passive margin because of collision with an island arc, event called Antler orogeny, first of many orogenies there - Life underwent radical changes, new species in the sea such as gastropods and bivalves replaced species that disappeared in mass extinction - Vascular plants with woody tissues, seeds, and veins rooted for the first time, plants could grow much larger, swampy forests with tree-sized relatives of mosses and ferns

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Spiders, scorpions, insects, crustaceans, jawed fish like sharks At very end of Devonian, first amphibians crawled out

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Late Paleozoic Era (Carboniferous-Permian Periods, 359-251 Ma) - Climate cooled, seas retreated from continents, sees with epicontinental limestone reefs became coastal areas and river deltas w accumulated debris - Laurentia was by the equator so it enjoyed tropical conditions that rew lush plant debris, transformed into coal after burial - Succession of continental collisions, culminating in Pangaea - Largest collision was when Gondwana rammed into Laurentia and Baltica, causing Alleghanian orogeny of North America, final stage in Appalachians development, eastern North America rammed into northwestern Africa, and present-day Gulf Coast region squashed against northern margin of South America - Vast mountain belt grew, rocks deformed during these events are visible in Appalachian and Ouachita Mountains, wide band of deformation called Appalachian fold-thrust belt formed - Faults merged with near-horizontal sliding surface - Preexisting faults in continental crust across North America became active - Ancestral Rockies: late Paleozoic uplifts of the Rocky Mountain region - Plants and animals continued to evolve, fixed-wing insects like dragonflies in coal swamps, club mosses - By end of Carboniferous Period, cockroach, gymnosperms, cycads, amphibians, reptiles (marked evolution of new component in animal reproduction, eggs with a protective covering, permitting reptiles to reproduce without returning to water) - Ended with mass extinction event, 95% of marine species disappeared - Result of extraordinary volcanic activity in present-day Siberia, basalt sheets extruded during the event are known as Siberian traps - Eruptions clouded atmosphere, acidified oceans, disrupted food chain

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Mesozoic Era was when dinosaurs ruled Early and Middle Mesozoic Era (Triassic-Jurassic Periods, 251-145 Ma) - Panagaea existed for 100 million years until rifting happened during late Triassic/early Jurassic and the continent broke up, by end of Jurassic North America had split from Europe and Africa, Mid Atlantic Ridge formed, North Atlantic Ocean grew - Warm climate but during late Jurassic and early Cretaceous, climate cooled, Panagaea interior was nonmarine, by middle Jurassic sea level rose and a shallow sea submerged the Rockies, convergent margin tectonics, subduction

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generated volcanic island arcs North America grew in area from fragments of crust from elsewhere, called exotic terranes Major continental volcanic arc, Sierran arc, grew on western margin of North America During early Mesozoic, variety of new planet and animal species, reptiles swam in oceans, new corals in reefs, gymnosperms and reptiles diversified, first turtles and flying reptiles, at end of Triassic first dinosaurs evolved - Differed from reptiles, legs under bodies instead of at side, maybe warm blooded By end of Jurassic, gigantic dinosaurs and first feathered birds were there Earliest ancestors of mammals appeared at end of Triassic, small rat-like

Late Mesozoic Era (Cretaceous Period, 145-65 Ma) - Climate shifted to warmer, sea level rose, seaways flooded continents, breakup of Pangaea continued, opening of South Atlantic Ocean and separation of South America and Africa from Antarctica/Australia - India broke away from Gondwana and headed toward Asia - Passive-margin basins developed with thick layers of sediment - Sierran arc continued to be active, current day Sierra Nevada Mountains - Sevier orogeny: compressional stresses along western North American convergent boundary activated large thrust faults east of the arc, produced belt remnants in Canadian Rockies and western Wyoming - By end of period, continued compression caused slip on large faults in southwestern US, penetrated deep into continental rocks, movement generated uplifts - Laramide orogeny: overlying layers of Paleozoic strata warped into large monoclines, folds whose shapes resembled carpet on stairs, formed structure of present Rocky Mountains in US - Sea floor spread 3x faster than today, Cretaceous mid-ocean ridges occupied more volume than they today, displaced sea water, caused level to rise, huge submarine plateaus formed from basalts erupted at hot spot volcanoes - So there were active mantle plumes/superplumes at base of lithosphere - All this volcanism released CO2 into atmosphere, leading to global rise of atmospheric temperature, which caused sea water to expand and polar ice sheets to melt, making sea level go up more, continents flooded and large epicontinental seas formed during this era - Modern fish, huge reptiles, conifers, dinosaurs reached peak of success, social herds of dinosaurs roamed, mammals diversified but remained rat-like - K-T boundary event (Cretaceous-Tertiary): abrupt change in fossil assemblages,

faunal turnover happened instantaneously, dinosaurs simply vanished along with 90% of plankton species and 75% of plants - Huge meteorite, evidence is shale from this time had high concentrations of iridum, which comes from meteorites, and other unusual materials like tiny glass spheres and coesite, also huge crater in Mexico which was from the time of K-T boundary, impact blasted debris in air and generates tsunamis that generated forest fires, perpetual night and winter-like cold, acid rain, photosynthesis would stop so food chain was broke and extinction happened -

Cenozoic Era (last 65 million years) - alpine-Himalayan chain: collisions of former Gondwana with Europe/Asia - India collided with Asia to form Himalayas and Tibetan Plateau - Africa collided with Europe to produce the Alps - Convergent boundary activity built the Andes in South America - In North America, a transform boundary replaced convergent boundary and San Andreas Fault system formed because volcanism/compression ceased - Convergent-boundary tectonics continue only in Washington, Oregon, and northern California today where subduction of the Juan de Fuca Plate generates the volcanism of the Cascade volcanic chain - Basin and Range Province: broad continental rift whose development has stretched the region to twice its original width, south of Cascades - Climate became cooler, Antarctic glaciers reappeared, formation of grasslands in temperate climates - Pleistocene Ice Age: continental glaciers expanded and retreated, each time they grew sea level fell so much the continental shelf became exposed to air - Climate warmed 11,000 years ago and this is what we are still experiencing - After K-T boundary, plant life recovered with forests of angio/gymnosperms - Mammals diversified, Age of Mammals, huge mammals appeared but became extinct during past 10,000 years likely due to hunting by humans (mammoths, giant beavers, giant bears, giant sloths) - Ape-like primates diversified in Miocene Epoch (20 Ma), first human-like primates appeared 4 Ma, first human genus homo at 2.4 Ma (homo erectus at 1.6 Ma, made tools), homo sapiens diverged from neanderthals 500,000 years ago, modern people 200,000 years ago, last Neanderthals and Denisovans died 25,000 years ago...