Geology Needs a Time Scale

Transcription

1 Geological Time

2 Geology Needs a Time Scale Rocks record geologic events and changing life forms of the past Interpreting Earth history is a prime goal of geology, based on clues found in rocks The geologic time scale was developed and Earth s history was discovered to be exceedingly long

3 Relative Dating: Key Principles Placing rocks (and events) in their proper sequence of formation Law of superposition Developed by priest/geologist Nicolaus Steno in the 17 th century In an undeformed sequence of sedimentary rocks, each bed is older than the one above, younger than the one below

4 Relative Dating: Key Principles Principle of original horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed Principle of cross-cutting relations Younger features (dykes, faults) cut across older features (pre-existing rocks) Inclusions An inclusion is a piece of rock that is enclosed within another rock Rock containing the inclusion is younger Unconformities An unconformity is a break in the rock record caused by erosion and/or nondeposition of rock units When there is no break in the rock record, the rocks are considered conformable

5 Relative Dating: Key Principles Unconformities Types of Unconformities Angular unconformity tilted rocks are overlain by younger flat-lying rocks Disconformity strata on either side of the unconformity are parallel Nonconformity separates older metamorphic or igneous rocks from younger sedimentary strata

6 Correlation of Rock Layers Correlation by physical criteria Matching rocks of similar ages and features in different regions is known as correlation Fossils and correlation William Smith (late 1700s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content Principle of fossil succession fossil organisms succeed one another in a definite and determinable order; any time interval can be recognized by its fossils Index fossils: they are widespread geographically and limited to a short span of geologic time

7 Dating with Radioactivity Provides numeric ages specifying the actual number of years that have passed since an event occurred (also known as absolute age dating) Radioactivity Each atom has a nucleus of protons (+) and neutrons, orbited by electrons (-) Isotope Variant of the same parent atom Differs in the number of neutrons Some isotopes are unstable and their nuclei spontaneously break apart; this process is called radioactive decay or radioactivity Unstable isotopes are called parents Isotopes formed by the decay of parents are called daughter products

8 Dating with Radioactivity Half-Life The time required for half of the radioactive nuclei in a sample to decay Also the time elapsed when quantities of parent and daughter are equal (ratio 1:1) Together with the parent/ daughter ratio, half-life (rate of decay) is used to calculate the numeric age of a sample

9 Dating with Radioactivity

10 Dating with Radioactivity Radiometric dating Sources of error A closed system is required If temperature is too high, daughter products may be lost and the age underestimated To avoid potential problems, only fresh, unweathered rock samples should be used Dating with carbon-14 (radiocarbon dating) Used to date very recent geologic events Half-life of only 5730 years Carbon-14 is produced in the upper atmosphere and absorbed by organisms Useful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth history Importance of radiometric dating Radiometric dating is a complex procedure that requires precise measurement Rocks from several localities have been dated at more than 3 billion years Confirms the idea that geologic time is immense

11 The Geologic Time Scale Subdivides geologic history into units Originally created using relative dates Structure of the time scale Eon the greatest expanse of time Phanerozoic ( visible life ) the most recent eon, began just over 540 million years ago Earlier Precambrian eons include Proterozoic, Archean, and Hadean Era subdivision of an eon Eras of the Phanerozoic eon Cenozoic ( recent life ) Mesozoic ( middle life ) Paleozoic ( ancient life ) Eras are subdivided into periods Periods are subdivided into epochs

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13 Difficulties in Dating the Time Scale Not all rocks can be dated by radiometric methods Grains comprising detrital sedimentary rocks are not the same age as the rock in which they occur The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages Dates change as brackets become narrower and methods refined; e.g., base of Triassic is now 251 Ma, base of Permian is now 299 Ma, base of Cambrian is 542 Ma

14 Deep Time The Earth is Old and continually evolving True Scale: Geologic Time is dominated by the Precambrian (87% of all time to now)

15 Early Earth ( billion years ago) Prisocoan Eon Earth coalesces from material captured in proto-solar system

16 Early Earth ( billion years ago ) Prisocoan Eon Very hot liquid rock for much of this time magma oceans! Heavy bombardment phases many water-rich comets and iron-rich meteorites, including very large impacts probably

17 Early Earth ( billion years ago ) Prisocoan Eon Slow sinking of many heavy metals to form iron rich core

18 Early Earth ( billion years ago ) Prisocoan Eon Too hot for plate tectonics or life probably (atmosphere also oxygen poor and toxic)

19 Archean Earth ( billion years ago) Slowly cooling Earth 1st definite continent rock masses and water oceans, but still pretty hot and ocean chemistry very different (e.g. very iron rich) Atmosphere still very toxic and oxygen poor

20 Archean Earth ( billion years ago) First life by about billion years cyanobacteria only (like blue-green algae)

21 Life strongly influences the composition of the Atmosphere Our atmosphere formed by outgassing (gases released from solidifying rocks) which continues to this day Water vapour formed condensation and led to rain creating water bodies Bacteria and later other life forms generated free oxygen

22 Archean Earth ( billion years ago) Algal blooms flourish, produce oxygen by photosynthesis in oceans and use up much of ocean Fe

23 Archean Earth ( billion years ago) First good evidence of plate tectonics by about 3.2 billion years suggests stable mantle with convection happening by then

24 Proterozoic Earth (2.5 billion to 540 million years ago) Free oxygen in atmosphere by about 2.2 billion years

25 Proterozoic Earth (2.5 billion to 540 million years ago) First cold climate evidence (glaciations) probably climate becoming similar to present Earth with cold (icehouse) and warm (greenhouse) cycles; though perhaps more extreme and overall a very warm Earth Plate tectonics much like today s Earth

26 Proterozoic Earth (2.5 billion to 540 million years ago) Still just bacterial life until about 700 million years, then more complex lifeforms evolve (though still softbodied, leave impressions only Ediacara fauna)

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28 Paleozoic Earth ( million years) First complex lifeforms with skeletons rapid radiation of types stable Earth with climate very similar to present

29 Paleozoic Earth ( million years) Mostly stable climate

30 Ordovician Ma

31 Ordovician Ma Erosion of emerging Taconic Mountains to the NE Muds deposited in shallow tropical seas 420Ma

32 Silurian Ma

33 Silurian Ma Climate in Ohio was tropical similar to presentday Bahamas Silurian sea was largely mud-free Large reefs separated shallower waters across much of Ohio from deepter waters in the Michigan Basin to N and Appalachian Basin to E

34 Devonian Ma

35 Devonian Ma Early Devonian Shallow tropical seas cover Ohio Thick deposits of limestones on sea floor End of Devonian Erosion of the rising Appalachian Mountains to the E Muds deposited into oxygen-poor sea

36 Paleozoic Earth ( million years)

37 Paleozoic Earth ( million years) All life in oceans until about 420+ million years, then rapid evolution of land plants and fauna Why?

38 What do you know about the Ozone Hole?

39 Formation of Ozone

40 Sustaining Ozone

41 The Ozone Layer and Chlorofluorocarbons (CFCs) Ozone (O 3 ) is a chemically out of place pollutant CFCs attack ozone and can destroy, or thin, the ozone layer CFCs build up their concentration in the upper atmosphere, have a residence time of about 100 years, and breakdown into various by-products including chlorine monoxide (ClO) One ClO can destroy many ozone molecules

42 The Ozone Layer and Chlorofluorocarbons (CFCs) Ozone (O 3 ) is a chemically out of place pollutant In upper atmosphere (stratosphere) the ozone layer absorbs harmful ultraviolet radiation Ozone forms as O 2 + ½ O 2 = O 3 Ozone layer is beneficial to life

43 The Ozone Layer and Chlorofluorocarbons (CFCs)

44 The Ozone Layer and Chlorofluorocarbons (CFCs) Global Ozone Recovery Predictions

45 Carboniferous Ma

46 Carboniferous Ma Early Carboniferous (Mississippian) Extensive marine deposits of muds and silts from eroding Appalachians to the east Late Carboniferous (Pennsylvanian) Development of large deltaic systems, ie nearshore marine to terrestrial conditions and only rarely oceanic conditions Coal-forming swamps present in many parts of E and SE Ohio 345 Ma 325 Ma

47 Carboniferous Ma Large parts of western North America were covered by water Terranes of various sizes were drifting towards the westcoast Carbonate sediments were deposited in shallow sea Eventually added to western North America, forming the Rocky Mountains 310 Ma

48 Paleozoic Earth ( million years) Major extinction event at end of Paleozoic (Permian extinction) 85-90% of all organisms wiped out (cause much debated, probably extreme climate change due to major volcanic events, possibly meteorite impact?)

49 Mesozoic Earth ( million years ago) Rise of reptiles Dinosaurs and marine reptiles flourish and dominate large animal forms

50 Mesozoic Earth ( million years ago) First mammals (about million years ago) small and very minor species only through Mesozoic

51 Mesozoic Earth ( million years ago) A hot climate throughout no glaciations, probably no polar ice a very long Greenhouse climate

52 Jurassic Ma Atlantic Opens as Gondwana and Laurasia separate; ie Pangaea breaks up Starts from the north 195 Ma 180 Ma 150 Ma

53 Cretaceous Ma

54 Cretaceous Ma Western Interior Seaway Early to mid-cretaceous Arctic Ocean transgresses onto the continent and is joined by water from Gulf of Mexico from South separating the continent in half Shallow sea with abundant wildlife 115 Ma 75 Ma 65 Ma

55 Mesozoic Earth ( million years ago) Rapid extinction of dinosaurs and many other species at 65 million years Extreme climate change, due to Meteorite impact and/or major volcanic events

56 Chicxulub Impact Structure, Mexico 65 Ma, same age as mass extinction Meteorite would have been ~10 km in diameter Iridium layer observed in clays in many places around the world

57 Consequences of large impact >1 km A base surge, like a volcanic PF, generated by impact Terrestrial impact, rock pulverized and/or vaporized huge amounts of dust into the stratosphere Huge amounts of water will be vaporized runaway hurricanes, hypercanes - winds to 1,000 km/hr? Global tsunamis, Earthquakes

58 Consequences of large impact >1 km Suspended dust & soot will cause global winter & global darkness Acid rains, Greenhouse effects Catastrophic crop failure MASS EXTINCTIONS

59 Cenozoic Earth (65 million years to present) Rapid evolution of mammals to fill extinct dinosaur and marine reptile niches

60 Cenozoic Earth (65 million years to present) Variable climate mostly warmer than today except last 2.5 million years

61 Tertiary Ma Oceans of today take shape Large parts of Eurasia are flooded Ocean separates North from South America Himalayas form as Tethys disappears Antartica becomes isolated as first Australia then South America drift north ---> Southern Ocean

62 Tertiary Ma North & South America connect as subduction to the west forms volcanic islands; leads to island chain First islands above sealevel about 15 Ma Isthmus of Panama closes around 3 Ma changing sea circulation as until then Atlantic and Pacific exchanged water freely Greenland still connected to NA 8 Ma

63 Oceans of today Caribbean forms as North & South America connect and subduction takes place at eastern edge Due to massive icesheets, sea-level is much lower for much of the Quaternary by m exposing the shelves Quaternary 2.6 Ma - present

64 Cenozoic Earth (65 million years to present) Quaternary climate (2.5 million years to present) cyclic oscillations between icehouse and greenhouse climates glacial vs. non-glacial. Have been mostly in an intraglacial climate for last 12,000+ years and continuing.

65 Last Glacial Maximum

66 Deglaciation of the Lower Mainland

67 Cenozoic Earth (65 million years to present) First Homo genus (direct Homo Sapien ancestors) about million years ago (lots of debate still controversial evidence for as old as 6 million) Homo Sapiens by about 130,000 years ago in Africa (could be older)

68 Human evolution

69 Homo Homo is the genus that includes modern humans and species closely related to them; 2.3 to 2.4 Ma ago Homo sapiens is the only non-extinct species of its genus Some of these other species might have been our ancestors, but many were likely our "cousins", having speciated away from our ancestral line No consensus as to which of these groups are species or subspecies; partly due to lack of fossils or due to the slight differences used to classify species in the Homo genus Homo arrived at same time as first evidence of stone tools Due to large number of morphological similarities exhibited, Homo is closely related to several extinct hominid genera, and at this stage none of them is universally accepted as the confirmed direct ancestor of Homo

70 Homo sapiens

71 Homo sapiens

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74 Deep Time The Earth is Old and continually evolving True Scale: Geologic Time is dominated by the Precambrian (87% of all time to now)

75 Archean Earth ( billion years ago) First life by about billion years cyanobacteria only (like blue-green algae)

76 Proterozoic Earth (2.5 billion to 540 million years ago) Still just bacterial life until about 700 million years, then more complex lifeforms evolve (though still softbodied, leave impressions only Ediacara fauna)

77 Paleozoic Earth ( million years) All life in oceans until about 420+ million years, then rapid evolution of land plants and fauna

78 Mesozoic Earth ( million years ago) Rise of reptiles Dinosaurs and marine reptiles flourish and dominate large animal forms

79 Cenozoic Earth (65 million years to present) Rapid evolution of mammals to fill extinct dinosaur and marine reptile niches