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Geobiology 2006 Introductions Rationale The interactive system: in geologic, environmental and climate change throughout Earth . Since life began it has continually shaped and re- shaped the , , and the solid earth.

‘Geobiology’ introduces the concept of 'life as a geological agent' and examines the interaction between biology and the earth system during the roughly 3.5 billion since life first appeared. 12.007 GEOBIOLOGY 2007

Instructor: Roger Summons Guest Lecturers: Professor Richard Binzel Professor Ed Boyle Dr D’Arcy Meyer-Dombard

Lectures: Tues. & Thurs. 11-12:30

Course Description: The interactive Earth system: biology in geologic, environmental and climate change throughout Earth history. Grading: 15% Participation in class discussions 20% Problem Sets/Assignments 20% Final Paper & Oral Presentation 20% Midterm Exam 25% Final Exam 1 Lecture 1 • Introduction and requirements • Scales; Some introductory ; How to Make a Habitable : Big Bang; Origin of The elements; How we things Lecture 2 • Origin of the , Earth and , early Earth segregation, atmosphere and hydrosphere; characteristics of the ‘habitable zone’

Week 2 Æ What is Life? Theories about the origin of Life 1&2 Assignment

Essay: What is the Universe made of? 4 pages incl. figures Check recent literature on…….. ‘Ordinary matter’ (~4%); we know mostly H, He What and where is the rest and how was it made? ‘exotic matter’ = dark matter (~23%) and ‘dark ’ (~73%) OR: Essay: What is meant by the concept of Galactic Habitable Zones 4 pages incl. figures Making a Habitable Planet

• The right kind of and a rocky planet • A benign cosmological environment • Matter, temperature where liquid water stable, energy • And many more…………see WS Broecker, How to Build a Habitable Planet Cosmic Time Scales

Oxygen First trees and atmosphere 1 12 23 reptiles Origin of the Universe Origin of 2 13 24 our galaxy First dinosaurs 3 14 25

April 4 15 26 5 16 27 6 17 28 Origin of solar system 7 18 29 Dinosaurs wiped out, First land plants 8 19 30 mammals take over First primates 9 20 31 All of human Life on Earth history Origin of sex Neanderthals 10 21 December atmosphere 11 22

Early homo sapiens last 10 The cosmic calender - the history of the universe compressed to one . All of recorded history (human civilization) occurs in last 21 !

Figure by MIT OCW. Avg. human life span=0.15 s Image removed due to copyright restrictions. See illustration in Des Marais, D. J. ": When Did Emerge on Earth?" 289 (2000): 1703-1705. Time T(K) E Density What’s Happening? The standard cosmological model of the formation of the universe: Figure removed due to copyright restrictions. See http://hyperphysics.phy-astr.gsu.edu/hbase/astro/bbang.html “The Big Bang” New NASA Speak: The theory of The Big Bang •From: The First Three Minutes, by Steven Weinberg Evidence for the Big Image removed due to copyright restrictions. Bang #1: An Illustration of the raisin bread model of expanding universe. Expanding Universe

•The galaxies we see in all directions are moving away from the Earth, as evidenced by their red shifts (Hubble). •The fact that we see all moving away from us does not imply that we are the center of the universe! •All stars will see all other stars moving away from them in an expanding universe. •A rising loaf of raisin bread is a good visual model: each raisin will see all other raisins moving away from it as the loaf expands. Evidence for the Big Bang Image removed due to copyright restrictions. #2: The 3K See http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod/bkg3.gif.

Cosmic Microwave Background

•Uniform background radiation in the microwave region of the spectrum is observed in all directions in the sky. •Has the wavelength dependence of a Blackbody radiator at ~3K. •Considered to be the remnant of the radiation emitted at the time the expanding universe became transparent (to radiation) at ~3000 K. (Above that T matter exists as a plasma (ionized ) & is opaque to most radiation.) Science Magazine: Breakthrough of the Year 2003

• Wilkinson Microwave Anisotropy Probe (WMAP) produced data to indicate the abundances and sizes of hot and cold spots in the CMB. • Universe is very strange • Universe not just expanding but CREDIT: GSFC/NASA Image courtesy of NASA. accelerating • Universe is 4% ordinary matter, 23% ‘exotic matter = dark matter’ and 73% dark energy • is 13.7± .2 b.y. and expanding • It’s flat Evidence for the Big Bang #3: H-He Abundance

Image removed due to copyright restrictions. See http://hyperphysics.phy-astr.gsu.edu/hbase/astro/imgast/hyhel.gif.

•Hydrogen (73%) and He (25%) account for nearly all the nuclear matter in the universe, with all other elements constituting < 2%. •High % of He argues strongly for the big bang model, since other models gave very low %. •Since no known process significantly changes this H/He ratio, it is taken to be the ratio which existed at the time when the deuteron became stable in the expansion of the universe. Nucleosynthesis

Image courtesy of Wikimedia Commons. Nucleosynthesis I: Fusion Reactions in Stars

Fusion Ignition T Reaction Process (106 K) Hydrogen Produced in H-->He,Li,Be,B 50-100 Burning early universe

Helium Burning He-->C,O 200-300 3He=C, 4He=O

Carbon Burning C->O,Ne,Na,Mg 800-1000

Neon, Oxygen Ne,O-->Mg-S 2000 Burning Fe is the end of the Silicon Burning Si-->Fe 3000 line for E-producing fusion reactions... Hydrogen to Iron •Elements above iron in the periodic cannot be formed in the normal nuclear fusion processes in stars. •Up to iron, fusion yields energy and thus can proceed. •But since the "iron group" is at the peak of the binding energy curve, fusion of elements above iron dramatically absorbs energy.

Fe

The 'iron group' yield from 8 of are the nuclear fission most tightly bound. r

a 62 V e Ni (most tightly bound) l e

c 28 u M

58 n

Fe n 6 Elements heavier r i

e 26 ) 56 p than iron can yield n Fe o y

e 26 energy by nuclear g l

r have 8.8 MeV c e

u fission. n

n per nucleon e (

g yield from e 4 binding energy. l n i c i

d nuclear fusion t n r i a B p

2 Average mass of fission fragments 235 is about 118. U

50 100 150 200 Mass Number, A

Figure by MIT OCW. Nuclear Binding Energy •Nuclei are made up of protons and neutrons, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. •The difference is a measure of the nuclear binding energy which holds the nucleus together. •This energy is released during fusion.

•BE can be calculated from the relationship: BE = Δmc2 •For α particle, Δm= 0.0304 u, yielding BE=28.3 MeV **The mass of nuclei heavier than Fe is greater than the mass of the nuclei merged to form it.** Elements Heavier than Iron

•To produce elements heavier than Fe, enormous amounts of energy are needed which is thought to derive solely from the cataclysmic explosions of supernovae.

•In the supernova explosion, a large flux of energetic neutrons is produced and nuclei bombarded by these neutrons build up mass one unit at a time (neutron capture) producing heavy nuclei.

•The layers containing the heavy elements can then be blown off be the explosion to provide the raw material of heavy elements in distant hydrogen clouds where new stars form.

Image courtesy of NASA. Neutron Capture & Radioactive Decay •Neutron capture in supernova explosions produces Image removed due to copyright restrictions. Illustration of Cd undergoing neutron capture until an unstable some unstable is produced, at which point it undergoes radioactive decay into a new nuclei. element; see http://abyss.uoregon.edu/~js/images/neutron_capture.gif

•These nuclei radioactively decay until a stable isotope is reached. Cosmic Abundance n o e l c u N

of the Elements Fusion

r e p

y g r Fission •H (73%) & He (25%) account for e n E 98% of all nuclear matter in the Fe universe. Atomic number = Number of protons

•Low abundances of Li, Be, B due H 10 to high combustibility in stars. 9 He

•High abundance of nuclei w/ mass 8 O 4 C 7 divisible by He: e Ne l a

c N

s Si 6 Fe

C,O,Ne,Mg,Si,S,Ar,Ca g S o l Ar The "cosmic" abundance of the e 5 c Ca Ni elements is derived from •High Fe abundance due to max n a spectroscopic studies of the d 4 n

u supplemented by chemical analyses of Zn binding energy. b K

a 3 chondritic .

e F v i •Even heavy nuclides favored over t 2 Cu a Li l e R 1 B Sc odd due to lower “neutron-capture Sn Pb cross-section” (smaller target = 0 Pt Be -1 Bi higher abundance). Au -2 Th •All nuclei with >209 particles U (209Bi) are radioactive. 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Figure by MIT OCW. Basics of Geology & Asthenosphere

Mantle and Crust Lithosphere/Asthenosphere Outer 660 km divided into two layers based on mechanical properties Lithosphere Rigid outer layer including crust and upper mantle Averages 100 km thick; thicker under Asthenosphere Weak, ductile layer under lithosphere Lower boundary about 660 km (entirely within mantle) The Core Outer Core Earth’s Interior: How do we know its ~2300 km thick structure? Liquid Fe with Ni, S, O, and/or Si Avg density of Earth (5.5 g/cm3) is evidence of flow Denser than crust & mantle Density ~ 11 g/cm3 Inner Core Composition of meteorites ~1200 km thick Seismic wave velocities Solid Fe with Ni, S, O, and/or Si Laboratory experiments Density ~13.5 g/cm3 Chemical stability Earth’s magnetic field Earth’s Surface

Principle Features of Earth’s Surface

Continent Shield--Nucleus of composed of rocks Continent-Ocean Transition Continental shelf--extension of continent Continental slope--transition to ocean basin

Ocean basin--underlain by ocean crust Why do oceans overlie basaltic crust? Mid-ocean ridge Mountain belt encircling globe Ex: Mid-Atlantic Ridge, East Pacific Rise Deep-ocean trenches Elongate trough Ex: Peru-Chile trench Earth’s Crustal Evolution: 2

3°Crust = Formed from slow, continuous distillation by on a geologically active planet (I.e., ). •Results in highly differentiated magma distinct from basalt--the low- density, light-colored granite. •Earth may be the only planet where this type of crust exists. •Unlike 1° & 2° crusts, which form in < 200 M.y., 3° crusts evolve over billions of years.

Image removed due to copyright restrictions. See figure in McLennan, S. M., and S. R. Taylor. “Heat Flow and the Chemical Composition of Continental Crust.” J Geol 104 (1996): 369-377. Igneous Rocks

Basalt (2° Crust; Oceanic crust) Image removed due to copyright restrictions. Photographs of basalt and granite rocks, from Stanley (course text).

Granite (1° Crust; Continental Crust)

Stanley (1999) The Crust Ocean Crust 3-15 km thick The Crust Basaltic rock & Mantle Young (<180 Ma) Density ~ 3.0 g/cm3 Continental Crust 35 km average thickness Granitic rock Old (up to 3.8 Ga) Density ~ 2.7 g/cm3 Crust "floating" on "weak" mantle The Mantle ~2900 km thick Comprises >82% of Earth’s volume Mg-Fe silicates (rock) Two main subdivisions: Upper mantle (upper 660 km) Lower mantle (660 to ~2900 km; "Mesosphere") Structure of Earth Image removed due to copyright restrictions. Cutaway image of Earth, showing crust, mantle, outer and inner core layers. Figure 1-14 in Stanley textbook.

From Stanley (1999)

Image removed due to copyright restrictions. Illustration of the structure of Earth’s crust and mantle. Figure 1-15 in Stanley textbook. Why is Continental Crust “Elevated Relative to Oceanic Crust?

•High-density Basalt sinks into mantle more than low-density Granite. •Volcanism continually produces highly

Image removed due to copyright restrictions. differentiated continental crust on Earth. See figure in McLennan, S. M., and S. R. Taylor. “Heat Flow and the Chemical Composition of •Venus surface appears to be all basalt. Continental Crust.” J Geol 104 (1996): 369- 377. •Plate tectonics & volcanism do not appear to be happening on Venus (or Mars, Moon). •So Earth may be unique in Solar System. And plate tectonics & volcanism likely critical in determining habitability.

Taylor & McLennan Sci. Am. (1996) Lithospheric Plates From Stanley (1999) •8 large plates (+ add’l. small ones) •Average speed: 5 cm/yr •3 types of motion result in 3 types of boundaries: sliding toward (subduction zones), sliding away (ridge axes), skiding along (transform faults)

Image removed due to copyright restrictions.

Illustration of lithospheric plates. Figure 1-17 in Stanley text. Convection Drives Plate Movements

Image removed due to copyright restrictions.

Figure 1-18 from Stanley textbook.

From Stanley (1999) Tectonic Activity in the South Atlantic

Image removed due to copyright restrictions.

Figure 1-19 from Stanley textbook.

From Stanley (1999) Rock Image removed due to copyright restrictions. Basics Figure 1-7 in Stanley textook.

Igneous + metamorphic = Crystalline Rocks

From Stanley (1999) The Rock Cycle Igneous Rock

Image removed due to copyright restrictions.

Figure 1-9 in Stanley textook.

From Stanley (1999) •Felsic: Si-,Al-rich. Light-colored, low-density. Feldspar (pink) &

quartz (SiO2)-rich. Most continental crust. Granite most abundant. Igneous •Mafic: Mg-, Fe-rich. Dark-colored, high-density. Most oceanic crust. Ultramafic rock (more dense) forms mantle below crust. Rocks •Extrusive: cools rapidly; small crystals 101 •Intrusive: cools slowly; large crystals

Basalt (Oceanic Image removed due to copyright restrictions. Crust) Photographs of basalt and granite rocks. Figure 2-10 in Stanley textbook.

Granite (Continental Crust)

Stanley (1999) ME MI FE FI • Slab of lithosphere is subducted, melted & incorporated into asthenosphere Plate Tectonics • Convection carries molten material upward where & the Rock it emerges along a spreading zone as new lithosphere. Cycle

Image removed due to copyright restrictions.

Figure 1-20 from Stanley textbook.

•Subducted sediment melts at a shallower depth where it contributes to magma emitted from an island arc volcano and a mountain chain volcano •Erosion of volcanic rock provides sediment sediment to complete cycle From Stanley (1999) Sedimentary Rocks Image removed due to copyright restrictions. Represent Illustration from Taylor, S. Ross and Scott M. McLennan. "The Evolution of Continental Crust." Scientific American, 1996. Homogenous Mixture of Continental Crust Geologic Time

A major difference between and most other is their attitude about time. A "long" time may not be important unless it is > 1 million years. Comparing Individual 206Pb/238U analyses for SHRIMP and ID-TIMS 680 SHRIMP weighted mean 0.112 206 238 680 Pb/ U date: _ 621 + 7Ma 660 0.108 660 MSWD = 1.13 U ID-TIMS single 238 0.104 640 grain analyses Pb/ 640 620 206 0.100

600 0.095 Concordant Pb-loss 580 620 0.092 560

0.088 0.4 0.6 0.8 1.0 600 Time (millions of years) (millions Time

lonprobe (SHRIMP) spot analyses 580 ID-TIMS weighted mean 206Pb/238U date: 632.50 +_ 0.48 MSWD = 0.38 560

0.1045 Figure by MIT OCW.

636 0.1035

632 Concordant analyses 0.1025 628

624 0.1015 Analysis with apparent Pb-loss

0.1005 0.844 0.848 0.852 0.856 0.860 0.864 0.868 0.872 207Pb/235U Figure by MIT OCW. Absolute Calibration:

• Add numbers to the stratigraphic column based on . • Based on the regular radioactive decay of some chemical elements. Radioactive

Image removed due to copyright restrictions.

Rubidium-87 parent nucleus begins with 37 protons and 50 Decay of neutrons. One neutron decays into a constituent proton and electron, creating strontium-87, with 38 protons and 49 neutrons in its nucleus. Rubidium to Strontium

Fig. 9.14 Proportion of Parent Atoms Remaining as a of Time 1 t f e l s m o t a f o

n 1/2 o i t r o p o r P 1/4 1/8 1/16 1/32 1 2 3 4 5 Time, in half - lives

Figure by MIT OCW. Fig. 9.15 Isotopic dating • Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters). • The rate at which this decay occurs is constant and knowable. • Therefore, if we know the rate of decay and the amount of parent and daughter, we can calculate how long this reaction has been proceeding. Major Radioactive Elements Used in Isotopic Dating

ISOTOPES HALF-LI FE EFFECTIVE AND OTHER OF PARENT DATING MATERIALS THAT PARENT DAUGHTER (YEARS) RANGE (YEARS) CAN BE DATED

Uranium-238 Lead-206 4.5 billion 10 million- Zircon 46 billion Uraninite

Potassium-40 Argon-40 1.3 billion 50,000 - Muscovite 4.6 billion Biotite Hornblende Whole volcanic rock Rubidium-87 Strontium-87 47 billion 10 million - Muscovite 4.6 billion Biotite Potassium feldspar Whole metamorphic or igneous rock Carbon-14 Nitrogen-14 5730 100 -70,000 Wood,charcoal, peat Bone and Shell and other Groundwater, ocean water, and glacier ice containing dissolved

Table 9.1 Figure by MIT OCW. Geologically Useful Decay Schemes

Parent Daughter Half-life (years) 235U 207Pb 0.71 x 109 238U 206Pb 4.5 x 109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x 109 14C 14N 5730 From dendrochronology to geochronology • Tree rings can be dated with 14C to calibrate them • Radiocarbon can only be used to date organic material (plant or ) younger than ~ 60,000 yrs • For rocks and older material, we need other methods: e.g. /lead

Courtesy of Henri D. Grissino-Mayer. Used with permission.

http://web.utk.edu/~grissino/ Two ways to date geologic events

1) relative dating (fossils,structure) 2) absolute dating (isotopic, tree rings, etc.) Time in years Process Timekeeping Device or

Only micro One billion Age of the earth Radioactive decay 109 Time for mountain fossils Amount of Time years range to be uplifted 3000m fossils at 0.2mm/year One million 106 Time for the years Atlantic ocean to widen 1 km at Required for 4cm/year One thousand 103 years Human Lifetime Historical records Measurable erosion One year 100 Some Geologic of rivers and One shorelines One Floods 10-3 One Processes and Earthquake waves One 10-6 go through and around earth One Events 10-9 One thousand Time for one sound wave detectable of a second by human ears 10-12

Nuclear Processes 10-15 Some geologic processes can be documented Image removed due to copyright restrictions. using historical Map with some land shaded more darkly. records (brown is new land from 1887-1988) Ammonite Fossils Petrified Wood

Chip Clark Fig. 9.4 Tom Bean Steno's Laws

Nicolaus Steno (1669) • Principle of Superposition • Principle of Original Horizontality • Principle of Lateral Continuity

Laws apply to both sedimentary & volcanic rocks. Principle of Superposition

In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom. Principle of Superposition

Photograph removed due to copyright restrictions. Image showing a striated mountainside with older layers towards the bottom and newer layers towards the top.

Jim Steinberg/Photo Researchers Fig. 9.3b Principle of Original Horizontality

Layered strata are deposited horizontal or nearly horizontal or nearly parallel to the Earth’s surface. Principles of original horizontality and superposition

Image removed due to copyright restrictions.

Illustration of lake or sea ; younger layers of sediment in the lakebed are formed on top of older layers. Principle of Lateral Continuity

Layered rocks are deposited in continuous contact. Using Fossils to Correlate Rocks

OUTCROP A OUTCROP B

I I

II II II e m i

III T

III

Outcrop may be separated by a long distance

Figure by MIT OCW. Unconformity

A buried surface of erosion Formation of a Disconformity

D C B A

Sedimentation of beds A-D beneath the sea Uplift above sea level and exposure of D to erosion

E C B A

Unconformity

Continual erosion strips D away completely Subsidence below the sea and sedimentation of E over and exposes C to erosion C; erosion surface of C preserved as an unconformity

Figure by MIT OCW. Fig. 9.6 South rim of the Grand Canyon South rim of the Grand Canyon 250 million years old

PaleozoicPaleozoic StrataStrata

550 million years old 1.7 billion years old PrecambrianPrecambrian South rim of the Grand Canyon 250 million years old

550 million years old 1.7 billion years old Nonconformity TheThe GreatGreat UnconformityUnconformity ofof thethe GrandGrand CanyonCanyon

Geoscience Features Picture Libraryc Fig. 9.7 Angular Unconformity at Siccar Point Sedimentation of Beds A-D Beneath the Sea

D C B A

Sedimentation of beds A-D beneath the sea

Figure by MIT OCW.

Fig. 9.8 Deformation and Erosion During Mountain Building

Image removed due to copyright restrictions. Uniformitarianism The present is the key to the . —— JamesJames HuttonHutton Natural laws do not change— however, rates and intensity of processes may. Many methods have been used to determine the age of the Earth 1) Bible: In 1664, Archbishop Usher of Dublin used of the Book of Genesis to calculate that the world began on Oct. 26, 4004 B.C.

2) Salt in the Ocean: (ca. 1899) Assuming the oceans began as fresh water, the rate at which rivers are transporting salts to the oceans would lead to present salinity in ~100 m.y. Many methods have been used to determine the age of the Earth

3) Sediment Thickness: Assuming the rate of deposition is the same today as in the past, the thickest sedimentary sequences (e.g., Grand Canyon) would have been deposited in ~ 100 m.y.

4) Kelvin’s Calculation: (1870): Lord Kelvin calculated that the present geothermal gradient of ~30°C/km would result in an initially molten earth cooled for 30 – 100 m.y. Oldest rocks on Earth Slave Province, Northern Canada • Zircons in a metamorphosed granite dated at 4.03 Ga by the U-Pb method Yilgarn block, Western • Detrital zircons in a sandstone dated at 4.4 Ga by U-Pb method. Several other regions dated at 3.8 Ga by various methods including Minnesota, Wyoming, Greenland, South Africa, and Antarctica. The geologic timescale and absolute ages

Isotopic dating of intebedded volcanic rocks allows assignment of an absolute age for transitions The big assumption

The half-lives of radioactive isotopes are the same as they were billions of years ago. Test of the assumption

Meteorites and Moon rocks (that are thought to have had a very simple history since they formed), have been dated by up to 10 independent isotopic systems all of which have given the same answer. However, scientists continue to critically evaluate this data. Frequently used decay schemes have half-lives which vary by a factor of > 100 parent238U daughter206Pb half 4.5 life x 10 (years)9 235U 207Pb 0.71 x 109 40K 40Ar 1.25 x 109 87Rb 87Sr 47 x 109 147Sm 144Nd 106 x 109 Minerals with no initial daughter • 40K decays to 40Ar (a gas)

• Zircon: ZrSiO4 ion radius (Å) Zr4+ 0.92 U4+ 1.08 Pb2+ 1.37 World’s Oldest Rock: Acasta Gneiss Acasta Zircon (Ages in My) Zircons: ’s Time Capsules The

• Divisions in the worldwide stratigraphic column based on variations in preserved fossils • Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages Image removed due to copyright restrictions.

Illustration: “Eras of the Phanerozoic”, a graph of geologic time versus , based on work by John Phillips, 1860. 25 60 Millions of Years 0 11 40 70 135 180 225 270 305 350 400 440 500 600 0 6 Pliocene 21 42 Miocene Oligocene 50 60 90 100 110 Paleocene Cretaceous 161 150 Jurassic Thousands of Feet

Thousands of Feet 205 200 Triassic 235 Permian 254 250 Upper 274 Carboniferous Lower 300 300 Devonian 330 Silurian 350 372 Ordovician 400 412 Cambrian 452 450 Precambrian 476 675 0 100 200 300 400 500 600 700 Millions of Years A Revised Geological Time-Scale

Figure by MIT OCW. Image removed due to copyright restrictions.

Generalized Stratigraphic Section of Rocks Exposed in the Grand Canyon

Image removed due to copyright restrictions.

Illustration from Beus, Stanley S. and Michael Morales. Grand Canyon Geology. New York, NY: Oxford University Press, 1990. ISBN: 9780195050141.

after: Beus & Moral (1990) Some of the Geologic Units Exposed in the Grand Canyon

Michael Collier The study of life in the past based on fossilized plants and . Fossil: Evidence of past life Fossils preserved in sedimentary rocks are used to determine: 1) Relative age 2) Environment of deposition Trilobites (Cambrian) Fossil Fern (Pennsylvanian) Fossil Sycamore-like Leaf (Eocene) • Tree rings can be counted and dated with 14C to calibrate them • Radiocarbon can only be used to date organic material (plant or animal) younger than ~ 60,000 yrs • For rocks and older material, we need other methods: e.g. uranium/lead

Isotopic Dating • Radioactive elements (parents) decay to nonradioactive (stable) elements Courtesy of Henri D. Grissino-Mayer. Used with permission. Proportion of (daughters). Parent Atoms • The rate at which this decay occurs is Remaining as a Figure by MIT OCW. Function of Time constant and knowable. 1 • Therefore, if we know the rate of decay t f e l

s and the amount present of parent and m o t a

f o

n 1/2 daughter, we can calculate how long o i t r o p o r this reaction has been proceeding. P 1/4 1/8 1/16 1/32 1 2 3 4 5 Time, in half - lives Frequently used decay schemes; half-lives vary by a factor of > 100

238U Æ 206Pb 4.5 x 109 235U Æ 207Pb 0.71 x 109 40K Æ 40Ar 1.25 x 109 87Rb Æ 87Sr 47 x 109 147Sm Æ 144Nd 106 x 109

Courtesy of the U.S. Geological Survey’s Cascades Volcano Observatory. Acasta: Worlds oldest rock: (Ages in My)

Zircons: Nature’s Time Capsules Origin and Early Evolution of Life

• The lost record of the origin of Life? Few crustal rocks from >3 Ga and half life of sediments 100-200Ma so most destroyed

High Temperature/Low Pressure Modern

Subduction Regime Island-Arc Regime t

100 s ) u e r u l Accretion of Earth C

a

80 l a V

t t

Oldest Found On Earth n n e e n

60 s (Zircon In Younger Sediments) i t e r n P o

f

Oldest Rocks 40 C

o

f t

(Acasta Gneiss) o

n e e

Major Episode of Growth 20 c m r e u l P o (

0 V 4 3 2 1 0 Geological Age (Billions of Years Before Present)

CRUSTAL GROWTH has proceeded in episodic fashion for billions of years. An important growth spurt lasted from about 3.0 to 2.5 billion years ago, the transition between the Archean and Proterozoic eons. Widespread melting at this time formed the granite bodies that now constitute much of the upper layer of the continental crust.

Figure by MIT OCW.