geobiology - massachusetts institute of...

Geobiology 2006Introductions

RationaleThe interactive Earth system: biology in geologic, environmental and climate change throughout Earth history. Since life began it has continually shaped and re-shaped the atmosphere, hydrosphere, cryosphereand 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 years since life first appeared.

12.007 GEOBIOLOGYSPRING 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 discussions20% Problem Sets/Assignments20% Final Paper & Oral Presentation20% Midterm Exam25% Final Exam

Week 1Lecture 1• Introduction and requirements• Time Scales; Some introductory Geology; How to Make

a Habitable Planet: Big Bang; Origin of The elements; How we date things

Lecture 2 • Origin of the Solar System, Earth and Moon, early Earth

segregation, atmosphere and hydrosphere; characteristics of the ‘habitable zone’

Week 2 What is Life? Theories about the origin of Life

Weeks 1&2 Assignment

Essay: What is the Universe made of?4 pages incl. figuresCheck recent literature on……..‘Ordinary matter’ (~4%); we know mostly H, HeWhat and where is the rest and how was it made? ‘exotic matter’ = dark matter (~23%) and ‘dark energy’ (~73%)

OR:Essay: What is meant by the concept of

Galactic Habitable Zones4 pages incl. figures

Making a Habitable Planet

• The right kind of star 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

Avg. human life span=0.15 s

January

February

March

April

May

June

July

August

September

October

November

December

Origin of theUniverse

Origin ofsolar system

Oxygenatmosphere

Oxygenatmosphere

Origin ofour galaxy

Life on EarthOrigin of sex

First land plants

December

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3

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5

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31

First trees andreptiles

First dinosaurs

Dinosaurs wiped out,mammals take overFirstprimates

last 10 minutesEarly homo sapiens

Neanderthals

All of humanhistory

The cosmic calender - the history of the universe compressed to one year. All of recorded history (human civilization) occurs in last 21 seconds!

Figure by MIT OCW.

Image removed due to copyright restrictions.See illustration in Des Marais, D. J. "Evolution: When Did

Photosynthesis Emerge on Earth?" Science 289 (2000): 1703-1705.

Time T(K) E Density What’s Happening?

The standard cosmological model of the formation of the universe:

“The Big Bang”

•From: The First Three Minutes, by Steven Weinberg

New NASA Speak: The

theory of The Big Bang

Figure removed due to copyright restrictions.See http://hyperphysics.phy-astr.gsu.edu/hbase/astro/bbang.html

Evidence for the Big

Bang #1: An 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 stars 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.

Image removed due to copyright restrictions.Illustration of the raisin bread model of expanding universe.

Evidence for the Big Bang #2: The 3K

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 atoms) & is opaque to most radiation.)

Image removed due to copyright restrictions.See http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod/bkg3.gif.

http://hyperphysics.phy-astr.gsu.edu/hbase/imgmod/bkg3.gif

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

accelerating• Universe is 4% ordinary matter, 23%

‘exotic matter = dark matter’ and 73% dark energy

• Age is 13.7± .2 b.y. and expanding • It’s flat

CREDIT: GSFC/NASA

Image courtesy of NASA.

Evidence for the Big Bang #3: H-He Abundance

•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.

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

http://hyperphysics.phy-astr.gsu.edu/hbase/astro/imgast/hyhel.gif

Nucleosynthesis

Image courtesy of Wikimedia Commons.

Nucleosynthesis I: Fusion Reactions in Stars

Fusion Process Reaction Ignition T

(106 K)Hydrogen Burning H-->He,Li,Be,BHe,Li,Be,B 50-100

Helium Burning He-->C,O 200-300

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

Neon, Oxygen Burning Ne,O-->Mg-S 2000

Silicon Burning Si-->Fe 3000

Produced in early universe

Fe is the end of the line for E-producing fusion reactions...

3He=C, 4He=O

Hydrogen to Iron•Elements above iron in the periodic table 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 energycurve, fusion of elements above iron dramatically absorbs energy.

Fe

8

6

4

2

The 'iron group'of isotopes are themost tightly bound.

6228

5826

Fe5626

Fe

Ni (most tightly bound)

have 8.8 MeVper nucleonbinding energy.yield from

nuclear fusion

Elements heavierthan iron can yieldenergy by nuclearfission.

Average massof fission fragmentsis about 118.

yield fromnuclear fission

50 100 150 200Mass Number, A

235U

Bin

ding

ene

rgy

per n

ucle

arpa

rticl

e (n

ucle

on) i

n M

eV

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 some unstable nuclei.

•These nuclei radioactively decayuntil a stable isotope is reached.

Image removed due to copyright restrictions.

Illustration of Cd undergoing neutron capture until an unstable isotope is produced, at which point it undergoes radioactive decay into a new element; see http://abyss.uoregon.edu/~js/images/neutron_capture.gif

http://abyss.uoregon.edu/~js/images/neutron_capture.gif

Cosmic Abundance of the Elements

•H (73%) & He (25%) account for 98% of all nuclear matter in the universe.•Low abundances of Li, Be, B due to high combustibility in stars.•High abundance of nuclei w/ mass divisible by 4He: C,O,Ne,Mg,Si,S,Ar,Ca•High Fe abundance due to max binding energy.•Even heavy nuclides favored over odd due to lower “neutron-capture cross-section” (smaller target = higher abundance).•All nuclei with >209 particles (209Bi) are radioactive.

Ener

gy p

er N

ucle

on

Fusion

Fission

Atomic number = Number of protons

The "cosmic" abundance of the elements is derived from spectroscopic studies of the sunsupplemented by chemical analyses of chondritic meteorites.

10

9

8

7

6

5

4

3

2

1

0

-1

-2

H

He

Be

B

Li

N

CO

Ne

F

SiS

Ar

Sn

Ca

Fe

Fe

Sc

K

Ni

Cu

Zn

Bi

Pb

Pt

UTh

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Rel

ativ

e ab

unda

nce

log

scal

e

AuAu

Figure by MIT OCW.

Basics of Geology

Mantle and CrustLithosphere/Asthenosphere

Outer 660 km divided into two layers based on mechanical propertiesLithosphere

Rigid outer layer including crust and upper mantleAverages 100 km thick; thicker under continents

AsthenosphereWeak, ductile layer under lithosphereLower boundary about 660 km (entirely within mantle)

The CoreOuter Core

~2300 km thickLiquid Fe with Ni, S, O, and/or SiMagnetic field is evidence of flowDensity ~ 11 g/cm3

Inner Core~1200 km thickSolid Fe with Ni, S, O, and/or SiDensity ~13.5 g/cm3

Earth’s Interior: How do we know its structure?

Avg density of Earth (5.5 g/cm3)Denser than crust & mantleComposition of meteoritesSeismic wave velocitiesLaboratory experimentsChemical stabilityEarth’s magnetic field

Lithosphere & Asthenosphere

Principle Features of Earth’s Surface

ContinentShield--Nucleus of continent composed of Precambrian rocks

Continent-Ocean TransitionContinental shelf--extension of continentContinental slope--transition to ocean basin

Ocean basin--underlain by ocean crustWhy do oceans overlie basaltic crust?Mid-ocean ridge

Mountain belt encircling globeEx: Mid-Atlantic Ridge, East Pacific Rise

Deep-ocean trenchesElongate troughEx: Peru-Chile trench

Earth’s Surface

Earth’s Crustal Evolution: 23°Crust = Formed from slow, continuous distillation by volcanism on a geologically active planet (I.e., plate tectonics).•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

Granite(1° Crust; Continental Crust)

Basalt(2° Crust;Oceanic crust)

Stanley (1999)

Image removed due to copyright restrictions.Photographs of basalt and granite rocks, from Stanley (course text).

The CrustOcean Crust

3-15 km thickBasaltic rockYoung (<180 Ma)Density ~ 3.0 g/cm3

Continental Crust35 km average thicknessGranitic rockOld (up to 3.8 Ga)Density ~ 2.7 g/cm3

Crust "floating" on "weak" mantle

The Crust & Mantle

The Mantle~2900 km thickComprises >82% of Earth’s volumeMg-Fe silicates (rock)Two main subdivisions:

Upper mantle (upper 660 km)Lower mantle (660 to ~2900 km; "Mesosphere")

Structure of Earth

From Stanley (1999)

Image removed due to copyright restrictions.Cutaway image of Earth, showing crust, mantle, outer

and inner core layers. Figure 1-14 in Stanley textbook.

Image removed due to copyright restrictions.Illustration of the structure of Earth’s crust and mantle.

Figure 1-15 in Stanley textbook.

•High-density Basalt sinks into mantle more than low-density Granite.•Volcanism continually produces highly differentiated continental crust on Earth.•Venus surface appears to be all basalt.•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)

Why is Continental Crust “Elevated Relative to Oceanic Crust?

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.

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 (subductionzones), sliding away (ridge axes), skiding along (transform faults)


Illustration of lithospheric plates. Figure 1-17 in Stanley text.

Convection Drives Plate Movements

From Stanley (1999)


Figure 1-18 from Stanley textbook.

Tectonic Activity in the South Atlantic

From Stanley (1999)



Rock Basics

Igneous + metamorphic = Crystalline Rocks

From Stanley (1999)


Figure 1-7 in Stanley textook.

The Rock Cycle

From Stanley (1999)

Igneous Rock


Figure 1-9 in Stanley textook.

Igneous Rocks

101

•Felsic: Si-,Al-rich. Light-colored, low-density. Feldspar (pink) & quartz (SiO2)-rich. Most continental crust. Granite most abundant.•Mafic: Mg-, Fe-rich. Dark-colored, high-density. Most oceanic crust. Ultramafic rock (more dense) forms mantle below crust.

•Extrusive: cools rapidly; small crystals•Intrusive: cools slowly; large crystals

Granite(Continental Crust)

Basalt(Oceanic Crust)

ME MIFE FI

Stanley (1999)


Photographs of basalt and granite rocks. Figure 2-10 in Stanley textbook.

Plate Tectonics & the Rock

Cycle

From Stanley (1999)

• Slab of lithosphere is subducted, melted & incorporated into asthenosphere

• Convection carries molten material upward where it emerges along a spreading zone as new lithosphere.

•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



Sedimentary Rocks

Represent Homogenous Mixture of Continental

Crust


Illustration from Taylor, S. Ross and Scott M. McLennan. "The Evolution of Continental Crust." Scientific American, 1996.

Geologic Time

A major difference between geologists and most other scientists is their attitude about time.

A "long" time may not be important unless it is > 1 million years.

ID-TIMS singlegrain analyses

Concordant Pb-loss

lonprobe (SHRIMP) spot analyses

Comparing Individual 206Pb/238U analyses for SHRIMP and ID-TIMS680

660

640

620

600

580

560

Tim

e (m

illio

ns o

f yea

rs)

Figure by MIT OCW.

0.112

0.108

0.104

0.100

0.095

0.092

0.088

560

580

600

620

640

660

680

206 P

b/23

8 U

207Pb/235U

0.4 0.6 0.8 1.0

0.1045

0.1035

0.1025

0.1015

0.10050.844 0.848 0.852 0.856 0.860 0.864 0.868 0.872

624

628

632

636

Concordantanalyses

Analysis withapparent Pb-loss

SHRIMP weighted mean 621 7MaMSWD = 1.13

+_date:206Pb/238U

632.50 0.48MSWD = 0.38

+_date:206Pb/238U

ID-TIMS weighted mean

Figure by MIT OCW.

Absolute Calibration: Geochronology

• Add numbers to the stratigraphic column based on fossils.

• Based on the regular radioactive decay of some chemical elements.

Radioactive Decay of

Rubidium to Strontium

Fig. 9.14Fig. 9.14


Rubidium-87 parent nucleus begins with 37 protons and 50 neutrons. One neutron decays into a constituent proton

and electron, creating strontium-87, with 38 protons and 49 neutrons in its nucleus.

Fig. 9.15Fig. 9.15

Proportion of Parent Atoms

Remaining as a Function of

Time

Time, in half - lives

Prop

ortio

n of

ato

ms l

eft

1

1

1/2

1/4

1/81/161/32

2 3 4 5

Figure by MIT OCW.

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 present of parent and daughter, we can calculate how long this reaction has been proceeding.

Major Radioactive Elements Used in Isotopic Dating

Table 9.1Table 9.1

ISOTOPES

PARENT DAUGHTER

HALF-LI FEOF PARENT (YEARS)

EFFECTIVE DATING RANGE (YEARS)

MINERALS AND OTHER MATERIALS THAT CAN BE DATED

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

Zircon

Argon-40Potassium-40 1.3 billion 50,000 - 4.6 billion

MuscoviteBiotiteHornblendeWhole volcanic rock

Carbon-14

Strontium-87

Nitrogen-14

Rubidium-87 47 billion 10 million - 4.6 billion

100 -70,000

MuscoviteBiotitePotassium feldsparWhole metamorphic or igneous rockWood,charcoal, peatBone and tissueShell and other calcium carbonateGroundwater, ocean water, and glacier ice containing dissolvedcarbon dioxide

5730

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 Radiocarbon can only be used to date organic material (plant or date organic material (plant or animal) younger than ~ 60,000 yrsanimal) younger than ~ 60,000 yrs

•• For rocks and older material, we For rocks and older material, we need other methods: e.g. need other methods: e.g. uranium/leaduranium/lead

http://web.utk.edu/~grissino/

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

Two ways to date geologic events

1) relative dating (fossils,structure)

2) absolute dating (isotopic, tree

rings, etc.)

Amount of Time Required for

Some Geologic Processes and

Events

Time in years

One billionyears

109

One million years

106

One thousand years

103

100

10-3

10-9

10-12

10-15

One yearOne monthOne day

One hour

One minuteOne second

10-6

One thousand of a second

Time for mountainrange to be uplifted 3000mat 0.2mm/year

Age of the earth

Time for the Atlantic oceanto widen 1 km at 4cm/year

Human LifetimeMeasurable erosion of rivers and shorelines

Floods

Earthquake waves go throughand around earth

Time for one soundwave detectableby human ears

Nuclear Processes

Calendars

Clocks

Historical records

Only microorganismfossils

fossils

Radioactive decay

Timekeeping DeviceProcess or Event

Some geologic processes can be

documented using historical

records(brown is new land from 1887-1988)

Image removed due to copyright restrictions.Map with some land shaded more darkly.

Fig. 9.4Chip Clark

Ammonite Fossils Petrified Wood

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.

Fig. 9.3bFig. 9.3bJim Steinberg/Photo Researchers

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.

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


Illustration of lake or sea sedimentation; 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 RocksOUTCROP A OUTCROP B

I

IIII

III

I

II

III

Tim

e

Outcrop may be separated by a long distance

Figure by MIT OCW.

Unconformity

A buried surface of erosion

Formation of a Disconformity

Fig. 9.6Fig. 9.6

ABCD

ABCE

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

Continual erosion strips D away completely and exposes C to erosion

Subsidence below the sea and sedimentation of E overC; erosion surface of C preserved as an unconformity

Unconformity

Figure by MIT OCW.

South rim of the Grand Canyon

South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years old1.7 billion years old1.7 billion years old

Paleozoic StrataPaleozoic Strata

PrecambrianPrecambrian

South rim of the Grand Canyon250 million years old250 million years old

550 million years old550 million years old

Nonconformity1.7 billion years old1.7 billion years old

Fig. 9.7Fig. 9.7

The Great Unconformity of the Grand CanyonThe Great Unconformity of the Grand Canyon

Geoscience Features Picture Libraryc

Angular Unconformity at Siccar Point

Fig. 9.8Fig. 9.8

Sedimentation of Beds A-D Beneath the Sea

ABCD

Sedimentation of beds A-D beneath the sea

Figure by MIT OCW.

Deformation and Erosion During Mountain Building


UniformitarianismThe present is the key to the past.

—— James HuttonJames Hutton

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 chronology 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 EarthSlave Province, Northern Canada• Zircons in a metamorphosed granite dated

at 4.03 Ga by the U-Pb methodYilgarn block, Western Australia• 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 intebeddedvolcanic rocks allows assignment of an absolute age for fossil 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 schemeshave half-lives which vary by

a factor of > 100parent daughter half life (years)238U 206Pb 4.5 x 109

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: Nature’s Time Capsules

The Geologic time scale

• 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


Illustration: “Eras of the Phanerozoic”, a graph of geologic time versus biodiversity, based on work by John Phillips, 1860.

0

50

100

150

200

250

300

350

400

450

0 100 200 300 400 500 600 700 Millions of Years

Precambrian 476675

452

412

372

330

300274254235

205

161

11090

60422160

Cambrian

Ordovician

Silurian

Devonian

Carboniferous

PermianTriassic

Jurassic

Cretaceous

UpperLower

Thou

sand

s of F

eet

PlioceneMioceneOligocene

1125 60

40 70 135 180 225 270 305 350 400 440 500 600Millions of Years

Thou

sand

s of F

eet Eocene

A Revised Geological Time-Scale

Paleocene

Figure by MIT OCW.

Generalized Stratigraphic

Section of Rocks Exposed in the Grand Canyon

after: after: BeusBeus & Moral (1990)& Moral (1990)


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

1990. ISBN: 9780195050141.

Some of the Geologic Units Exposed in the Grand Canyon

Michael Collier

PaleontologyThe study of life in the past based on

fossilized plants and animals.

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)

Proportion of Parent Atoms Remaining as a Function of Time

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 present of parent and daughter, we can calculate how long this reaction has been proceeding.

• Tree rings can be counted and dated with 14C to calibrate them

•• Radiocarbon can only be used to date Radiocarbon can only be used to date organic material (plant or animal) younger organic material (plant or animal) younger than ~ 60,000 yrsthan ~ 60,000 yrs

•• For rocks and older material, we need other For rocks and older material, we need other methods: e.g. uranium/leadmethods: e.g. uranium/lead

Time, in half - lives

Prop

ortio

n of

ato

ms l

eft

1

1

1/2

1/4

1/81/161/32

2 3 4 5

Figure by MIT OCW.

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

Acasta: Worlds oldest rock: (Ages in My)

Zircons: Nature’s Time Capsules

Frequently used decay schemes; half-lives vary by a factor of > 100

238U 206Pb 4.5 x 109

235U 207Pb 0.71 x 109

40K 40Ar 1.25 x 109

87Rb 87Sr 47 x 109

147Sm 144Nd 106 x 109

Courtesy of the U.S. Geological Survey’s Cascades Volcano Observatory.

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

100

80

60

40

20

001234

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.

Volu

me

of C

ontin

enta

l Cru

st(P

erce

nt o

f Pre

sent

Val

ue)

Accretion of Earth

Oldest Mineral Found On Earth(Zircon In Younger Archean Sediments)

High Temperature/Low PressureSubduction Regime

ModernIsland-Arc Regime

Oldest Rocks(Acasta Gneiss)

Major Episode of Growth

Geological Age (Billions of Years Before Present)

Figure by MIT OCW.

geobiology - massachusetts institute of...

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