Geobiology - Massachusetts Institute of...
Transcript of 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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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.
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.
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
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)
Image removed due to copyright restrictions.
Illustration of lithospheric plates. Figure 1-17 in Stanley text.
Convection Drives Plate Movements
From Stanley (1999)
Image removed due to copyright restrictions.
Figure 1-18 from Stanley textbook.
Tectonic Activity in the South Atlantic
From Stanley (1999)
Image removed due to copyright restrictions.
Figure 1-19 from Stanley textbook.
Rock Basics
Igneous + metamorphic = Crystalline Rocks
From Stanley (1999)
Image removed due to copyright restrictions.
Figure 1-7 in Stanley textook.
The Rock Cycle
From Stanley (1999)
Igneous Rock
Image removed due to copyright restrictions.
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)
Image removed due to copyright restrictions.
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
Image removed due to copyright restrictions.
Figure 1-20 from Stanley textbook.
Sedimentary Rocks
Represent Homogenous Mixture of Continental
Crust
Image removed due to copyright restrictions.
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
Image removed due to copyright restrictions.
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
Image removed due to copyright restrictions.
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
Image removed due to copyright restrictions.
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
Image removed due to copyright restrictions.
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.
Image removed due to copyright restrictions.
Generalized Stratigraphic
Section of Rocks Exposed in the Grand Canyon
after: after: BeusBeus & Moral (1990)& Moral (1990)
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.
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.