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Unformatted text preview: HISTORICAL GEOLOGY
GEOLOGY 106 Observe the Earth and it shall teach thee.
(Job 12:8) ORIGIN OF THE EARTH
ACCRETIONARY ORIGIN OF THE EARTH
begins with BIG BANG THEORY
of origin of universe
that produces matter
then, SOLAR NEBULA THEORY
of solar system origin
leads to formation of planets 1 STAR FORMATION
The BIG BANG THEORY explains radial movement of
galaxies from central point and points to origin of matter
at 13 billion years ago (red shift)
Explosive origin produces nuclear particles, atoms
(mostly Hydrogen), and energy
- determines amount of hydrogen & helium in universe
- radiant energy still present as 5°K background radiation
Millions years after explosion, matter aggregates into
stars, nebulae, and galaxies, including nebular clouds
that form stars and planetary systems.
Our Sun is a late generation star
- Later generation stellar systems are more enriched in
heavy elements Why is Solar System enriched in heavy elements? SECONDARY ENRICHMENT OF
Why is solar system enriched in heavy elements?
Heavy elements formed by
thermonuclear fusion reactions in interior of stars A series of fusion reactions:
H - He (main energy source)
He - C at higher temps
C cycle produces N & O, F, Ne, Na, Mg, P, S, and Fe
Fe fusion at extreme temps
Heavier elements formed by neutron capture
“Waste products” of fusion are heavy elements 2 NUCLEAR FUSION IN STARS
Iron (56Fe) is the end product
of ‘normal’ fusion in big stars
(with some cobalt and nickel
produced as well)
Heavier elements formed
during nova explosions NOVA
Nova & supernova explosions of stars blast gas into
space, spreading nuclear waste through galaxy
Enrich nebular clouds with heavy elements.
Heavy elements may also form in some nebulae by
high energy radiation of lighter elements 3 ACCRET
SYSTEM ORIGIN OF SOLAR SYSTEM
Sun &planets form 4.56 b.y.ago
Formed in nebula cloud of dust and gas
- Sun is a small mass star
Earth, Sun, & planets form at same time
- all have similar rotation & all orbit in same plane 4 SOLAR NEBULA THEORY
1) Nebular cloud contract to central ball with 90% mass
& turbulent, rotating outer disk
2) Solids condense in nebula
- heavy elements condense to refractory
rocks in inner area (high temps)
- lighter elements condense to ices in
outer areas (low temp)
- ‘snow line’ at 2.7 astronomical units (AU)
3) Sun forms: gravitational contraction heats and starts
thermonuclear fusion reactions
4) Sun produces explosive blast driving light materials
out of inner areas of nebula SOLAR NEBULA THEORY
5) Magnetic ﬁelds of Sun & nebula interact and
slow Sun's rotation
6) Planets accrete from solids
7) Residual material present in outer edge of Solar System
- Kuiper belt beyond Neptune with small icy planetoids
like Pluto; have irregular orbits;
some captured by Jovian planet as moons
- Oort cloud in outermost zone contains comets;
orbits easily disturbed and changed 5 PLANETARY ACCRETION ACCRETION OF THE EARTH
Earth formed by accretion of refractory materials
- metals (Fe, Mg, Al, Ni, Ca, Na, K)
much more abundant than elsewhere in galaxy
contains 0.03 weight% water (rock & hydro)
H2 & He gases nearly lacking
- Earth's gravity cannot retain
Neon, argon, krypton & xenon rare compared to cosmos
Neon - 1:5,000,000,000
Xenon - 1:5,000,000 6 EARLY EARTH ATMOSPHERE
Original atmosphere of Earth lost by solar blasting
Replacement atmosphere formed by volcanic
emissions (N2, CO2, H2O, HCl)
(a unique feature of the Earth)
Earth was cool enough for water vapor to condense
and form oceans.
(<100° C on surface; <75 ° C at top troposphere) HCl (acid) would dissolve in water to form acidic
oceans and rain, that in turn would rapidly react with
exposed rocks (weathering). Resulting chemistry
produce ocean water similar to modern oceans. Zonal Structure of Earth HEAT FLOW Driving force for Earth interior processes
Internal heat: drives motion of mantle and crust
(mantle plumes; plate tectonics)
Internal heat result of:
Radioactive decay: main source of longterm internal heat
4 b.y. ago: 50% of Earth heat from radioactive decay;
now only 10%
Fluid Earth: Solar heat: drives ﬂow of air and water
Earth heat budget now mostly from solar radiation 7 ZONAL STRUCTURE OF EARTH
Initial heating of Earth to 2000°C
Earth melted - volatiles lost (degassing)
Melting and separation of immiscible materials
Iron & nickel melt sink to center
to form CORE
Ca, K, and Na compounds ﬂoat to top
to form CRUST
(also much O, Si, Al, & U)
Residual Fe & Mg silicates and metal oxides
form MANTLE ZONAL STRUCTURE OF EARTH MANTLE:
Asthenosphere: weak plastic
Mesosphere: strong plastic CRUST: too thin to show
on this diagram 8 PROPERTIES OF LAYERS Continental crust: Low density layer ﬂoating on mantle
(covers 30% of Earth surface)
Oceanic crust: Chilled outer margin of mantle rock FORMATION OF CORE
Core formation dominant event of the early Earth.
Core of Earth form within 50 million years of Earth origin.
Moved most metals from mantle to center of Earth.
(Iron and "siderophile" elements.)
Composed of Fe with 5% Ni and 2-10% S.
Density of liquid (outer) core lower than the density
of pure Fe+Ni.
Settling of iron to center of Earth releases a tremendous
amount of heat energy, accelerating process of melting.
(enough to melt entire Earth twice) 9 CONTINENTAL CRUST
Melting of planets 4.5-4.4 b.y.
Probably no magma oceans on Earth Crust segregation
- lithosphere form by 4.4 b.y., with water-laden surface
form felsic (diorite, granite) continental crust
plate tectonic movement starts
mantle plumes dominant
small size tectonic plates Geologic dating AGE OF EARTH
Oldest rocks in solar system (meteorites, lunar rocks)
Oldest rocks on Earth are 4.03 b.y.
(but some zircon crystals in metamorphics are
Use RADIOMETRIC DATING to determine age 10 GEOLOGIC TIME SCALE GEOLOGIC TIME SCALE 11 METHODS FOR AGE DATING
Numerical age dating - quantitative measures of time
(e.g., radioactive decay) that involve time-dependent
processes; no addition or loss of parent or decay material
Relative age dating - identify unique intervals of
geologic time (e.g., dating with fossils)
Inferential age dating - determine age by comparison to
known sequence of events (e.g., magnetostratigraphy) RADIOMETRIC DATING
Numerical age determination of materials containing
Concept: Naturally occurring radioactive atoms change
to other atoms by spontaneous decay.
The decay process occurs in a time-predictable manner.
examples: parent - daughter
40K 238U 92 → 19 →
90 → 206Pb
82 Nearly all elements have radioactive isotopes. 12 DECAY PROCESSES
Alpha decay - release of alpha particle (4∝2) is same as
release of a helium nucleus without any electrons
- atomic number decreases by 2;
atomic weight decreases by 4
example: 235U 231Th → 92 90 Beta emission - release of an electron
(but it comes from the nucleus)
[neutron can be visualized as proton plus electron]
n = p+ + e- atomic number goes up by 1;
atomic weight unchanged (0)
example: 14C → 6 14N 7 DECAY PROCESSES Electron capture - electron is captured by a proton,
turning it into a neutron
p+ + e- → n
- atomic number goes down one
[atomic weight unchanged]
example: 40K 19 → 40Ar
18 Spontaneous ﬁssion - disintegration of a heavy nucleus
into large fragments
example: 238U 92 → Ba56 + Kr36 + 3 n 13 DECAY PROCESSES RADIOACTIVE DECAY Decay of radioactive isotopes governed by law:
atomic disintegration = decay X atoms
present Decay constant (l) : the fraction of atoms decaying per unit time Half-life: time needed for
daughter product 1/2 of parent to decay to Half-lives range from milliseconds to billions of years
1) Uranium-235 → Lead decay series 0.7 b.y. 2) Uranium-238 → Lead decay series 4.5 b.y. 3) Potassium → Argon electron capture 4) Carbon-14 (Radiocarbon) β emission 1.3 b.y.
5568 years 14 ISOTOPIC HALFLIFE U235 DECAY SERIES
In 235U 92 decay series: Isotope particle emitted
stable half life of isotope
7.13 x108 years
3.43 x104 years (34,300 years)
1.83 x 10-3 seconds
4.76 minutes Sum of half-lives = 0.7 billion years 15 U238 DECAY SERIES URANIUM SERIES METHODS
Decay of U-235 & U-238 to isotopes of lead
Compare ages of co-occurring U-235→Pb-207 and U-238
→Pb-206; both ages should be the same (be concordant)
[some migration of lead atoms may occur] Also,
Decay of intermediate isotopes
Thorium 230: in decay series of U-238→Pb-206
Protactinium 231: in decay series U-235→Pb-207
In water, secreted in shell, thus isolating it from uranium.
Forms closed system, with Th and Pa as parent isotopes.
Half-lives of 75,000 years and 33,000 years.
Dating limit about 300,000 years 16 K-40→Ar-40 DATING METHOD
(An accumulation clock method)
Potassium contains 0.01% 40K isotope Method best for dating volcanic rock and minerals.
1) Pre-existing argon lost from magma.
2) At cooling, new (radiogenic) argon is retained
in mineral or rock
3) As rock cools below closure temperature it forms
Age: the time since material began to retain argon.
Subsequent heating allows argon gas to escape - so,
can date time of metamorphism of heated rocks.
Although 89% of K-40 decays to Ca-40, this decay not often used for dating. 40Ar/39Ar method: a modiﬁed, more precise method. C14 DATING METHOD
(A numerical decay clock method)
Half-life for C-14 is 5568 years
Useful limit for C-14 dating 60,000 years. [Useful range for age dating is about 10 half-lives of the isotope used.] C-14 formed from N-14 in atmosphere,
by cosmic ray bombardment, via electron capture. C-14 is then used by living organisms.
(Method dates once-living material). When organism alive, C-14 level nearly constant (continual
intake of C-14). At death, C-14 level begins to decrease.
Measure C-14 remaining in sample to determine age.
Assume constant production of C-14.
(only minor ﬂuctuations in past 60,000 years) 17 FISSION TRACK DATING
(a numerical radiation damage dating method) Heavy radioactive decay particles (ﬁssion fragments)
produce tunnel of damage in mineral or glass.
These ﬁssion tracks are up to 15 microns long.
Seen by acid etching a polished surface of material. FISSION radiation damage dating method)
Heavy radioactive decay particles (ﬁssion fragments)
produce tunnel of damage in mineral or glass.
These ﬁssion tracks are up to 15 microns long.
Seen by acid etching a polished surface of material.
Age determined by:
1) counting number of ﬁssion tracks per unit area
(= daughter product)
2) annealing surface to remove tracks
3) irradiate to induce ﬁssion of remaining radioactives
4) count number of new ﬁssion tracks (= parent material)
Method works well with zircon and glass.
Material must remain below annealing temperature. 18 COSMOGENIC decay clock method) DATING
(A numerical Measures accumulation of new radioactive isotopes,
produced by exposure to cosmic radiation. Cosmic rays
affects materials within a few metres of ground surface.
Dates ice, lake sediments,and exposure surfaces on rock.
Useful cosmogenic nuclides:
β+ (positron) emission
53Mn electron capture half-life 1,510,000 years
half-life 716,000 years
half-life 3,700,000 years Best for dating exposure surfaces is ratio of 26Al/10Be. Dating limit for exposure surfaces is the saturation level,
when new nuclei balanced by loss.
Saturation level occurs after a few half-lives. DENDROCHRONOLOGY
Age dating using annual
Growth bands record
environmental variation in
addition to age 19 SCLEROCHRONOLOGY
Determining age, growth rate and environmental
variation using periodic (annual, monthly, daily) growth
increments in skeletons.
Corals, molluscs, brachiopods Bivalve growth bands Coral growth banding Inferential dating MAGNETOSTRATIGRAPHY
Inferential dating method Layered rocks contain history of Earth's magnetic ﬁeld
Earth's magnetic ﬁeld is global in extent.
- changes polarity
- reversal occurs quickly - a global time horizon
- intervals between reversals have different duration
Match local record against global standard record to
determine age. (is pattern matching)
Other dating methods used to verify assumed ages. 20 EARTH’S MAGNETIC FIELD P
E 1n BRUNHES NEOGENE RECORD 0.78
1 Jaramillo 1r 2n
2 2r MATUYAMA Olduvai
Reunion Record of reversals in direction of
Earth’s magnetic ﬁeld during last
5 million years
(changes in polarity of the dipole ﬁeld) 2.58
E 2An Kaena
Mammoth Black shows times of normal polarity;
white shows times of reversed polarity 3.58
3r Thvera 21 Geologic Time Scale RELATIVE DATING METHODS
PRINCIPLE OF BIOTIC SUCCESSION
Fossils and fossil assemblages change through a
stratigraphic section and do not repeat
PRINCIPLE OF SUPERPOSITION
In an undisturbed sequence of sediments, younger
deposits occur at at the top and older at the bottom
A rock unit which intrudes another is younger than the
surrounding rock unit PRINCIPLE OF BIOTIC SUCCESSION
William Smith, 1815 Def: Assemblages of fossils change regularly through a
stratigraphic section and do not repeat, so each
stratigraphic unit contains a unique set of fossils.
Each species is limited to a part of the geologic column.
The tool used for deﬁning units of geologic time scale.
Based on the
Principle of Organic Evolution.
Def: Organic evolution is an irreversible process and
each species is unique and different from all others and
has a unique time range. 22 GEOLOGIC TIME SCALE PRINCIPLE OF SUPERPOSITION
Def: In an undisturbed sequence of sedimentary strata,
the oldest strata lie at the bottom and the youngest
strata lie at the top.
Correlary 1 -- Principle of original horizontality Sediment strata are deposited as horizontal layers.
Correlary 2 -- Principle of original lateral continuity Strata originally extended in all directions until they
thinned to zero at the edge of original area (or basin)
of deposition. 23 EARTH FORMED OF LAYERS
general belief that whole Earth formed of layers
G. Arduino, 1759: proposed names for layers
Tertiary (or Alluvial)
loose materials in low areas, with young fossils
layered, fossiliferous rocks in mountains; tilted
hard crystalline rocks in centers of mountains
This was the basis for GEOLOGICAL TIME SCALE
- A method of deﬁning units of geologic time NAMING GEOLOGIC TIME UNITS
Compiled in early 1800's, using biotic succession .
Story: A. Sedgewick & R. Murchison,1835-1841
Murchison trip to eastern Europe tested units (to see if
they were useful “worldwide”). On trip, saw and proposed
Personal quarrel and feud over early Paleozoic units. 24 GEOLOGIC TIME SCALE UNITS
Time units recognized by contained fossils - units of
geologic time scale - are relative time units.
These are chronostratigraphic units.
Boundaries of units are time horizons.
Hierarchy of units (in geologic time scale)
Eras -groups of systems [Paleozoic, Mesozoic, Cenozoic]
System (or Period)
Series (Epoch) - subdivisions of systems Inferential age dating EUSTATIC SEALEVEL CURVES
Changes in sealevel are global.
Historic record of changes is eustatic sealevel curve.
This curve charts transgressions and regressions of
sedimentary cycles (T-R cycles).
- reversals on curve are global-synchronous events
which can be used to correlate stratigraphic sections
Causes of eustatic changes in sealevel:
1) Storage of water on land in continental glaciers
2) Change in mid-ocean ridges by changes inmantle. 25 Inferential age dating SEALEVEL
TECTONICS SEQUENCE UNITS
The sedimentary unit Unconformity-bound units containing a cycle of deposits;
they extend from basin center to highlands
Are T-R (transgressive-regressive) cycles: are the result of
deposition during sealevel or baselevel change in a basin.
Most are global; produced from eustatic changes
Unconformities correspond to tectonic or climatic events;
Contain characteristic groups of deposits (system tracts):
transgressive, highstand, regressive and lowstand sets of
deposits 26 TRANSGRESSION & REGRESSION
migration of a shoreline away from center of an ocean
basin, covering land with water.
Sedimentary environments (facies) shift landward;
seaward facies overlie more landward facies.
migration of shoreline towards center of ocean basin,
exposing more land.
Sedimentary environments (facies) shift seaward;
landward facies overlie more seaward facies. T-R CYCLES
Transgression & regression occur because:
1) sealevel rises or falls
2) land sinks or rises
3) shorelines are ﬁlled in with sediment deposits
Produce a sedimentary cycle (T-R cycle) with vertical
a) change in water depth, associated with
b) change in sediment type 27 UNCONFORMITIES
The boundaries of units Gaps in the rock record Def: An unconformity represents a break in deposition
for a long time interval; shows relation between
tectonism, erosion, and sedimentation.
Surface created by erosion or non-deposition.
Sediments overlie igneous/metamorphic rocks.
Sediments overlie tilted strata. UNCONFORMITIES IN GRAND CANYON 28 FACIES The components of a sedimentary unit The way a sedimentary deposit is related to other
sediments of the same age is the basis for facies.
Facies: Lateral changes in sediment type of age-equivalent
sedimentary deposits that result from deposition in
different depositional environments.
Different types of sediments are deposited in different
Facies changes occur in predictable patterns along
1) elevation - depth
2) climate gradient FACIES
The reason for deﬁning facies In working with sediments, we want to know the age of
the deposits and the environment in which they
accumulated. There is a rich mosaic of depositional
environments on the surface of the Earth and sediments
contain indicators of those environments in the form of
contrasting lithology, sedimentary structures, fossils and
chemistry. The deposits of these many environments are
known as facies. 29 WALTHER’S LAW
Lateral and vertical relationships of facies.
Deﬁnition: For sediments deposited during a transgression
or regression, laterally adjacent facies will occur above or
below the other in a conformable vertical sequence of
Change in vertical trend similar to change in lateral trend.
Used to predict type of sediment present in unsampled
areas. Sediments SEDIMENTARY UNITS
They occur in characteristic units Sediments contain a record of conditions at the Earth
surface through time, but the record may be
discontinuous. In the stack of sediments, the missing
intervals occur at a surface called an unconformity.
Unconformities of wide extent separate related groups
of sediment that are called sequences.
Sequence units contain sediments deposited in a mosaic
of depositional environments. The sediments of adjacent
depositional environments are called facies.
When depositional environments (facies) are mapped in
the sediment record, a picture of the Earth surface can be
mapped. To do this requires determining the geologic age
of the sediments. 30 ...
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This note was uploaded on 11/22/2010 for the course GEOL 106 taught by Professor Yancey during the Spring '08 term at Texas A&M.
- Spring '08