Download Chapter 8: Geologic Time

Chapter 8: Geologic Time
 PowerPoint
Presentation
 Stan Hatfield . SW Illinois College
 Ken Pinzke . SW Illinois College
 Charles Henderson . University of
Calgary
 Tark Hamilton . Camosun College
Copyright (c) 2005 Pearson Education
Canada, Inc.
1
Geological
Time is of
Vast Duration
Sediments were
buried under a
Mountain Range,
metamorphosed,
half the crust was
uplifted & eroded,
glaciers carried it,
lake storms
removed all but
the biggest rocks.
Geologic Time


Relative age dates – placing rocks and events in
their proper sequence of formation. e.g. Tertiary
is younger than Cretaceous (rocks, fossils, climate)
Numerical dates – specifying the actual number
of years that have passed since an event occurred
(known as absolute age dating using isotope clocks)

Geologic time scale – Earth’s history is ~4.5 Ga
long and written disproportionately by the rocks
formed and processes which operated at different
times and places.
Ordovician Strata: Chute Vernal, PQ
Relative Dating – Key Principles

Law of Superposition
 Developed
by the Danish physician Nicolaus Steno
working in Italy in 1669
 In an undeformed sequence of sedimentary rocks
(or layered igneous rocks), the oldest rocks are on
the bottom
 Most sedimentary & volcanic rocks are deposited in
sequences of essentially flat lying beds
Relative Dating – Key Principles

Principle of Original Horizontality
 Layers
of sediment are generally deposited in a
horizontal position
 Rock layers that are flat have not been disturbed

Principle of Cross-Cutting Relationships
 Younger

features cut across older features
Principle of Inclusions
 Younger
rocks/features include older ones
 All 3 types of rocks can include older rocks and
minerals
Relative Dating – Key Principles:
Horizontality & Law of
Superposition
Correlation
Permian Strata, South Rim
Lower Paleozoic Strata: Devon, UK
Fold Hinge
Deposition, Deformation
At the Closing of Iapetus
Formation of Sedimentary Inclusions
Relative Dating – Key Principles
 Unconformities
Types
of Unconformities
 Disconformity
– strata on either side of
the unconformity are parallel (least missing
time)
 Angular
unconformity – tilted rocks are
overlain by flat-lying rocks (10’s of Ma missing)
 Nonconformity – metamorphic or igneous
rocks in contact with sedimentary strata
(most missing time, eroded down to crystalline
basement rocks)
Siccar Point, Scotland &
The development of an angular unconformity.
Figure out the sequence of geological events,
Oldest on the bottom, youngest on the top.
m
h
g
i
j
e
c
k
l
f
d
What kind of
unconformity is “i”?
What kind of
Contact is “m”?
Dyke A
Dyke B
Intr-l
Fault B
k
j
I
Fault A
h
g
fh
e g
d
c
Figure out the sequence of geological events,
Oldest on the bottom, youngest on the top.
m
i
j
e
c
k
l
f
d
“i” is a disconformity
“m” is intrusive,
& baked margins.
Which type of unconformity here at the
South Rim represents the greatest amount
of missing time and why?
Correlation of Rock Layers


Matching of rocks of similar ages in different
regions is known as correlation
Correlation often relies upon fossils
 William
Smith (late 1700s and early 1800s) noted
that sedimentary strata in widely separated areas
could be identified and correlated by their
distinctive fossil content
 Other than fossils we often rely on widespread
instantaneous events like volcanic ash falls
 Unconformities bound packages of stratigraphy
Correlation
across
slightly
overlapping
stratigraphy
Arizona to
Utah.
Correlation of Rock Layers
 Correlation
 Principle
often relies upon fossils
of Fossil (Faunal) Succession – fossil
organisms succeed one another in a definite and
determinable order that documents the evolution of
life; therefore any time period can be recognized by
its fossil content
 Index fossils – represent best fossils for correlation;
they are widespread ecologically & geographically
and are limited to a short time span (i.e., they
evolved rapidly)
Time from Concurrent Range Zones
Dating with Radioactivity
 Reviewing
 Isotope
Basic Atomic Structure
(only a few of these are radioactive)
 Variant
of the same parent atom, same atomic
number
 Differs in the number of neutrons (weight)
 Results in a different mass number than the
common type of atom for this element
 Too many additional neutrons makes a nucleus
unstable and prone to radioactive decay
3 kinds
of
radioactive
decay
Dating with Radioactivity
Parent – an unstable radioactive isotope
 Daughter product – the isotopes that result
from the decay of a parent
 Half-Life – the time required for one-half of
the radioactive nuclei in a sample to decay
 Half-life differs for every Parent-Daughter
pair of isotopes

Exponential Decay of Parent Isotopes
(& exponential growth of daughters)
The Decay
Equation:
A = A0 e –λt
A0 is the initial
amount of the
parent isotope
Dating with Radioactivity
 Radiometric
Principle
 The
Dating
of Radioactive Dating
percentage of radioactive atoms that
decay during one half-life is always the
same (50 percent)
 However, the actual number of atoms that
decay continually decreases
 Comparing the ratio of parent to daughter
yields the age of the sample
Dating with Radioactivity
 Radiometric
dating
Useful
radioactive isotopes for
providing radiometric ages
Rubidium-87  Strontium-87 ~Precambrian
 Feldspar, micas, amphiboles
 Thorium-232  Lead-207 ~Pc
 Zircon
 Two isotopes of uranium (235 and 238) to Lead (208 &
206), 4.5 Ga & 713 Ma respectively, ~Pc to Mesozoic
 Zircon
 Potassium-40  Argon-40 or relative to Argon-39 >2 Ma
 Feldspar, micas, volcanic glass

Slow U
238U
 210Pb
(½ life = 4.5 Ga)
Used for
Ancient
Zircons
&
<200a
210Pb
Dating with Radioactivity
 Rules
for Radiometric Dating
 Mineral
contains both parent & daughter
as for Zircon with 238U and 206Pb or
feldspar with 87Rb and 87Sr
 Formed at time of event to be dated as for
a lava flow, dyke or contact
metamorphism
 The mineral is a closed system and has
neither gained nor lost parent & daughter
Dating with Radioactivity
 Radiometric
Sources
A
dating
of error
closed system is required
 If temperature is too high, daughter
products may be lost
 To avoid potential problems, only fresh,
unweathered rock samples should be used
Production of 14C in the upper atmosphere
(& decay once organisms die)
Cosmic rays
Expel neutrons from
Atmospheric gases (N,O).
This expels a proton
From nitrogen
Forming radioactive 14C.
The extra neutron
in radioactive 14C
decays back to 14N
With a ½ life of
5730 years.
Dating with Radioactivity
 Dating
dating)
with carbon-14 (radiocarbon
 Half-life
of only 5730 years
 Used to date very recent events
 Carbon-14 is produced in the upper
atmosphere
 Useful tool for anthropologists,
archaeologists, historians, and geologists
who study very recent Earth history
 Only works for plant or animal tissue, not
rocks
The Geological Time Scale
Geologic Time Scale
 Structure
 Names
of the geologic time scale
of the eons
 Phanerozoic
(“visible life”) – the most recent
eon, began just over 540 million years ago
 Proterozoic
 Archean
 Hadean – the oldest eon
The Geological Time Scale
Geologic Time Scale
 Structure
 Era
of the geologic time scale
– subdivision of an eon
 Eras of the Phanerozoic eon
 Cenozoic (“recent life”)
 Mesozoic (“middle life”)
 Paleozoic (“ancient life”)
 Eras are subdivided into periods
 Periods are subdivided into Epochs
 Precambrian
time
Nearly
4 billion years prior to the
Cambrian period
Not divided into smaller time units
because the events of Precambrian
history are not known in great enough
detail
 Also,
first abundant fossil evidence does not
appear until the beginning of the Cambrian
 Difficulties
time scale
in dating the geologic
Not
all rocks can be dated by
radiometric methods
 Grains
comprising detrital sedimentary rocks are
older than the rock in which they formed
 The age of a particular mineral in a
metamorphic rock may not necessarily represent
the time when the rock formed as
porphyroblasts may grow for many Ma
 The rock needs minerals with the right parent
and daughter isotope pairs and in the right age
span to be measureable.
 Difficulties
in dating the geologic
time scale
Datable
materials (such as volcanic ash
beds and igneous intrusions) are often
used to bracket various episodes in
Earth history and arrive at ages
Dates change as brackets become
narrower and methods refined; e.g.,
base of Triassic is now 252 Ma, base of
Permian is now 299 Ma, base of
Cambrian is 543 Ma…
Igneous events permit
Absolute dating
Of sedimentary rocks.
Dating Terrains On other Worlds
Crater
counts
& cross
cutting
relations
mostly
Precambrian
Dendrochronology (wiggle matching)
For wood in 14C realm <70Ka
Extinctions: Dinosaur Graveyards
Bolide impact
& Deccan Trap
Volcanism both
occur at 65Ma.
Biodiversity
dwindled for 10 Ma
prior to this.
The K/T massive
extinction is
debated.