radioactive decay. - Wiley

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Chapter 11: Geologic Time And The
Rock Record

Introduction


The concept that most geologic processes happen
very slowly was proposed by James Hutton (1726
-
1797).


Geologists sort Earth’s history into a sequence of
events.


Position in that sequence identifies
relative age.


Numerical age

can be determined through analysis of the
products of radioactive decay


Reading The Record Of Layered Rocks


Layered sedimentary or volcanic rocks contain
important clues about past environments at and near
Earth’s surface.


Their sequence and relative ages provide the basis
for reconstructing much of Earth’s history.


The study of strata is called
stratigraphy
.

Figure 11.1

The Laws of Stratigraphy


Most sediment is laid down in the sea, generally in
relatively shallow waters, or by streams on the land.


Each new layer is laid down horizontally over older
ones.


The
law of original horizontality

states that water
-
laid sediments are deposited in strata that are
horizontal or nearly horizontal.


Stratification, Superposition, and the
Relative Ages of Strata (1)


The principle of
stratigraphic superposition

states
that any sequence of sedimentary strata was
deposited from bottom to top.


Charles Lyell and other geologists of the nineteenth
century speculated that it might be possible to
determine numerical ages by using stratigraphic
record.

Figure 11.2

Stratification, Superposition, and the
Relative Ages of Strata (2)


Two assumptions must be correct for the method to
work:


It must be assumed that the rate of sedimentation was
constant throughout the time of sediment accumulation.


It must be assumed that all strata exhibit conformity,
meaning they have been deposited layer after layer
without interruption.

Stratification, Superposition, and the
Relative Ages of Strata (3)


The first assumption is false because it can be
observed today that sedimentation rates vary widely
from place to place and time to time.


The second and even more important assumption is
false because sedimentation can be disrupted
periodically by major environmental changes, such
as sea level changes and tectonic activity that lead
to intervals of erosion or non deposition.

Kinds of Unconformities (1)


An
unconformity

is a substantial break or gap in a
stratigraphic sequence.


Three important kinds of unconformities are found
in sedimentary rocks:


Angular unconformity
.


The older strata were deformed and then cut off by erosion
before the younger layers were deposited across them.

Figure 11.3

Kinds of Unconformities (2)


Disconformity.


It is an irregular surface of erosion between parallel strata.


A disconformity implies a cessation of sedimentation and
erosion, but not tilting.


It is often hard to recognize, because the strata above and
below are parallel
.


Nonconformity.


Strata overlie igneous or metamorphic rock
.

Figure 11.4

The Significance of Unconformities


The many unconformities exposed in rocks of
Earth’s crust are evidence that former seafloors
were uplifted by tectonic forces and exposed to
erosion.


Preservation of a surface of erosion occurs when
later tectonic forces depress the surface.


The surface, in turn, becomes a site of deposition of
sediment.

Stratigraphic Classification (1)


A
rock
-
stratigraphic unit

is any distinctive
stratum that differs from the strata above and below.


The basis of rock stratigraphy is the
formation.


A formation is a collection of similar strata that are
sufficiently different from adjacent groups of strata so
that on the basis of physical properties they constitute a
distinctive, recognizable unit that can be used for
geologic mapping over a wide area.


Stratigraphic Classification (2)


Each of the boundaries of a
time
-
stratigraphic
unit
, upper and lower, is uniformly the same age.


The primary time
-
stratigraphic unit is a
system
,
which is chosen to represent a time interval
sufficiently great so that such units can be used all
over the world.

Figure 11.6

Stratigraphic Classification (3)


The primary unit of geologic time is a
geologic
period
, which is the time during which a geologic
system accumulated.


Correlation

is the determination of equivalence in
time
-
stratigraphic or rock
-
stratigraphic units of the
succession of strata found in two or more different
places.

How Correlation Is Accomplished


Correlation involves two main tasks:


Determining the relative ages of units exposed within a
local area being studied (identifying the same formation
wherever it crops out).


Establishing the ages of the local rock units relative to a
standard scale of geologic time.


Distinctive fossils (
index fossils
)are especially useful for
this purpose. If a distinctive index fossil is recognizable at
an outcrop, a rapid and reliable means of correlation is
available.

Figure 11.7

Figure 11.9

The Geologic Column and the Geologic
Time Scale


In the nineteenth century, geologists began to
assemble a geologic column, which is a composite
column containing, in chronological order, the
succession of known strata, fitted together on the
basis of their fossils or other evidence of relative
age.


The corresponding column of time is the geologic
time scale.

Figure 11.10

Eons


An
eon

is the largest interval into which geologic
time is divided.


There are four eons.


The
Hadean Eon

is the oldest


Some of the samples brought back from the moon were formed
during the Hadean Eon.


The
Archean Eon

follows the Hadean.


Archean rocks, which contain primitive microscopic life forms
are the oldest rocks we know of on the Earth.


The
Proterozoic Eon

follows the Archean.


The
Phanerozoic Eon

is the most recent of the four eons.


Eras (1)


Each of the eons is subdivided into shorter time
units called eras.


The Phanerozoic Eon is divided into the:


Paleozoic (old life).


Mesozoic (middle life).


Cenozoic (recent life).

Eras (2)


In the
Paleozoic Era
, early land plants appeared,
expanded and evolved. Developing animal life
included marine invertebrates, fishes,
amphibians,and reptiles.


The
Mesozoic Era

saw the rise of the dinosaurs,
which became the dominant vertebrates on land.
Mammals first appeared during the Mesozoic Era as
did flowering plants.


Mammals dominated the
Cenozoic Era
. Grasses
evolved during the Cenozoic Era, and became an
important food for grazing mammals
.


Periods


The Eras of the Phanerozoic Eon are divided into
periods
.


The periods are defined on the basis of the fossils
contained in the equivalent rocks.


The two Periods are the Quaternary Period and the
Tertiary Period


Epochs


Periods are further subdivided into
epochs

on the
basis of the fossil record.


The Tertiary Period is divided into these epochs:


Paleocene.


Eocene.


Oligocene.


The Quaternary Period is divided into these epochs:


Holocene.


Pleistocene.


Early Attempts to Measure Geologic
Time Numerically (1)


Early attempts to measure geologic time
numerically were inaccurate.


Edmund Halley suggested, in 1715, that sea salt might be
used to date the ocean.


John Joly finally made the necessary measurements and
calculations in 1889. His determination of the ocean’s
age, 90 million years, was not correct.


Salts are added both by erosion and by submarine
volcanism, but salts are also removed by solution.


Early Attempts to Measure Geologic
Time Numerically (2)


Lord Kelvin, a physicist, attempted to calculate the
time Earth has been a solid body.


By measuring the thermal properties of rock and
estimating the present temperature of Earth’s
interior, he calculated the time for the Earth to cool
to its present state.


His estimate of 100 million years is incorrect.


The Earth’s interior is cooling so slowly that it has a nearly
constant temperature over periods as long as hundreds of
millions of years
.


Radioactivity (1)


In 1896, the discovery of radioactivity provided the
needed method to measure the age of the Earth
accurately.


Different kinds of atoms of an element that contain
different numbers of neutrons are called isotopes.


Most Isotopes of the chemical elements found in Earth
are generally stable and not subject to change.

Figure 11.11

Radioactivity (2)


A few isotopes, such as
14
C, are radioactive.


Radioactivity arises because of instability within an
atomic nucleus.


If the ratio of the number of neutrons (n) to the number of
protons (p) is too high or too low, the atomic nucleus of a
radioactive isotope will transform spontaneously to a
nucleus of a more stable isotope of a different chemical
element.

Radioactivity (3)



The process is called
radioactive decay.


An atomic nucleus undergoing radioactive decay is said
to be the parent.


The product arising form radioactive decay is called a
daughter.

Kinds of Radioactive Decay (1)


Radioactive decay can happen in five ways:


1. Beta decay: emission of an electron from the nucleus.


2. Positron emission: emission of a particle with the same
mass as an electron but with a positive charge.


3. Electron capture: by capture into the nucleus of one of
the orbital electrons, a process that decreases the number
of protons in the nucleus by one.

Kinds of Radioactive Decay (2)


4. Alpha decay: emission from the nucleus of a heavy
atomic particle consisting of two neutrons and two
protons called an
α (alpha) particle.


5. Gamma ray emission: emission of γ rays (gamma
rays), which are very short
-
wavelength, high
-
energy
electromagnetic rays.


Gamma rays have no mass, so gamma ray emission does
not affect either the atomic number or the mass number of
an isotope.

Figure 11.12

Rates of Decay and the Half
-
Lives of
Isotopes (1)


The rate at which radioactive decay occurs varies
among isotopes.


Decay rates are unaffected by changes in the
chemical and physical environment.


The decay rate of a given isotope is the same in the
mantle or in a sedimentary rock.


In radioactive decay, the proportion

fraction or
percentage

of parent atoms that decay during each
unit of time is always the same.

Rates of Decay and the Half
-
Lives of
Isotopes (2)


The rate of radioactive decay is measured in terms
of
half
-
life
, the amount of time needed for the
number of parent atoms to be reduced by one half.


At the end of each unit of time (half
-
life), the
number of parent atoms has decreased by exactly
one
-
half.

Figure 11.13

Using Radioactivity to Measure Time


Radioactivity in a mineral is like a clock.


The length of time this clock has been ticking is the
mineral’s radiometric age.


Many natural radioactive isotopes can be used for
radiometric dating, but six predominate in geologic
studies:


Two radioactive isotopes of
uranium

plus radioactive isotopes
of
thorium
,
potassium
,
rubidium

and
carbon

are used.


In practice, an isotope can be used for dating samples that are
no older than about six half
-
lives of the isotope.

Radiocarbon Dating (1)


14
C is especially useful for dating geologically
young samples.


The half
-
life of radiocarbon is short

5730 years

by comparison with the half
-
lives of most isotopes
used for radiometric dating.


Radiocarbon is continuously created in the
atmosphere through bombardment of
14
C by
neutrons created by cosmic radiation.

Figure 11.14

Radiocarbon Dating (2)


Though some variations have been identified, the
proportion of
14
C is nearly constant throughout the
atmosphere and biosphere.


Living organisms have the same proportion of
14
C

In their bodies as exists in their environment.


No carbon is added after death, so by measuring the
radioactivity remaining in an organic sample, we
can calculate how many half
-
lives ago the organism
died.

Radiometric Dating and the Geologic
Column


Through various methods of radiometric dating,
geologists have determined the dates of
solidification of many bodies of igneous rock.


“Moon dust” brought back by astronauts, is 4.55
billion years old.


The Earth was formed approximately 4.55 billion
years ago.

Figure 11.15

Figure B01

Figure B02

Magnetic Polarity Time Scale (1)


Certain rocks become permanent magnets as a result
of the way they form.


Magnetite and certain other iron
-
bearing minerals
can become permanently magnetized.


Above a certain temperature (called the
Curie
point
), the thermal agitation of atoms is such that
permanent magnetism is impossible.


Below that temperature, however, the magnetic
fields of adjacent iron atoms reinforce each other.

Figure 11.16

Figure 11.17

Magnetic Polarity Time Scale (2)


As solidified lava cools, the temperature will drop
below 580
o
C, the Curie point for magnetite.


When the temperature drops below the Curie point,
all the magnetite grains in the rock become tiny
permanent magnets with the same polarity as
Earth’s field.


All lava formed at the same time records the same
magnetic polarity information.

Figure 11.18

Magnetic Polarity Time Scale (3)


The Earth’s polarity has shifted in the past. A period in which polarity
remains stable is called a
magnetic chron.



The four most recent chrons have been named for scientists
who made great contributions to studies of magnetism. The
four chrons below occurred during the last 4.5 million
years. From the most recent to the oldest:


Brunhes.


Matuyama.


Gauss.


Gilbert.


Figure 11.19

Primordial Gasses


Studies of volcanic gases provide other clues to the
age of the Earth.


Three gases,
40
Ar (daughter of
40
K),
3
He, and
36
Ar (both
primordial gases trapped in Earth from the solar nebula),
are being released, but they are not being recycled.


Because they accumulate in the atmosphere, their
growing proportion can be used to estimate the age of the
Earth.