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 14C, 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)

14C 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 14C by neutrons created by cosmic radiation.

Figure 11.14

Radiocarbon Dating (2)

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

Living organisms have the same proportion of 14C

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 580oC, 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, 40Ar (daughter of 40K), 3He, and 36Ar (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.