ch01 introduction to earth science

Why Study Earth History?The Earth has changed through time.. • Changes to life on Earth through time - sometimes slow, sometimes swift and deadly... Two major branches of geology: • Phy

Chapter 1

The Science of Historical

Geology

Introduction

The Earth formed about 4.6 billion years

ago

Homo sapiens appeared on Earth between

about 300,000 and 150,000 years ago

Humans ask questions about their surroundings.

How did the Earth form?

Why do earthquakes occur?

What lies beneath the land and below the ocean floor?

Curiosity leads to exploration

Why Study Earth History?

The Earth has changed through time

Understanding past geologic events will help us predict future geologic events

Past geologic events include:

• Earthquakes

• Volcanic eruptions

• Continents flooded by inland seas

• Drifting and colliding continents

• Glaciers have covered large parts of continents

• Meteorite and asteroid impacts

• Changes in chemistry of oceans and atmosphere

• Changes to life on Earth through time - sometimes

slow, sometimes swift and deadly

Geology

Geology is the study of the Earth

Two major branches of geology:

• Physical Geology - deals with Earth

materials and processes

• Historical Geology - deals with origin and

changes of Earth and life through time and space

What do Geologists Do?

• Study the structure of mountain ranges

• Attempt to predict geologic hazards like

earthquakes and volcanic eruptions.

• Identify minerals in meteorites to learn how Earth

formed.

• Study rivers, floods, glaciers, and underground

water.

• Look at results of past events and work backward

in time to discover causes of those events

• Search for fossil fuels and mineral resources

Scientific Method in Geology

Science operates through the use of the Scientific Method

The scientific method is a method for finding

answers to questions and solutions to problems Scientists work like detectives to gather data , to try to figure out what happened

The data may be obtained through observations

and/or experiments , which can be repeated and verified by others

Summary of Scientific Method

1 A question is formulated

2 Observations (collect data)

3 Develop multiple working hypotheses (ideas to explain the observations)

4 Test the hypotheses by experimenting and either accept ,

reject , or modify the hypothesis.

The simplest explanation is best

5 When a hypothesis has considerable experimental or

observational support, it is accepted and others are

rejected, and it may become a theory

6 A theory ultimately may become a scientific law

What is a Theory?

A hypothesis that survives repeated challenges , and

is supported by a large body of evidence , may be elevated to the status of a theory

A theory is not just an wild idea or a guess.

Theories have survived close examination, and can

be accepted with confidence

A theory has a very high probability of being correct

Examples of theories include the theory of relativity, plate

tectonics theory, evolutionary theory, and atomic theory.

Major Themes in Earth History

1 Deep time

2 Plate tectonics

3 Evolution of life

Deep Time

• Recognition of immensity of geologic time

is geology's most important contribution to human knowledge

• The science that deals with determining

the ages of rocks is called geochronology

Methods of Dating Rocks

Quantifying the age of the rock or mineral

in years

are older and which are younger

Absolute Age

The discovery of radioactivity in 1896 gave us the tools to find the absolute age of a rock.

Radiometric dating involves analysis of the

breakdown of unstable radioactive elements in rocks

Radioactive elements decay by releasing

subatomic particles from their nuclei Through this process, the unstable radioactive element is converted to a stable "daughter" element

Example: Uranium-235 decays to form lead-207

Radioactive Decay

Many radioactive elements can be used as geologic clocks Each radioactive element decays at its own nearly constant rate

The rate of decay can be measured

Once this rate is known, geologists can

determine the length of time over which

decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter

elements

Example: Uranium-235 has a half-life of about

704 million years

235 decays to form lead-207

Uranium-235 has a half-life of about 704 million years

1 After 704 million years, only half (50%) of the

uranium atoms in the mineral remain (The rest have decayed to lead-207.)

2. After another 704 million years, only half of

that amount (or 25%) of the uranium atoms

remain

3 So, a rock with 25% uranium-235 and 75%

lead-207 must be 1,408 million years old (or

1.408 billion years old)

Using radiometric dating, some rocks found

in Canada's Northwest Territories have

been dated at 4.04 billion years old

Relative age

• Determining which rocks are older and which are

younger “Rock unit A is older than rock unit B".

• The geologic time scale was developed through

relative dating.

• Relative age determinations provide a

framework or geologic time scale in which to

place events of the geologic past.

• Using radiometric dating, actual dates in years

have been determined for the geologic time

scale

Major Themes in Earth History

1 Deep time

2 Plate tectonics

3 Evolution of life

Plate Tectonics

The theory of plate tectonics has

revolutionized the understanding of

geology Plate tectonics explains many large scale patterns in the Earth's

geological record

It is a "great unifying theory" in geology

Plate Tectonics

The Earth's surface or lithosphere is divided into

plates (about 7 large plates and 20 smaller ones).

Plate Tectonics

The lithosphere is about 100 km thick and

consists of the rigid, brittle crust and

The plates are moving, but their rates and

directions of movement vary

Plate Movements

Plate movement is due to convectional flow

(circular movement of the asthenosphere

due to hot material rising and cooler

material sinking)

The plates only move a few millimeters per year, about the rate at which your

fingernails grow

Types of plate boundaries:

• Divergent - where plates move apart from one

Major Themes in Earth History

1 Deep time

2 Plate tectonics

3 Evolution of life

Evolution of Life

In biology, evolution is the

"great unifying theory" for understanding

the history of life

Evolution of Life

As a result of evolution, plants and animals living today are different from their ancestors They

differ in appearance, genetic characteristics,

body chemistry, and in the way they function.

These differences appear to be a response to

changes in the environment and competition for food

Fossils record the changes in organisms over time

Natural Selection

Charles Darwin and Alfred Wallace were the first scientists to assemble a large body of convincing observational evidence in

support of evolution

They proposed a mechanism for evolution which Darwin called natural selection

Natural selection is based on the following observations:

• Any given species produces more offspring than

can survive to maturity

• Variations exist among the offspring

• Offspring must compete with one another for

food and habitat

• Offspring with the most favorable characteristics

are more likely to survive to reproduce

• Beneficial traits are passed on to the next

generation

Lines of evidence for evolution

cited by Darwin

• Fossils provide direct evidence for changes in

life in rocks of different ages

• Certain organs or structures are present in a

variety of species, but they are modified to

function differently ( homologous structures ).

• Modern organisms contain vestigial organs that

appear to have little or no use These structures had a useful function in ancestral species

• Animals that are very different, had

similar-looking embryos

Other lines of evidence for evolution come from the fields of:

• Genetics (DNA molecule)

• Biochemistry (Biochemistry of

closely-related organism is similar, but very

different from more distantly related

organisms)

• Molecular biology (sequences of amino

acids in proteins)

Organic Evolution

These discoveries indicate that plants and animals of each geologic era arose from earlier species by the process we call

"organic evolution"

Organic evolution refers to changes that have occurred in organisms with the

passage of time