Warm Up 1/10/08Warm Up 1/10/081. Fossils are the ____.

a. oldest layers of rock in a regionb. objects that people of long ago left behind as artifactsc. living creatures with habitats in or around rockd. remains or traces of prehistoric life

2. Radiocarbon dating could be used to date which of the following?a. 60,000-year-old metamorphic rockb. 60,000-year-old mammoth bonec. 15-million-year-old lava flowd. 65-million-year-old meteorite

3. Radioactivity is produced when unstable nuclei ____.a. break apart c. bond togetherb. become coolerd. expand

Answers: 1) d. 2) b. 3) a.

The Geologic Time The Geologic Time ScaleScale

Chapter 12, Section 4Chapter 12, Section 4

Based on their interpretation of the rock Based on their interpretation of the rock record, geologists have divided Earth’s record, geologists have divided Earth’s 4.56-billion-year history into units that 4.56-billion-year history into units that represent specific amounts of timerepresent specific amounts of time

Geologic Time Scale –Geologic Time Scale – the division of the division of Earth history into blocks of time—eons, Earth history into blocks of time—eons, eras, periods, and epochs; the time scale eras, periods, and epochs; the time scale was created using relative dating was created using relative dating principlesprinciples

Since the time scale was created in the Since the time scale was created in the 1919thth Century, when radiometric dating was Century, when radiometric dating was unavailable, it wasn’t until the 20unavailable, it wasn’t until the 20thth Century Century that radiometric dates were incorporatedthat radiometric dates were incorporated

Geologic Time ScaleGeologic Time Scale

EonsEons represent the greatest expanse of time represent the greatest expanse of time EonsEons are divided into are divided into eraseras Each Each eraera is subdivided into is subdivided into periodsperiods PeriodsPeriods are divided into still smaller units called are divided into still smaller units called

epochsepochs The The eoneon that began about 540 million years ago that began about 540 million years ago

is the Phanerozoic, Greek for “visible life”is the Phanerozoic, Greek for “visible life” There are three There are three eraseras within the Phanerozoic: the within the Phanerozoic: the

Paleozoic (“ancient life”), the Mesozoic (“middle Paleozoic (“ancient life”), the Mesozoic (“middle life”), and the Cenozoic (“recent life”)life”), and the Cenozoic (“recent life”)

Each Each eraera is subdivided into is subdivided into periodsperiods, each of which , each of which is characterized by a somewhat less profound is characterized by a somewhat less profound change in life forms as compared with the change in life forms as compared with the eraseras

The The periods periods of the Cenozoic are divided into still of the Cenozoic are divided into still smaller units called smaller units called epochsepochs

The The epochsepochs of other of other periods periods are not usually are not usually referred to by specific namesreferred to by specific names

Structure of the Time ScaleStructure of the Time Scale

Concept CheckConcept Check

What is the geologic time scale What is the geologic time scale divided into?divided into?

Eons, eras, periods, and epochsEons, eras, periods, and epochs

Precambrian TimePrecambrian Time

The detail of the geologic time scale does The detail of the geologic time scale does not begin until the start of the Cambrian not begin until the start of the Cambrian Period (540 million years ago)Period (540 million years ago)

The 4 billion years prior to the Cambrian The 4 billion years prior to the Cambrian are divided into eons, and generally called are divided into eons, and generally called the Precambrianthe Precambrian

The Precambrian represents about 88% of The Precambrian represents about 88% of Earth’s history, but little is known about itEarth’s history, but little is known about it

During Precambrian time, there were During Precambrian time, there were fewer life forms; these life forms are more fewer life forms; these life forms are more difficult to identify and the rocks have difficult to identify and the rocks have been disturbed oftenbeen disturbed often

Precambrian FossilsPrecambrian Fossils

Difficulties With the Geologic Time ScaleDifficulties With the Geologic Time Scale For a radiometric date to be useful, all minerals in For a radiometric date to be useful, all minerals in

the rock must have formed at about the same timethe rock must have formed at about the same time Radioactive isotopes can be used to determine when Radioactive isotopes can be used to determine when

minerals in an igneous rock crystallized and when minerals in an igneous rock crystallized and when pressure and heat made new minerals in pressure and heat made new minerals in metamorphic rockmetamorphic rock

A sedimentary rock may contain particles that A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are contain radioactive isotopes, but these particles are not the same age as the rock in which they occurnot the same age as the rock in which they occur

The sediments that are eventually cemented The sediments that are eventually cemented together into a sedimentary rock have been together into a sedimentary rock have been weathered from older rocksweathered from older rocks

The age of a particular mineral in a metamorphic The age of a particular mineral in a metamorphic rock does not necessarily represent the time when rock does not necessarily represent the time when the rock first formed; the date may indicate when the the rock first formed; the date may indicate when the rock was metamorphosedrock was metamorphosed

Absolute Dating of Sedimentary RocksAbsolute Dating of Sedimentary Rocks

Concept CheckConcept Check

How can geologists overcome the How can geologists overcome the problem of sedimentary rocks and problem of sedimentary rocks and dating the time units of the geologic dating the time units of the geologic time scale?time scale?

Geologists can date igneous rocks, Geologists can date igneous rocks, then determine how the igneous then determine how the igneous rocks relate to nearby sedimentary rocks relate to nearby sedimentary rocks.rocks.

DendrochronologyDendrochronology

AssignmentAssignment

Read Chapter 12, Section 4 Read Chapter 12, Section 4

(pg. 352-355)(pg. 352-355) Do Section 12.4 Assessment #1-7 (pg. Do Section 12.4 Assessment #1-7 (pg.

355)355) Study for Chapter 12 Quiz!Study for Chapter 12 Quiz!