The Rock Record

The Rock RecordThe Rock RecordChapter 32 Earth Science BookChapter 32 Earth Science Book

read pages 596-609read pages 596-609restate & answer Topic Questions 1-14restate & answer Topic Questions 1-14

Telling TimeTelling TimeTopic 1: Page 597 Earth Textbook

James Hutton (1726-1797).

Proposed that most geologic

processes happen very slowly.

Relative Time - Position in that sequence; older or younger than nearby layers

Absolute Time - Numerical Age can be determined through analysis of the products of radioactive decay

Layered rocks contain clues about past environments at/near surface.

Sequence and relative ages provide basis for reconstructing Earth’s history.

The study of strata is called stratigraphy.

StratigraphyStratigraphyTopic 1: Page 597 Earth Textbook

The basis of rock stratigraphy is the formation.- a collection of similar strata that are sufficiently

different from adjacent groups - basis of physical properties they constitute a

distinctive, recognizable unit that can be used for geologic mapping over a wide area.

Stratographic ClassificationStratographic ClassificationTopic 1: Page 597 Earth Textbook

19th century – geologists assembled a

geologic columncomposite column

containing, in chronological order, the succession of known strata, fitted together

based of fossils or other evidence of relative age.

This is the geologic time scale.

Geologic Column & Geologic TimescaleGeologic Column & Geologic TimescaleTopic 1: Page 597 Earth Textbook

Relative ages: younger or olderRelative ages: younger or olderRelative TimeRelative Time

Topic 2: Page 597 Earth Textbook

*Law of Superposition

*Law of Cross-cutting Relationships

*Law of Included Fragments

*Unconformities

Law of SuperpositionLaw of Superposition


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Law of Superposition:Law of Superposition:Grand Canyon StrataGrand Canyon Strata

Topic 2:

Most sediment is laid down in the sea, in shallow waters, or in streams.

Each new layer is laid down

horizontally over older ones

Principle of original horizontality - sediments are deposited in strata that are horizontal or nearly horizontal

Law of Superposition: SedimentsLaw of Superposition: SedimentsTopic 2: Page 597 Earth Textbook

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Law of Superposition: HorizontalityLaw of Superposition: HorizontalityTopic 2: Page 597 Earth Textbook

A fault is a break or crack in Earth’s crust where rocks shift their positions.

Law of cross-cutting relationships = a fault or igneous intrusion is always younger than the rock layers it cuts through

Law of Cross-Cutting RelationshipsLaw of Cross-Cutting Relationships


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Law of Cross-Cutting RelationshipsLaw of Cross-Cutting RelationshipsTopic 2: Page 597 Earth Textbook

Law of Law of Cross-Cutting Cross-Cutting RelationshipsRelationships

Igneous intrusion

Fault

1) Based on the law of superposition, what is the oldest layer pictured?

Law of Cross-Cutting RelationshipsLaw of Cross-Cutting RelationshipsTopic 2:

Igneous intrusion

Fault

1) Based on the law of superposition, what is the oldest layer pictured? A is the oldest, D is youngest


Igneous intrusion

Fault


2) Based on the law of crosscutting relationships, which is older, the fault or the igneous intrusion?


Igneous intrusion

Fault


2) Based on the law of crosscutting relationships, the igneous intrusion is older than the fault ?


List the labeled formations from youngest to oldest.

List the labeled formations from youngest to oldest.

C B A E D

Pieces of one rock found in another rock must be older than the rock in which they are found.

“ingredients are older than the rock”

Law of Included FragmentsLaw of Included Fragments

included fragment

s are older

than the rock


Law of Included

Fragments

Unconformities are breaks in the geologic record that show deposition stopped for a period of time, rock was removed by erosion, and deposition resumed.

Angular unconformity

UnconformitiesUnconformitiesTopic 2: Page 597 Earth Textbook

Evidence of former seafloors uplifted by tectonic forces and exposed to erosion.

Later tectonic forces depress the surface.

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

The Significance of UnconformitiesThe Significance of UnconformitiesTopic 2: Page 597 Earth Textbook

summary of major events in Earth’s Historysummary of major events in Earth’s HistoryGeologic TimetableGeologic Timetable


5 Eras of Earth’s History1. Archean: 4.5-2.5 Billion Years Ago

2. Proterozoic: 2.5 BYA-570 MYA3. Paleozoic: 570-250 MYA4. Mesozioc: 250-65 MYA

5. Cenozoic: 65 MYA- Today

*Eras broken into Periods, Periods into Epochs

Chapter 8

Chapter 8

Geologic Time and the Rock RecordGeologic Time and the Rock RecordTopic 3: Page 598 Earth Textbook

The divisions

of the geologic

time scale.

Topic 3:

Figure 11.10

Topic 3:

Scale of the Geologic Time

Topic 3:

Topic 3:

Fossil FormationFossil FormationTopic 4: Page 602 Earth Textbook

*Any evidence of earlier life preserved in rock

1.original remains: unchanged organisms2. replaced remains: become calcite, silica

3. molds, imprints, casts or hollows4. trace fossils: trails burrows, footprints

Why Do We Study Fossils Found in Rocks?

What is a Fossil?

Definition: The evidence or remains of once-living plants or animals

A. To provide evidence of the past existence of life formsB. To provide information about past environmental conditionsC. To provide evidence that populations have undergone change

over time due to environmental changes (evolution)

Studying The PastTopic 4:

Original PreservationDescription: plant or animal remains that have not undergone

change since death.a. Uncommon because frozen, extremely dry, or oxygen-

free environments are required to form these fossilsb. Examples:

• Mummified humans• Frozen organisms (Ice Man)• Mammoths & cats in La Brea Tar Pits• Fossilized insects in tree sap (amber)

Fossil FormationFossil FormationTopic 4:

Replaced Hard PartsDescription: all organic material has been removed and the

hard parts of the organism have been changeda. Minerals seep in slowly and replace the original

organic tissue, forming a rock-like fossilb. The fossil has the same shape as the original object,

but is chemically more like a rock!c. Examples:

• Petrified wood• Recrystallized shells


Molds and CastsDescription: Fossils that do not contain any shell or bone

a. A mold is formed when original shell parts are weathered and eroded, leaving an impression of the shell.

b. This cavity might later become filled with minerals or sediment to create a cast.

c. Examples: • Common with shellfish


E. Trace Fossils1. Description: Indirect evidence of plant and animal life

a. Provide information about how an organism lived, moved or obtained food

b. Examples: • worm trails• burrows• footprints


How Fossils

Are Formed

Freezing (refrigeration) Best means of preservation of ancient materials. Rare - continually frozen from death til discovery. Mammoths and wooly rhinoceros found in ice from

Alaska and Siberia. Specimens with flesh, skin, and hair intact have been

found. suggest that they were flash frozen, with food still in the

mouth and stomach.

Topic 4:

Drying (desiccation)

Mummified bodies discovered in arid parts Soft tissues preserved if completely dried.


Asphalt La Brea Tar Pits - 100 pits filled with sticky asphalt or tar. formed by crude oil seeping through fissures in the earth. lighter elements evaporated leaving thick sticky asphalt. pits are famous for Pleistocene fossils The fossils date between 10 and 40 thousand years old. Asphalt is an excellent preservative.

Topic 4:

Amber (Unaltered preservation) Insects, spiders, and even small lizards have been found,

nearly perfectly preserved in amber. Over millions of years, sap with our fly inside is

polymerized and hardened into amber.


Permineralization (Petrification) most commonMinerals fill the cellular spaces and crystallize. Shape of the original plant or animal is

preserved as rock. Sometimes the original material is dissolved away leaving the form and structure but none of the organic material remains.

CarbonizationPlant leaves, and some soft body parts of fish,

reptiles, and marine invertebrates decompose leaving behind only the carbon.

This carbon creates an impression in the rock outlining the fossil, sometimes with great detail.

Topic 4:

Casts & Molds molds and casts of organisms which have dissolved or rotted

away, leaving only a trace of their existence. Casts and molds are types of fossils where the physical

characteristics of organisms have been impressed onto rocks. buried or trapped in mud, clay, or other materials which

hardened around them - leaving molds of the organism. There are two types of molds: external and internal.

Topic 4:

Fossils: Evidence for EvolutionFossils: Evidence for EvolutionTopic 5: Page 603 Earth Textbook

The process of gradual change that produces new life forms over geologic time.

Natural Selection: survival of the fittest.

Relative-Age Dating: 1. Definition: Dating rocks and fossils by placing them in

chronological order without exact dates.2. Geologic Principles (used in this dating process):

a. Original Horizontality• Sedimentary rocks are deposited in horizontal layers

b. The Law of Superposition • in an undisturbed sequence the oldest rocks are at the

bottom and each successive layer is youngerc. Principle of Cross-Cutting Relationships:

• an intrusion or a fault is younger than the rock it cuts across

Fossils: Evidence for EvolutionFossils: Evidence for EvolutionTopic 5: Page 603 Earth Textbook

Index Fossils & Key BedsIndex Fossils & Key BedsTopic 6: Page 604 Earth Textbook

Description: Remains of unique species that can be used to correlate rock layers or to date a particular rock layera. Must be easily recognized, abundant, and widely

distributed geographicallyb. Must have lived during a relatively short time periodc. Examples:

• The mollusk Ecphora has a distinctive shape

Rock CorrelationRock CorrelationTopic 7: Page 604 Earth Textbook

matching of rock layers from one area to another

Rock Correlation


Other Uses of FossilsOther Uses of FossilsTopic 8: Page 605 Earth Textbook

*fossils indicate past climate

*micro fossils used in oil exploration

Absolute Time: Tree RingsAbsolute Time: Tree RingsTopic 9: Page 606 Earth Textbook

Absolute-Age Dating: Definition: Dating rocks and fossils by using techniques

to determine their actual age. (actual numbered dates)Methods:

Tree Rings and Seasonal Climatic Changes• Each tree ring represents 1 year of growth• Good growth years are thicker• Although accurate, this method can be used to date very far back in time.

Absolute Time: VarvesAbsolute Time: VarvesTopic 10: Page 606 Earth Textbook

Varves are annual layers found in some sedimentary rocks that can be counted like tree rings.

Winter: dark fine sediment

Summer: Thick light colored layers

Absolute Time: RadioactivityAbsolute Time: RadioactivityTopic 11: Page 607 Earth Textbook

Atoms of the same element have different numbers of neutrons (isotopes)

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

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

3. 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.

Kinds of Radioactive DecayKinds of Radioactive DecayTopic 11: Page 607 Earth Textbook

Figure 11.12

Figure 11.14

A few isotopes, such as 14C, are radioactive.instability within the

nucleus.transform

spontaneously to a nucleus of a more stable isotope of a different chemical element.

The process is called radioactive decay.

Radioactive DecayRadioactive DecayTopic 11: Page 607 Earth Textbook

Absolute Time: Half-LifeAbsolute Time: Half-LifeTopic 12: Page 607 Earth Textbook

The time it takes for half of a radioactive element to decay to a stable end product

After 1half-life haspassed, half the atoms have changedor emitted energy.

After a 2nd half-life has passed half ofthe remaining atomshave also changed

Half-LifeHalf-LifePage 607 Earth Textbook

Suppose a rock sample has 1/64th of the expected amount of radioactive material.

1 1/2 1/4 1/8 1/16 1/32 1/64 This means that the sample has been through 6 half-

lives. If each half-life is 100,000 years then the rock layer

is 600,000 years old


Absolute Time: Radiocarbon DatingAbsolute Time: Radiocarbon Dating


14C is useful for dating young samples.Half-life of 14C is short - 5730 yrs 14C continuously created in the atmosphere

through bombardment of 14C by neutrons created by cosmic radiation.

Other Radiometric MethodsOther Radiometric MethodsTopic 14: Page 608 Earth Textbook

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.

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

The Earth was formed approximately 4.55 billion years ago.

Age of the EarthAge of the EarthTopic 14: Page 608 Earth Textbook

The Rock RecordThe Rock RecordChapter 32 Earth Science BookChapter 32 Earth Science Book

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The Rock Record

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