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The Rock Record. read pages 596-609. restate & answer Topic Questions 1-14. Chapter 32 Earth Science Book. Topic 1:. Page 597 Earth Textbook. Telling Time. James Hutton (1726-1797). Proposed that most geologic

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The Rock Record

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The Rock Record

read pages 596-609

restate & answer Topic Questions 1-14

Chapter 32 Earth Science Book


Topic 1:

Page 597 Earth Textbook

Telling Time

  • 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.

Topic 1:

Page 597 Earth Textbook

Stratigraphy


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.

Topic 1:

Page 597 Earth Textbook

Stratographic Classification


0072


19th century –

geologists assembled a geologic column

composite 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.

Topic 1:

Page 597 Earth Textbook

Geologic Column & Geologic Timescale


Topic 2:

Page 597 Earth Textbook

Relative Time

*Law of Superposition

*Law of Cross-cutting Relationships

Relative ages: younger or older

*Law of Included Fragments

*Unconformities


Topic 2:

Page 597 Earth Textbook

Law of Superposition


Law of Superposition:

Grand Canyon Strata

Topic 2:

9


Topic 2:

Page 597 Earth Textbook

Law of Superposition: Sediments

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


Topic 2:

Page 597 Earth Textbook

Law of Superposition: Horizontality

11


Topic 2:

Page 597 Earth Textbook


Topic 2:

Page 597 Earth Textbook

Law of Cross-Cutting Relationships

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


Topic 2:

Page 597 Earth Textbook

Law of Cross-Cutting Relationships

14


Law of

Cross-Cutting

Relationships


Topic 2:

Law of Cross-Cutting Relationships

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

Igneous intrusion

Fault


Topic 2:

Law of Cross-Cutting Relationships

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

Igneous intrusion

Fault


Topic 2:

Law of Cross-Cutting Relationships

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

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

Igneous intrusion

Fault


Topic 2:

Law of Cross-Cutting Relationships

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

  • Based on the law of crosscutting relationships, the igneous intrusionis older than the fault?

Igneous intrusion

Fault


List the labeled formations from youngest to oldest.


List the labeled formations from youngest to oldest.

C B A E D


Topic 2:

Page 597 Earth Textbook

Law of Included Fragments

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”

included fragments are older than the rock


Law of Included Fragments


Topic 2:

Page 597 Earth Textbook

Unconformities

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


0071


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.

Topic 2:

Page 597 Earth Textbook

The Significance of Unconformities


Topic 3:

Page 598 Earth Textbook

Geologic Timetable

5 Eras of Earth’s History

1. Archean: 4.5-2.5 Billion Years Ago

2. Proterozoic: 2.5 BYA-570 MYA

3. Paleozoic: 570-250 MYA

4. Mesozioc: 250-65 MYA

5. Cenozoic: 65 MYA- Today

summary of major events in Earth’s History

*Eras broken into Periods, Periods into Epochs


Topic 3:

Page 598 Earth Textbook

Geologic Time and the Rock Record

Chapter 8

Chapter 8


The divisions of the geologic time scale.

Topic 3:


Figure 11.10


Topic 3:


Topic 3:

Scale of the Geologic Time


Topic 3:


Topic 4:

Page 602 Earth Textbook

*Any evidence of earlier life preserved in rock

Fossil Formation

  • original remains: unchanged organisms

  • replaced remains: become calcite, silica

  • molds, imprints, casts or hollows

  • trace fossils: trails burrows, footprints


Topic 4:

Studying The Past

What is a Fossil?

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

Why Do We Study Fossils Found in Rocks?

To provide evidence of the past existence of life forms

To provide information about past environmental conditions

To provide evidence that populations have undergone change over time due to environmental changes (evolution)


Fossil Formation

Topic 4:

Original Preservation

Description: plant or animal remains that have not undergone change since death.

  • Uncommon because frozen, extremely dry, or oxygen-free environments are required to form these fossils

  • Examples:

    • Mummified humans

    • Frozen organisms (Ice Man)

    • Mammoths & cats in La Brea Tar Pits

    • Fossilized insects in tree sap (amber)


Fossil Formation

Topic 4:

Replaced Hard Parts

Description: all organic material has been removed and the hard parts of the organism have been changed

  • Minerals seep in slowly and replace the original organic tissue, forming a rock-like fossil

  • The fossil has the same shape as the original object, but is chemically more like a rock!

  • Examples:

    • Petrified wood

    • Recrystallized shells


Fossil Formation

Topic 4:

Molds and Casts

Description: Fossils that do not contain any shell or bone

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

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

  • Examples:

    • Common with shellfish


Fossil Formation

Topic 4:

E.Trace Fossils

  • Description: Indirect evidence of plant and animal life

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

    • 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.

Topic 4:

Page 602 Earth Textbook


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.

Topic 4:

Page 602 Earth Textbook


Permineralization (Petrification)

most common

Minerals 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.


Carbonization

Plant 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:


Topic 5:

Page 603 Earth Textbook

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

Fossils: Evidence for Evolution

Natural Selection: survival of the fittest.


Topic 5:

Page 603 Earth Textbook

Fossils: Evidence for Evolution

Relative-Age Dating:

  • Definition: Dating rocks and fossils by placing them in chronological order without exact dates.

  • Geologic Principles (used in this dating process):

    • Original Horizontality

      • Sedimentary rocks are deposited in horizontal layers

    • The Law of Superposition

      • in an undisturbed sequence the oldest rocks are at the bottom and each successive layer is younger

    • Principle of Cross-Cutting Relationships:

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


Topic 6:

Page 604 Earth Textbook

Description: Remains of unique species that can be used to correlate rock layers or to date a particular rock layer

  • Must be easily recognized, abundant, and widely distributed geographically

  • Must have lived during a relatively short time period

  • Examples:

    • The mollusk Ecphora has a distinctive shape

Index Fossils & Key Beds


Topic 7:

Page 604 Earth Textbook

matching of rock layers from one area to another

Rock Correlation


Topic 7:

Page 604 Earth Textbook

Rock Correlation


Topic 8:

Page 605 Earth Textbook

*fossils indicate past climate

Other Uses of Fossils

*micro fossils used in oil exploration


Topic 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: Tree Rings


Topic 10:

Page 606 Earth Textbook

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

Absolute Time: Varves

Winter: dark fine sediment

Summer: Thick light colored layers


Topic 11:

Page 607 Earth Textbook

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

Absolute Time: Radioactivity


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.

Topic 11:

Page 607 Earth Textbook

Kinds of Radioactive Decay


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.

Topic 11:

Page 607 Earth Textbook

Radioactive Decay

  • The process is called radioactive decay.


Topic 12:

Page 607 Earth Textbook

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

Absolute Time: Half-Life


Page 607 Earth Textbook

Half-Life

After a 2nd

half-life has

passed half of

the remaining atoms

have also changed

After 1

half-life has

passed, half

the atoms

have changed

or emitted

energy.


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

Topic 12:

Page 607 Earth Textbook

Absolute Time: Half-Life


Topic 12:

Page 607 Earth Textbook

Absolute Time: Half-Life


Topic 13:

Page 608 Earth Textbook

  • 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.

Absolute Time: Radiocarbon Dating


Topic 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.

Other Radiometric Methods


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

The Earth was formed approximately 4.55 billion years ago.

Topic 14:

Page 608 Earth Textbook

Age of the Earth


The Rock Record

read pages 596-609

restate & answer Topic Questions 1-14

Chapter 32 Earth Science Book


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