Ch. 23.6: Interpreting the Rock Record

OBJECTIVE:

Use principles of relative and absolute dating to determine a sequence of events (climate, tectonic, & environmental) in Earth’s history.

Key terms: Law of Superposition; Principle of Horizontality; Unconformities; Crosscutting Relationships; Index fossils; Radiometric dating; isotopes; half-life

Earth’s Age Up until the 1700s E’s age was estimated to be ~ 6,000 years old

Today: E’s age is estimated to be 4.6 billion years old.

Determined by absolute dating or radiometric isotopes (we’ll get back to)

• Paleoenvironment & Climate Was this place a swamp? Coral reef? Desert? Tropical forest? Covered in ice?

• Rates of Climate ChangeHas Earth rapidly warmed or cooled before? What’s Earth’s normal?

• Document EvolutionFossil record

• Major Events: Meteroid impact; Mountain building (uplift); Rifting; Glaciation

Importance of Rock Record

Relative Dating of Earth’s Layers

• Allows you to determine the SEQUENCE OF EVENTS

• Order that rock layers formed (1st, 2nd, etc.)

• No specific date

Relative Age1. Law of Superposition

A sedimentary rock layer is older than the layer above; younger than layer below

* Undeformed layers

Sediments are deposited on top of existing layers and lithified.

Relative Age2. Principle of Horizontality

Sedimentary rock layers started out HORIZONTAL.

If layers are TILTED or CURVED, tectonics deformed them (Mt. Building or Faulting)

Relative Age3. Unconformities

Breaks in geologic record = Missing Time

Deposition stopped or Rock layers were removed (usually after uplift and erosion)

Relative AgeTypes of UnconformitiesLook for erosional surfaces; tilted layers; or igneous

intrusions

Left: Nonconformity = Igneous or metamorphic rock is uplifted, exposed, and eroded. Sed layers deposited on top.

Middle: Angular Unconformity = layers are folded or tilted, then eroded. New layers sed layers deposited on top.

Right: Disconformity = Horizontal layers are uplifted and eroded. New sed. Layers deposited on top.

Relative Age4. Crosscutting Relationships

If a fault or igneous intrusion cuts across a layer … it happened after that layer

•Which happened first: faulting or igneous intrusion?

•Write a summary of events for this region (oldest --> most recent).

Relative Age: Index Fossils

Fossils that narrow age of rock to a geologic period or era (millions of years)

Requirements:

1. Abundant - found in many regions

2. Lived during “short” , specific span of time

3. Distinguishing features

Relative Age: Index Fossils

Example: Ammonite fossils in layer 4 formed in rocks 108 - 206 mya

Problem 1

1. Sequence the order of rock layers (oldest --> youngest)

2. All of the numbered layers are sedimentary except for ___ and _____.

3. There is an unconformity present. Where is it? What does this mean?

Problem 1

4. What evidence is there that a tectonic event affected this area in the past? Describe and interpret this evidence.

5. What happened first: Faulting (B) or Intrusion (3)?

Problem 21. Label youngest and oldest

sedimentary layers (bottom drawing).

2. Describe the tectonic setting that would produce the folded layers.

3. Why are the tops of the folded layers cut off? How did this happen?

Problem 3 1. List sequence of events in relative order (oldest --> youngest)

Events may include:

•Deposition of sedimentary layers

•Intrusion of igneous rock

•Tectonics: Uplift; folding; faulting

•Erosion

Problem 4

1. Put sedimentary layers in order.

2. Indicate when the intrusion happened.

Absolute Age: Radiometric Dating

• Uses Radioactive Isotopes

• Compares relative % of parent:daughter

• Gives specific age of rock

Isotopes = Atoms of the same element with different # of neutrons. Ex: 12 C (6 protons + 6 neutrons), 14 C (6 protons + 8 neutrons)

Radioactive Isotopes = Atoms that have nuclei that break apart (unstable) naturally.Release energy & particles

Absolute Age: Radiometric Dating

Nucleus = Particles w/Mass

Protons (+), determine element identity

Neutrons (no charge), can vary

Unstable PARENT Isotope breaks down to stable DAUGHTER Isotope (& releases energy)

Decay happens at a constant rate (not changed by Temp., Pressure, or environmental conditions).

Absolute Age: Radioactive Decay

Absolute Dating: Radiometric Decay

The time it takes for 1/2 the mass of PARENT --> DAUGHTER.

Half life of 14C = 5,730 years 100 g 14 C -----> 50g 14 C +

50g 14N after 5,730 years

Absolute Age: Half Life

Half- Life of U 238 = 4.5 billion years

Absolute Dating: Half Life

Complete the chart belowTime Parent

isotope (g)Daughter isotope (g)

Remaining Parent

Time (Years)

Rock cyrstallizes

(forms)

100 0 100% 0

1 half-life 50 50 50% (1/2) 700

2 half - lives 25 75 25% (1/4) 1400

3 half-lives 12. 5 87. 5 12. 5% (1/8) 2100

4 half-lives 6.25 93.75 6.25% (1/16)

2800

Complete the chart belowTime Parent

isotope (g)Daughter isotope (g)

Remaining Parent

Time (Years)

Rock cyrstallizes

(forms)

100 0 0

1 half-life 50% (1/2) 700

2 half - lives 25 25% (1/4)

3 half-lives 87. 5 2100

4 half-lives 6.25 6.25% (1/16)

Absolute Dating: Carbon Dating

Used for dating organic matter found in younger rocks (< 70,000 years)

Wood, bones, shells

14 C made by cosmic radiation & incoporated into plants via photosynthesis (plants take in CO2 from air)

Alive - Organisms have constant ratio of 12C: 14C

Dead - 14C decays and 14N increases

Answers to Quick Lab p.196

1. Parent Isotope 4. After 3 intervals: 12.5%

After 6 intervals: 1. 5%

After 9 intervals: 0.195%2. Daughter Isotopes created by decay

3. 20 seconds 5. No new parent (paper) added or removed; cut at constant rate (half-life)