Presentation on theme: "Ch. 23.6: Interpreting the Rock Record"— Presentation transcript:
1Ch. 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
2Earth’s AgeUp until the 1700s E’s age was estimated to be ~ 6,000 years oldToday: E’s age is estimated to be 4.6 billion years old.Determined by absolute dating or radiometric isotopes (we’ll get back to)
3Importance of Rock Record Paleoenvironment & ClimateWas 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 recordMajor Events: Meteroid impact; Mountain building (uplift); Rifting; Glaciation
4Relative Dating of Earth’s Layers Allows you to determine the SEQUENCE OF EVENTSOrder that rock layers formed (1st, 2nd, etc.)No specific date
5Relative Age 1. Law of Superposition A sedimentary rock layer is older than the layer above; younger than layer below* Undeformed layersSediments are deposited on top of existing layers and lithified.
6Relative Age 2. Principle of Horizontality Sedimentary rock layers started out HORIZONTAL.If layers are TILTED or CURVED, tectonics deformed them (Mt. Building or Faulting)
7Relative Age 3. Unconformities Breaks in geologic record = Missing TimeDeposition stopped or Rock layers were removed (usually after uplift and erosion)
8Relative Age Types of Unconformities Look for erosional surfaces; tilted layers; or igneous intrusionsLeft: 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.
9Relative Age 4. Crosscutting Relationships If a fault or igneous intrusion cuts across a layer … it happened after that layerWhich happened first: faulting or igneous intrusion?Write a summary of events for this region (oldest --> most recent).
10Relative Age: Index Fossils Fossils that narrow age of rock to a geologic period or era (millions of years)Requirements:Abundant - found in many regionsLived during “short” , specific span of timeDistinguishing features
11Relative Age: Index Fossils Example: Ammonite fossils in layer 4 formed in rocks mya
12Problem 1Sequence the order of rock layers (oldest --> youngest)2. All of the numbered layers are sedimentary except for ___ and _____.There is an unconformity present. Where is it? What does this mean?
13Problem 1What 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)?
14Problem 2Label youngest and oldest sedimentary layers (bottom drawing).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?
15Problem 3List sequence of events in relative order (oldest --> youngest)Events may include:Deposition of sedimentary layersIntrusion of igneous rockTectonics: Uplift; folding; faultingErosion
16Problem 4 Put sedimentary layers in order. Indicate when the intrusion happened.
17Absolute Age: Radiometric Dating Uses Radioactive IsotopesCompares relative % of parent:daughterGives specific age of rock
18Absolute Age: Radiometric Dating Nucleus = Particles w/MassProtons (+), determine element identityNeutrons (no charge), can varyIsotopes = 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
19Absolute Age: Radioactive Decay 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).
27Absolute Dating: Carbon Dating Used for dating organic matter found in younger rocks (< 70,000 years)Wood, bones, shells14 C made by cosmic radiation & incoporated into plants via photosynthesis (plants take in CO2 from air)Alive - Organisms have constant ratio of 12C: 14CDead - 14C decays and 14N increases
29Answers to Quick Lab p.196 1. Parent Isotope After 3 intervals: 12.5%After 6 intervals: 1. 5%After 9 intervals: %2. Daughter Isotopes created by decayseconds5. No new parent (paper) added or removed; cut at constant rate (half-life)