Presentation on theme: "Ch. 23.6: Interpreting the Rock Record"— Presentation transcript:
1 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
2 Earth’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)
3 Importance 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
4 Relative Dating of Earth’s Layers Allows you to determine the SEQUENCE OF EVENTSOrder that rock layers formed (1st, 2nd, etc.)No specific date
5 Relative 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.
6 Relative Age 2. Principle of Horizontality Sedimentary rock layers started out HORIZONTAL.If layers are TILTED or CURVED, tectonics deformed them (Mt. Building or Faulting)
7 Relative Age 3. Unconformities Breaks in geologic record = Missing TimeDeposition stopped or Rock layers were removed (usually after uplift and erosion)
8 Relative 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.
9 Relative 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).
10 Relative 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
11 Relative Age: Index Fossils Example: Ammonite fossils in layer 4 formed in rocks mya
12 Problem 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?
13 Problem 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)?
14 Problem 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?
15 Problem 3List sequence of events in relative order (oldest --> youngest)Events may include:Deposition of sedimentary layersIntrusion of igneous rockTectonics: Uplift; folding; faultingErosion
16 Problem 4 Put sedimentary layers in order. Indicate when the intrusion happened.
17 Absolute Age: Radiometric Dating Uses Radioactive IsotopesCompares relative % of parent:daughterGives specific age of rock
18 Absolute 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
19 Absolute 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).
27 Absolute 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
29 Answers 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)