Time and Geology
The Present is the Key to the Past
– Catastrophism-October 23, 4004 BC @ 9:00AM• James Ussher, Bishop of Ireland
– Uniformitarianism- James Hutton (1795)• uniform rate of geologic processes
– Actualism• gradualism and catastrophism shape geologic processes
• Relative Time– Sequence of Geologic Events
• Absolute Age
Principles Used to Determine Relative Age
• Superposition- Nicolas Steno– Progressively younger from bottom => top
• Lateral Continuity• Original Horizontality- Steno• Contacts• Formations• Cross-cutting Relationships- Lyell
– Truncated units (dikes and faults)
Correlation• Physical Continuity of Lithologic Units• Similarity of Rock Types• Superposition• Correlation by Fossils
– Principle of Faunal Succession– Index Fossil– Fossil Assemblage
Correlation• Correlation by Fossils
– Principle of Faunal Succession• Wm. Smith & Georges Cuvier• Organism succeed one another in lithologic strata• One organism will NOT be found in rocks of widely different ages
– Index Fossil• Wide geographic distribution• Of limited life span as a species• Easily identifiable
– Fossil Assemblage
Standard Geologic Time Scale• Established in the 19th century- Sedgwick & Murchinson
• Based on Fossil Assemblages• Eons, Eras, Periods, and Epochs
– PHANEROZOIC since 544 m.y.b.p.• Paleozoic Era 544-245 m.y.b.p.• Mesozoic Era 245-65 m.y.b.p.• Cenozoic Era 65 m.y.b.p.-present
– Tertiary & Quaternary Periods» Recent (Holocene) Epoch last 10,000 years
• Precambrian- All time before Paleozoic
Unconformities
• Unconformity - surface that represents a gap in the geologic record– Disconformity - contact representing missing
parallel beds of sedimentary beds– Angular unconformity - younger strata overlie
an erosion surface on tilted or folded layers– Nonconformity - erosion surface on plutonic or
metamorphic rock
Unconformities
• Unconformity - surface that represents a gap in the geologic record– Disconformity - contact representing missing
parallel beds of sedimentary beds– Angular unconformity - younger strata overlie
an erosion surface on tilted or folded layers– Nonconformity - erosion surface on plutonic or
metamorphic rock
Unconformities
• Unconformity - surface that represents a gap in the geologic record– Disconformity - contact representing missing
parallel beds of sedimentary beds– Angular unconformity - younger strata overlie
an erosion surface on tilted or folded layers– Nonconformity - erosion surface on plutonic or
metamorphic rock
Unconformities
• Unconformity - surface that represents a gap in the geologic record– Disconformity - contact representing missing
parallel beds of sedimentary beds– Angular unconformity - younger strata overlie
an erosion surface on tilted or folded layers– Nonconformity - erosion surface on plutonic or
metamorphic rock
Absolute Age• Isotopic Dating• Isotopes and Radioactive Decay
– Isotopes - Differing number of neutrons, but same number of protons
– Radioactive Decay - Spontaneous nuclear disintegration of unstable isotopes
• Daughter Product• Half-life
– Various techniques (e.g. U-Pb, K-Ar, radiocarbon dating)
Absolute Age• Uses of Isotopic Dating
– Igneous Rocks• Reliability of Isotopic Dating
– Accuracy and precision of analytical instrumentation
Isotopic Decay• Unstable Nucleus
– spontaneously change into another type of atom
• Decay through Beta emission (Rb>Sr), Positron emission(N>C), Electron capture (K>Ar), Alpha emission (U>Th), Spontaneous fission
• Decay Rate is Constant, it depends on the particular isotope
Radioactive Decay Series
Age of the Earth• Early speculation• Dating of meteorites and moon rocks• Between 4.5 & 4.6 billion years old• Comprehending Geologic Time