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