Geologic Time: How do we determine age (relative & absolute) What is the age of the earth? How do we know? Hutton s Outcrop at Siccar Point Hutton looked at this outcrop and reasoned that the Earth was very old. He formulated the Principle of Uniformitarianism and opened the door for Darwin s Origin of Species. 1
Uniformitarianism: The Earth processes that act to shape the Earth today are the same processes that acted to shape the Earth in the past. The present is the key to the past. Periodic Chart of the Elements U and Th - in the Ac contraction series Elements defined by number of protons in nucleus Some elements have unstable nuclei and spontaneously decay : Protons turn into Neutrons and vise versa. 2
How does isotopic age dating work? Unstable isotope nuclei decay Rate of decay is a constant function of the number of Parent isotopes present This is what allows us to use it as a clock! Emit particle and Heat! This is a major source of heat in the crust and mantle Now for some fun math 3
Half-life & Parent-Daughter ratio As time proceeds, number of Daughters grow, parents diminish Thus the ratio of parents to daughters (P/D) is a function of time! Need lots of Parents and Daughters to measure P/D Not all rocks can be dated: need to have enough P and D. Which is to say they need to have the right composition and the right age. Why can t all rocks be dated? b/c not all rocks are attractive Not all rocks contain appropriate isotopes (composition) Age must be within magnitude of half-life or there won t be enough parent or daughter isotope present to measure. Many rocks are Open Systems - they loose parent or daughter during their long history. Dating Sedimentary rocks = age of sediment NOT age of the rock. 4
So, what is the age of Earth? We cannot date rocks from Earth to obtain an age of the Earth because tectonics resurfaces the Earth over time! Slope of isochron is a function of time Iron meteorites ~ initial composition of Earth (terrestrial planets) Stony meteorites ~ Earth (terrestrial planet) closed to isotope loss Marine Lead = homogenous Earth Pb! Shows that Earth falls on meteorite isochron and validates the argument! What we learn from isotopes: Age of Earth When Earth differentiated into layered structure 4.3 Ga detrital Zircon = oceans & crust 4.03 Ga Acasta gneiss = plate tectonics Age and timing (rates) of mountain building events 5
Absolute age date = number age Relative age = sequence of events Chicxulub impact site: 6 km diameter meteorite impact. Produced Ir-rich clay layer world wide at K-T boundary (relative dating). Ar-Ar dating of igneous rocks from impact melting give age = 65 Ma All rocks cannot be dated. So, how do we know when something happened? Combine isotopic age dating with relative age to determine age of events! Relative age dating - How do we place rocks in sequence? Let s review the principles 6
Principle of Original Horizontality Steno: 1600s Sediment (& lava) deposited horizontally in sheets. Thus, inclined/folded beds are disturbed. Principle of Superposition: Steno - 1600s If sediment is deposited in layers, then Oldest layers are on bottom provided they have not been disrupted (e.g. faulted) 7
Principle of Continuity The layers of sediments (and lavas) are continuous over great distances. Thus, can infer that they continue across eroded distances. Principle Cross-cutting relationships Faults & intrusions cut older rocks (i.e., faults and igneous intrusions are younger than the rocks they cut) 8
Principle of Inclusion If portions of pre-existing rocks are entrained in a rock (inclusions), Then the inclusions are older than the rock containing them. Principle of Fossil Succession Fossils are remnants of previously living organisms preserved in rock or sediment Smith - late 1700s noticed: Fossils occur in groups of species (assemblage) Fossil organisms succeed one another in definite order vertically through a sediment column. He correctly reasoned that one could group/correlate rock units over distance based on their common fossil assemblage i.e., Fossil assemblages are distinct in time Loss of a fossil organism from an assemblage = extinction (Curvier, early 1800s) 9
Fossil succession & relative time Given that fossil assemblages change over time due to evolution and extinctions, And, fossils assemblages correlate over distances Then, distinct fossil assemblages can be used to assign relative age of the rocks in which they are preserved. Unconformities = Gap in record or loss of recorded time animation! Disconformity Nonconformity 10
Angular Unconformity: What is implied or what can you infer from unconformities? Loss of recorded time! (uplift and erosion). Given Principles and Unconformities we can apply relative age dating to rocks in a given area. Can you list these numbered units according to relative age? (relative age animation) 11
We then use principles to correlate rocks over regional distances Geologic Time Scale: Current Ice Age Major Extinction Major Extinction Cambrian Explosion of macroscopic life How do we know when something happened? Combine isotopic age dating with correlations (relative time scale) to determine age of events! 12
More interesting events 90% of Earth History in Precambrian Dinosaurs evolve from reptiles 3.8 Ga 4.3 Ga 13