1LESSON 10 – Earth’s History: Estimating Geologic Time Modified fromHow do you reconstruct geologic history with rocks?History
2Determining geological ages Geologists use rocks to tell time in two waysIn the field, they can look at a landscape and decipher the order of events that produced itTo actually know “when” an event occurred, or to know when a rock formed, requires laboratory analysis
3This field sketch shows observations of a landscape This field sketch shows observations of a landscape. How do we ascertain the order in which the rocks were placed there? By first determining the rocks’ relative ages.
4How do you determine the order of events? Relative age – the ordering of objects or features from oldest to youngest. Things that happened first, then next, and lastAbsolute age – establishing the date of an event (in years before the present time).
5How are geologic events placed in relative order? By the application of a set of geological “rules”First is the Principle of Superposition – This states that when rocks were placed, newer ones were placed or formed on top of the older onesNew RocksSo here at the Grand Canyon – the old rocks are on the bottom of the pile, and the new ones are on top.Old Rocks
6How are geologic events placed in relative order? Principle of Original Horizontality – sedimentary layers are (more or less) horizontal when they form.(These Flat layers that are no longer horizontal)
7Principle of Original Horizontality – sedimentary layers are (more or less) horizontal when they form.Older rock layersNewer rock layers(These Flat layers that are no longer horizontal)
9How are geologic events placed in relative order? Principle of Cross-Cutting Relationships – geologic features that cut across rock must be younger than the rock they cut through.Older RockNewer Rock
10How are geologic events placed in relative order? Principle of Inclusions – This states that objects enclosed in rock must be older than the time of rock formation
11Review: Principles of Relative Dating Law of SuperpositionIn an undeformed sequence of surface-deposited rocks, the oldest rocks are on the bottom.Includes sedimentary rocks, lava flows, ash deposits and pyroclastic strata.Does not include intrusive rocks, which intrude from below.
12Review: Principles of Relative Dating Principle of original horizontalityLayers of sediment are generally deposited in a horizontal positionRock layers that are flat have not been disturbedPrinciple of cross-cutting relationshipsYounger features cut across older features (faults, intrusions etc)Principle of InclusionObjects enclosed in rock must be older than the time of rock formation
13Fossil RecordEvidence of organisms preserved in the sedimentary rock record.Typically hard shelled organisms or bones of other organisms are the only evidence preserved as fossils.Organisms must be buried quickly after they die to be preserved.
14Principle of Fossil Succession: Fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil contentDrawn from theory of evolutionFossil organisms become more similar to modern organisms with geologic timeExtinct fossils organisms never reappear in the fossil record
17How have scientists determined the age of Earth? Radiometric dating (radioactive decay) provided a measure for the absolute ages of rocks, and the 4.5-billion-year age of Earth.(Image from
18How have scientists determined the age of Earth? RadioactivityIsotopes – atoms of the same element with the same number of protons, but a different number of neutronsRadioactive decay – a change in the number of protons, neutrons, or both that transforms an unstable isotope towards a stable oneFig 7.16
19How have scientists determined the age of Earth? Radioactive decay of specific elements can be used to determine the Absolute Age (the actual age of a rock or event)Some elements decay (change) into other elements over a specific amount of time (half-life).
20How is the absolute age of a rock determined? Use Half-Life to measure the isotopic abundanceAn unstable “parent” isotope decays towards a stable “daughter” isotope.Assuming each daughter comes from a parent isotope, and that we know the average time it takes to convert, we can use the ratio of the two to calculate the age of the rock they are in.Half-life – the amount of time it takes for ½ of the parent to turn into the daughter isotope.Fig 7.17
211st Half LifeFigure 18.14After one half-life, 50% of the parent isotope will have become daughter isotope, regardless of the sample size.2nd Half LifeAfter two half-lives 50% of the remaining parent isotope will have become daughter isotope. This means 75% of the original parent isotope will have changed.
22Half-Life ExampleA sample with an original amount of 40g of carbon-14 is found and now contains 2.5g of carbon-14, how many years old is this sample?40g 20g 10g 5g 2.5gThis sample has undergone 4 half-lives, and the half-life of carbon-14 is 5700 years:5700 yrs. x 4 = 22,800 years!