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Unit 2: History of Earth
Geologic Time, Evidence, Simultaneous Coevolu<on of Earth’s Systems and Biological life
² Earth Forma<on ² Geologic Time-‐ Crea<ng a <meline
² Evidence to support it ² Coevolu<on of Earth’s Systems and Biological life
Unit 2 Overview ² First, how old do you think Earth is? How do you think it was formed? ² Rewrite these two ques<ons on your bell ringer sheet and answer them.
How old is Earth?
² Precambrian-‐ Informal <me unit that spans nearly 90% of Earth’s History
² Includes these Eons:
² Hadean (hot, volcanic, “hell-‐like”) ² Archean (early signs of life) ² Proterozoic (earliest cells with nuclei)
Age of Earth and Evidence for it ² Evidence we know:
² Crustal Rock ² mineral Zircon by Abs. Age Da<ng
² Solar System ² Meteorites, Moon samples
Age of Earth and Evidence for it
² Formed very early in Earth’s history when a massive solar system body the size of Mars collided with Earth ² called the Giant Impact Hypothesis
How was the Moon formed? ² Extremely hot, (Hadean)
² 3 Sources of HEAT/ENERGY: ² Earth’s gravita<onal contrac<on ² Radioac<vity ² Bombardment by asteroids, meteorites, and other solar system bodies
Early Earth…What was it like? ² Accumula<on of small, rocky bodies in orbit around the Sun
² Earth started to collect these bodies and it grew in size and mass. ² MASS MATTERS! Increased mass, increased gravity ² Raised Earth’s internal temperature
Earth’s GravitaLonal ContracLon
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² Decay of radioac<ve isotopes
² Certain ones were more abundant in Earth’s past ² Uranium-‐238
² More radioac<ve isotopes, hoaer it is
RadioacLvity ² Stuff= asteroids, meteorites, objects from solar system ² Asteroids: Carbon or mineral-‐rich
² Now most are in Kuiper Belt ² Meteorites: Hit the Earth (Atmosphere helps)
² Impacts from these objects generated a ton OF THERMAL ENERGY ² Moon forma<on caused parts of Earth to melt
² Cooling has helped form atmosphere, oceans
Bombardment of STUFF ² How do you think density relates to “Early Earth” ² Write down your answer in complete sentences on your Bell Ringer Sheet
“Bell Ringer” 10/15
Early Earth h$p://www.youtube.com/watch?v=_mcC8kFacrk
² There are two ways to relate <me in geology:
² Rela<ve Age Da<ng
² Absolute Age Da<ng
Geologic Time
RelaLve Age DaLng
² Placing events in a sequence based on their posi<on in the rock record
² Younger rocks on top of old rocks ² Index Fossils are fossils of plants or animals that are specific to only one <me period
² By studying the type of rocks and type of fossils imprinted within those rocks, scien<sts can es<mate what environment was like during that <me period long ago.
² Do all animals and plants leave behind fossils? ² If not, how can this lead to misleading info?
RelaLve Age DaLng
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RelaLve Age DaLng ² In a sequence of undeformed sedimentary rocks, the oldest beds are on the boaom and the youngest are on top
Law of SuperposiLon ² Sedimentary layers are horizontal, or nearly so, when originally deposited.
² Layers that are not horizontal have been deformed by movements of the Earth’s crust.
Law of Original Horizontality
² Groups of fossil plants and animals occur in the geologic record in a definite and determinable order.
² A period of geologic <me can be recognized by its respec<ve fossils.
Principle of Faunal Succession ² Geologic features, such as faults, and igneous intrusions are younger than the rocks they cut.
Principle of CrosscuZng RelaLons ² A rock body that contains inclusions of preexis<ng rocks is younger that the rocks from which the inclusions came from.
Principle of Inclusion
² There are two ways to relate <me in geology:
² Rela<ve Age Da<ng
² Absolute Age Da<ng
Geologic Time
Absolute Age DaLng
Absolute Age DaLng
² Placing a specific <me or date to an event or rock
² Radioac<ve Decay
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RadioacLve Decay ² Over <me, radioac<ve elements break down into other isotopes or other elements.
² The <me it takes for these materials to break down to half its original amount is called HALF LIFE.
RadioacLve Decay
² Isotope ² Forms of chemical elements that differ in the number of neutrons in their atomic nuclei
² Half Life ² Time required for half of an element's atoms in a sample to change to the decay product.
² In each half-‐life only half of the remaining radioac<ve atoms decay, no maaer how large the sample is.
Abs. Age DaLng Vocab ² Decay of radioac<ve isotopes
² Certain ones were more abundant in Earth’s past ² Uranium-‐238
² More radioac<ve isotopes, hoaer it is
² hap://www.youtube.com/watch?v=cpnIxlDVmHw
RadioacLvity ² How does radioac<ve decay work?
² If an isotope is radioac<ve, it will break down naturally into a lighter element called a decay product.
² This process occurs at a predictable rate and can be used to determine how old an object is.
RadioacLve decay + Half Life
² Look at the diagram which represents the radioac<ve decay of uranium-‐238.
² The shaded area represents the decay product which is lead-‐206.
² The half-‐life of uranuim-‐238 is 4.5 billion years, since this object has gone through two half-‐lives it is 9 billion years old.
Uranium-‐ 238 RadioacLve Decay PracLce
² If there is 20 pounds of a radioac<ve material whose half life is 5 years, how many years old is the material if there is 5 pounds when scien<sts measure how much is leo?
RadioacLve Decay Data
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² Barium-‐122 has a half-‐life of 2 minutes. A fresh sample weighing 80 g was obtained.
² If it takes 10 minutes to set up an experiment using barium-‐122, how much barium-‐122 will be leo when the experiment begins?
Half Life PracLce ² A SKELETON IS FOUND TO CONTAIN 1/8TH OF ITS ORIGINAL RADIOACTIVE MATERIAL. ² How many half lives have gone by?
Example
1. If 100 grams of pure Carbon-‐14 starts to decay, how much Nitrogen-‐14 (N-‐14) will be created in 11,400 years?
2. A 200 gram rock sample is found to contain
25 grams of Potassium-‐40 (K-‐40.). Half Life of Potassium is 1.3 billion years. àWhat is the age of this rock?
Half Life PracLce
² Using both Rela<ve Age and newer Absolute Age Da<ng techniques, scien<sts have put together a <me scale of the history of the Earth.
Geologic Time Scale
Co-‐evoluLon of Biologic Species