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Geologic Time and Origins of the Earth Thinking about time • • • • • Geologic time vs. “normal” time What is the date today? What is the day? What is the time? Why do we care? Geologic time • Thinking in terms of “millions of years” • Try it in seconds, just for fun: – 1,000,000 seconds • How many minutes is this? – 1,000,000 seconds / 60 seconds per minute – This = 16,667 minutes • How many hours is this? – 16,667 / 60 minutes per hour = 278 hours • How many days? – 278 hours / 24 hours per day = 11.6 days • So, 1 million seconds ~ 12 days How does 1 million compare to 1 billion? • Again, in seconds just for fun: – 1,000,000,000 seconds • How many minutes is this? – 1,000,000,000 seconds / 60 seconds per minute – This = 16,666,667 minutes • How many hours is this? – 16,666,667 / 60 minutes per hour = 277,778 hours • How many days? – 277,778 hours / 24 hours per day = 11,574 days Keep going… • 11,574 days--how many weeks is this? • 11,574 / 7 days per week = 1,653 weeks – Great, now how many years is this? • • • • • 1,653 weeks / 52 weeks per year This equals 32 years So, one billion seconds = 32 years And one million seconds = 11 days Now instead of seconds, lets think in YEARS Geologic time • Video clip: Carl Sagan and the Cosmic Calendar (5-7 minutes) – – – – From the “Cosmos” Public Television Series, 1981 Episode 1, “The Shores of the Cosmic Ocean” About 10 minutes before the end Available at the UTEP Library for viewing Geologic Time • • • • • • The big divisions are: Proterozoic time before 544 m.y.a. Paleozoic time from 544 - 245 m.y.a. Mesozoic time between 245 - 66 m.y.a. Cenozoic time between 66 - 2.5 m.y.a. Even MORE divisions Geologic time scale Earth Origins • AFTER “Big Bang” • Earth Origins are described by the Nebular Hypothesis • RECAP: Scientific Terms – Hypothesis = provisional explanation for observations; subject to continued testing & modification – Theory = An explanation for some natural phenomenon that has a large body of supporting evidence; testable & repeatable Nebular Hypothesis 1 • Solar system did not exist 5 billion years ago • A giant cloud of dust and gas occupied this area of space – Many times larger than the present solar system – A lot of space between individual atoms of gas, metal and ions • Why do we think this? – We observe large gas clouds in space, which contain basic building blocks of our solar system Nebular Hypothesis 2 • Something caused this cloud to condense • Maybe a nearby star exploded, sending its mass out into space • This mass collided with our dust cloud – Forcing atoms and dust grains to collide (accretion) • Each atom & grain has its own gravity • The bigger the grain, the bigger its gravity • The bigger the gravity, the bigger the chance for accretion • (“snowballing” effect like positive feedback) N.H. 2, continued • Nearby supernova caused cloud to condense • Accretion began • Why do we think this? – We have seen supernovae – We have seen the remainder of star mass ejected into space – We can mathematically model what happens when this mass collides with gas clouds Nebular Hypothesis 3 • • • • Cloud continues to condense Center of cloud attracts the most matter Cloud flattens into a disk and begins to rotate Incredible amount of heat generated in the center – So much heat that atoms are fused together, and the sun “turns on” • Why do we think this? – Explains the similar rotation of all planets – Explains how the planets are all orbiting in one plane (DISK) – Explains location of sun – Explains motion that we see in other gas clouds Nebular Hypothesis 4 • A new sun in the center of a disk of dust • More volatile elements move away from the sun – H, He, Methane, Ammonia • Accretion continues in the disk – Planets form by collision • Why do we think this? – Explains why inner planets are rocky & outer planets are gaseous – Explains old craters on planetary surfaces Nebular Hypothesis 5 • Accretion continued until most of the matter accreted into planets • Accretion occurred in stages, with the “Late Heavy Bombardment” happening ~3 b.y.a. • Why do we think this? – Explains relative emptiness between planets – Explains different apparent ages of craters on moon, Mars – Explains old craters on planetary surfaces Nebular Hypothesis • Video Clip: Carl Sagan’s Cosmos Series – Episode 8, “Travels in Space & Time” – Available for viewing at the UTEP library – About 15 minutes before the end of the episode Why is the N.H. a hypothesis and not a theory? • • • • Observations are indirect We were not there to see it What we can see is very slow So… – we do not try to produce a solar nebula model that exactly describes our Solar System and the planetary orbits and their masses – rather we look for a model that describes the characteristic properties of the planets at their observed locations in the Solar System Making the Earth 1 • Accretion acts over an extended area (the disk) and for a extended period of time • Solid grains condense out of the nebula’s gas – This is a chemistry process • Grains accrete into larger bodies (planetesimals) – This is a dynamic, collisional process • Planetesimals collide to produce protoplanets • Protoplanets accrete more material and become genuine planets Making the Earth 2 • Earth cools a bit: – Elements, minerals condense/crystallize • Iron, nickel, then rocky material • Layered Earth forms: – Stays molten due to heat of radioactive elements and L.H.B. • Iron and nickel sink to center, rocky material floats outward (density sort). • This is called differentiation Layered Earth