Geologic Time and Origins of the Earth

1. Geologic Time and Origins of the Earth

2. 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?

3. 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

4. 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

5. 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

6. 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

7. 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

8. Geologic time scale

9. 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

10. 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

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. Layered Earth