This document summarizes key concepts about measuring geological time and dating rocks and fossils. It explains that Earth's history is divided into eons, eras, periods, and epochs on the geologic time scale (GTS). It describes the major eras - Precambrian, Paleozoic, Mesozoic, and Cenozoic - and the distribution of life over time. The document also discusses relative dating methods based on principles like superposition and cross-cutting relationships, and absolute dating using radioactive isotopes to determine the precise ages of rocks and fossils.

1. Fossils and the Rock Record by Annie Cloutier Sandwich High School 2011 copyright Chapter 21 Earth & Space Science GTS

2. How can we measure time ? Imagine…. no clocks no books no TV no radio no Internet no cell phones

3. Earth’s History is Measured in GTS Geologic Time Scale Eon – longest time unit measured in billions of years * The longest Era known is the ARCHEAN EON 4,600 billion years before present Era – next longest span of time measured in hundreds of millions to billions of years Period – are defined by the life forms that were abundant or became extinct during the time in which specific rocks were deposited –tens of millions tohundreds of million years Epoch – Shortest unit of time from millions to tens of millions of years

4. MYBP MYBP = millions of years before present BYBP = billions of years before present Today we live in the . . . Recent Epoch In the Quaternary Period Cenozoic Era - 1.6 MYBP to present day Within the Phanerozoic Eon

5. LIFE

6. Percent of Time 90% of Earth’s GTS was in the PrecambrianTime 6 % Paleozoic 4% Mesozoic 1% Cenozoic

7. paleo means “ancient” zoic means “life” Paleozoic means “ancient life” Meso means “ middle” Ceno means “recent”

8. Relative Dating is not going to a movie with Grandma…

9. Relative dating of rocks is what James Hutton’s hypothesized in 1770 …theory of Uniformitarian' s - states that the processes occurring today have always been occurring since Earth was formed. this is not an exact science of dating rocks, more like an estimate of time with no exact dates “sort of a ball park figure”

10. Rocks are never left alone. Superpositioning states that in an undisturbed rock sequence the oldest rocks are on the bottom. Before we do the laundry, the oldest dirty clothes are on the bottom of the basket.

11. Original horizontality is the natural layering of sedimentary rocks deposited in a horizontal or nearly horizontal layer. Think of the Grand Canyon the oldest rocks are at the bottom layer of the canyon. Vishnu Group rocks at the very bottom are from the Precambrian

12. Exceptions to the rule Sometimes lava or magma in volcanic dikes or sills, intrude in a rock formations, in cross-cutting this can be the case so an intrusion is younger than the original rock.

13. Unconformity is when an erosion surface might become buried by the deposition of younger rocks. Disconformity is when horizontal sedimentary rocks overlay other horizontal sedimentary rocks James Huttons illustration of rock layers

14. Absolute Age of Rocks STOP let me see you license I don’t believe how old you are Scientist can actually date the true age of many rocks and fossils. This is called Absolute –Age. By measuring the decay rates of radioactive isotopes. Isotopes are found in igneous and metamorphic rocks, some fossils and , organic remains (once living) Radioactive substances emit nuclear particles at a constant rate, the protons and neutrons change with each nuclear emission As the number of protons and neutrons change with each nuclear emission, the element changes by converting into a different element. The original element is the “parent” The new element is the “daughter”

15. Uranium 238 converts into Lead 206

16. CSI crime scene investigation uses Carbon Carbon has a half life of 5730 years. Decay product Nitrogen -14 Uranium 238 has a half life of 4,5 billion years Decay product Lead 206 “ if it wasn’t for Carbon 14, I would not date at all.”

17. Time is relative to the life forms existing and to the significant objects in the Universe. People in their 40’s are at the half way point in life The Sun is at the estimated half way point in it’s life…3.5 billion years old

18. The end of measuring the passing of time time machine time machine