Discovering Earth’s History By 1. Geologic Time 2. Fossil 3. Stratum or Rock Units or Rock layers
How do geologists determine how old rocks are? 1.Relative age -- Is the age of an object or event compared to another object or event 2. Absolute age -- use radiometric dating techniques to determine how long ago the rock formed in the exact number of years *Not all rocks can be dated absolutely, so combinations of techniques are used. 1.Geologic Time
Relative age Is the age of an object or event compared to another object or event determine whether the rock is older or younger than other rocks using The Geologic time scale. However, it cannot tell you the rock’s age in years.
Absolute age Use naturally-occurring radioactive isotopes Rate of Radioactive Decay Radioisotopes decay at a constant rate. Rate of decay is measured by half-life Decay products Radioisotopes may decay to form a different isotope or a stable isotope. Common isotopes used in age dating Carbon 14 -- half-life of C-14 is 5730 yrs
The Geologic time scale Geologists have divided Earth’s 4.5-billion-year history into units that represent specific amounts of time. It also summarizes changes in Earth’s surface, climate, and organisms over time. Eons ( บรมยุค ) Eras ( มหายุค ) Period( ยุค ) Epoch( สมัย ) Ages ( ช่วงอายุ )
There are 3 Eras within 1.the Phanerozoic era: the Paleozoic, which means “ancient life,” 2.the Mesozoic, which means “middle life,” 3.the Cenozoic, which means “recent life.”
The Geologic Time Scale Origin of the Earth 4.55 Billion years First multi- celled organisms Age of Fishes Age of Coal Formation Major Mass Extinction Age of Dinosaurs Major Mass Extinction “Humans” arrive Most recent “Ice Age”
2. Fossils: Evidence of Past Life Fossils are the remains or traces of organisms that lived in the past. They are important components of sediment and sedimentary rocks. Some remains of organisms—such as teeth, bones, and shells—may not have been altered, or may have changed hardly at all over time. The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried. Unaltered Remains
Altered Remains The remains of an organism are likely to be changed over time. Fossils often become petrified or turned to stone. Molds and casts are another common type of fossil. Carbonization is particularly effective in preserving leaves and delicate animals. It occurs when an organism is buried under fine sediment.
2.2 index fossil Index fossils are fossils found only in rock layers of a certain geologic age. To be an index fossil must, 1.Be present in rocks found over a large region 2.Have features that make it clearly different from other fossils 3.Be from an organism that lived during a short span of geologic time 4.Be found with many other fossils of the same organism
index fossil 1.Trilobite 2. Graptolite 3. Fusulinid
3. Stratum or Rock Units Stratum (plural: strata) is a layer of sedimentary rock or rock units. Rocks record geological events and changing life forms of the past. We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today.
The law of superposition states that in an sequence of sedimentary rocks, A layer