2Discovering Earth’s History By 1. Geologic Time 2. Fossil 3 Discovering Earth’s History By 1. Geologic Time 2. Fossil 3. Stratum or Rock Units or Rock layers
3How do geologists determine how old rocks are? 1.Geologic TimeHow do geologists determine how old rocks are?Relative age -- Is the age of an object or event compared to another object or event2. 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.
4Relative ageIs the age of an object or event compared to another object or eventdetermine 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.
5Absolute age Use naturally-occurring radioactive isotopes Rate of Radioactive DecayRadioisotopes decay at a constant rate.Rate of decay is measured by half-lifeDecay productsRadioisotopes may decay to form a different isotope or a stable isotope.Common isotopes used in age datingCarbon half-life of C-14 is 5730 yrs
7The 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)
11There are 3 Eras withinthe Phanerozoic era: the Paleozoic, which means “ancient life,”the Mesozoic, which means “middle life,”the Cenozoic, which means “recent life.”
12The Geologic Time Scale Most recent “Ice Age”“Humans” arriveMajor Mass ExtinctionAge of DinosaursMajor Mass ExtinctionAge of Coal FormationAge of FishesFirst multi-celled organismsThe Geologic Time ScaleOrigin of the Earth 4.55 Billion years
132. 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.The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried.Unaltered Remains• Some remains of organisms—such as teeth, bones, and shells—may not have been altered, or may have changed hardly at all over time.
16 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.
242.2 index fossilIndex 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
323. Stratum or Rock UnitsStratum (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.
33The law of superposition states that in an sequence of sedimentary rocks, A layer