3.4 Earth’s Geologic History

Uniformitarianism Uniformitarianism means the forces and processes we observe today have been at work for a very long time. “The present is the key to the past”

Relative Dating – Key Principles Relative dating tells us the sequence in which events occurred, not how long ago they occurred. Law of superposition States that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it

Law of Superposition

Relative Dating – Key Principles Principle of Original Horizontality Layers of sediment are initially deposited in a horizontal position Principle of cross-cutting relationships when a fault cuts through rock layers or magma intrudes other rocks, we can assume the fault or intrusion is younger than the rocks it affects

Relative Dating – Key Principles Inclusions Inclusions are rocks contained within other rocks Rocks containing inclusions are younger than the inclusions they have

Relative Dating – Key Principles Unconformities A long period which deposition stopped, erosion removed previously formed rocks, and then deposition resumed 3 types of unconformities: Angular unconformity– during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred Nonconformity – when sedimentary rock lies above and was deposited on pre- existing and eroded metamorphic or igneous rock. Disconformity - when two sedimentary rock layers are separated by an erosional surface.

Dating with Radioactivity Radioactivity is the spontaneous decay of certain unstable atoms Half-life is the amount of time necessary for one-half of the atoms in a sample to decay to a stable isotope (version) Isotopes are different versions of the same element, just have a different amount of neutrons

Dating with Radioactivity Radioactive isotopes decay at a constant rate since the formation of the rock they occur in Radiometric dating is the procedure of calculating absolute ages of rocks/minerals containing radioactive isotopes As radioactive (parent) isotope decays, atoms of the stable (daughter) product form

Radioactive Isotopes Frequently Used in Radiometric Dating

Dating with Radioactivity Example - Dating with Carbon-14 Radiocarbon dating is the method for finding age by comparing amount of carbon-14 (parent) to amount of carbon-12 (daughter) in a sample When an organism dies, the amount of C-14 decreases as it decays. By looking at ratio of C-14 to C-12 in a sample, age can be determined Importance of Radiometric dating It has supported ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.

Fossil Formation Fossils are the remains or traces of prehistoric life. They are important parts of sediment/sedimentary rocks The type of fossil that is formed is determined by the conditions which an organism died and how it was buried.

Fossil Formation Unaltered Remains Some remains of organisms—such as teeth, bones, and shells— may not have been altered (hardly changed at all over time)

Fossil Formation Altered Remains The remains of an organism are likely to be changed over time Fossils often become petrified (organic parts replaced with minerals) or turned to stone Molds – organism dies, covered by sediments, its flesh and bone decays, and leaves a cavity of what it looks like Casts – created if the hollow spaces of a mold are filled with mineral matter Carbon films – organic material is compressed, leaving only a carbon residue

Fossil Formation Indirect Evidence Conditions Favoring Preservation Trace fossils are indirect evidence of prehistoric life (footprints, burrows, coprolite) Conditions Favoring Preservation Two conditions are important for preservation: Rapid burial, and possession of hard parts.

Fossils and Correlation Principle of fossil succession: Fossil organisms succeed (come after) one another in a definite order. Therefore any time period can be figured out by its fossil content Index fossils are widespread on Earth, are limited to a short span of time, and occur in large numbers Tool for determining age of rock layers

Fossils and Correlation Interpreting Environments Fossils can also be used to interpret and describe ancient environments For example, suppose geologists found fossil clam shells in limestone. They can infer that the region was once covered by a shallow sea.

Structure of the Time Scale Geologic time scale is a timeline that divides Earth’s history into units representing specific intervals of time Eons represent the greatest expanses of time. Eons are divided into eras. Each era is subdivided into periods Periods are divided into smaller units called epochs

Structure of the Time Scale There are three eras in the Phanerozoic eon: Paleozoic – means “ancient life” Mesozoic – means “middle life” Cenozoic – means “recent life Each period within an era is defined by less significant changes in life forms compared to changes that occur in an era The periods of the Cenozoic era are divided into smaller units called epochs, which have even less significant changes in life forms

Precambrian Time – Vast and Puzzling Time frame: 4.56 BYA to 540 MYA Rocks were large masses of metamorphic rock (Shields) Atmosphere was water vapor, CO2, NO OXYGEN Oxygen began to accumulate 2.5 BYA due to evolution of photosynthetic plants Most common Precambrian fossils are stromatolites - layered mounds of CaCO3 made from algae

Paleozoic Era – Life Explodes Time frame: 540 MYA to about 250 MYA By end of Paleozoic, all continents had fused into supercontinent Pangaea Life in early Paleozoic was restricted to seas Late Paleozoic Some 400 MYA, plants had adapted to live at water’s edge, began to move inland, becoming land plants Amphibians rapidly diversified because they had minimal competition from other land dwellers

Mesozoic Era: Age of Reptiles Time frame: About 250 MYA to about 65 MYA Major tectonic event broke up Pangaea Seed-producing plants (gymnosperms) quickly became dominant With perfection of shelled egg, reptiles quickly became dominant land animals At end of Mesozoic, many reptile groups became extinct Bye- bye dinosaurs!

Cenozoic Era – Age of Mammals Time frame: about 65 MYA to present Plate tectonics caused many events of mountain building, volcanism, and earthquakes Mammals replaced reptiles as dominant land animals Angiosperms – flowering plants with covered seeds – replaced gymnosperms as dominant land plants With extinction of dinosaurs, mammals diversified forming many new groups including bats and whales