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Chapter 12. Geologic Time. Studying Earth’s History. 12.1 Discovering Earth’s History. Three Main Ideas: The rock record provides evidence of geological events and life forms of the past. Processes observed on Earth in the present also acted in the past.
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Chapter12 Geologic Time
Studying Earth’s History 12.1 Discovering Earth’s History Three Main Ideas: • The rock record provides evidence of geological events and life forms of the past. • Processes observed on Earth in the present also acted in the past. • Earth is very old and has changed over time.
A Brief History of Geology 12.1 Discovering Earth’s History • Formulated by James Hutton, The principle of uniformitarianism simply states that physical, chemical and biological laws that operate today have also operated in the geologic past. • Uniformitarianism means that the forces and processes that we observe today have been at work for a very long time • To understand the past, we must first understand present-day processes and results.
Relative Dating—Key Principles 12.1 Discovering Earth’s History Relative dating tells us the sequence in which events occurred, not how long ago they occurred. Law of Superposition • The 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.
Relative Dating—Key Principles 12.1 Discovering Earth’s History Principle of Original Horizontality • The principle of original horizontality means that layers of sediment are generally deposited in a horizontal position.
Relative Dating—Key Principles 12.1 Discovering Earth’s History Principle of Cross-Cutting Relationships • The principle of cross-cutting relationships states that when a fault cuts through rock layers, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected. Inclusions • Inclusions are rocks contained within other rocks. • Rocks containing inclusions are younger than the inclusions they contain.
Relative Dating—Key Principles 12.1 Discovering Earth’s History Unconformities • • A surface that represents a break in the rock record. • An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed. • An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.
Relative Dating—Key Principles 12.1 Discovering Earth’s History Unconformities • A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks. • A disconformity is when two sedimentary rock layers are separated by an erosional surface.
Correlation of Rock Layers 12.1 Discovering Earth’s History • Correlation is establishing the equivalence of rocks of similar age in different areas. • Geologists correlate layers by noting position of a distinctive layer in a sequence of layers, and if they find the same layer in another location can infer that the same layer once covered both locations.
Fossil Formation 12.2 Fossils: Evidence of Past Life Fossils are the remains or traces of prehistoric life preserved from the geologic 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.
Fossil Types 12.2 Fossils: Evidence of Past Life Unaltered Fossils--Some remains of organisms—such as teeth, bones, and shells—may not have been altered, or may have changed hardly at all over time. .
Fossil Formation 12.2 Fossils: Evidence of Past Life Altered Remains The remains of an organism are likely to be changed over time. • • Petrified Fossils– Means “turned into stone” ; mineral rich water soaks into cavities and pores of the organism, minerals precipitate and fill the spaces. • Molds and casts are another common type of fossil. Occur when an organism is buried in sediment and dissolved by the underground water. Molds reflect the shape and surface markings, Casts are created if the mold later becomes filled with minerals.
Fossil Formation 12.2 Fossils: Evidence of Past Life Altered Remains continued: • Carbonization or Carbon films is particularly effective in preserving leaves and delicate animals. It occurs when an organism is buried under fine sediment and over time the liquids and gases are squeezed out leaving behind a thin film of carbon. • Often found in Black Shale. • .
Fossil Formation 12.2 Fossils: Evidence of Past Life • Preserved Remains: when all or part of the organism is saved with relatively little change • Examples include: Mammoths in permafrost; insects in amber, Mastodons in the Le Brea Tar Pits in LA, CA •
Fossil Formation 12.2 Fossils: Evidence of Past Life Indirect Evidence • Trace fossils are indirect evidence of prehistoric life. Includes tracks or footprints, worm burrows, coprolites (dung fossils), and gastroliths. Conditions Favoring Preservation • Two conditions are important for preservation: rapid burial and the possession of hard parts.
Fossils and the History of life 12.2 Fossils: Evidence of Past Life Two major scientific developments helped scientists explain fossil record: the principle of fossil succession and the theory of evolution. • Fossil Succession: The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content
Fossils and Correlation 12.2 Fossils: Evidence of Past Life • Theory of Evolution: Life forms change over time or evolved, by means of Natural Selection. Evidence for this adapation is found in the fossil record. Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.
Fossil Formation 12.2 Fossils: Evidence of Past Life Interpreting Environments • Fossils can also be used to interpret and describe ancient environments because animals evolve with adaptations suited to their environments. Examples: tooth type might indicate type of vegetation present, limestone on clam shells indicate shallow sea, coral fossils indicate a warm environment, etc.
Radioactivity 12.3 Dating with Radioactivity • Radioactivity is the spontaneous decay of certain unstable atomic nuclei. • Radioactive isotopes result from the decay of an unstable parent element and continues until a nonradioactive isotope is formed. • Example: Radioactive, unstable U-238 decays to form stable, nonradioactive Pb-206.
Half-Life 12.3 Dating with Radioactivity • A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope. • Example: if the half-life of an unstable isotope is 1 million years and 1/16th of the parent isotope remains, the amount indicates that 4 half-lives have passed and the sample must be 4 million years old.
Radiometric Dating 12.3 Dating with Radioactivity Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. • Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes. • The ratio between the radioactive parent isotope and the daughter products is measured in the sample to be dated. The older the sample, the more daughter product it contains.
Radiometric Dating 12.3 Dating with Radioactivity As a radioactive isotope decays, atoms of the daughter product are formed and accumulate. An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation.
Dating with Carbon-14 12.3 Dating with Radioactivity Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample. When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.
Importance of Radiometric Dating 12.3 Dating with Radioactivity Radiometric dating has supported the ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.
The Geologic Time Scale is a timeline that divides the Earth’s history into units representing specific intervals of time. Structure of the Time Scale Eons represent the longest intervals of geologic time. Eons are divided into eras, Eras into periods, and periods into epochs. In general, breaks in the units represent a major geologic event and/or change in life form. 12.4 The Geologic Time Scale
Structure of the Time Scale 12.4 The Geologic Time Scale • There are three eras within the Phanerozoic eon: • the Paleozoic, which means “ancient life” • the Mesozoic, which means “middle life” • the Cenozoic, which means “recent life”
Structure of the Time Scale 12.4 The Geologic Time Scale Each period within an era is characterized by somewhat less profound changes in life forms as compared with the changes that occur during an era. The periods of the Cenozoic era are divided into still smaller units called epochs, during which even less profound changes in life forms occur.
Precambrian Time 12.4 The Geologic Time Scale During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often.
Chapter13 Earth’s History
Precambrian History 13.1 Precambrian Time: Vast and Puzzling The Precambrian encompasses immense geological time, from Earth’s distant beginnings 4.56 billion years ago until the start of the Cambrian period, over 4 billion years later. .
13.1 Precambrian Time: Vast and Puzzling Earth forms as gravity pulled together dust, rock and Ice in space. Earth’s Atmosphere Evolves: Earth’s original atmosphere was made up of gases similar to those released in volcanic eruptions today—water vapor, carbon dioxide, nitrogen, and several trace gases, but no oxygen. Later, primary plants evolved that used photosynthesis and released oxygen. Oxygen began to accumulate in the atmosphere about 2.5 billion years ago. The Oceans Form as the planets cool, water vapor forms clouds, rains begin, evaporate & cool surface, and torrential rains fill the oceans.
Precambrian Rocks • Shields are large, relatively flat expanses of ancient metamorphic rock within the stable continental interior • Much of what we know about Precambrian rocks comes from ores mined from shields
13.1 Precambrian Time: Vast and Puzzling Precambrian Fossils • The most common Precambrian fossils are stromatolites. • Stromatolites are distinctively layered mounds or columns of calcium carbonate. They are not the remains of actual organisms but are the material deposited by algae. • Many of these ancient fossils are preserved in chert—a hard dense chemical sedimentary rock.
During the Precambrian time the earliest life • forms were: • Prokaryotic (bacteria called cyanobacteria) • Single celled Eukaryotes • (Red and Green Algae) • Multicellular Eukaryotes ( worms, jellyfish and • coral-like organisms) 13.1 Precambrian Time: Vast and Puzzling
Early Paleozoic 13.2 Paleozoic Era: Life Explodes Following the long Precambrian, the most recent 540 million years of Earth’s history are divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
