Chapter 12 Geologic Time Sedimentary Rocks Rocks formed
- Slides: 117
Chapter 12 Geologic Time Sedimentary Rocks: Rocks formed from the weathered products of preexisting rocks that have been transported, deposited, compacted, and cemented.
Chapter 12 Geologic Time Igneous Rocks: Rocks formed by the crystallization of molten magma
12. 1 Discovering Earth’s History Studying Earth’s History Major John Wesley Powell- Civil war vet and geologists- led scientific expedition down the Colorado River, through the Grand Canyon. To him, the walls of the Canyon provided evidence of Earth’s long history. His work led scientists to believe geologic time grew from thousands to many millions of years.
12. 1 Discovering Earth’s History Studying Earth’s History In studying Earth’s history, geologists make use of three main ideas: 1. the rock record provides evidence of geological events and life forms of the past 2. Processes overserved on Earth in the present also acted in the past 3. Earth is very old and has changed over geologic time.
12. 1 Discovering Earth’s History Studying Earth’s History Late 1700’s- Scottish physician- James Hutton published his Theory of the Earth. Hutton put forth the fundamental principal of Uniformitarianism: means that the physical, chemical, and biological processes that we observe today have been at work for a very long time.
12. 1 Discovering Earth’s History Relative Dating—Key Principles 1800’s- William Hutton- worked to interpret Earth’s rock record, known as relative dating Method used to place rocks in chronological order. Identifies which rock units formed first, second, etc.
12. 1 Discovering Earth’s History Relative Dating—Key Principles In relative dating, geologists follow several principals: Law of Superposition Principle of Original Horizontality Principle of Cross-Cutting Relationship.
12. 1 Discovering Earth’s History Relative Dating—Key Principles 1636 - 1686 : Nicolaus Steno- Danish geologists Steno developed the Law of Superposition: States that in an under-formed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it.
Ordering the Grand Canyon’s History Law of Superposition
12. 1 Discovering Earth’s History Relative Dating—Key Principles Steno also developed the Principle of Original Horizontality: States that layers of sediment are generally deposited in a horizontal position. If rock layers appear flat, it means they’ve remained in their original horizontal position.
Disturbed Rock Layers Principle of Original Horizontality
12. 1 Discovering Earth’s History Relative Dating—Key Principles 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. Fault: a fracture in Earth along which movement has occurred Dike: a tabular-shaped intrusive igneous feature that cut across preexisting rock layers.
Applying Cross-Cutting Relationships Fault: a fracture in Earth along which movement has occurred Dike: a tabular-shaped intrusive igneous feature that cut across preexisting rock layers.
12. 1 Discovering Earth’s History Reading the Rock Record Applying Steno’s principles, geologists read • geologic features to reconstruct Earth’s history. Geologists use three methods to interpret rock records: Inclusions Unconformities Correlate rock layers
12. 1 Discovering Earth’s History Reading the Rock Record Inclusions: are pieces of one rock unit that are contained within another. Inclusions made of older igneous rock can be found within younger sedimentary rock layers on top of the weathered igneous rock.
Formation of Inclusions
12. 1 Discovering Earth’s History Relative Dating—Key Principles Unconformities • represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.
A Record of Uplift, Erosion, and Deposition
Formation of an Angular Conformity 3 types of unconformities: 1. angular- layers of sedimentary rock form over older sedimentary rock that are tilted or folded. 2. disconformities – two sedimentary rock layers are separated by an erosional surface. 3. nonconformities – an erosional surface separates older metamorphic or igneous rocks from younger sedimentary rocks.
12. 1 Discovering Earth’s History Correlation of Rock Layers Correlation is establishing the equivalence of rocks of similar age in different areas.
Correlation of Strata at Two Locations
12. 2 Fossils: Evidence of Past Life Fossil Formation An extinct organism is one that no longer exists on Earth. Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks. The type of fossil that is formed is determined by the conditions under which organism died and how it was buried.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Petrified Fossils: Organisms remains become petrified. Petrified means “turned into stone. ” Process: 1. Mineral-rich water soaks into small cavities and pores of the original organism. 2. Minerals precipitate from the water and fill the spaces. 3. Minerals sometimes replace the cell walls or solid material of the organism.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Petrified
12. 2 Fossils: Evidence of Past Life Types of Fossils: Molds and Casts Molds Process: 1. Mold is created when a shell or other structure is buried in sediment and then dissolved by underground water. Mold reflects only the shape and surface marking of the organism. Cast fossils are created if the hollow spaces of a mold are later filled with mineral matter.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Molds and Casts
12. 2 Fossils: Evidence of Past Life Types of Fossils: Carbon Films Carbon films preserve delicate details of leaves and animal parts. Process: 1. An organism is buried under fine sediment 2. Over time, pressure squeezes out liquids and gases to leave behind a thin film of carbon. Sometimes impressions of the organism remain.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Carbon Films
12. 2 Fossils: Evidence of Past Life Types of Fossils: Preserved Remains Fossilization preserves ALL or PART of an organism with little change to its remains. Preservation can occur in amber- the hardened resin, or sap, of an ancient tree. Preservation can also occur in tar- a thick petroleum that collects in pools at the surface.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Preserved Remains
12. 2 Fossils: Evidence of Past Life Types of Fossils: Trace Fossils Indirect evidence of prehistoric life Some of the oldest known trace fossils are believed to be worm burrows. Coprolites are fossils of dung (poop) and stomach contents (throw-up). Gastroliths are highly polished stomach stones that were used in grinding of food by extinct reptiles.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Trace Fossils
Types of Fossilization
12. 2 Fossils: Evidence of Past Life Conditions for Fossilization Two conditions that favor preservation of an organism as a fossil are rapid burial and the possession of hard parts. Dead organisms with shells, bones, and teeth (hard parts) have a better chance at being preserved. Dead organisms without hard parts, unless covered by sediment immediately, were likely to be eaten or decomposed by bacteria.
12. 2 Fossils: Evidence of Past Life Fossils and the History of Life 1700 -1800’s: Two major developments helped scientists explain fossil records: 1. The Principle of Fossil Succession 2. Theory of Evolution
12. 2 Fossils: Evidence of Past Life Fossils and the History of Life Fossil Succession: Developed by William Smith states that fossil organisms succeed one another in a definite and determinable order. - Smith found that fossils weren’t randomly distributed through rock layers. - Rather, each layer contained an assortment of fossils that did not occur in the layer above or below it.
12. 2 Fossils: Evidence of Past Life Fossils and the History of Life Theory of Evolution: Developed by Charles Darwin 1859 - Theory of Evolution- states that life forms have changed over time, or evolved, from simpler to more complex forms. Natural Selection- individuals that are better adapted to their environment are more likely to survive and reproduce than others. Adaptations- traits possessed by organisms that affect their ability to survive and reproduce. (drives evolution)
12. 2 Fossils: Evidence of Past Life Interpreting the Fossil Record Geologists used fossils to improve the correlation of rock layers and reconstruct past environments. Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers. The presence of an index fossil in rock layers at different locations means that the layers are roughly the same age.
Overlapping Ranges of Fossils
12. 2 Fossils: Evidence of Past Life Fossils and Past Evidence Interpreting Environments • Fossils can be used to interpret and describe ancient environments. • Fossils can also be used to indicate the former temperature of the water in which organisms lived.
12. 3 Dating with Radioactivity Today, geologists can determine the absolute age of a rock. A rock’s absolute age is the approximate number of years before the present that the rock formed. EX) Earth is about 4. 56 byo EX) Dinosaurs became extinct 65 mya
12. 3 Dating with Radioactivity Basic Atomic Structures Orbiting the nucleus are electrons, which are negative electrical charges. Atomic number is the number of protons in the atom’s nucleus. Mass number is the number of protons plus the number of neutrons in an atom’s nucleus.
12. 3 Dating with Radioactivity What is Radioactivity? Radioactivity is the process by which atoms decay, causing a releasing of energy. Recall: each atom has a nucleus made up of protons and neutrons. During radioactive decay, unstable atomic nuclei spontaneously break apart, or decay, releasing energy.
Common Types of Radioactive Decay
12. 3 Dating with Radioactivity Half-Life & Isotopes An isotope is a different form of an element. Half-Time is a common way of expressing the rate of radioactive decay. A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.
The Half-Life Decay Curve
12. 3 Dating with Radioactivity Radiometric Dating Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. Radiometric dating is the way of calculating the absolute ages of rocks and minerals that contain radioactive isotopes. In radioactive dating, scientists measure the ratio between the radioactive parent isotope and the daughter products in a sample.
12. 3 Dating with Radioactivity Radiometric Dating 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.
Radioactive Isotopes Frequently Used in Radiometric Dating
12. 3 Dating with Radioactivity Dating with Carbon-14 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.
12. 3 Dating with Radioactivity Importance of Radiometric Dating Radiometric dating has supported the ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.
12. 4 The Geologic Time Scale Structure of the Time Scale Based on their interpretations of the rock record, geologists have divided Earth’s 4. 56 billion-year history into units that represent specific amounts of time. Taken together, these time spans make up the geologic time scale.
12. 4 The Geologic Time Scale Structure of the Time Scale Eons represent the greatest expanses of time. Eons are divided into eras. Each era is subdivided into periods. Finally, periods are divided into smaller units called epochs. There are three eras within the Phanerozoic eon: the Paleozoic, which means “ancient life, ” the Mesozoic, which means “middle life, ” and the Cenozoic, which means “recent life. ”
12. 4 The Geologic Time Scale Structure of the 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.
12. 4 The Geologic Time Scale Precambrian Time During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often.
The Geologic Time Scale
12. 4 The Geologic Time Scale Difficulties With the Geologic Time Scale A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are not the same age as the rock in which they occur. The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock was first formed. Instead, the date may indicate when the rock was metamorphosed.
Using Radiometric Methods to Help Date Sedimentary Rocks
Chapter 12 Geologic Time Sedimentary Rocks: Rocks formed from the weathered products of preexisting rocks that have been _____, ________, and ______.
Chapter 12 Geologic Time Igneous Rocks: Rocks formed by the crystallization of ____________
12. 1 Discovering Earth’s History Studying Earth’s History Major __________- Civil war vet and geologists- led scientific expedition down the Colorado River, through the Grand Canyon. To him, the walls of the Canyon provided evidence of Earth’s long history. His work led scientists to believe geologic time grew from _____ to many ______of years.
12. 1 Discovering Earth’s History Studying Earth’s History In studying Earth’s history, geologists make use of three main ideas: 1. the rock record provides evidence of ________ and life forms of ______ 2. Processes overserved on Earth in the present also acted in the past 3. Earth is very old and has changed over geologic time.
12. 1 Discovering Earth’s History Studying Earth’s History Late 1700’s- Scottish physician- ______ published his Theory of the Earth. Hutton put forth the fundamental principal of ___________: means that the physical, chemical, and biological processes that we observe today have been at work for a very long time.
12. 1 Discovering Earth’s History Relative Dating—Key Principles 1800’s- William Hutton- worked to interpret Earth’s rock record, known as ___________ Method used to place rocks in chronological order. Identifies which rock units formed first, second, etc.
12. 1 Discovering Earth’s History Relative Dating—Key Principles In relative dating, geologists follow several principals: Law of ___________ Principle of _________________.
12. 1 Discovering Earth’s History Relative Dating—Key Principles 1636 - 1686 : _______ Danish geologists Steno developed the Law of Superposition: States that in an under-formed sequence of sedimentary rocks, each layer is _____ than the one above it and ____ than the one below it.
Ordering the Grand Canyon’s History Law of Superposition
12. 1 Discovering Earth’s History Relative Dating—Key Principles Steno also developed the Principle of Original Horizontality: States that layers of sediment are generally deposited in a _______________. If rock layers appear flat, it means they’ve remained in their original horizontal position.
Disturbed Rock Layers Principle of Original Horizontality
12. 1 Discovering Earth’s History Relative Dating—Key Principles 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. Fault: __________________. Dike: __________________.
Applying Cross-Cutting Relationships Fault: a fracture in Earth along which movement has occurred Dike: a tabular-shaped intrusive igneous feature that cut across preexisting rock layers.
12. 1 Discovering Earth’s History Reading the Rock Record Applying Steno’s principles, geologists read • geologic features to reconstruct Earth’s history. Geologists use three methods to interpret rock records: __________________
12. 1 Discovering Earth’s History Reading the Rock Record Inclusions: are _______________________________ Inclusions made of older igneous rock can be found within younger sedimentary rock layers on top of the weathered igneous rock.
Formation of Inclusions
12. 1 Discovering Earth’s History Relative Dating—Key Principles __________ • represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.
A Record of Uplift, Erosion, and Deposition
Formation of an Angular Conformity 3 types of unconformities: 1. ______- layers of sedimentary rock form over older sedimentary rock that are tilted or folded. 2. _______ – two sedimentary rock layers are separated by an erosional surface. 3. ________ – an erosional surface separates older metamorphic or igneous rocks from younger sedimentary rocks.
12. 1 Discovering Earth’s History Correlation of Rock Layers Correlation is establishing the _____ of rocks of similar age in different areas.
Correlation of Strata at Two Locations
12. 2 Fossils: Evidence of Past Life Fossil Formation An ______ organism is one that no longer exists on Earth. ______ are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks. The type of fossil that is formed is determined by the conditions under which the organism _____ and _______.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Petrified Fossils: Organisms remains become petrified. Petrified means “_________” Process: 1. _________ soaks into small cavities and pores of the original organism. 2. Minerals ______ from the water and fill the spaces. 3. Minerals sometimes _____ the cell walls or solid material of the organism.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Petrified
12. 2 Fossils: Evidence of Past Life Types of Fossils: Molds and Casts ______ Process: 1. Mold is created when a shell or other structure is ________ and then dissolved by underground water. Mold reflects only _____ and _______ marking of the organism. ________are created if the hollow spaces of a mold are later filled with mineral matter.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Molds and Casts
12. 2 Fossils: Evidence of Past Life Types of Fossils: Carbon Films _______ preserve delicate details of leaves and animal parts. Process: 1. An organism is buried under ______ 2. Over time, pressure squeezes out _______ and ____ to leave behind a thin film of carbon. Sometimes impressions of the organism remain.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Carbon Films
12. 2 Fossils: Evidence of Past Life Types of Fossils: Preserved Remains Fossilization preserves _____ or _______ of an organism with little change to its remains. Preservation can occur in amber- ______________________. Preservation can also occur _______- a thick petroleum that collects in pools at the surface.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Preserved Remains
12. 2 Fossils: Evidence of Past Life Types of Fossils: Trace Fossils Indirect evidence of _________ Some of the oldest known trace fossils are believed to be _________. ______ are fossils of dung (poop) and stomach contents (throw-up). ______ are highly polished stomach stones that were used in grinding of food by extinct reptiles.
12. 2 Fossils: Evidence of Past Life Types of Fossils: Trace Fossils
Types of Fossilization
12. 2 Fossils: Evidence of Past Life Conditions for Fossilization Two conditions that favor preservation of an organism as a fossil are _____ and the ____________. Dead organisms with shells, bones, and teeth (hard parts) have a better chance at being preserved. Dead organisms without hard parts, unless covered by sediment immediately, were likely to be eaten or ______________________.
12. 2 Fossils: Evidence of Past Life Fossils and the History of Life 1700 -1800’s: Two major developments helped scientists explain fossil records: 1. The Principle of ___________ 2. Theory of _______
12. 2 Fossils: Evidence of Past Life Fossils and the History of Life Fossil Succession: Developed by ______ states that fossil organisms _________ in a definite and determinable order. - Smith found that fossils weren’t randomly distributed through rock layers. - Rather, each layer contained an assortment of fossils that did not occur in the layer ________________.
12. 2 Fossils: Evidence of Past Life Fossils and the History of Life Theory of Evolution: Developed by ________________ 1859 - Theory of Evolution- states that life forms have changed over time, ______, from simpler to more complex forms. _________- individuals that are better adapted to their environment are more likely to survive and reproduce than others. ________- traits possessed by organisms that affect their ability to survive and reproduce. (drives evolution)
12. 2 Fossils: Evidence of Past Life Interpreting the Fossil Record Geologists used fossils to improve the __________ and reconstruct __________. Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers. The presence of an index fossil in rock layers at different locations means _____________________.
Overlapping Ranges of Fossils
12. 2 Fossils: Evidence of Past Life Fossils and Past Evidence Interpreting Environments • Fossils can be used to interpret and describe ____________. • Fossils can also be used to indicate the former _______________ in which organisms lived.
12. 3 Dating with Radioactivity Today, geologists can determine the ________ of a rock. A rock’s absolute age is _________________________. EX) Earth is about ______ byo EX) Dinosaurs became extinct ____ mya
12. 3 Dating with Radioactivity Basic Atomic Structures Orbiting the nucleus are electrons, which are _____ electrical charges. _______ is the number of protons in the atom’s nucleus. ________is the number of protons plus the number of neutrons in an atom’s nucleus.
12. 3 Dating with Radioactivity What is Radioactivity? Radioactivity is the process by which _________________. Recall: each atom has a nucleus made up of protons and neutrons. During radioactive decay, unstable atomic nuclei spontaneously ______________________.
Common Types of Radioactive Decay
12. 3 Dating with Radioactivity Half-Life & Isotopes An_____ is a different form of an element. _______ is a common way of expressing the rate of radioactive decay. A half-life is the amount of time necessary for one-half of the nuclei in a sample to ___________.
The Half-Life Decay Curve
12. 3 Dating with Radioactivity Radiometric Dating Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. Radiometric dating is the way of calculating the absolute ages of rocks and _________________. In radioactive dating, scientists measure the ratio between the radioactive parent isotope and the daughter products in a sample.
12. 3 Dating with Radioactivity Radiometric Dating 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.
Radioactive Isotopes Frequently Used in Radiometric Dating
12. 3 Dating with Radioactivity Dating with Carbon-14 Radiocarbon dating is the method for determining age by comparing the amount of _____ to the amount of _______ 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.
12. 3 Dating with Radioactivity Importance of Radiometric Dating Radiometric dating has supported the ideas of ____________, and others who inferred that geologic time must be immense.
12. 3 Dating with Radioactivity Carbon Dating Video Go to www. brainpop. com Login as: gslschool pswd: saints Search: Carbon Dating Watch Video Take the graded quiz until you score a 10/10. Then email your Results to alisha. kimbro@stjoeshs. org
12. 4 The Geologic Time Scale Structure of the Time Scale Based on their interpretations of the rock record, geologists have divided Earth’s 4. 56 billion-year history into units that represent specific amounts of time. Taken together, these time spans make up the geologic time scale.
12. 4 The Geologic Time Scale Structure of the Time Scale Eons represent the greatest expanses of time. Eons are divided into eras. Each era is subdivided into periods. Finally, periods are divided into smaller units called epochs. There are three eras within the Phanerozoic eon: the Paleozoic, which means “ancient life, ” the Mesozoic, which means “middle life, ” and the Cenozoic, which means “recent life. ”
12. 4 The Geologic Time Scale Structure of the 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.
12. 4 The Geologic Time Scale Precambrian Time During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often.
The Geologic Time Scale
12. 4 The Geologic Time Scale Difficulties With the Geologic Time Scale A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are not the same age as the rock in which they occur. The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock was first formed. Instead, the date may indicate when the rock was metamorphosed.
Using Radiometric Methods to Help Date Sedimentary Rocks