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Geologic Times
Rocks record geological events and changing life forms of the past.
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.
Uniformitarianism - the forces and processes that we observe today have been at work for a very long time.
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.
Principle of Original Horizontality - means that layers of sediment are generally deposited in a horizontal position.
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.
Unconformities
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.
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
Correlation - is establishing the equivalence of rocks of similar age in different areas.
Fossil Formation
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 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.
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.
Indirect Evidence
Trace fossils are indirect evidence of prehistoric life.
Conditions favoring Preservation
Two conditions are important for preservation: rapid burial and the possession of hard parts.
Fossils and Correlation
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.
Index Fossils - are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.
Interpreting Environments - Fossils can also be used to interpret and describe ancient environments.
Dating with Radioactivity
Basic Atomic Structures
â—† 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.
Radioactivity - is the spontaneous decay of certain unstable atomic nuclei.
Half-life - is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.
Radiometric Dating
- is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.
* Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.
* 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
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.
* Radiometric dating has supported the ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.
Evolution
Theme: Evolution drives diversity and unity of life
â—‹ It refers to the cumulative change in a population of organism over time
Principles of Geology ( Charles Lyell)
(1830-33)
- revived Hutton's principle of slow geological change, and presented a classic explanation of development over millions of years.
Natural Selection:
Each species show variation
There is competition within each species for food, living space, water, mates, etc.
The “better adapted” members of these species are more likely to survive “Survival of the fittest”
These survivors will pass on their better genes to their offspring who will also show this beneficial variation
Theory of Natural Selection:
Descent w/ modification and adaption to the environment is the basis of natural selection
Inherited better traits /adaptions, produce more offspring that reach reproductive age.
4 Main Points:
* Adaption
* Variation
* Overproduction
* Descent/ modification
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.”
â—† 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.
â—† 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
- During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often.
Difficulties with the Geologic Time Scale
â—† 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.
â—† 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.
Precambrian History
- 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.
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.
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.
Precambrian Fossils
Stromatolites - The most common fossils. They 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.
Early Paleozoic
Following the long Precambrian, the most recent 540 million years of Earth’s history are divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
Early Paleozoic History
• During the Cambrian, Ordovician, and Silurian periods, the vast southern continent of Gondwana encompassed five continents (South America, Africa, Australia, Antarctica, and part of Asia).
• Life in early Paleozoic time was restricted to the seas.
Late Paleozoic History
Laurasia - is the continental mass that formed the northern portion of Pangaea, consisting of present-day North America and Eurasia.
* By the end of the Paleozoic, all the continents had fused into the supercontinent of Pangaea.
• Some 400 million years ago, plants that had adapted to survive at the water’s edge began to move inland, becoming land plants.
• The amphibians rapidly diversified because they had minimal competition from other land dwellers.
The Great Paleozoic Extinction
◆ The world’s climate became very seasonal, probably causing the dramatic extinction of many species.
â—† The late Paleozoic extinction was the greatest of at least five mass extinctions to occur over the past 500 million years.
Mesozoic Era
â—† Dinosaurs were land-dwelling reptiles that thrived during the Mesozoic era.
Mesozoic History
• A major event of the Mesozoic era was the breakup of Pangaea.
Mesozoic Life
• Gymnosperms are seed-bearing plants that do not depend on free-standing water for fertilization.
• The gymnosperms quickly became the dominant plants of the Mesozoic era.
The Shelled Egg
• Unlike amphibians, reptiles have shell-covered eggs that can be laid on the land.
• The elimination of a water-dwelling stage (like the tadpole stage in frogs) was an important evolutionary step.
Reptiles Dominate
• With the perfection of the shelled egg, reptiles quickly became the dominant land animals.
• At the end of the Mesozoic era, many reptile groups became extinct.
Cenozoic North America
â—† The Cenozoic era is divided into two periods of very unequal duration, the Tertiary period and the Quaternary period.
â—† Plate interactions during the Cenozoic era caused many events of mountain building, volcanism, and earthquakes in the West.
Cenozoic Life
Mammals - animals that bear live young and maintain a steady body temperature— replaced reptiles as the dominant land animals in the Cenozoic era.
Angiosperms - flowering plants with covered seeds—replaced gymnosperms as the dominant land plants.
Mammals Replace Reptiles
• Adaptations like being warm blooded, developing insulating body hair, and having more efficient heart and lungs allow mammals to lead more active lives than reptiles.
Large Mammals and Extinction
• In North America, the mastodon and mammoth, both huge relatives of the elephant, became extinct. In addition, saber-toothed cats, giant beavers, large ground sloths, horses, camels, giant bison, and others died out on the North American continent.
• The reason for this recent wave of extinctions puzzles scientists.
Geologic Times
Rocks record geological events and changing life forms of the past.
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.
Uniformitarianism - the forces and processes that we observe today have been at work for a very long time.
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.
Principle of Original Horizontality - means that layers of sediment are generally deposited in a horizontal position.
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.
Unconformities
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.
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
Correlation - is establishing the equivalence of rocks of similar age in different areas.
Fossil Formation
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 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.
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.
Indirect Evidence
Trace fossils are indirect evidence of prehistoric life.
Conditions favoring Preservation
Two conditions are important for preservation: rapid burial and the possession of hard parts.
Fossils and Correlation
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.
Index Fossils - are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.
Interpreting Environments - Fossils can also be used to interpret and describe ancient environments.
Dating with Radioactivity
Basic Atomic Structures
â—† 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.
Radioactivity - is the spontaneous decay of certain unstable atomic nuclei.
Half-life - is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.
Radiometric Dating
- is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.
* Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.
* 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
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.
* Radiometric dating has supported the ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense.
Evolution
Theme: Evolution drives diversity and unity of life
â—‹ It refers to the cumulative change in a population of organism over time
Principles of Geology ( Charles Lyell)
(1830-33)
- revived Hutton's principle of slow geological change, and presented a classic explanation of development over millions of years.
Natural Selection:
Each species show variation
There is competition within each species for food, living space, water, mates, etc.
The “better adapted” members of these species are more likely to survive “Survival of the fittest”
These survivors will pass on their better genes to their offspring who will also show this beneficial variation
Theory of Natural Selection:
Descent w/ modification and adaption to the environment is the basis of natural selection
Inherited better traits /adaptions, produce more offspring that reach reproductive age.
4 Main Points:
* Adaption
* Variation
* Overproduction
* Descent/ modification
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.”
â—† 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.
â—† 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
- During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often.
Difficulties with the Geologic Time Scale
â—† 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.
â—† 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.
Precambrian History
- 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.
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.
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.
Precambrian Fossils
Stromatolites - The most common fossils. They 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.
Early Paleozoic
Following the long Precambrian, the most recent 540 million years of Earth’s history are divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
Early Paleozoic History
• During the Cambrian, Ordovician, and Silurian periods, the vast southern continent of Gondwana encompassed five continents (South America, Africa, Australia, Antarctica, and part of Asia).
• Life in early Paleozoic time was restricted to the seas.
Late Paleozoic History
Laurasia - is the continental mass that formed the northern portion of Pangaea, consisting of present-day North America and Eurasia.
* By the end of the Paleozoic, all the continents had fused into the supercontinent of Pangaea.
• Some 400 million years ago, plants that had adapted to survive at the water’s edge began to move inland, becoming land plants.
• The amphibians rapidly diversified because they had minimal competition from other land dwellers.
The Great Paleozoic Extinction
◆ The world’s climate became very seasonal, probably causing the dramatic extinction of many species.
â—† The late Paleozoic extinction was the greatest of at least five mass extinctions to occur over the past 500 million years.
Mesozoic Era
â—† Dinosaurs were land-dwelling reptiles that thrived during the Mesozoic era.
Mesozoic History
• A major event of the Mesozoic era was the breakup of Pangaea.
Mesozoic Life
• Gymnosperms are seed-bearing plants that do not depend on free-standing water for fertilization.
• The gymnosperms quickly became the dominant plants of the Mesozoic era.
The Shelled Egg
• Unlike amphibians, reptiles have shell-covered eggs that can be laid on the land.
• The elimination of a water-dwelling stage (like the tadpole stage in frogs) was an important evolutionary step.
Reptiles Dominate
• With the perfection of the shelled egg, reptiles quickly became the dominant land animals.
• At the end of the Mesozoic era, many reptile groups became extinct.
Cenozoic North America
â—† The Cenozoic era is divided into two periods of very unequal duration, the Tertiary period and the Quaternary period.
â—† Plate interactions during the Cenozoic era caused many events of mountain building, volcanism, and earthquakes in the West.
Cenozoic Life
Mammals - animals that bear live young and maintain a steady body temperature— replaced reptiles as the dominant land animals in the Cenozoic era.
Angiosperms - flowering plants with covered seeds—replaced gymnosperms as the dominant land plants.
Mammals Replace Reptiles
• Adaptations like being warm blooded, developing insulating body hair, and having more efficient heart and lungs allow mammals to lead more active lives than reptiles.
Large Mammals and Extinction
• In North America, the mastodon and mammoth, both huge relatives of the elephant, became extinct. In addition, saber-toothed cats, giant beavers, large ground sloths, horses, camels, giant bison, and others died out on the North American continent.
• The reason for this recent wave of extinctions puzzles scientists.
NoteStudied by 6 peopleNoteStudied by 29 peopleNoteStudied by 24 peopleNoteStudied by 33 peopleNoteStudied by 11 peopleNoteStudied by 52 people