Lecture 9: Geologic Time and Relative Age Dating
Veronica McCann・2 minutes read
The lecture outlines key concepts of geologic time and dating methods while detailing the structure of upcoming assignments and activities related to plate tectonics and relative age dating. It emphasizes the Earth's extensive history, highlighting significant events such as extinction periods and the emergence of multicellular life, supported by fossil evidence and geological principles.
Insights
- The lecture on geologic time and relative age dating has been streamlined into a single session, with deadlines for summaries and replies set for July 9th and July 11th, respectively, in preparation for an upcoming exam, highlighting the importance of timely engagement in the learning process.
- Students will collaborate on a group activity focused on plate tectonics, presenting their findings using Google Slides or Google Docs, which emphasizes the integration of technology in education and the necessity for students to effectively communicate complex scientific concepts.
- The Earth, estimated to be around 4.56 billion years old, is divided into eons, eras, and periods, with significant events like the Cambrian explosion and the Great Dying marking critical points in the evolution of life, illustrating the vast timescales and transformative events that have shaped the planet's biological history.
- The principles of geological dating, such as superposition and cross-cutting relationships, provide essential frameworks for understanding rock layers and their histories, enabling geologists to piece together the Earth's past while recognizing gaps in the geological record that can complicate interpretations.
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Recent questions
What is geological time scale?
The geological time scale is a system used by geologists and paleontologists to describe the timing and relationships of events in Earth's history. It divides Earth's 4.56 billion-year history into eons, eras, periods, and epochs, allowing scientists to organize and understand the complex history of life and geological changes. The current eon, the Phanerozoic, is crucial for studying the development of complex life and is further divided into three eras: Paleozoic, Mesozoic, and Cenozoic. Each era is characterized by significant events, such as the emergence of multicellular life during the Cambrian explosion or the age of dinosaurs in the Mesozoic. This scale helps researchers correlate rock layers and fossils across different regions, providing a framework for understanding the evolution of life and the Earth's geological processes over time.
How do fossils help in dating rocks?
Fossils play a critical role in dating rocks through the principle of faunal succession, which states that certain fossil organisms are characteristic of specific geological time periods. By identifying and correlating these fossils within rock layers, geologists can determine the relative ages of the rocks. Fossils from organisms that existed for a short duration, such as ammonites or trilobites, are particularly useful for this purpose, as they provide precise markers for specific intervals in geological history. Additionally, the presence of fossils can indicate past environments and help identify unconformities, or gaps in the geological record, where certain time periods may be missing. This method of dating rocks through fossils allows scientists to reconstruct the timeline of life on Earth and understand the evolutionary history of various species.
What is an unconformity in geology?
An unconformity in geology refers to a gap in the geological record where rock layers are missing due to erosion or non-deposition. This phenomenon can occur between different types of rocks, such as sedimentary and igneous or metamorphic rocks, and is crucial for understanding the geological history of an area. There are three main types of unconformities: nonconformity, which occurs between older igneous or metamorphic rocks and overlying sedimentary layers; disconformity, which indicates a break in sedimentary deposition; and angular unconformity, where tilted layers are eroded before new layers are deposited. Recognizing unconformities helps geologists interpret the processes that shaped the Earth's surface over time and provides context for the chronological order of geological events.
What is relative age dating?
Relative age dating is a method used by geologists to determine the chronological order of rock layers and geological events without assigning specific numerical ages. This technique relies on several principles, such as the principle of superposition, which states that in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest at the top. Other principles include original horizontality, which asserts that sedimentary layers are deposited in horizontal layers, and cross-cutting relationships, which indicate that geological features like faults or intrusions are younger than the rocks they cut through. By applying these principles, geologists can construct a relative timeline of geological history, helping to understand the sequence of events that have shaped the Earth over millions of years.
What is the significance of the Cambrian explosion?
The Cambrian explosion, which occurred around 542 million years ago, marks a pivotal event in Earth's history characterized by a rapid diversification of life forms. During this period, a significant increase in multicellular organisms took place, leading to the emergence of various invertebrates and early vertebrates. This explosion of biodiversity is crucial for understanding the evolution of complex life, as it set the stage for the development of many major animal groups that still exist today. Fossils from this time provide critical evidence of the types of organisms that thrived in ancient oceans, indicating a dramatic shift in the complexity and variety of life. The Cambrian explosion is often viewed as a key moment in the history of life on Earth, highlighting the dynamic processes that drive evolution and the intricate relationships between organisms and their environments.
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Summary
00:00
Geologic Time and Age Dating Overview
- The lecture on geologic time and relative age dating has been consolidated into one session, with a summary due by July 9th and replies due by July 11th, due to an upcoming exam.
- The second half of the relative age dating and absolute age dating will be combined into one lecture, with a summary due by July 16th and replies by July 18th, as part of a group activity on plate tectonics.
- Students will present their understanding of plate tectonics using Google Slides or Google Docs, with examples provided during the week of the activity.
- Resources for geologic time and age dating will be available on the course page, including YouTube videos explaining relative age dating principles and a demonstration of radiometric age dating.
- The Earth is approximately 4.56 billion years old, and geologic time is divided into eons, eras, and periods, similar to how human time is divided into centuries, decades, and years.
- The current eon is the Phanerozoic, which is divided into three eras: Paleozoic (development of multicellular life), Mesozoic (age of dinosaurs), and Cenozoic (age of mammals and birds).
- Each era is further divided into periods, such as Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary.
- The Cambrian explosion, occurring around 542 million years ago, marked a significant increase in multicellular organisms, with fossils indicating the emergence of invertebrates and early vertebrates.
- The Carboniferous period was characterized by high levels of CO2 and oxygen, leading to the development of large amphibians and extensive coal deposits, particularly in North America, where it is referred to as the Mississippian and Pennsylvanian.
- The Permian period ended with the Great Dying, an extinction event that wiped out approximately 95% of Earth's life, likely caused by volcanic activity, greenhouse gas emissions, and rising ocean temperatures over millions of years.
23:42
Impact of Climate Change on Biodiversity
- A significant climate change of six degrees Celsius could potentially wipe out 85-90% of ocean life and 80-85% of land life, highlighting the fragility of biodiversity and its importance to human existence.
- The extinction event that led to the rise of dinosaurs occurred approximately 65 million years ago due to a meteorite impact near the Yucatan Peninsula, which caused drastic temperature changes and ultimately the extinction of dinosaurs.
- The Mesozoic Era, known as the age of dinosaurs, also saw the emergence of early birds and mammals, with evidence suggesting that some dinosaurs had feathers, linking them closely to modern birds.
- The geological time scale is divided into eons and eras, with the Phanerozoic Eon (542 million years ago to present) being crucial for understanding the development of complex life, including the Paleozoic, Mesozoic, and Cenozoic eras.
- The Cenozoic Era is further divided into three periods: the Paleogene, Neogene, and Quaternary, with the current period being the Holocene, which began approximately 11,700 years ago.
- The Anthropocene is a proposed epoch that reflects significant human impact on Earth's geology and ecosystems, with scientists advocating for its inclusion in the geological time scale due to human-induced changes.
- The Earth formed about 4.56 billion years ago, with the oldest rocks dating back to approximately 4.1 billion years, found in regions like the Canadian Shield, which includes some of the oldest known geological formations.
- The Cambrian period, marking the start of the Phanerozoic Eon, began around 541 million years ago and is characterized by the rapid diversification of life, including the first complex multicellular organisms.
- Geological time can be conceptualized in a single year analogy, where significant events like the formation of the Earth occur on January 1st, the emergence of dinosaurs on December 21st, and humans evolving just one minute before midnight on December 31st.
- James Hutton, often referred to as the father of modern geology, was the first to systematically study rocks and their formation, laying the groundwork for the field, with his ideas later popularized by Charles Lyell.
45:21
Geological Dating Principles Explained Through Examples
- The discussion begins with the concept of relative age dating, using examples like the Grand Canyon to illustrate how geologists identify rock layers based on their characteristics, such as limestone or volcanic rock, to determine their relative ages, with older rocks at the bottom and younger rocks at the top.
- The principle of original horizontality states that sedimentary layers are deposited in horizontal layers, similar to stacking pages, and if they are not horizontal, it indicates that geological events have occurred after their deposition.
- The principle of superposition asserts that in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest are at the top, akin to building a lasagna with layers of sauce, noodles, and cheese.
- A practical example of superposition is provided using the Grand Canyon, where the Sakai group layer is the oldest, followed by the Hermit shale, Coconino sandstone, Toro weep formation, and the Kaibab limestone, each representing millions of years of geological history.
- The law of lateral continuity explains that rock layers extend in all directions until they reach a boundary, allowing geologists to correlate rock types over large distances, similar to pancake batter spreading in a pan.
- The principle of faunal succession, or biostratigraphy, utilizes fossils to correlate rock layers, indicating that certain fossils can signify specific geological time periods, helping to identify missing time or unconformities in the rock record.
- An unconformity is defined as a gap in the geological record where rock layers are missing, which can occur due to erosion or non-deposition, and geologists label these gaps without making assumptions about their causes.
- Fossils that are useful for relative age dating include those from organisms that existed for a short time, such as the pass Keystone, which lived during the Devonian period, and ammonites, which were present during the Mesozoic era.
- The principle of cross-cutting relationships states that geological features, such as faults or igneous intrusions, are younger than the rocks they cut through, illustrated by the analogy of a jelly donut where the filling (igneous intrusion) can only occur after the donut (rock) exists.
- The summary concludes with the idea that rock fragments found within other rocks indicate that the original rock must have existed before being incorporated, reinforcing the principles of geological dating and interpretation.
01:05:26
Geological Concepts of Time and History
- Included fragments in geology refer to older rock pieces found within younger rock formations, similar to cake frosting containing bits of cake, indicating that the fragments predate the rock they are in.
- Cross-cutting relationships state that geological features like fault lines or igneous intrusions are younger than the rocks they cut through, aiding geologists in determining the relative ages of rock layers.
- Correlation in geology involves analyzing rock layers across different locations to reconstruct geological history, with sedimentary rocks providing snapshots of past environments, igneous rocks indicating eruption types, and metamorphic rocks revealing tectonic activity.
- Unconformities represent gaps in geological time, where sedimentary layers may be missing due to erosion or non-deposition, complicating the interpretation of geological history.
- A realistic geological scenario might include various sediment types at different locations, such as silt, mud, ash, and limestone, which can indicate past riverbanks, floodplains, or volcanic activity, helping to build a geological narrative.
- Fossils, such as trilobites and brachiopods, found in rock layers provide critical evidence of past environments, indicating the presence of ancient oceans and helping to identify missing time through unconformities.
- The Grand Canyon, Zion, and Bryce Canyon are examples of locations where geological correlation can be observed, with similar rock layers found across these areas despite their distance, indicating a shared geological history.
- Three types of unconformities are identified: nonconformity (between igneous/metamorphic and sedimentary rocks), disconformity (missing time between sedimentary layers), and angular unconformity (tilted layers eroded before new layers are deposited).
- Nonconformity involves a gap between older igneous or metamorphic rocks and overlying sedimentary layers, while disconformity indicates a break in sedimentary deposition, and angular unconformity shows erosion between tilted and horizontal layers.
- Understanding these unconformities is crucial for geologists, as they provide context for interpreting the geological history of an area, allowing for more precise discussions about the processes that shaped the Earth's surface over time.
01:28:02
Geological Unconformities and Missing Time Explained
- The concept of unconformity in geology is illustrated through examples such as sedimentary layers with missing time, where fossils help identify these gaps; specifically, the ungulate unconformity features tilted beds with horizontal layers above, indicating significant geological time loss.
- A notable example is Siccar Point in Scotland, where James Hutton observed tilted rocks beneath horizontal layers, leading him to contemplate the vast time required for these formations, represented by a line marking the unconformity between the two rock types.
- Angular unconformities can occur even with slight tilting of beds; the text emphasizes that the presence of tilted beds beneath horizontal layers signifies missing geological time, which can be interpreted through fossil evidence, such as trilobites from different time periods.
- The analysis of fossil sequences reveals potential missing time, with trilobites like the Owenella and the Phycopus indicating gaps of over 100 million years, as certain geological periods, such as the Ordovician and Silurian, are not represented in the rock layers.
- The Great Unconformity in the Grand Canyon exemplifies a significant geological gap, where the Tonto Group, formed during a marine transgression, sits atop the Unpair Group, indicating a large amount of missing time due to changes in sea level.
- Practical application of these concepts involves backtracking through sedimentary rock formations, identifying igneous intrusions, and using relative age dating techniques to determine the chronological order of rock layers, with specific tasks assigned for students to practice these skills.
