Lesson 2
SG: How do geologists (stratigraphers) define a new division on the geologic time scale?
"Golden spike" from the GSSP; usually latkes millions of years to form
How old is the Earth (geologic time)?
4.54 Ga
What is an index fossil?
Geographically widespread, existed for a limited time and indicate the age of rocks that contain them
What changed within technology to allow scientists to reconsider the scale?
In the 1950s absolute dating techniques based on radioactive decay of isotopes in minerals allowed for numerical ages
What's the difference between intrusive and extrusive rocks?
Intrusive (formed inside something) vs extrusive (protrudes front he surface) rocks
What's a possible start to the Anthropocene?
Start of the Industrial Revolution (increase in coal burning, plasticonglomerate)
How is absolute age calculated?
(number of half lives) x (half life)
SG: What are some options for defining the start of the Anthropocene based on the assigned readings? Which of these options has been agreed upon by the Anthropocene Working Group and is currently being searched for to designate the Global boundary Stratotype Section and Point (GSSP) a.k.a. "the golden spike"?
1945 - nuclear warfare Industrial Revolution
What dictates a change in the geologic time scale?
A change in the fossil record
What's the geologic column?
A composite geologic column, representing relative ages, was assembled from multiple, incomplete sections from all over the world. Represents almost all of Earth history Bottom to top → oldest to youngest Names come from the location of the type section of rock Reflect the process of assembling
What's the Anthropocene?
A proposed new division in the geological timescale defined by the observation that the "human imprint on the global environment has now become so large and active that it rivals some of the great forces of Nature in its impact on the functioning of the Earth system"; based on human impact
What is important to note in relation to mass extinctions?
A rapid growth in diversity (positive slope) occurs directly after mass extinctions
What's the principle of inclusions?
A rock containing an inclusion must be younger than the inclusion
What's a marker bed?
A unique layer present over a broad area that aids correlation (example: layer of volcanic ash)
SG: What is the difference between absolute and relative age? Give an example of each. How do they each play a role in the construction of the geologic time scale?
Absolute age is a numerical number assigned to an object, whether fossil or rock. Relative age is a value (non-numerical) given in regard to another object. An example of absolute age could be derived through radiometric dating, such that would reveal a certain fossil to be 2 GA old. An example of relative age could be revealed through the principle of superposition which would assign the relative age of one rock strata to be older than the rock strata above.
What's absolute age?
Age of something in numeric years (determined from radioactive decay of isotopes in minerals)
What's relative age?
Age of units relative to one another (gathered from observed contact between rock relationships and the fossil record)
What's the principle of baked contacts?
An igneous intrusion "bakes" (metamorphoses) surrounding rocks so the rock that has been baked must be older than the intrusion
Review: the anatomy of an atom
Atoms are made of protons (+) and neutrons (0) in the nucleus and orbiting electrons (-) Elements are arranged on the periodic table according to their properties (atomic number: number of protons) Atomic mass: the number of protons + the number of neutrons
What's an isotope?
Atoms of a single element that have different number of neutrons but the same number of protons in their nuclei. Isotopes of a single element have different atomic mass but same atomic number (and similar properties). Isotopes have different number of neutrons, same number of protons
How do these differing preservation potentials affect the rock record?
Biased record of identifying fossils: those which have hard parts or have lived along continental shelf
How is igneous rock classified?
Classified according the crystal size and composition
What are the different types of sedimentary rock?
Clastic Biochemical Chemical
SG: Describe how an isotopic system's closure temperature can reflect the age of the mineral/rock. Discuss how multiple closure temperatures of different minerals and different isotopic systems in a single rock can yield information about the rock's thermal evolution. Consult the assigned readings in our textbook for support.
Closure temperature: temperature by which atoms are no longer free to move When specifying a radioisotopic date, we are defining the time at which the mineral cooled below its closure temperature If we date minerals in sedimentary rock, we determine only when these minerals first crystallized as part of an igneous or metamorphic rock
What are the 5 major mass extinctions?
Cretaceous/tertiary; end triassic; permo/triassic' late Denovian; late Ordovician
Events are verbs:
Deposition of Intrusion of Metamorphism of Weathering and erosion of
What's stratigraphic correlation?
Examine fossil assemblages, lithology, and the relative order of the rock sequence over broad areas to determine lateral extent and thickness
What are some issues in establishing the Anthropocene?
Finding an appropriate marker preserved - Deep oceans - 0 years of sediment is less than 70 years Scientific process - Deductive approach (top down) The effects of humans on the Earth system prior to 1945 - Pre-1945 is pre-Anthropocene - Does it make sense to define the Anthropocene after deforestation? - Is the Anthropocene when humans changed the functioning of the earth system, not when they first played a significant role in shaping it?
In looking to understand the scale of geologic time, what are some important events if you were to view 4.54 Ga as a single year?
First bacteria: Feb 21 Invertebrates with hard parts: Oct 25 Amphibians: Nov 20 Mammals/birds: Dec 15 Age of the mammals: Dec 25 Homo sapiens: Dec 31, 11 pm Recorded human history: last 30 seconds
How are fossil assemblages used?
Fossil assemblages indicate the geologic age for the rocks that contain them. Fossil assemblages can be used to correlate geographically separate rock packages that are great distances apart (regional to global). Lithology (rock type) can be used to correlate geographically separate rock packages that are closer in proximity (regional).
What's important to note in using fossils and radiometric dating?
Fossils: gives an age range; radiometric dating gives a specific number
SG: Describe how the geologic column was created and how it later became the geologic time scale. You should incorporate relative and absolute time concepts in your answer.
Geologic column: the representation that geologists use to divide geologic time into named intervals Radioisotopic dating (radiometric dating) that allows geologists to determine the numerical age of rocks: the age of rocks in years Numeric ages added to the geologic column to produce the geologic time scale Geologic column: composite stratigraphic column that represents the entirety of Earth history Divided into segments, each which represent a specific interval of time How Do We Determine Numerical Ages? Radioactivity provided the key to determine numerical ages Radioisotopic dating (radiometric dating): technique used to determine numerical age The geologic time scale → Discovery of new data causes boundaries dictating the time period to change Geologic time scale assigns numerical ages to the intervals on the geologic column
What's the connection between the geologic time scale and the geologic column?
Geologic time scale come from the geologic column and our understanding of radiometric dating. The time scale divisions are defined by abrupt changes in the fossil record
SG: How did geologists reconstruct Earth's history if there is not a single stratigraphic section that represents it? How do fossils and lithology aid in this? Incorporate the terms marker bed and index fossil into your discussion.
Geologists create stratigraphic columns that depict the order of the units relative to the thickness of formations and rock types within. This is coined stratigraphic correlation in that geologists are able to determine the age of strata in one location through respect of another. Along with this, geologists utilize lithologic correlation in which they identify similarities of rock type in a location. Within this, there may be marker beds which refer to a unique layer that provides a definitive basis for correlation. Lastly, to correlate rock units over broad areas, they must rely on fossils to define the relative ages of sedimentary units, namely fossil correlation. Index fossils allow this through being used to associate strata with a particular time interval.
What are important half life ratios to remember?
Half life #, parent #, daughter #, ratio 0, 100, 100, -- 1, 50, 50, 1:1 2, 25, 75, 1:3 3, 12.5, 87.5, 1:7 4, 6.25, 93.75, 1:15 Half of parent - add to daughter count
What dictates different preservation potential of fossils?
Hard parts - more likely to be preserved Soft parts - more likely to be destroyed Plus: need certain conditions to increase the likelihood - rapid burial plus a lack of groundwater flow, metamorphism and tectonism to preserve fossils Best place to preserve organisms: continental shelf - weathering and erosion don't really happen under water
SG: Discuss the relevance of Hutton's observations at Siccar Point, Scotland to the development of the principle of uniformitarianism.
Hutton's general conclusion from his observations at Siccar point indicated that Earth processes were the same today as they were a (very) long time ago. By referencing current processes, Hutton concluded that the Earth was much older than previously accepted, introducing the principle of uniformitarianism that states that practices operating today were operating in the past at about the same rate.
SG: Discuss how a numeric age could be artificially too old or too young.
If daughter isotopes are added and parent isotopes are taken away, the age is too old. If daughter isotopes are taken away and parent isotopes are added, the age is too young.
What's the principle of cross-cutting relations?
If one geologic feature cuts across another, the feature that has been cut is older
What's the principle of superposition?
In a sequence of sedimentary rock layers, each layer must be younger than the one below for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect; bottom layer = oldest
What are some limitations of isotopic dating?
It can only be appleid if assumed that no parent or daughter isotopes are lost or are added to the mineral. The detection limits of mass spectrometers: not too few or too many half lives. Increasingly small numbers for parent isotopes; mass spec may not be able to accurately record - may be an inaccurate ratio (10 half-lives <). Using both relative help narrow down range of rock Must select a system based on the presence of the isotope (certain isotopes are found in certain minerals )and an appropriate half life for the estimated age of the material being dated
What is the principle pf uniformitarianism in Hutton's words?
James Hutton assumed that practices operating today were operating in the past at about the same rate. Earth processes (lithification) and the development of geologic features (mountain ranges) take a long time and have operated prior to human history. This challenged the previously accepted theory of catastrophism; contingent that the Earth was very young and centered on Biblical principles
Who first formed the principle of uniformitarianism? Where?
James Hutton, Siccar Point, Scotland
What is the impact of lost or added parent and daughter isotopes?
Lose daughter, add parent - younger than should be Add daughter, lose parent - older than should be
How is half life determined?
Measure parent and daughter isotopes with a mass spectrometer; determine parent/daughter ratios; indicate the number of half lives passed
What's an angular unconformity?
Missing time represented by sediments that have been laid down horizontally, tilted, eroded, and then covered by younger sediments.
What are the different types of unconformities?
Nonconformity, disconformity, angular unconformity
What's the principle of uniformitarianism?
Physical processes that we observe today also operated in the past, roughly at comparable rates - "the present is the key to the past"
What principle are igneous rocks associated with?
Principle of baked contacts
What principle are metamorphic rocks associated with?
Principle of baked contacts (contact metamorphism)
SG: Explain in your own words how isotopes allow geologists to calculate the numeric age of a mineral/rock. Incorporate the following terms correctly: parent isotope, daughter product, radioactive decay, half-life.
Radioisotopic dating (radiometric dating) is a technique used to determine numerical age. Half-life is constant; we can measure the age of a mineral by measuring the ratio of parent to daughter atoms in the mineral. They do this through collecting the unweathered rocks and dissolving them in acid and counting the ratio using a mass spectrometer.
What are usually features of a time scale division?
Reflect global, long-term (several million years) change Be marked by a globally identifiable boundary Global standard Stratotype Section and Point (GSSP) aka a "golden spike"
How are relative and numerical ages used?
Relative and numerical dating techniques are used in conjunction to decipher the geologic history of an area and place it in the broader context of the geologic past
How was the geologic time scale developed in the 18th and 19th centuries?
Relative dating techniques based on observed rock relationships and fossil assemblages
What are metamorphic rock?
Result from solid-state changes in texture, mineralogy, and/or chemical composition in response to changes in heat and/or pressure and/or chemical environment; any pre-existing rock can be metamorphosed
What is igneous rock?
Rock formed from magma (from below), created by the melting of pre-existing rock crystallizing in the crust OR when it reaches the surface through fractures and extrudes as lava (once it reaches the surface)
What are sedimentary rocks?
Rocks formed by the cementation of fragments (grains) broken off pre-existing rocks or by the precipitation of mineral crystals out of water solutions at or near the Earth's surface; tend to form pancake layers
What's a nonconformity?
Sedimentary rocks overlie igneous or metamorphic Always two different rock types Missing time represented by a body of plutonic rock or metamorphic rock overlain by sedimentary rocks
What's the principle of lateral continuity?
Sediments generally accumulate in continuous sheets within a given region
What's the principle of original horizontality?
Sediments on Earth settle out of fluids in a gravitational field; surfaces on which they accumulate = horizontal - leads to conclusion that folding, tilting and faulting of sedimentary must occur post deposit
SG: Explain how relative and absolute dates are used in tandem to discern a geologic history of an area.
See above mainly; absolute dating drawn from radioisotopic dating (determining half-life and rate of radioactive decay) is used to further the knowledge garnered from geologic principles (listed under #4); either confirm or deny - constantly evolving, used together
SG: Discuss the issue social scientists have with the scientific use of "the Anthropocene" and how it's at odds with how geologists (stratigraphers) propose to scientifically define "the Anthropocene".
Social scientists believe that the Anthropocene should include a more holistic perspective to defining the era as opposed to one which simply focuses on the stratigraphy of the rocks.
SG: What characteristics distinguish an index fossil from a typical fossil?
Some fossil species are widespread but survived only short period of time Index fossils (guide fossils) - can be used to associate strata with a particular time interval
What's absolute time?
Specific age of a unit in years
What's the principle of fossil succession?
Specific fossil assemblages succeed one another in the rock record in a definite order (evolution); relies on the idea that species have changed through time; once a fossil disappears, it will not appear higher in the strata
What are the extinction recovery stages?
Survival: adaptation and reproduction Diversification: ecological vacuum with open ecological niches = greater opportunity for expression of genetic variations = rapid radiation and an increase in diversity Stabilization: increased competition reduces the rate of evolution and a new ecosystem is created
What's closure temperature?
Temperature below which there is no diffusion of isotopes, parent or daughter, out of a mineral When minerals in igneous rocks crystallize below their closure temperatures, isotopes are trapped in their crystal structures
How are new divisions usually determined?
The Anthropocene Working Group (AWG), a subcommittee on Quaternary Geology, evaluating the issue for the International Commission on Stratigraphy (ICS)
What's radioactive decay?
The change in which unstable parent isotopes turn into stable daughter isotopes
What's the relationship between parent and daughter isotopes?
The decay curves for parent and daughter isotopes are inverse of each other but only if we assume no other parent or daughter isotopes were lost or added.
What's an unconformity?
The interface between two rock units of different ages that represents missing time due to erosion
What's a half life?
The time it takes for half of the present parent material to decay to the daughter product at any given time. Half life is different for various isotopic systems but is always constant within a specific system.
What represents missing time?
Unconformities
What's a disconformity?
Weathering and erosion from rising and falling sea level Only sedimentary Missing time represented by the surface between parallel bedded sediments Difficult to recognize, fossil evidence is often required Can be marked by channels
How is the geologic time scale structured?
Youngest at the top; oldest at the bottom (geologists think about time vertically, not horizontally)
