Geology Chapter 9: Geologic time

What is meant by the correlation of rock units? Identify different criteria (e.g. lithology, sedimentary structures, color, fossils, etc.) that can be used to correlate rocks from one exposure to another.

Correlation involves matching a particular rock layer (formation) in one exposure with its counterpart at a different locality. Lithology is rock type Fossils allow for correlation of rocks from different places

Correlation

Correlation involves matching a particular rock layer (formation) in one exposure with its counterpart at a different locality. Correlation can determine the original geographic extent of different rocks prior to removal or burial. Geologic maps can then be constructed

Numerical (absolute) dating

Determine actual ages of rocks and events Utilizes isotopes and radiometric dating

Relative dating

Determine sequence or order that geologic events and processes occurred No actual dates attached, just the order Principles of relative dating: Law of superposition, Principle of original horizontality, Principle of cross-cutting relationships, Inclusions, Unconformities

Which Eon contains most of earth's fossils? Is more known about the Precambrian or Phanerozoic Eon? Why?

Eon: Cenozoic More known about Phanerozoic - relatively little is known about Precambrian, maybe because its fossil record is poor.

Era

Eons subdivide into eras; eras divide into periods

Cenozoic Era

Era of the Phanerozoic Eon that began about 66 million years ago, known as the "Age of Mammals" or "recent life"

Mesozoic Era

Era of the Phanerozoic Eon, also called "middle life" (245-144 million years ago); rise of mammals and dinosaurs; the rise of birds; extinction of dinosaurs, rise of flowering plants

Period

Eras are divided into periods, which are divided into epochs.

Given a parent/daughter ratio (percent, proportion or number of atoms) and a half-life, be able to calculate the age of the rock or mineral.

Example: assume that the half-life of a hypothetical unstable isotope is 1 million years old, and the parent/daughter ratio is 1:15. This ratio indicates that four half-lives have passed and thus, the sample must be 4 million years old.

Fossil

Fossils are traces or remains of ancient life now preserved in rock. Fossils are generally found in sediment or sedimentary rock. Paleontology is the study of fossils. Geologically, fossils are important because they: • Aid in interpretation of the geologic past • Serve as important time indicators • Allow for correlation of rocks from different places

What are fossils? Describe at least five different types of fossils.

Fossils are traces or remains of prehistoric life now preserved in rock. They are generally found in sediment or sedimentary rock. Types of fossils include: molds and casts (molds reflect only the shape and surface marking of the organism and if those hollow spaces where the internal structure used to be are filled with mineral matter, casts are created); carbonization and impressions (particularly effective at preserving leaves and delicate animals); amber (one way in which some insects have been preserved—the hardened resin of ancient trees); and trace fossils (tracks, burrows, coprolites, gastroliths).

What do unconformities represent in terms of rock units and geologic time? How do unconformities form? Be able to sketch and describe the following: Angular unconformity, disconformity and nonconformity.

Unconformities represent erosional surfaces or intervals of missing strata within a rock sequence. In lengthier terms, it represents a long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed. Unconformities are important features because they represent significant geologic events in Earth history. Moreover, they help geologists identify what intervals of time are not represented by strata and thus are missing from the geologic record. Go to textbook for examples of the unconformities.

Unconformity

Unconformities represent old erosional surfaces or intervals of missing strata within a rock sequence • Three types of unconformities: ̶ Angular unconformity ̶ Nonconformity ̶ Disconformity

Geologic Time Scale

a "calendar" of Earth history: Subdivides geologic history into units, originally created using relative dating. Absolute dates assigned later after the discovery of radioactivity in 1896 subdivided into various units of time: Eons (largest divisions), Eras, Periods, Epochs (smallest divisions)

What are inclusions? If a sedimentary rock layer contains inclusions of an adjacent igneous rock, what can we say about the relative ages of these rock units?

Inclusions are fragments of a rock unit enclosed in another rock unit. In the scenario described, we can say that the rock with the inclusions is younger than the adjacent igneous rock from which the inclusions came from.

What are isotopes? Distinguish between a parent and daughter isotope. Which one is more stable? Can a parent isotope decay into a daughter isotope of the same element?

Isotopes are each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei. Parent is the original unstable radioactive isotope. Daughter is the new, more stable isotope formed as a result of radioactive decay of parent. Daughter is more stable. Over time, unstable parent isotopes decay to more stable daughter isotopes at a rate measured in half lives.

Isotopic dating of sedimentary rocks?

Isotopic dating of a sedimentary rock is possible if it contains authigenic minerals or material (e.g. glauconite, K-feldspar, organic matter) that formed during burial and diagenesis in the sedimentary basin

The radioactive isotopes uranium-238, uranium-235, and thorium-232 eventually decay to different, stable, daughter isotopes of ___________.

Lead

Precambrian

Name for the time in earths early history that accounts for ninety percent of earth's time, but only cellular organisms lived.

Inclusions

One of the principles of relative dating. Inclusions are fragments of one rock unit that have been enclosed within another. The rock containing inclusions is YOUNGER than the inclusions.

Epoch

Periods are divided into epochs, the smallest subsection of geologic time.

Phanerozoic Eon

Phanerozoic ("visible life"), most recent Eon, began about 542 million yrs ago Eras of the Phanerozoic Eon: - Cenozoic ("recent life") - Mesozoic ("middle life") - Paleozoic ("ancient life")

Where are we on the Geologic Time Scale? Where on the time scale is the origin of earth represented and what is the age (in millions of years)?

Phanerozoic = us precambrian = beginning of earth, 4.6B years

Define each of the following: Principle of Superposition, Principle of Original Horizontality, and Principle of Cross-cutting Relationships. How are each of these principles applied to relative dating?

Principle of Superposition: in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below. Principle of Original Horizontality: layers of sediment are generally deposited in a horizontal position. If resulting sedimentary rock is folded or faulted, then the deformation occurred after formation. Principle of Crosscutting Relationships: if a fault or a magma intrudes rocks, the fault/magma are younger than the rocks. Faulting and intrusion occurred after the sediments were deposited and lithified

Superposition, principle of

Principle of relative dating. In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below

Principle of original horizontality

Principle of relative dating. States that sediments are initially deposited as horizontal layers; resulting sedimentary rocks should also be horizontally layered. If sedimentary rocks are folded or faulted, then the deformation occurred after formation

Carbon-14 dating

Process of estimating age of once living material by measuring the amount of radioactive isotope of carbon present in material. Carbon-14 is the radioactive isotope of carbon. The source of natural carbon-14 is cosmic ray collisions and neutron-capture involving atmospheric nitrogen

What is radioactivity? What types of atoms undergo radioactive decay?

Radioactivity is the spontaneous decay of certain unstable atomic nuclei.

Distinguish between relative and absolute (numerical) dating.

Relative dating is used to know which object or item is older in comparison to the other one. Sequences dates of time. Absolute dating tells about the exact age of the artifact or the site using the methods like carbon dating. Absolute dating is also known as numerical dating as it comes up with the exact numerical age of the item (in millions of yrs). In relative dating, techniques like stratigraphy and biostratigraphy are used to know which object is older. In absolute dating, methods like radiometric dating, carbon dating, and trapped electron method are used.

Hadean Eon

The earliest period in Earth's history (4600 - 4000 m.y. ago), the "hellish era," when the planet's formation was still ongoing and was unsuited to life.

How does metamorphism affect the isotopic date of a mineral? If the whole-rock age of a metamorphic rock is older than its mineral age as determined by radiometric dating, then what might each of these ages represent (see my power point slides)?

The isotopic clock can be reset by metamorphism when temperatures become high enough for daughter isotopes to escape the minerals where they accumulated. If these ages are different as described above, the mineral age would give the age of the last metamorphic event (implying that a metamorphic event reset the isotopic clock). Whole rock age gives the age of initial crystallization.

Eon

The largest division of geologic time, subdivide into eras. Eons constitute the largest divisions: Phanerozoic ("visible life"), most recent Eon, began about 541 million years ago and continues to present Proterozoic (2500 - 541 m.y. ago) Archean (4000 - 2500 m.y. ago) Hadean - oldest (4600 - 4000 m.y. ago)

Proterozoic Eon

The most recent of the Precambrian eons, (2500 - 541 m.y. ago)

Radioactivity

The process in which some substances spontaneously emit radiation. Nuclei spontaneously break apart, or decay. Importance of radiometric dating: - Radiometric dating is a complex procedure that requires precise measurement - Rocks from several localities have been dated at more than 3 billion years - Confirms the idea that geologic time is immense

principle of fossil succession

fossil organisms succeed one another over time in a definite and determinable order. Therefore any time period can be recognized by its fossil content

Conformable

layers of rock that have been deposited essentially without interruption. No place on Earth has a complete set of conformable strata.

Given an outline of the time scale, be able to place the names of eons and eras in the appropriate spaces. Know in general what divisions represent periods and epochs, but do not memorize the names.

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Given a diagram showing sedimentary strata, faults, igneous intrusions, inclusions, and unconformities, rank each of these rock units and geologic events (e.g. faulting, erosion, etc.) in order of oldest to youngest.

ok... textbook? videos?

Paleozoic Era

the oldest era of the Phanerozoic eon - immediately after Precambrian; organisms developed hard parts also called "ancient life"

Radiometric dating

the process of measuring the absolute age (in millions of years) of geologic material by measuring the concentrations of radioactive isotopes and their decay products If the following are known: - Half life of the isotope being measured - Parent/daughter ratio in the sample Then the age of the sample can be calculated

Paleontology

the study of fossils

Half-life

the time required for half of the nuclei in a sample to decay is called the half-life of the isotope. Half-life is a common way of expressing the rate of radioactive disintegration. When the quantities of parent and daughter are equal (ratio 1:1), we know that one half-life has transpired. 2 half-lives: 1:3 ratio 3 half-lives: 1:7 ratio 4 half-lives: 1:15 ratio

What is the age (in millions of years) assigned to the time line marking the boundary between the Precambrian and Phanerozoic Eons? What percentage of earth history falls into the Precambrian?

542M years 88%

Disconformity

A disconformity is an old erosional surface where layered sedimentary rocks occurring above and below the unconformity are parallel. Disconformities forms by deposition of horizontal layers; weathering erodes surface; eroded surface covered by later sediment

What is a formation? What are the different ways (criteria) by which rocks can be grouped into formations?

A formation is the most basic local unit of stratigraphy. A particular rock unit is grouped into a formation based on a distinctive appearance and/or other characteristics (e.g., lithology, color, sedimentary structures, certain dispositional patters, textural properties, mineral composition, organic remains, outcrop character, thickness.... it just depends on what the geologists have to work with as to how they define the formation of a rock)

Do different parent-daughter isotopes (e.g. uranium 238 - lead 206, potassium 40 - argon 40, etc.) have different half-lives?

After one half-life, half of the radioactive atoms have decayed to the stable daughter product. The number of radioactive parent atoms = stable daughter product. After a 2nd half-life, half the remaining radioactive atoms will decay, compared to the beginning there are only 1/4 as many. After a 3rd half-life, the fraction of the original radioactive atoms is 1/8 as many.

Describe the principles behind carbon-14 dating. What types of materials can be dated using carbon-14? Is carbon-14 useful for dating most geological samples? Why or why not?

To date relatively recent events, carbon-14 dating is used. This dating can only be used to date material less than 80,000 years old, because there is hardly any carbon-14 left after that amount of time. By comparing proportions of carbon-14 and carbon-12 (the stable and most common isotope of carbon) in a sample, radiocarbon dates can be determined. Carbon-14 can only be used to date organic materials, such as wood, charcoal, bones, flesh, and cloth. Only useful for dating the last small fraction of geologic time.

Index fossil

these fossils are widespread geographically, but limited to a short span of geologic time. Thus, their presence provides an important method of matching rocks of the same age.

What goes up the chimney down but not down the chimney up?

umbrella

What types of rocks can be effectively dated using isotopic dating techniques? Why is isotopic dating not useful for most sedimentary rocks? How can the approximate ages of these sedimentary rocks be determined?

- Igneous/ metamorphic rocks - Clastic sedimentary rocks do not give meaningful ages b/c minerals were derived from other sources. Clastic minerals in detrital sedimentary rocks only provide the age(s) of their parent source rocks. These ages do not reflect the time of sedimentation - Sedimentary rocks can be bracketed by dating underlying/overlying igneous and metamorphic rocks, interlaced volcanic ash deposits and cross-cutting igneous dikes. Watch video 9C for this.

Nonconformity

A nonconformity is an old erosional surface separating younger sedimentary rocks above from older igneous or metamorphic rocks below. Nonconformities imply crustal movements.

Cross-cutting relationship

A principle or law of geology stating that the geologic feature which cuts another is the younger of the two features. It is a relative dating technique in geology.

Angular unconformity

An angular unconformity is an old erosional surface separating tilted or folded sedimentary rocks below from flat-lying sedimentary rocks above. This unconformity represents that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.

Archean Eon

Eon extending from the time that life originated 4000 - 2500 m.y. ago). The oldest eon of Earth's history.

What is the Principle of Fossil Succession? How is this principle useful for correlating rocks from one exposure to another? What are the criteria for a good index fossil?

The Principle of Fossil Succession says that fossil organisms succeed one another over time in a definite and determinable order—therefore any time-period can be determined by its fossil content. While correlation over short distances is often achieved by noting the position of a bed in a sequence of strata, this does not work when trying to correlate between widely separated areas or between continents—when geologists must rely on fossils to correlate. Index fossils are widespread geographically and are limited to a short span of geologic time, so their presence provides an important method of matching rocks of the same age. A good index fossil is one with four characteristics: it is distinctive, widespread, abundant and limited in geologic time.

Which of the following describes radioactive decay by beta particle emission?

The atomic number of the daughter isotope is one more than the parent; the mass numbers are the same.

Formation

The basic local unit of stratigraphy is the formation A particular rock unit is grouped into a formation based on a distinctive appearance and/or other characteristics: - Lithology (rock type) - Color - Sedimentary structures - Certain depositional patterns Lithostratigraphy (formations)