geology

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Geologic Time

Chapter 8 Lecture

Natalie Bursztyn Utah State University

Foundations of Earth Science Eighth Edition

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• Explain the principle of uniformitarianism. • Discuss how it differs from catastrophism.

Focus Questions 8.1

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• Mid-1600s – James Ussher stated Earth was only a few thousand years

old • Catastrophism

– Belief that Earth’s landscapes were formed by great catastrophes

– Prevalent during the 1600s and 1700s – Used to fit the rate of Earth’s processes to prevailing ideas

of Earth’s age

A Brief History of Geology

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• Late 1700s – James Hutton published Theory of the Earth

• Uniformitarianism – States that the physical, chemical, and biological laws that

operate today have also operated in the geologic past – To understand ancient rocks, we must understand

present-day processes – Geologic processes occur over extremely long periods of

time

A Brief History of Geology

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• Distinguish between numerical and relative dating. • Apply relative dating principles to determine a time

sequence of geologic events.

Focus Questions 8.2

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• Efforts to determine Earth’s age during the 1800s and 1900s were unreliable

• Today radiometric dating allows scientists to accurately determine numerical ages for rocks representing important events in Earth’s past

• Relative dates are determined by placing rocks in the proper sequence of formation

Creating a Timescale — Relative Dating Principles

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• Principle of superposition – Developed by Nicolas Steno in the mid-1600s – Studied sedimentary rock layers in Italy

• In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below – Also applies to lava flows and ash beds

Creating a Timescale — Relative Dating Principles

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Creating a Timescale — Relative Dating Principles

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• Principle of original horizontality – Layers of sediment are generally deposited in a horizontal

position – Rock layers that are flat have not been disturbed – Folded or inclined rocks must have been disrupted after

deposition

Creating a Timescale — Relative Dating Principles

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• Principle of lateral continuity – Sedimentary beds originate as continuous layers that

extend in all directions – Identical strata on two sides of a canyon were continuous

before the canyon was carved

Creating a Timescale — Relative Dating Principles

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• Principle of cross-cutting relationships – Geologic features that cut across rocks must form after the

rocks they cut through – Faults, igneous intrusions

Creating a Timescale — Relative Dating Principles

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• Inclusions – Fragments of one rock

unit enclosed within another

• Rock that contains inclusions is younger than the rock that provided the inclusions

Creating a Timescale — Relative Dating Principles

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• Layers of rock that have been deposited without interruption are called conformable – A complete set of conformable strata for all of Earth history

does not exist • Interrupting the deposition of sediment creates a

break in the rock record called an unconformity – Represents a period when deposition stopped, erosion

occurred, and then deposition resumed – Generally, uplift causes deposition to stop and subsidence

causes deposition to resume

Unconformities

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• Angular unconformity – Consists of tilted or folded sedimentary rocks overlain by

younger, more flat lying strata – Deformation occurred during the time that deposition

stopped

Unconformities

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Unconformities

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• Disconformity – A break in sedimentary

rock strata representing a time when erosion occurred

– Difficult to identify because layers are parallel

– Evidence of erosion (buried stream channel)

Unconformities

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• Nonconformity – Younger sedimentary rocks on top of older metamorphic or

intrusive igneous rocks – Imply period of uplift of deeply buried rocks

Unconformities

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Unconformities

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Applying Relative Dating Principles

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• Define fossil. • Discuss the conditions that favor the preservation of

organisms as fossils. • List and describe various fossil types.

Focus Questions 8.3

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• Fossils – The remains or traces of prehistoric life

• Paleontology – The scientific study of fossils

Fossils: Evidence of Past Life

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Fossils: Evidence of Past Life

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• Fossils can be preserved in many ways • Some remains may not be altered at all

– Teeth, bones, shells – Entire animals including flesh are not common

• Mammoths frozen in Arctic tundra • Mummified slots in a dry cave in Nevada

Types of Fossils

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• Permineralization – Mineral-rich groundwater permeates porous tissues – Petrified wood is permineralized with silica – “Petrified” means “turned to stone”

• Molds – Form where a structure buried in sediment was dissolved

by groundwater – Only the outside shape and surface marking is preserved;

no internal structure – If hollow spaces are filled with mineral matter, a cast is

formed

Types of Fossils

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• Carbonization – Remains are encased in sediment; pressure squeezes

out all liquid and gas until only a thin residue of carbon remains

– Effectively preserves leaves and delicate animals – Impressions may show considerable detail

• Amber – The hardened resin of ancient trees – Seals organisms from atmosphere and water – Preserves delicate organisms like insects

Types of Fossils

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• Trace Fossils – Indirect evidence of organisms

• Tracks • Burrows • Coprolites • Gastroliths

Types of Fossils

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Types of Fossils

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• Only a very small fraction of organisms are preserved as fossils

• Rapid burial and hard parts favor preservation – Soft parts are eaten or decomposed – Sediment protects organisms from destruction – Shells, bones, and teeth are much more common in

the fossil record • Fossil record is biased

Conditions Favoring Preservation

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• What types of organisms are most likely to be missing from, or are very rare, in the fossil record? How might this bias our picture of what life on Earth was like in the past? – Hint: Think about the organisms themselves, but also

their ecological context and depositional environment.

Conditions Favoring Preservation

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• Explain how rocks of similar age that are in different places can be matched up.

Focus Question 8.4

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• Correlation is matching up rocks of similar age in different regions – Reveals a more comprehensive picture of the

sedimentary rock record • Correlation by walking along outcropping edges is

possible within limited areas – Rock layers made of distinctive material can be

identified in other places – Widely separated areas require the use of fossils

Correlation of Rock Layers

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Correlation of Rock Layers

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• William Smith – 1700s to 1800s – Noted that rock formations in canals contained fossils

unlike the fossils in the beds above and below • Distinctive fossils can be used to identify and

correlate widely separated sedimentary strata • Principle of fossil succession

– Fossil organisms succeed one another in a definite and determinable order, therefore any time period can be recognized by its fossil content

– Fossils document the evolution of life through time

Correlation of Rock Layers

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• Index fossils – Geographically widespread and limited to a short span

of geologic time – Important for correlation

• Fossil assemblage – Can be used when there aren’t index fossils

• Fossils are useful environmental indicators

Correlation of Rock Layers

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Correlation of Rock Layers

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• Discuss three types of radioactive decay. • Explain how radioactive isotopes are used to

determine numerical dates.

Focus Questions 8.5

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• Each atom is made up of protons, neutrons, and electrons – Protons have a positive charge – Electrons have a negative charge – Neutrons are neutral

• Elements are identified by atomic number – Number of protons in the nucleus

Reviewing Basic Atomic Structure

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• 99.9% of an atom’s mass is in the nucleus – Electrons have almost no mass

• Number of protons + number of neutrons in an atom = the mass number

• An isotope has a different number of neutrons in the nucleus – Different mass number

Reviewing Basic Atomic Structure

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• Some isotopes have unstable nuclei with bonds that are not strong enough to hold the protons and neutrons together

• These nuclei will break apart (decay) in a process called radioactivity

Dating with Radioactivity

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• Three common types of radioactive decay: – Alpha particle = 2 protons and 2 neutrons

• Mass number reduced by 4 and atomic number decreased by 2

– Beta particle = electron from the neutron • Neutron is actually a proton and electron combined • Mass number remains the same, but atomic number

increases by 1 – Electron capture

• Captured by the nucleus and combined with a proton to form a neutron

• Mass number remains the same, but atomic number decreases by 1

Dating with Radioactivity

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Dating with Radioactivity

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• Parent Isotope – Unstable radioactive

isotope • Daughter Product

– Isotope resulting from radioactive decay

Dating with Radioactivity

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• Radiometric dating – Reliable method of calculating ages of rocks – Rate of decay for many isotopes does not vary – Rate of decay has been precisely measured – Daughter product has been accumulating at a known

rate since rocks were formed

Dating with Radioactivity

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• Half-life – Time required for one-half of the nuclei in a sample to

decay – One half-life has transpired when quantities of parent

and daughter are equal (1:1 ratio) • If half-life of an isotope is known and

parentdaughter ratio can be measured, then age can be calculated.

Dating with Radioactivity

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Dating with Radioactivity

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• Five radioactive isotopes are important in geology: – Rubidium-87 – Uranium-238 – Uranium-235 – Thorium-232 – Potassium-40

• Only useful if the mineral remained in a closed system – No addition of loss of parent or daughter isotopes

Dating with Radioactivity

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Dating with Radioactivity

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• Radiometric dating methods have been used to determine the age of the oldest rocks on Earth – 3.5-billion-year-old rocks found on all continents – Oldest rocks: 4.28 billion years old (Quebec, Canada) – 3.7 to 3.8 billion years old in western Greenland – 3.5 to 3.7 billion years old in the Minnesota River Valley

and northern Michigan – 3.4 to 3.5 billion years old in southern Africa – 3.4 to 3.6 billion years in western Australia

Dating with Radioactivity

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• Radiocarbon dating – Using the carbon-14 isotope to date very recent events – Half-life of carbon-14 is only 5,730 years

• Only useful for dating events from historic past and very recent geologic history – Carbon-14 is present in small amounts in all organisms

Dating with Radioactivity

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• Distinguish among the four basic time units that make up the geologic time scale.

• Explain why the time scale is considered to be a dynamic tool.

Focus Questions 8.6

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• Geologic history divided into units of variable magnitude – Developed during the nineteenth century – Based on relative dating

• Eons represent the greatest span of time – Phanerozoic Eon began about 542 million years ago

• Eons divided into eras – Phanerozoic includes Paleozoic, Mesozoic, and

Cenozoic – Bounded by profound worldwide changes in life-forms

• Eras divided into periods • Periods divided into epochs

The Geologic Time Scale

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The Geologic Time Scale

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• Most detail in the geologic time scale begins at 542 million years ago

• 4 billion years before the Cambrian is known as the Precambrian – Divided into Archean and Proterozoic eons – Together are divided into seven eras – Represents 88% of geologic time

The Geologic Time Scale

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• Some “unofficial” terms are associated with the geologic time scale – Precambrian = eons and eras before the Phanerozoic – Hadean = earliest eon of Earth history (before the

oldest known rocks)

The Geologic Time Scale

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• Geologic time scale must be updated periodically to include changes in unit names and boundary age estimates – A few years ago, Cenozoic divided into Tertiary and

Quaternary periods – Today, former Tertiary is divided into Paleogene and

Neogene periods

The Geologic Time Scale

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• Explain how reliable numerical dates are determined for layers of sedimentary rock.

Focus Question 8.7

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• Rocks can only be radiometrically dated if all minerals formed at the same time – Works for igneous and metamorphic rocks – Sedimentary rocks contain particles of many ages

• Must be related to datable igneous masses

Determining Numerical Dates for Sedimentary Strata

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Determining Numerical Dates for Sedimentary Strata