1.1 tiempo y geología

8/3/2019 1.1 Tiempo y geologa

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Time and Geology

Chapter 8


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Objectives

Know what Hutton and Lyell did

Know the Laws and Principles

Understand the Unconformities

Know index fossils and fossil assemblages

Understand correlation of fossils

Understand radiometric dating: half life, problems w/ dating,and parent and daughter isotopes.


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The Age of the Earth

Prior to the 19th century, accepted age of Earth based on religious

beliefs~6,000 years for Western culture (Biblical)Old beyond comprehension (Chinese/Hindu)

James Hutton Father of Geology went against current thinking

Uniformitarianismgeologic processes operating at present arethe same processes that operated in the past. UNIFORM?

Actualism fits better both slow and violent processes canoccur together

Charles Lyell (early 1800s) pushed Huttons theory and gainedwidespread attentionLyell also impacted Charles Darwin ideas of life science


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Determining Relative Age

Certain principals are used to order events

Stratigraphy layers of rocks four main principals Original Horizontality

Superposition

Lateral continuity

Cross-cutting relationships


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Relative vs. Numerical Age

After radioactivity was found, the clock helped geologistsdetermine ageRelative age- the order of events or objects, from first (oldest) tolast (youngest)

Determined by applying simple principles, including originalhorizontality, superposition, lateral continuity, cross-cuttingrelationships, inclusions, unconformities, and correlationof rock

units and fossilsNumerical age- the age of events or objects, expressed as anumber or numbers

Determined using radiometric dating(determining how muchradioactive decay of a specific element has occurred since a

rock formed or an event occurred)


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Relative Age Determination

Formations- bodies of rock ofconsiderable thickness with recognizablecharacteristics allowing them to bedistinguished from adjacent rock layers

Original horizontality- beds ofsediment deposited in water are initially

formed as horizontal or nearlyhorizontal layers

Contacts- surfaces separatingsuccessive rock layers (beds)


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Superposition- within an undisturbedsequence of sedimentary or volcanic rocks,layers get younger from bottom to topLateral continuity- original horizontallayer extends laterally until it tapers or thinsat its edges


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Cross-cutting relationships- a disruptedpattern is older than the cause of the

disruptionIntrusionsand faultsare younger than therocks they cut through

Baked contacts- contacts between igneousintrusions and surrounding rocks, where

surrounding rocks have experienced contactmetamorphismInclusions fragments of one rock unit thathave been enclosed w/in another.


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Unconformities

Law of Unconformities- a surface (orcontact) that represents a gap in the

geologic record that formed whereverlayers were not deposited for a time orlayers were removed by erosion.

Three Types -1. Disconformities

2. Angular unconformity3. Nonconformity


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Disconformity- an unconformity in which the contact representingmissing rock layers separates beds that are parallel to each other


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Angular unconformity- an unconformity in which the contactseparates overlying younger layers from eroded tilted or folder layers


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Nonconformity- an unconformity in which an erosion surface onplutonic or metamorphic rock has been covered by younger sedimentaryor volcanic rock


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Correlation

Correlation- determining the time-equivalency of rock unitsWithin a region, a continent, between continents

Physical continuity

Physically tracing a continuous exposure of a rock unitEasily done in Grand CanyonSimilarity of rock types

Assumes similar sequences of rocksformed at same timeCan be inaccurate if very common

rock types are involvedKey beds = very distinctive beds (volcanic ash)

Correlation by fossils

Fossil species succeed one another throughthe layers in a predictable order (faunal succession)Similar fossil assemblages(groups of different fossil species) usedPrincipal of Fossil Succession fossil organisms succeed one another ina definite and determinable order, and therefore any time period can berecognized by its fossil content.


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Fossil Correlations Continued

Index fossil used by geologists to correlate fossils foundanywhere in the world.

Index fossil characteristics must be very short lived

geologically widespread

and known to exist during a specific geologic period.

Fossil Assemblage several different fossils in a given rock layer.


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Geologic Time Scale

Standard geologic time scaleWorldwide relative time scale

Subdivides geologic time basedon fossil assemblagesDivided into eons, eras, periods,and epochs

Precambrian- vast amount of

time prior to the Paleozoic era; fewfossils preserved prior to massextinction (glaciation?)Paleozoic era-old life

appearance of complex life;

many fossils. Precambrianfossils didnt preserve due tolack of shells. These developedduring the Paleozoic era. Endedwith possible mass extinction ofglaciation?


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Geologic Time ScaleMesozoic era- "middle life"

Dinosaurs abundant on landPeriod ended by mass extinction due to

possible meteorite and/or volcaniceruptions

Cenozoic era- "new life"Mammals and birds abundantWe are currently in the Recent

(Holocene) Epochof the QuaternaryPeriodof the Cenozoic EraMost recent ice ages occurred duringthe Pleistocene Epochof the QuaternaryPeriod

Geologic Time ScaleP.7 in old book

Divided into four EonsHadean,Archean, Proterozoic,Phanerozoic

Precambrian (all time prior toPhanerozoic) represents 87% of

geologic time)


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Numerical Age Dating

Numerical dating- puts absolute values (e.g.,millions of years) on the ages of rocks and

geologic time periodsUses radioactive decayofunstable isotopesOnly possible since radioactivity wasdiscovered in 1896Radioactive isotopes decay in predictablemanner, giving a characteristic half-life(time

it takes for a given amount of radioactiveisotope to be reduced by half)


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RadioactivityEach atom has a nucleus: w/ protons, neutrons orbited by electrons.

Atomic number is the number of protons in the nucleusAtomic mass is the sum of protons and neutrons

the number of neutrons can vary = isotopes w/ different mass numbers

Forces that bind protons and neutrons together are strong, but someisotopes have weaker isotopes that are unstable. The unstable

nuclei break apart, or decay thru radioactivity.

Three types of decay are:1. Alpha particles 2 P & 2 N are emitted. A.M. lower by 4

238 U to 234 Th (A.N. 90)

2. Beta particles an electron is given off. A.N. +1 A.M. no change

234 Th to 234 Pa (A.N. 91)3. Electron capture an electron is captured. A.N.1 A.M. no change

40 K to 40Ar (A.N. 19)

These all decay at a fixed rate and do not vary under physical conditions.The radiometric clock is always reset when a rock is altered.


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Radioactivity

Unstable radioactive isotope is called the parent

The isotope resulting from the decay is the daughter product.

The time required for half of the nuclei in a sample to decayis called the Half-lifeTo determine the age of a object, it needs atoms ofa radiometric decay pair that originated when it formed.A ratio of 1:1 = one half-lifeA ratio of 1:3 = two half-lifeA ratio of 1:7 = three half-life

Errors can happen if the system is not a closed system


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Radioactivity

Problems with radiometric dating:

Sedimentary rocks rarely able to date, most grains in a rock are not the sameage as the rock, due to weathering differentiation. Unless the scientistscan relate them to igneous masses from volcanic ash beds.

Metamorphic rocks age of a particular mineral does not necessarily represent

the time when the rock initially formed.


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Age of the Earth

Numerical dating gives absolute agefor Earth of about 4.56 billion years

Oldest age obtained formeteorites, believed to havebeen unchanged since theformation of the solar systemEarth and rest of solar systemvery likely formed at this time

Geologic(deep) timeisvastA long human lifetime (100years) represents only about0.000002%of geologic time


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End


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Figure 8.1

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Figure 8.2

gure 8 3


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gure 8.3


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Figure 8.4


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Figure 8.5


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Figure 8.6


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Figure 8.7

Fi 8 8


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Figure 8.8


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Figure 8.9

Fi 8 10


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Figure 8.10


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Figure 8.11

Fi 8 12


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Figure 8.12


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Fi 8 14 d


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Figure 8.14a-d


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Figure 8.14e


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Figure 8.14f


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gure


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gure .


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Figure 8.17

Figure 8 18


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Figure 8.18


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Figure 8.19


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Subterraneanworld

Ancient

Life


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Figure 8.25

1.1 tiempo y geología

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