Chapter 21 Fossils & the Rock Records

Produced by S. Koziol11-4-2014

21.1 The Geological Time ScaleObjectives • Describe the geological

time scale• Distinguish among the

following geological time scale divisions: eons, era, period, and epochs

Geological time scale

The geological time scale divides Earth’s history into units from its origin to the present.

Remember scale issues….

Geologic timeGeologic time in order from shortest to longest epoch, period, era, eon

Geologic time – divisions?

Periods are defined by the abundance or extinction of life-forms.

Geological time (continued)

On the geologic time scale, the smallest unit of time is called an epoch.

We are currently in the Holocene epoch of the Quaternary period.

Greater collection of fossils has been found and paleontologists have more detailed knowledge of the events that occurred during those times.

Geological Time – most recentThe most recent period is the Quaternary

The Anthropocene is an informal geologic chronological term for the proposed epoch that began when human activities had a significant global impact on the Earth's ecosystems.

Geological Time - longer

The Archean and Proterozoic are examples of eons.

Eras vs Periods

ErasEras are longer spans of time, measured in hundreds of millions to billions of years. They are defined by differences in life-forms found in rocks.

PeriodsPeriods are usually measured in terms of tens of millions of years to hundreds of millions of years. They are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited.

Both are units of time on the geologic time scale.

Geological Time Scale

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21.2 Relative-age Dating of Rocks

Objectives • Apply the principle for determining

relative age to interpret rock sequences.

• Describe an unconformity and how it is formed within the rock record.

Laws of Superposition

Student guided , remember the foam boards

Undisturbed rock sequenceThe oldest rock layer in an undisturbed rock sequence occurs at the bottom of the sequence.

Which book was laid down 1st?

Which rock layer was laid down 1st?

The principle of superposition

The principle of superposition states that, in an undisturbed sequence, the oldest rocks are at the bottom of the sequence and successive layers are younger than those below them.

Cross-cutting relationship

You can use the principle of cross-cutting relationships to infer that a fault or an intrusion is younger than the rock it cuts across.

CorrelationThe matching of rock layers from one geographic area with those of another area is known as correlation.

Uniformitarianism

The principle of uniformitarianism states that the processes occurring today have been occurring on Earth since it formed. However, the rate, intensity, and scale with which these processes occur have changed.

UnconformityWhen part of the rock record is destroyed, the erosional gap that forms is an unconformity.

Angular unconformity

The gap in the rock record that occurs between folded or uplifted rock layers and a sedimentary rock layer on top of them is called an angular unconformity.

NonconformityA buried erosional surface between a non-sedimentary rock and a sedimentary rock is called a nonconformity.

A nonconformity in the Wind River gorge of Wyoming.

21.3 Absolute-age Dating of RocksObjectives • Explain the several different methods used

by scientist to determine absolute age.• Describe how objects are dated by the use

of selected radioactive elements.• Explain how annual tree rings and glacial

varves are used to date geological events.

Dating techniquesRelative-age datingRelative-age dating places the

ages of rocks and the events that formed them in order, without exact dates. This is done by comparing one event with another or one rock with another rock.

Absolute-age datingIn contrast, absolute-age

dating determines the actual age of a rock, a fossil, or an object. This is done through radiometric dating, a process that determines the ratio of parent material to daughter product in a given sample of rock or fossil.

Half-life

The amount of time it takes for one-half of the original amount of an isotope to decay is known as its half-life.

Parent Daughter t1/2 Useful Range Type of Material

238U 206Pb 4.5 b.y

>10 million years

Igneous Rocks and Minerals

235U 207Pb 710 m.y

232Th 208Pb 14 b.y

40K 40Ar & 40Ca 1.3 b.y >10,000 years

87Rb 87Sr 47 b.y >10 million years

14C 14N 5,730 y 100 - 70,000 years Organic Material

Radiometric datingRadiometric dating is used to determine the absolute

age of a rock. This is done using the idea of half-life.

Key BedA key bed contains distinctive material that

geologists can easily recognize in the rock record and use as a time marker.

DendochronologyThe science of dendochronology uses the annual growth of tree

rings to date events and environmental changes.

PermineralizationIn the process of permineralization, pore spaces

within an organism’s shell are filled in with mineral substances.

Objectives

21.4 Remains of organisms in the rock records

• Define fossil.• Explain several methods by which fossils

can be preserved.• Describe the characteristics of an index

fossil.• Discuss how fossils can be used to

interpret Earth’s past physical and environmental history.

AmberFossil insects can be found imbedded in amber,

the hardened sap of prehistoric trees.

Mold

When the original parts of an organism in a sedimentary rock are weathered and eroded, a hollowed-out impression called a mold forms.

CastsNatural casts can be formed when minerals are

deposited within the mold.

FossilsMolds, casts, coprolites, and petrified wood are

all example of fossils

Original Preservation (rare)Mummified animals found in dry caves can be

examples of fossils with original preservation.Discovered in 1977, this six-to-eight-month-old woolly mammoth baby named Dima is pictured in situ near Kirgiljach River in northeast Siberia.

5,300 year old Iceman discovered in Austrian Alps, 1991

FossilsFossils are the remains or evidence of once-living plants or animals.

They provide clues about Earth’s past environmental conditions and evolutionary changes in organisms over time. They also help to correlate rock layers from one area to another.

Index fossilsGeologists use index fossils to correlate rock layers over large

geographic areas or to date a particular rock layer. To be useful, an index fossil must be easily recognized, abundant, and widely distributed geographically. It must also have lived over a short period of time.

Index fossils (continued)Index fossils are useful to geologists if the fossils

have lived over a short period of time

Evolution

The adaptation of life-forms to changes in the environment is known as evolution.

Evolution (continued)Evolution is the process by which organisms

adapt to changes in their environments.

End of the Precambrian

The end of the Precambrian is marked by the appearance of organisms with hard parts.

Remember this is speaking to the end of the pre-Cambrian