The Changing EarthChapter 4:

Views of Earth’s Past

Section 4.1

Earth’s Past is Revealed in Rocks and Fossils

Rocks, fossils, and original remains give clues about the

past.• Fossils: traces or remains of living things

from long ago. • Examples: preserved bones, imprints in

rock, petrified wood, carbon films, original remains.

• They provide direct evidence of organisms

• They are found in places where conditions prevent normal decomposition, such as ice, amber, or tar.

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• Fossils form in several different ways. Molds and casts form in sedimentary rock.• Mold: visible impression of a decayed

organism.• Cast: forms when the mold later fills with

sediment that hardens into stone.

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Some fossils are not original remains, but rather impressions or traces made of rock. They provide indirect evidence of the organism (example: shoe print)

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• Petrified Wood is the stone fossil of a tree. It forms when minerals take the place of the tree’s cells.

• Carbon films are detailed prints of organism’s left behind by the carbon contained in the organism’s cells after the organisms decay.

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• Trace fossils are pieces of indirect evidence of an organism’s presence, such as preserved footprints, trails, animal holes, and feces.

Fossils and other natural evidence show changes in life and the environment.

• Tree rings: vary in width, according to a tree’s yearly growth, which is determined by environmental conditions such as amount of rainfall.• Scientists can create

an accurate history of weather patterns over time.

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• Ice cores: tubular samples of layers of snow and ice that built up over thousands of years.• By analyzing the air and dust trapped in an

ice core, scientists can determine past events such as volcanic eruptions and changes in climate.

Section 4.2

Rocks Provide a Timeline for Earth

Layers of sedimentary rocks show relative age.

• Relative age: the age in relation to other objects or events.

• Sedimentary rock layers indicate the order of life, oldest on the bottom.

• Igneous rock intrusions can be relatively dated using sedimentary layers.

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• Index fossils are fossils of organisms that:• Were common (abundant).• Lived in many different geographical

areas.• Existed only during specific spans of

time.• Index fossils are particularly useful for

estimating the age of rocks in which they are found.

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Radioactive dating can show absolute age.

• Absolute age is the actual age. It can be determined by radioactive dating, the process of measuring (in half lives) the rate of change of a radioactive element.

• A half-life is the length of time it takes for half of the atoms in a sample of radioactive element to change from an unstable form into another form.

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Common Radioactive Elements Used for Dating Rocks

Element Half-LifeDecay

ProductK-40Potassium

1.3 billion years Ar-40Argon

U-238Uranium

4.5 billion years Pb-206Lead

Th-232Thorium

14.1 billion years Pb-208Lead

Rb-87Rubidium

47.0 billion years Sr-87Strontium

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Scientists use the breakdown or radioactivity of atoms to determine relative ages of rocks.

Section 4.3

The Geologic Time Scale Shows Earth’s Past.

Earth is constantly changing.

• The theory of uniformitarianism states that: Earth is always changing, and the same forces of change at work today were at work in the past.

• Some changes occur rapidly: volcanic eruptions, earthquakes, etc.

• Some changes occur gradually: erosion, weathering, etc.

• James Hutton, the father of modern geology, proposed the theory in the 18th century.

Geologic time scale divides Earth’s history.

• Intervals are defined by major events or changes on Earth. The divisions include:• Eons: the largest unit of time. • Eras: subunit of an eon.• Periods: subunits of eras.• Epochs: subdivisions of periods.

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