Chapter 13: Clues to Chapter 13: Clues to Earth’s PastEarth’s Past

Paleontologists study fossils and Paleontologists study fossils and reconstruct the appearance of animals.reconstruct the appearance of animals.

Fossils: remains, imprints, or traces Fossils: remains, imprints, or traces of prehistoric organismsof prehistoric organisms

Fossils can form if the organism is Fossils can form if the organism is quickly buried by sediments.quickly buried by sediments.

Organisms with hard parts are more Organisms with hard parts are more likely to become fossils than likely to become fossils than organisms with soft parts.organisms with soft parts.

Types of preservationTypes of preservation

Fossils in which spaces inside are filled Fossils in which spaces inside are filled with minerals from groundwater are with minerals from groundwater are

called permineralized remains.called permineralized remains.

Carbon film results when a thin film or Carbon film results when a thin film or carbon residue forms a silhouette of the carbon residue forms a silhouette of the

original organism; carbonized plant original organism; carbonized plant material becomes coal.material becomes coal.

Mold: cavity in rock left when the Mold: cavity in rock left when the hard parts of an organism decay.hard parts of an organism decay.

If sediments wash into a mold, they If sediments wash into a mold, they can form a cast of the original can form a cast of the original

organismorganism

Occasionally original remains are Occasionally original remains are preserved in a material such as amber, preserved in a material such as amber,

ice, or tar.ice, or tar.

Trace fossils: evidence of an organism’s Trace fossils: evidence of an organism’s activitiesactivities

Trace fossils can be footprints left in Trace fossils can be footprints left in mud or sand that became stone.mud or sand that became stone.

Trace fossils can be burrows made Trace fossils can be burrows made by worms and other animals.by worms and other animals.

Index fossils: abundant, geographically Index fossils: abundant, geographically widespread organisms that existed for widespread organisms that existed for

relatively short periods of time.relatively short periods of time.

Fossils can reveal information about Fossils can reveal information about past land forms and climate.past land forms and climate.

Section 2: Relative Ages of RockSection 2: Relative Ages of Rock

Principle of superposition: process of Principle of superposition: process of reading undisturbed rock layers.reading undisturbed rock layers.

Oldest rocks in the bottom layerOldest rocks in the bottom layer Younger rocks in the top layerYounger rocks in the top layer

How old something is in comparison How old something is in comparison with something else is its “relative with something else is its “relative

age.”age.”

The age of undisturbed rocks can be The age of undisturbed rocks can be determined by examining layer determined by examining layer

sequences.sequences.

The age of disturbed rocks may have The age of disturbed rocks may have to be determined by fossils or other to be determined by fossils or other

clues.clues.

Unconformities: drastic changes in Unconformities: drastic changes in rock layersrock layers

““Angular unconformity”: rock layers are tilted, and Angular unconformity”: rock layers are tilted, and younger sediment layers are deposited horizontally on top younger sediment layers are deposited horizontally on top

of the eroded and tilted layersof the eroded and tilted layers..

DISCONFORMITY: 2 SEPARATE REGIONS DISCONFORMITY: 2 SEPARATE REGIONS OF HORIZONTAL ROCK LAYERS OF HORIZONTAL ROCK LAYERS

INTERUPTED BY UPLIFTING, EROSION FOR INTERUPTED BY UPLIFTING, EROSION FOR A PERIOD OF TIME THEN MORE A PERIOD OF TIME THEN MORE

DEPOSITIONDEPOSITION

Nonconformity: sedimentary rock forms over Nonconformity: sedimentary rock forms over eroded “metamorphic or igneous rock.”eroded “metamorphic or igneous rock.”

The same rock layers can be found in different The same rock layers can be found in different locations; fossils can be used to correlate those locations; fossils can be used to correlate those

rock layersrock layers..

Section 3: Absolute Ages of RockSection 3: Absolute Ages of Rock

Absolute age: age, in years, of a rock Absolute age: age, in years, of a rock or other object; determined by or other object; determined by

properties of atoms.properties of atoms.

Unstable isotopes break down into Unstable isotopes break down into other isotopes and particles in the other isotopes and particles in the

process of radioactive decay.process of radioactive decay.

The time it takes for half the atoms The time it takes for half the atoms in an isotope to decay is the isotope’s in an isotope to decay is the isotope’s

half-life.half-life.

Calculating the absolute age of a Calculating the absolute age of a rock using the ratio of parent isotope rock using the ratio of parent isotope to daughter product and the half-life to daughter product and the half-life

of the parent is called radiometric of the parent is called radiometric dating.dating.

Potassium-argon dating is used to Potassium-argon dating is used to date ancient rocks millions of years date ancient rocks millions of years

old.old.

Carbon-14 dating is used to date Carbon-14 dating is used to date bones, wood, and charcoal up to bones, wood, and charcoal up to

75,000 years old.75,000 years old.

Earth is estimated to be about 4.5 billion Earth is estimated to be about 4.5 billion years old; the oldest known rocks are years old; the oldest known rocks are

about 3.96 billion years old.about 3.96 billion years old.

Uniformitatianism: Earth processes Uniformitatianism: Earth processes occurring today are similar to those occurring today are similar to those

that occurred in the past.that occurred in the past.