Science 6 Grade Chapter 8 Lesson 2

Front 1

Do you wonder how scientists can know a rock’s age?

Back 1

They can tell how old some rocks are by measuring the amounts of radioactive atoms in them

Front 2

Because sedimentary rocks form from older rocks, it is difficult to date them by using the radioactive atom method.

Back 2

Instead, scientists often use the fossils found in these rocks as an indication of their age

Front 3

Sedimentary rocks often form layers. In a group of undisturbed rock layers, the oldest layer will be on the bottom because it was deposited first, and the youngest layer will be on the top.

Back 3

Scientists use this fact to help them determine the relative age of each layer.

Front 4

______ lived before the dinosaurs walked the earth.

Back 4

trilobites

Front 5

In a stack of rocks representing all life on earth, _________ would be found on the bottom.

Back 5

trilobites

Front 6

____________ were common in the oceans at the time of the dinosaurs.

Back 6

ammonites

Front 7

________ are likely to be found in limestone and chalk.

Back 7

ammonites

Front 8

In rock layers representing the whole history of life on earth _________ would be close to the middle.

Back 8

ammonites

Front 9

Younger fossils are found in the upper layers of undisturbed rock

Back 9

mastodons

Front 10

_______ are so young that they are not found in rock but in sediment that has not yet become rock.

Back 10

mastodons

Front 11

_________ are fossils of organisms that lived during a relatively short time span.

Back 11

Index fossils

Front 12

Fossils that are common and are found in many places on Earth make the _______ index fossils.

Back 12

best

Front 13

A fossil that is rare or is found in only one place is ______ useful as an index.

Back 13

not

Front 14

Knowing when an index fossil organism lived tells scientists the age of the rock in which it is found.

Back 14

the age of the rock in which it is found

Front 15

A fossil that __________ is not useful as an index.

Back 15

is rare or is found in only one place

Front 16

__________ provide clues about how Earth’s surface has changed over time.

Back 16

fossils

Front 17

Fossils of temperate plants and animals have been found in Antarctica. This shows that Antarctica was once much warmer than it is today.

Back 17

Antarctica was once much warmer than it is today.

Front 18

____________ process explains that the continents are slowly moving.

Back 18

continental drift