Geologic Time Scale & Fossil Dating

75 fill-in-the-blank flashcards covering the geologic time scale, characteristics of each eon, Earth’s formation through accretion, fossil evidence, and carbon-14 dating.

Scientists use the __ to refer to large spans of Earth’s history.

geologic time scale

Earth’s history is divided first into four major __.

eons

Within the geologic hierarchy, eons are divided into __.

eras

Eras are further subdivided into __.

periods

Periods are finally broken down into __.

epochs

The earliest eon of Earth is called the __ Eon.

Hadean

The Hadean Eon lasted from 4,540 million years ago to __ million years ago.

4,000

During the Hadean Eon, Earth and its __ were formed.

Moon

The second eon is known as the __ Eon.

Archean

The Archean Eon ended about __ million years ago.

2,500

__ life forms first appeared during the Archean Eon.

Single-celled

Some early __ formed during the Archean Eon.

continents

The third eon is the __ Eon.

Proterozoic

The Proterozoic Eon ended around __ million years ago.

541

__ life forms emerged during the Proterozoic Eon.

Multicellular

The first plants and animals appeared later in the __ Eon.

Proterozoic

The current eon is the __ Eon.

Phanerozoic

The Phanerozoic Eon began approximately __ million years ago.

541

The __ Eon is the shortest eon so far.

Phanerozoic

Complex life has __ during the Phanerozoic Eon.

flourished

Around 300 million years ago, the supercontinent __ formed.

Pangaea

Before the Hadean, Earth’s layers formed through __.

accretion

__ is the process in which small particles collide and combine to form larger bodies.

Accretion

As Earth grew, it drew more __ into its gravitational field.

cosmic dust

The attraction of cosmic dust to Earth still continues __.

today

Scientists believe all life on Earth shares a __ ancestor.

common

Petrified remains of past organisms are called __.

fossils

Studying a rock layer’s composition and fossils helps scientists estimate its __.

age

Plant fossils occurring below animal fossils indicate that __ formed first.

plants

For relatively young specimens, scientists use __ dating.

carbon

Carbon dating is reliable only for organic matter younger than about __ years.

50,000

Carbon-14 is an __ isotope of carbon.

unstable

Carbon-14 has a half-life of about __ years.

5,000

To determine a fossil’s age, scientists compare its Carbon-14 content to that of __ tissue.

living

Older fossils generally represent more __ organisms.

basic

Similar body structures in fossils and living species suggest a recent __ ancestor.

common

Greater similarity in __ indicates organisms share a more recent common ancestor.

DNA

Because older material is deeper underground, scientists can infer the __ sequence of rock layers.

order

Earth formed from debris orbiting the __ during the Hadean Eon.

Sun

The geologic time scale spans from Earth’s formation to the __ day.

present

Understanding Earth’s formation has involved contributions from all branches of __ science.

natural

As Earth attracted more matter, it began forming __ layers.

rock

Deeper sediment layers were deposited __ than the layers above them.

earlier

Earth is estimated to have formed about __ million years ago.

4,540

The Moon was formed during the __ Eon.

Hadean

Some continental crust stabilized during the __ Eon.

Archean

During accretion, smaller particles collide and __ to make larger ones.

combine

As an object grows during accretion, its __ field becomes stronger.

gravitational

The four eons together represent the entire __ history of Earth.

geologic

The order Hadean, Archean, Proterozoic, Phanerozoic is a __ sequence.

chronological

During the Phanerozoic Eon, Pangaea eventually __ into separate continents.

broke apart

Evidence that plants evolved first is provided when plant fossils are found __ animal fossils.

below

Carbon dating is sometimes called __ dating.

radiocarbon

Because Carbon-14 is unstable, it gradually __ over time.

decays

A half-life measures the time required for half of a radioactive isotope to __.

decay

Using rock strata, scientists establish the __ order of historical events.

relative

Higher rock layers typically contain more __ fossils than lower layers.

complex

Earth’s internal layers began forming __ the Hadean Eon.

before

The constant pull of cosmic dust shows Earth is still gradually __ in mass.

growing

Fossil dating helps scientists assemble a general __ of life on Earth.

timeline

Comparing __ structures helps scientists trace evolutionary relationships.

body

The abbreviation “Ma” stands for “million __ ago.”

years

In the geologic hierarchy, __ come directly after eons.

eras

Periods are __ than eras but larger than epochs.

shorter

Epochs are the __ divisions mentioned in the geologic time scale.

smallest

The Archean–Proterozoic boundary is set at __ million years ago.

2,500

The Proterozoic–Phanerozoic boundary is set at __ million years ago.

541

Branches of __ science all contribute to understanding Earth’s formation.

natural

Fossils found in higher strata are generally __ than those below them.

younger

The term “petrified” means organic material has been replaced by __.

minerals

Because of its short half-life, Carbon-14 is useful only for dating relatively __ fossils.

recent

Carbon dating cannot be used on samples older than about __ thousand years.

fifty

Carbon-14 dating estimates the time since an organism __.

died

Accretion not only increased Earth’s size but also led to the formation of distinct __ such as core and mantle.

layers

The four major eons are Hadean, Archean, Proterozoic, and __.

Phanerozoic