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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.
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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