Activity 1a: Earth’s Mass Extinctions
Unit C: Earth’s History
Learning Target:
1a)
I can distinguish between mass extinction and background extinctions and describe
their occurrence in geologic history using data.
EARTH’S MASS EXTINCTIONS
Mass extinction occurs when 40 % or more of the living plant and animal species become extinct within
a short period. Mass extinctions occur on a world-wide scale, and there have been several mass
extinctions in the history of the Earth. In fact, scientists believe, that based on fossil records, about 99 %
of species that ever lived are now extinct.
The most recent mass extinction, the Cretaceous extinction event, took place 65 million years ago,
when nearly all dinosaur species were destroyed. In the past 540 million years, there have been five
major mass extinctions. These were at the end of the following periods: Triassic (about 208 million years
ago), Permian (about 245 million years ago), Devonian (about 360 million years ago) and the Ordovician
(about 438 million years ago).
Mass extinctions may occur for any of the following reasons:
 Climate change - When the climate changes, many habitats are altered or destroyed, causing
the death of many plants and animals. Climate change may be caused by volcanic eruptions,
changes in the Earth’s orbit, imbalances in the atmospheric gases and variations in solar output.
 Volcanic eruptions - These may be caused by movement of the continents and land masses.
Some of the larger volcanic eruptions lasted many years, pouring out lava that piled up
kilometers deep, killing everything it covered. Volcanoes also release large amounts of gases
into the atmosphere, changing the atmospheric gas balance, and emitting greenhouse gases,
which in turn leads to global warming and climate change. Climate change then leads to
environmental change.
 Changes in the Earth’s orbit - The way in which the Earth moves on its axis.
 Meteor/Asteroid Impact – A large meteor or asteroid impacting the Earth.
 Imbalances in atmospheric gases - Oxygen and carbon dioxide - changes in the oxygen levels in
the ocean.
 Variations in solar output - How much radiation and heat the sun is sending out.
 Changes in sea levels - During an ice age, sea levels drop globally as seawater is taken up in the
ice sheets. This is also called a glacial period. As the sea level drops, many sea animals are forced
to follow the retreating sea. If species are not able to move to a new, favorable habitat when
their old one is destroyed, they are simply wiped out.
When conditions are warmer, this is known as an interglacial period, the ice sheets melt causing sea
levels to rise. A sudden rise in sea levels causes flooding on lowlands, drowning everything in those
areas. Sea level changes are caused by climate changes as well as the movement of the Earth’s crust.
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DATA Analyze the data below describing mass extinction events.
Extinction
Percent of
Species Lost
MYA
Affected Species
85%
440mya
Marine Species
83%
360mya
Marine Species
96%
250 mya
Trilobites, and other
marine species, also
land plants,
amphibians,
and reptiles
Triassic
(Mesozoic Era)
80%
215
mya
Mostly marine
Species, but some
land species
Cretaceous/Tertiary
aka KT
(Mesozoic Era)
76%
65
mya
All Species, especially
dinosaurs
70%
In the
next
30
years
Affects species both
in the water and on
land
Ordovician
(Paleozoic Era)
Devonian
(Paleozoic Era)
Permian
(Paleozoic Era)
Quaternary
(Cenozoic Era)
Reported by 1,100 United
Nations scientists in 2002
Global Outlook 3
Hypotheses
Glaciation, oxygen
depletion
Meteor impact,
volcanism, glaciations,
Volcanism, meteor
impact, sea level
change, global warming,
glaciation
Global cooling, meteor
impact, sea-level
change, oxygen
depletion
Meteor impact,
volcanism, continental
drift, mammal
competition
Human caused effects
such as pollution, over
population, global
warming, over-fishing
and deforestation
GRAPH the % extinction vs. time period. Make sure you label axes and title your graph.
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ANALYSIS QUESTIONS
1. Which hypothesis is the most common over all the mass extinction?
2. What appears to be one criterion that scientists use when defining the timing of geologic
periods?
3. Which mass extinction is the greatest?
4. Which mass extinction is the least?
5. Extinction normally occurs at what rate (how frequently)?
6. After a mass extinction, which organisms usually survive? Why?
7. Predict what would have happened if all of the early mammals living 66 million years ago had
died out in the Cretaceous mass extinction?
8. Many biologists propose that we are currently in a sixth major extinction. If this is true, this mass
extinction event may be the first caused by one of the Earth’s inhabitants—humans. What
human actions do you think may be the cause of the increased rate of extinction today?
9. What data could you collect that would support the idea that we are currently at the start of the
sixth mass extinction?
10. In light of your answers to the previous questions, what would you predict to be the long-term
result of a sixth mass extinction?
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