8-1 Notes - Geologic Time and Mass Extinctions

Chapter 8, Lesson 1

Geologic Time Scale• The changes that have occurred in Earth’s

history are recorded on a time line called the geologic time scale.

• Each division is on the time scale is determined by the presence and then absence of index fossils.

Geologic Time Scale• An index fossil is the remains of a species

that existed over vast regions of Earth for a short period of time.

• Special criteria are used to identify index fossils.

Geologic Time Scale

Geologic Time Scale• No two divisions of time have the same

number of years.

• The largest divisions are eons, then eras, periods, and epochs.

eons

eras eras

periods periods periods periods

epochs epochs epochs epochs epochsepochs epochsepochs

Geologic Time Scale• We begin with

“Precambrian Time,” which includes 3 eons:– Hadean Eon– Archean Eon– Proterozoic Eon (ends 544

MYA with mass extinction)

Geologic Time Scale• Next is the Phanerozoic

Eon, which is divided into three eras:

Geologic Time Scale– Paleozoic Era: (ancient

life) dominated by invertebrate (no internal skeleton) marine organisms

Geologic Time Scale– Mesozoic Era: (middle

life) dinosaurs and mammals lived on land

Geologic Time Scale– Cenozoic Era: (recent

life) modern mammals and humans evolved; continues today

Geologic Time Scale• The Cenozoic Era is

divided into 3 periods: – Paleogene Period– Neogene Period– Quaternary Period (now)

Geologic Time Scale• The Quaternary Period

began 1.8 million years ago and is divided into 2 epochs:– Pleistocene Epoch (ended

about 8,000 years ago)– Holocene Epoch (what we

are in now).

Mass Extinctions

• Mass extinction is the dying off of many species at one time.

• A greater abundance of fossils in one rock layer compared to other layers indicates a mass extinction.

Mass Extinctions

• Mass extinctions happen over a great span of time.

• 99% of all species are now extinct.

Mass Extinctions• A catastrophic event causes drastic change

in the numbers of organisms of species over a short period in geologic time.

• Types of events that can cause mass extinction include changes in climate, volcanic eruptions, and asteroid impacts.

Climate Change• There is evidence that some

mass extinctions were caused by sudden climate change.

• Species that cannot survive a change in climate become extinct.

• Climate change can be caused by volcanic eruptions or asteroid impacts, and can result in global warming or cooling.

Coral core samples

Climate Change• Greenhouse gases (such

as carbon dioxide) in the atmosphere may contribute to global warming, which is a global increase in atmospheric temperature.

• Global warming causes a decrease in oxygen levels in water, and an increase in sea levels.May have been the cause for the Devonian mass extinction

Climate Change• Global cooling could lower sea levels as

more water is frozen in glacial ice.

• Less water means fewer warm, shallow-water environments.May have initiated the Ordovician mass extinction

Volcanoes• Eruptions can be explosive or non-explosive.

– The dust, ash, lava, and gas emitted from volcanic eruptions can affect climate and organisms.

A volcanic eruption is one hypothesis proposed to explain the mass extinction at the end of the Cretaceous period.

Mount Redoubt, Alaska

Volcanoes• Non-explosive floods of molten basalt emit

the largest volume of matter of any eruptions.

Geologic evidence shows that large basalt flows occurred during the Permian and Cretaceous mass extinctions.

Volcanoes• The gases produced by basalt flows cause a

volcanic haze effect.

• Sulfur dioxide gas is released and acid clouds form, preventing the Sun’s warming, ultraviolet rays from reaching Earth.

Volcanoes• Next, global cooling occurs over months

to tens of years.

• Over the next tens to thousands of years, heat is trapped in the atmosphere, and global temperatures increase.

Asteroid Impact• Scientists believe a 10 km-wide (6.2 mi)

asteroid impact in Mexico may have contributed to the Cretaceous mass extinction.

• http://www.spaceweather.com/

Asteroid Impact• Scientists propose that this impact sent

enough dust and materials into the atmosphere to block sunlight.

• Global cooling after the impact turned to global warming as carbon dioxide was released from burning plants.

Asteroid Impact• Evidence supports a mass extinction at the

end of the Cretaceous period because of asteroids.

• But the impact did not cause all the extinctions.

• Many species became extinct before the impact, and the rate of extinction was increasing before the impact.

Which describes the divisions of the geologic time scale in order of longest to shortest units of time?

A eons, eras, periods, epochs

B eons, epochs, eras, periods

C epochs, eons, periods, eras

D epochs, eons, eras, periods

8.1 Geologic Time and Mass Extinctions

What is the likely immediate outcome of an explosive volcanic eruption that sends dust into the atmosphere?

A global warming

B global cooling

C basalt flows

D volcanic haze

8.1 Geologic Time and Mass Extinctions

What type of eruption emits the largest volume of matter?

A basalt flows

B explosive eruptions

C non-explosive eruptions

D volcanic haze

8.1 Geologic Time and Mass Extinctions

Which is not an effect of an asteroid impact?

A sunlight blocked by dust

B basalt flows causing a complex sequence of events

C carbon dioxide causing global warming

D the death of primary producers causing mass extinctions

Why have some paleontologists proposed that asteroid impacts did not cause all the mass extinctions at the end of the Cretaceous period?

A There is no evidence of an asteroid impact at the end of the Cretaceous period.

B Volcanic haze is a more likely cause.

C Many species were already extinct by the time of the impact.

D The fossil record does not support a mass extinction at the end of the Cretaceous period.