Earth History

The geologic time scale is divided into four large units known as

Eons

The oldest eon is

Hadean

The Phanerozoic Eon is divided into how many eras?

3

Why do we use a geologic time scale?

To measure how long Earth has existed, describe how Earth has changed over time, and recognize major events in Earth’s history.

How do scientists determine the number and length of geologic time units?

By major events in Earth’s formation, physical changes, and the emergence or extinction of organisms.

Approximately how long ago did Earth form?

4.6 billion years ago

How would you describe Earth 4 billion years ago?

Extremely hot, covered with molten rock, and had no oxygen in the atmosphere.

What were the earliest life forms on Earth?

Anaerobic microbes

What was one result of the Great Oxygenation Event?

Oxygen caused many anaerobic microbes to die out and allowed oxygen-using organisms to evolve.

How was the Heavy Bombardment important to Earth’s formation?

Frequent asteroid and comet impacts shaped Earth’s surface and may have delivered water.

When did the earliest life forms emerge on Earth?

3.8 billion years ago

What life forms introduced oxygen into Earth’s atmosphere?

Cyanobacteria

Which major life form appeared last on Earth?

Humans

How did life change from Earth’s first billion years to the last 500 million years?

Early life was simple and unicellular, while later life became complex and diverse with plants and animals.

What is a mass extinction?

The loss of many species in a short period of time.

What is NOT a possible cause of ancient mass extinctions during the Phanerozoic Eon?

Humans

What most likely caused the death of the dinosaurs?

An asteroid impact

Compare and contrast the five major mass extinctions

All were caused by natural events and wiped out at least 50% of species, but different organisms were affected in each event.

What is a fossil?

The preserved remains or evidence of a once-living organism.

Fossils are usually found in what type of rock?

Sedimentary rock

Fossils found in deeper rock layers are ______ than fossils in shallower layers

Older

How do fossils help us study the past?

They provide evidence about ancient organisms, environments, and changes in life over time.

What can we infer about fossils found in the same rock layer?

They are generally the same age and lived during the same time period.

What is a mold fossil?

An impression of a once-living organism.

A mold fossil forms when what happens?

An organism leaves an empty impression in rock after it decays or dissolves.

A cast fossil is a filled-in what?

Mold fossil

How does a cast fossil form?

Minerals fill a mold and harden into a rock replica of the organism.

What type of fossil gives the most information about a once-living organism?

True fossil

Why do petrified fossils provide the most information?

They preserve the actual structure of the organism.

How is a trace fossil different from other fossils?

It preserves evidence of activity like footprints or burrows instead of the organism itself.

What is an index fossil?

A fossil that helps scientists date unknown fossils.

What characteristics does an index fossil have?

It is widely distributed and existed for a short period of time.

The unknown fossil must have existed during what time period?

100 to 75 million years ago

Why is it important for index fossils to be widely distributed?

So scientists can match rock layers in different locations and determine their ages.

What is relative dating?

A method for determining the relative age of a rock layer or fossil.

What does the Law of Superposition state?

The youngest rock layers are closest to the surface while older layers are deeper.

An igneous intrusion is ______ than the rock it cuts through.

Younger

Why is an igneous intrusion younger than surrounding rock?

It forms after the surrounding rock already exists.

Where did erosion take place?

At the exposed surface where rock layers were removed before new layers formed.

What is radioactive or radiometric dating?

A method for determining the actual age of a rock or fossil.

What is another term for radioactive decay?

Break down

What is a half-life?

The time it takes for half of a radioactive sample to decay into a stable product.

Carbon-14 problem: After 11,400 years, how much remains?

1 gram remains.

Carbon-14 problem: How much decayed after 11,400 years?

3 grams decayed.

Potassium-40 problem: After 2 half-lives, 20 grams becomes how many grams?

5 grams

What does a tree ring represent?

The growth of a tree during one growing season.

Dark-colored tree rings represent growth during what season?

Summer

What is a limitation of studying tree rings?

Tree growth can be affected by soil, sunlight, and competition.

What does a wide tree ring indicate about climate?

A warm and wet growing season with high precipitation.

What does a narrow tree ring indicate about climate?

A cold and dry growing season with low precipitation.

Where do scientists obtain ice cores from?

Polar ice caps

What information can scientists obtain from ice cores?

Data about atmospheric conditions, temperature, and precipitation.

The thickness of an ice layer provides information about what?

Precipitation rate

Why is the melt layer important in ice cores?

It helps scientists estimate past temperatures and seasonal changes.

What relationship exists between carbon dioxide and relative temperature?

As carbon dioxide levels increase, global temperatures also increase.