How can rocks be correlated?And

Geologic Time Scale

Interpreting Earth’s History

Index FossilsIndex fossils are: A.commonly foundB.widely distributed C.limited in time span. They help in dating other fossils found in the same sedimentary layer.

Humans will one day make excellent index fossils

Correlation- DO NOT WRITECorrelation- or CO-RELATION (two

or more things related) is a critical tool in the reconstruction of Earth’s history.

Geologists try to match similar rock strata in different locations to see if they formed at the same time or under similar conditions

Who’s got the TIME?- DO NOT WRITE! In notes from yesterdayRELATIVE: order/sequence known, but not the actual date of occurrence.

ABSOLUTE: actual date known. If 2 dates are known, then the RATE OF CHANGE can be known- such as Mountain Building.

First Things First…or…“How’d that get there?” (DO NOT WRITE)

In the 17th C., Nicolas Steno made an important observation:

"Sediments are usually deposited in horizontal layers." This was called: “ORIGINAL HORIZONTALITY”

REVIEW: DO NOT WRITESUPERPOSITION: sedimentary

sequence will be OLDEST on BOTTOM (if undisturbed).

CROSS-CUTTING: igneous rock is younger than rocks it has intruded (cut across).

INCLUDED FRAGMENTS: pieces of rock found IN another rock must be OLDER (formed first).

Geologic Time(WRITE THIS)Using our best data, the Earth is about 4.6 billion years old

That’s 4,600,000,000 years!

How do we know the Earth’s age, and which rocks are older?No one was around to tell us the age.

Many experiments using radiation found in rocks tell us.

The “Law of Superposition” can also help tell which rocks are older than other rocks.

DIVIDING UP THE EARTH’S AGEESRT page 8 (ESRT is behind page 264

in RRB)Just as we have hours and minutes, the

Earth’s time has also been broken up into units called ERAS.

The ERAS do not have a set amount of time like the hour. Some ERAS, then, are longer than others.

ERAS are broken down farther into PERIODS.

THE CENOZOIC ERAWe are currently living in the

Cenozoic Era.The Cenozoic Era has lasted for

about 63 million years.There are only two periods in the

Cenozoic Era:1.The Tertiary Period2.The Quaternary Period - the current

period

THE MESOZOIC ERAKnown as the “Age of the Dinosaur”Started about 265 million years

ago.Contained three periods:1.The Triassic2.The Jurassic3.The Cretaceous

THE PALEOZOIC ERAKnown as the “Age of Fish”Started about 570 million years

ago.Lasted about 340 million yearsToward the end of this era coal

began to form.

The PRECAMBRIANKnown as “Age of Bacteria”During much of this time there was no

life on earth.The earth was still hot The atmosphere was not like it is now.The Precambrian Era lasted for the first

4 billion years of Earth’s history.

EARTH’S GEOLOGIC ERAS- DO NOT WRITEPRECAMBRIAN -rocks form, atmosphere

PALEOZOIC -fish, amphibian,coal forms, Appalachian Mt. peak

MESOZOIC -dinosaur, birds, flowers

CENOZOIC -horses, humans, last ice age