How can rocks be correlated? And Geologic Time Scale Interpreting Earth’s History

Index Fossils Index fossils are commonly found, widely distributed fossils that are 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 Correlation- 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? RELATIVE: 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 17 th C., Nicolas Steno made an important observation: "Sediments are usually deposited in horizontal layers." This was called: “ORIGINAL HORIZONTALITY”

REVIEW: DO NOT WRITE SUPERPOSITION: 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.

THE CENOZOIC ERA We 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 Period 2. The Quaternary Period - the current period

THE MESOZOIC ERA Known as the “Age of the Dinosaur” Started about 265 million years ago. Contained three periods: 1. The Triassic 2. The Jurassic 3. The Cretaceous

THE PALEOZOIC ERA Known as the “Age of Fish” Started about 570 million years ago. Lasted about 340 million years Toward the end of this era coal began to form.

The PRECAMBRIAN Known 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 PRECAMBRIAN-rocks form, atmosphere PALEOZOIC-fish, amphibian,coal forms, Appalachian Mt. peak MESOZOIC-dinosaur, birds, flowers CENOZOIC-horses, humans, last ice age