READING THE ROCK RECORD

relative time: events are in sequence, but no actual dates absolute time: identifies actual date of event

Most geologic work is done using relative time!

Determining Relative Age of Rocks law of superposition: oldest rock layers are on the bottom and youngest rock layers are on top IF the layers have not been disturbed.

law of crosscutting: any geologic feature is younger than anything else it cuts across

unconformity: layers of rock are missing; a gap in the rock record

What is a fossil? fossil: the remains or traces of organisms that lived long ago

index fossil: fossils that are found over a wide geographic area but lived over a narrow range of time - help to identify the relative age of the rock in which they occur

Determining Absolute Age of Rocks radioactive decay: radioactive elements release a proton(s) to make a new, lighter, more stable element.

example: uranium-235 (U-235) OR Carbon-14

Radioactive elements decay at CONSTANT rates half-life: the time it takes for ½ of the atoms of a radioactive element to decay

ex. half-life of U-235 is 704,000,000 years

C-14 dating: used only to date things that were once alive - half-life is only 5800 years - can date more recent remains (up to about 50,000 years)

The Geologic Timetable era: the longest segments of geologic time Eras are broken down into segments called periods.

period: a subdivision of an era epoch: a subdivision of a period

PRECAMBRIAN TIME From beginning (4.6 billion years ago) to 545 million years ago (mya) Makes up 90% of Earth’s history

main life form was cyanobacteria

cyanobacteria added large amounts of oxygen (through photosynthesis) to the atmosphere... made it possible for animals to evolve

PALEOZOIC ERA “The Age of Invertebrates” From 545 mya to 245 mya (about 300 million years ago) warm, shallow seas

“Firsts”: land plants, fish, amphibians, reptiles and insects Appalachians form divided into seven periods

North America was at the equator; climate was very hot and humid Pennsylvania was underwater

Most of the limestone, coal and schist found in Pennsylvania formed during the Paleozoic Era

THE MESOZOIC ERA (“Age of Reptiles”) From 245 mya to 66 mya “Firsts”: mammals, birds and flowering plants

Dinosaurs evolve and later become extinct Pangaea breaks up!!!! Rocky Mountains form

Divided into three periods: - Triassic  Small Reptiles - Jurassic  Age of Dinosaurs - Cretaceous  dinosaurs become extinct

The extinction of dinosaurs marks the end of the Mesozoic Era and the beginning of…

CENOZOIC ERA “Age of Mammals” From 66 mya to present Most complete geologic record Mammals and flowering plants abundant

Alps and Himalayas form Grand Canyon Forms Homo sapiens (humans) evolve

Divided into 2 periods and each period is further divided into epochs We are living in the Holocene Epoch of the Quaternary Period