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 that are missing; a gap in the rock record

- most often caused by erosion

•Principle of Inclusions – fragments rock contained within a body of rock must be older than the “host” rock–Ex. Conglomerate

•Principle of Faunal Successions – specific fossils follow one another in a specific order–Ex. Dinosaur fossils in both Montana and China

Other laws of relative dating:

Determining Absolute Age of Rocks

radioactive decay: over time, radioactive elements release a proton(s) to make a new, lighter, more stable element.

Major isotopes used for Radiometric Dating:

U 238 Pb 206U 235 Pb 207Th 232 Pb 208

Radioactive elements decay at CONSTANT rates

half-life: the time it takes for ½ of the atoms of a radioactive element to decay

ParentDaughte

r Half-Life

U – 238 Pb – 206 4.5 billion years

U – 235 Pb – 207 713 million years

Th – 232 Pb – 208 14.1 billion years

Major isotopes used for Radiometric/Absolute Dating:

C-14 dating: used only to date things that were once alive

- half-life is only 5800 years; C-14 decays into N-14- can date more recent remains (up to about 50,000 years)

Other dating techniques:

• Dendrochronology: counting rings on trees–Rings look different in times of drought and other extreme climate conditions

• Varve chronology – looking at glacial sediments in lake beds –helps us find weather patterns for studying global warming

What is a fossil?

fossil: the remains or traces of organisms that lived long ago

Formation of a dinosaur fossil:

• Fossils can form in various ways.

• Typically, the body material is replaced by minerals.

Mold & Cast Formation Process

1.Organism becomes encased in sediment that is compressing to form a rock.

2.Water gradually dissolves organism.

3.This leaves a hole in the rock shaped like the organism – a mold

4.Water carries minerals that recrystallize in the mold making a cast

Replacement of Minerals:

1.Water partially or completely dissolves an organism, depositing minerals (like quartz) in its place.

2.Minerals are actually harder than the original bone.

trace fossil: evidence of life other than the

remains of plants or animals

ex. footprints, tracks, burrows

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

The Geologic Timetable

era: a long time segment defined by dominant life

forms

Eras are broken down into segments called periods.

period: a subdivision of an era

epoch: a subdivision of a period

The Earth is 4.6 billion years old

• How do we know?– Oldest “Earth” rocks found are

about 3.5 billion years old– Moon rocks (no plate tectonics/no

recycling of rock) taken during the lunar landing have been dated at 4.53 billion years old

– Meteorites (remnants of our early solar system) have been dated at 4.6 billion years

PRECAMBRIAN TIME

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

•many rocks eroded significantly

•main life form was cyanobacteria (photosynthetic bacteria)

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

PALEOZOIC ERA“The Age of

Invertebrates”

•From 545 mya to 245 mya (about 300 million years ago)

1. Cambrian 2. Ordovician3. Silurian4. Devonian – Age of Fishes

5. Carboniferous – Age of Amphibians

6. Permian – largest mass of extinction of recorded life

Divided into 6 periods

•In the US, the carboniferous period is divided into the –Upper Carboniferous or Pennsylvanian

–Lower Carboniferous or Mississippian

•warm, shallow seas•Pennsylvania was underwater

•North America was at the equator

Marine life forms: •trilobites - relative of horseshoe crab•brachiopods - look like clams•crinoids - relative of starfish

trilobite

brachiopod

crinoid

•“Firsts”: land plants, fish, amphibians, reptiles and insects

•Appalachians form•much of the limestone, coal and schist in PA formed during the Paleozoic Era

THE MESOZOIC ERA(“Age of Reptiles”)

•From 245 mya to 66 mya•“Firsts”: mammals, birds and angiosperms (flowering plants)

•Dinosaurs evolve and later become extinct

•Pangaea breaks up

•Rocky Mountains form

Divided into three periods:

- Triassic small reptiles

-Jurassic dinosaurs flourish

- 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 (100,000 yrs ago)

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