READING THE ROCK RECORD
Mar 30, 2015
relative time: events are in sequence, but no actual dates
absolute time: identifies actual date of event
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.
•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.
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
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.
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
•cyanobacteria added large amounts of oxygen (through photosynthesis) to the atmosphere...•made it possible for other life (plants and animals) to evolve
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
Marine life forms: •trilobites - relative of horseshoe crab•brachiopods - look like clams•crinoids - relative of starfish
•“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)
Divided into three periods:
- Triassic small reptiles
-Jurassic dinosaurs flourish
- Cretaceous dinosaurs become extinct
CENOZOIC ERA“Age of Mammals”
•From 66 mya to present•Most complete geologic record•Mammals and flowering plants abundant
•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