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Presentation on theme: "READING THE ROCK RECORD"— Presentation transcript:


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

3 Most geologic work is done using relative time!

4 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.

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

6 unconformity: layers of rock that are missing; a gap in the rock record - most often caused by erosion


8 Other laws of relative dating:
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

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

10 Major isotopes used for Radiometric Dating:
U 238  Pb 206 U 235  Pb 207 Th 232  Pb 208

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

12 Major isotopes used for Radiometric/Absolute Dating:
Parent Daughter 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

13 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)

14 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

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

16 Formation of a dinosaur fossil:
Fossils can form in various ways. Typically, the body material is replaced by minerals.

17 Mold & Cast Formation Process
Organism becomes encased in sediment that is compressing to form a rock. Water gradually dissolves organism.

18 This leaves a hole in the rock shaped like the organism – a mold
Water carries minerals that recrystallize in the mold making a cast

19 Replacement of Minerals:
Water partially or completely dissolves an organism, depositing minerals (like quartz) in its place. Minerals are actually harder than the original bone.

20 trace fossil: evidence of life other than the remains of plants or animals ex. footprints, tracks, burrows

21 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

22 The Geologic Timetable
era: a long time segment defined by dominant life forms Eras are broken down into segments called periods.

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

24 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

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

26 many rocks eroded significantly
main life form was cyanobacteria (photosynthetic bacteria)

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

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

29 Divided into 6 periods Cambrian Ordovician Silurian
Devonian – Age of Fishes Carboniferous – Age of Amphibians Permian – largest mass of extinction of recorded life

30 In the US, the carboniferous period is divided into the
Upper Carboniferous or Pennsylvanian Lower Carboniferous or Mississippian

31 warm, shallow seas Pennsylvania was underwater North America was at the equator

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

33 trilobite brachiopod crinoid

34 “Firsts”: land plants, fish, amphibians, reptiles and insects
Appalachians form much of the limestone, coal and schist in PA formed during the Paleozoic Era

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

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

37 Divided into three periods: - Triassic  small reptiles
Jurassic  dinosaurs flourish - Cretaceous  dinosaurs become extinct

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

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

40 Alps and Himalayas form
Grand Canyon Forms Homo sapiens (humans) evolve (100,000 yrs ago)

41 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



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