Geologic Time and Relative Dating

Review Geologic Time: begins when the Earth began 4.6 billion years ago Includes: Precambrian Time (4.6 billion years ago to 570 million years ago) Paleozoic Era (570 MYA to 225 MYA) Mesozoic Era (225 MYA to 65 MYA) Cenozoic Era (65 MYA to present) = TODAY!

Relative Dating Relative Dating: placing rocks (containing fossils) in their proper SEQUENCE OF FORMATION; compare one rock to another rock by examining their placement in the Earth; compare 1 st, 2 nd, 3 rd, etc. The layer on top is the youngest (unless it has been turned over) The layer below is older (unless it has been turned over)

Fossil Record Shows evidence for continental drift Shows evidence for mass extinction Shows evidence for evolution (slight changes in morphology = the way something looks) Fossils can be dated by looking at the layer of rock they are in compared to other layers = RELATIVE DATING

Mass Extinctions Mass Extinction: A large number of species on Earth are eliminated due to a drastic change in the environment or catastrophic event occurs Considers a MASS EXTINCTION when at least 50% of all of the species die out Each Era ended with a mass extinction (sort of) as well as many other extinctions throughout Earths history Scientists predict there will be a 6 th mass extinction to end this era…

Absolute Dating and the Fossil Record

Absolute Age Absolute Age: actual age of the rock Radioactive Decay: isotopes that emit particles and energy at a constant rate from their nuclei Change to new isotope of same element or new element all together Natural Clocks = measure how much original isotope is present compared to how much new isotope is present

Example - Uranium Example: Uranium emits 2 protons and 2 neutrons = RADIOACTIVE DECAY Uranium – parent element (unstable) Lead – daughter element (stable) Half-Life: time required for half of any given amount of radioactive material to decay into the daughter elements U-238 to decay into Pb- 206 is 4.5 billion years Example: 10 g of U- 238, after 4.5 billion years = 5 g of U- 238 and 5 g of Pb-206 After another 4.5 billion years – 2.5g of U-238 and 7.5 g of Pb- 206

Carbon Dating Carbon Dating: more recent samples = carbon – 14 decays to nitrogen – 14 Usually C-12 and C-14 are constant, but after death, C-12 and C-14 are not taken in, so amount of C-14 remaining breaks down to N-14 Half-life of C-14 is 5,730 years – find age of organic material (early humans, wood, bones, shells) Ages up to 70,000 years old

Fossils Preserve only the hard parts usually Burgess Shale Traces of Organisms Tracks, footprints, burrows Imprints of leaves or fish which shows the surface features; shells leave molds of empty cavities; sand or mud fills a mold producing a cast (replica of original) Waste material = castings, eating habits of animals Stones in digestive system (dinos)