3The Rock Record Geological Time Scale Dividing the Earth’s history into sections based on the fossils and rock evidence.
4How do we know?Principle of UNIFORMITARIANISM means that the natural processes in place today have been occurring since Earth formed.Processes in the past happened according to the same laws as processes do today.
5Dividing It Up Eon Era Period Epoch Longest time unit measured in BILLIONS of years.Eranext longest measured in HUNDREDS of MILLIONS of yearsPeriodmeasured in TENS of MILLIONS of yearsEpochmeasured in MILLIONS of years
6Historical Events Bacteria Form Trilobites are abundant, First fish appear, Appalachians begin to formFirst Land Plants and Insects appearReptiles appearAppalachians finish forming, glaciers retreat
7Important Events Pangaea breaks up, Mammals appear Dinosaurs dominate, mountain building in Western U.S.Flowering plants appear, Rockies form, Dinosaurs become extinctAlps and Himalayan Mountains form, mammals dominate0.01 million years ago – Homo Sapiens evolves.
9Geological Principles Original horizontalitysedimentary rocks were formed in HORIZONTAL LAYERSSuperpositionoldest rock layers are on the BOTTOMCross-cutting relationshipsIntrusions or Faults that cut across a layer are YOUNGER than the layer they cut through.Unconformitymissing layer of eroded rock
11Bending and Breaking Layers Rock layers are disturbed by geological processesFOLDING: Compression forces BEND the layersTILTING: Lifting forces lift one end of a layer of rockFAULTING: Shear forces shift some layers of the rock
18Absolute-Age DatingIn addition to learning about the relative age, scientists also have methods of determining the absolute-age.Absolute age = The rock is 2 million years oldRelative age = The rock is older than the other rock.
19Radiometric DatingScientists can use UNSTABLE atoms to determine the age of some rocks and fossils.Unstable atoms decay (release energy) at a predictable rate over time.The atoms change as they decay.Comparing the number of unchanged atoms (parent) and changed atoms (daughter), the age can be determinedThe rate these atoms decay is called the half-life.
20Radioactive Atoms and Their Use Carbon-14 is used for organic material less than 75,000 years old.The half-life of Carbon-14 is 5,730 yearsCarbon-14 has 6 protons and 8 neutrons and it decays to Nitrogen-14 with 7 protons and 7 neutronsOrganisms continually take in new carbon while they are alive but stop when they die, so the carbon-14 “clock” doesn’t start until after death.
22Other Radiometric Dating Uranium-235 – half-life of 0.7 billion yearsDecays to Lead-207Uranium-238 – half-life of 4.6 billion yearsDecays to Lead-208Potassium-40 – half-life of 1.3 billion yearsDecays to Argon-40These decay processes also have intermediate steps so we can use them to date rocks as young as 50,000 years to the oldest rocks in our solar system.
23DendrochronologyWe can examine the annual growth of trees (tree rings) to determine an absolute age of tree.Using tree rings to determine age is called dendrochronology.The width of the rings also can tell us if the climate was wet or dry and if any major forest fires occurred during the life of the tree.
25Other Age IndicatorsWe can also determine age by looking for indicators of climate changeVarves are bands of deposited sediments in sedimentary rocks. Thick bands mean more sediment was carried – so more water.Key beds are layers of distinct sediments formed by unique events – such as asteroid impacts
26Fossil Record Fossil – evidence or remains of once living organisms If you know the type of organism, it can tell you about the environmentIf you find a fossil shell in the desert, it was probably a marine environment when the fossil formed.If you have actual genetic material, you can use the DNA record and carbon dating to determine age.
27How do fossils form? Fossils usually only form in sedimentary rock. Sediments have to accumulate over the organism in order to preserve it.volcanic eruption, mudslide, tree sap or slow sediment accumulation on the bottom of a lakeMost fossils are marine in origin, or are land animals that fell into a water environment.This is because sediments easily accumulate in water environments (lakes, streams, oceans).
28Fossil RecordTrace fossils – fossils left by animal or plant ACTIVITY (footprints, worm burrows)Mold – Rock formed around an organism that later decays out, leaving a empty space in the shape of the organismCast – Molds that get filled up with later rockPermineralization – minerals from ground water seep into the organism and slowly fill in the pores in the animalOriginal Preservation – remains that have not changed since the organisms death (often frozen!)
29CAST FOSSIL This ammonite cast was discovered in the United Kingdom. PetrifiedFossilsMOLD FOSSILThis mold, or imprint, is of an extinct mollusk called an ammonite.CAST FOSSIL This ammonite cast was discovered in the United Kingdom.TraceFossilsPreservedRemainsCarbon Films
30Index FossilIndex fossils are remains of organisms that can be used to relate rock layers that spread over a large geographic area.Index fossils must beAbundantSpread out geographicallyLived only for a limited time frameIf you know when that organism was alive, you can determine when the rock formed.