2Earth’s History Planet Earth is approximately 4.5 X 109 years old Rocks of the crust provide clues to Earth’s pastBy analyzing these clues we can infer events from the past
3Earth’s History Principle of Uniformitarianism Major assumption in geologyEvents in the past occurred the same way that they are occurring today. Examples Include:Weathering/erosionDepositionVolcanismPlate tectonics
4Geologic TimePlate motions and mountain building events can be placed on the geologic time scale
5Geologic TimeGeologists have divided Earth’s history into time units based on the the fossil record
6Geologic Time A study of the fossil record shows A great variety of plants, animals, and simpler life forms have lived on Earth in the pastThat life forms have evolved through timeMost life forms of the geologic past have become extinct
12Law of SuperpositionSediments are laid down underwater in horizontal layers and form sedimentary rocks
13Law of SuperpositionIn a series of sedimentary rocks the bottom layer is the oldest and the top layer is the youngestLower layers must be in place before younger rocks can be deposited on top of themException: when something occurs to overturn layers
24Taconic Unconformity Upper Silurian Carbonates April 17Upper Silurian CarbonatesTilted Ordovician Shales and SandstonesWe stopped along the side of the road where engineers had cut into the rock formations to make the road. The unconformity was clearly visible showing banded carbonate rock overlying tilted shales and sandstones.unconformityTaconic UnconformityAllen
25Practice: what happened here? April 17Practice: what happened here?Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the next slide.Allen
311. Which layers are the same? 2. Which layer is older E or F?3. What is correct sequence of rock layers from oldest to youngest?4. An unconformity is represented by the interface between which 2 layers?
32Which fossil might be found in Devonian rock layers?
33Volcanic Ash FallsCan also be used to correlate rock layers over a large areaAsh is a good indicator because:The ash from one explosion has distinct characteristicsash can be deposited around the globeThe event occurs at one, geologically brief, time
34April 17FossilsNaturally occurring preserved remains or impressions of living thingsGenerally only hard parts get preservedBones, teeth, shellsHard parts are replaced by naturally occurring mineralsAllen
41Fossils Why don’t fossils exist in most igneous or metamorphic rock? April 17FossilsGenerally found in sedimentary rock layersWhy don’t fossils exist in most igneous or metamorphic rock?Allen
42April 17FossilsThe living organism that made the fossil lived during the time the rock layer was formingi.e. when the sediment was being depositedAllen
43Fossils Fossils can provide information about ancient environments Marine fossils indicate a marine environment, wooly mammoths indicate a cold environment, etc…
44Fossils A study of the fossil record shows: April 17FossilsA study of the fossil record shows:A great variety of plants, animals and simpler life forms have lived on Earth in the pastThat life forms have evolved through timeMost life forms of the geologic past have become extinctAllen
45Index Fossils Index Fossils--used to date rock layers (strata) Fossils from creatures that existed for a geologically short period of timeEx. less than 2.0 x 107Fossils from creatures that had a wide geographic distribution
46Which letter would make a good index fossil? Location ALocation BLocation CRock layer 1WW ZRock layer 2W ZYZRock layer 3W XXX ZWhich letter would make a good index fossil?