2 FOSSILS Paleontology is the study of life that existed in prehistoric times A fossil is any evidence of earlier life preserved in rock. Shells Bones Petrified trees Footprints Impressions made by leaves Burrows made by worms
3 FORMATION OF FOSSILS Fossils may form in several ways: From original remains From replaced remains Being preserved in molds and casts Being preserved as trace fossils Being preserved in carbonaceous film
4 ORIGINAL REMAINS In rare cases, organisms may be preserved in their entirety. Frozen wooly mammoths in Siberia and Alaska Prehistoric insects captured in resin Usually, soft body parts decay and only the hard parts are preserved. Bones teeth
5 REPLACED REMAINS The soft parts of plants and animals have decayed and the hard parts have been replaced by minerals Circulating groundwater removes the original organic material and replaces it with minerals such as calcite, silica, and pyrite. This results in an exact copy of the original plant or animal. Ex. Petrified wood
6 MOLDS AND CASTS Organisms can be buried in mud or other sediments. As the sediments become rock, the hard body parts become fossils. If the fossil dissolves out of the rock, a hollow depression in the rock called a mold results. Minerals may then seep into the mold and fill it, forming a cast, or copy, of the original fossil.
7 TRACE FOSSILS Indirect evidence of life that is left behind without being actual remains of the organism or plant is called a trace fossil. Trails, footprints, bite marks, burrows
8 CARBONACEOUS FILMS Decomposing organisms may be exposed to intense heat and/or pressure. Their tissues will undergo chemical reactions that sometimes leave behind a very thin film of carbon that resembles a silhouette of the organism. This is called a carbonaceous film.
9 RELATIVE TIME Most often, scientists have no way of knowing the exact dates of geological events on Earth. Therefore, they often use a process called relative dating. Relative dating does not assign exact dates to events but places events in a sequence, thus saying when they occurred in relation to other events.
10 THREE RULES OF RELATIVE DATING The Principle of Superposition In an undisturbed sequence of sedimentary strata, the oldest rock layer will be at the bottom and the youngest layer will be on the top. The Principle of Cross-Cutting Relationships An igneous intrusion is always younger than the rock it has intruded or cut across. Embedded Fragments Rocks that are embedded in another rock must be older than the rocks in which they are embedded.
11 Example #1
12 Example #2
13 Example #3
14 Grand Canyon Cross Section 1. Oldest layer 2. Youngest layer 3. Oldest organism 4. Place the following actions in the correct order of formation. (1-oldest, 5- youngest) -Granite dike intrusion, -folding of gray shale, -deposition of tan sandstone, -erosion of tan sandstone, -deposition of red sandstone
15 GAPS IN RELATIVE TIME: UNCOMFORMITIES An unconformity indicates where layers of rock are missing in the strata sequence means evidence is missing Angular unconformity Younger, flat strata are deposited on top of older strata older strata could have been tilted during uplift Disconformity All layers are horizontal, but some are missing due to erosion before the deposition of new layers Nonconformity Occurs when sedimentary layers are deposited on top of igneous or metamorphic rock
16 ROCK LAYER CORRELATION Correlation is the matching of rock layers from one area to those in another area Several methods are used to correlate rock layers: Walking the outcrop outcrop is the part of the rock layer that can be seen at Earth s surface Matching rock characteristics see if the rocks have the same appearance, color, and composition Using index fossils--(next slide) Fossils as Environmental indicators the presence of certain fossils can indicate climate and/or sea level changes Matching key beds a single rock layer that is unique, easily recognizable, and widespread Stratigraphic matching when the sequence of three of more layers or strata is repeated in two different areas
17 INDEX FOSSILS Index fossils are the remains of animals that lived and died within a particular time segment of Earth s history. Remains are buried and animals become extinct, so those rock layers can become distinctly tied to that time period. Index fossils have four characteristics: The fossils are easily recognizable. The fossils are abundant. The fossils are widespread in occurrence. The fossils occur only in a few rock layers because the organisms only existed for a brief time period
18 ABSOLUTE TIME Absolute time identifies the actual dates of events. Historical methods for measuring absolute time: Estimating rates of erosion and sedimentation Not very accurate because rates are not constant Counting tree rings Still used today One ring roughly equals one year Have dated trees back to 2000 BC Counting varves A varve is any sediment that is deposited on a yearly cylce Used most accurately in glacial lakes Have dated sediments back years
19 RADIOACTIVITY Recall that isotopes are atoms of the same element with different numbers of neutrons. Many elements have radioactive isotopes. Radioactive isotopes are those that exhibit radioactive decay by emitting or capturing tiny particles in order to try and achieve stability. Whenever, particles are emitted or captured, it can change the atomic mass and even the atomic number (identity) of the atom.
20 HALF-LIFE Radioactive isotopes decay at a constant rate. This rate is not affected by changes in temperature or pressure, or by the passage of time The amount of time it takes for exactly one-half of the radioactive atoms in a sample to decay to a stable product is called the half-life. After two half-lives, 25% of the original sample will remain. ( ½ x ½ = ¼). After three half-lives, only 12.5 % of the original sample will remain. ( ½ x ½ x ½ = 1/8
21 RADIOMETRIC DATING Scientists measure the amounts of a parent and a daughter isotope within a rock or mineral and use the ratio to find the age of the rock Carbon-14 is used to date organic material. Uranium-lead, Rubidium-strontium, and potassium-argon are used to date rocks and minerals.
22 RADIOCARBON DATING Radioactive carbon-14 and stable carbon-12 both exist in the tissues of living organisms. After death, the carbon-14 begins to decay. Scientists use the ratio of carbon-12 to carbon-14 to determine how long ago the plant or animal died. Two serious limitations: Can only be used to date things that were once alive Carbon-14 s half-live is 5730 years (short amount of time), so it can only be accurately used to date items that are years old or younger.
23 URANIUM-LEAD DATING Radioactive Uranium-238 decays to form stable lead-206. Uranium-238 is 4.5 billion years, so it is possible to date the oldest rocks of Earth s crust using this isotope. Is naturally found in zircon, which is in some igneous rocks very rarely found in sedimentary or metamorphic rocks
24 RUBIDIUM-STRONTIUM DATING Rubidium-87 decays to form strontium-87. The half-life of Rubidium-87 is 47 billion years (over 10x the age of Earth) Is the best method for dating the oldest of Earth s rocks Is found in common minerals like feldspars and micas.
25 POTASSIUM-ARGON DATING Potassium-40 decays to form Argon-40. The half-life of potassium-40 is 1.3 billion years. Are found in micas, feldspars, and amphiboles. Can be used to date sedimentary and metamorphic rocks as well as igneous ones.
26 Geologic Time Dinosaurs Mesozoic Era
27 Title: Geologic Timeline The Earth is about 4.6 byo based on the absolute da5ng of rock samples. The Earth s history is broken down into Eons, eras, periods and epochs. Each 5me frame does not last an even amount of 5me and the division of 5me is based on geologic and biologic events or evolu5ons. Precambrian Eon Today, Cenozoic Era
28 Geologic Time Defined by changes in species or major geologic events. Time frames are NOT uniform (NOT evenly divided!) Earth is 4.6 BYO Fossil evidence shows life at 3.5 BYA Life evolved between BYA
29 Precambrian ERA 4, mya 87% of the Earth s History Seas formed Mountains grew Oxygen built up in the atmosphere Cyanobacteria Life : bacteria, jellyfish, algae (Prokaryotes and Eukaryotes) No life on land yet, only in seas!
30 Precambrian Life Ediacara Stromatalites!
31 Paleozoic ERA mya Age of Fishes Appalachian Mountains form Warm, shallow seas cover the majority of North America
32 Periods 1. Cambrian Period Explosion of life Trilobites (skeletons, hard parts evolved) 2. Ordovician Period corals, sponges, first fish Mass ex5nc5on- 57% of marine species (glacia5on) 3. Silurian Period first land plants Paleozoic Era
33 Periods 4. Devonian age of fishes (armor- plated fish and amphibians) lungfish, coelacanths Ferns (seeds!) mass ex5nc5on! 50% of marine species- (glacia5on) 5. Mississippian Crinoids (Invertebrates) Many amphibians Paleozoic ERA
34 Paleozoic Era 6. Pennsylvanian Period Insects (cockroach, cen5pede, dragonfly) first rep5les Coal deposits 7. Permian abundant rep5les Mass ex5nc5on- 95% of land and marine species (regression)
35 Mesozoic Era my Age of Rep5les Mass ex5nc5ons at the beginning and end of this era Mississippi River System forms Rocky Mountains Atlan5c Ocean
36 Mesozoic Era my Periods 1.Triassic Pangaea breaks up 2. Jurassic First birds 3. Cretaceous first flowering plants Life Forms of Mesozoic Era Dinosaurs, First Mammals, and Modern Fish Mass ex5nc5on Meteorite, volcanism Wipes out dinos
37 Cenozoic ERA 65mya- today Age of Mammals Himalayan Mountains Alps Grand Canyon Great Lakes Chesapeake Bay
38 Periods 1. Ter5ary Mastodons, Mammoths, Primates Early humans Homonids (walk upright) Neanderthals ex5nc5on (45% of species) 2. Quaternary Civiliza5on begins10,000 years ago with end of most recent ice age and inven5on of agriculture Cenozoic Era
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