Presentation on theme: "Uniformitarianism Laws of nature DONT change with time Past events explained & estimated by modern processes The Present is the Key to the Past."— Presentation transcript:
Uniformitarianism Laws of nature DONT change with time Past events explained & estimated by modern processes The Present is the Key to the Past
sedimentary rocks form in horizontal layers parallel to Earths surface and will remain that way unless disturbed Original Horizontality CLAY SILT CLAY SAND SILT
Horizontal Layers of the Grand Canyon
Relative Time – determination of the age of a rock or geologic event by comparing with other rocks/geologic events He is Can you infer who is older?
3 Ways to Determine Relative Time………? Coming Right Up!!
In undisturbed sedimentary rock or lava flows: Law of Superposition Top = youngest layer In undisturbed sedimentary rock or lava flows Bottom = oldest layer
Law of Cross-Cutting Relationships Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts
Example of Cross- Cutting Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens
Examples of Cross-Cutting Example of Cross-cutting Extrusion - igneous rock that forms when lava flows on Earths surface and hardens
Example of Cross-cutting Inclusion - pieces of older rock trapped within younger rock
Examples of Cross-Cutting Faults, joints, tilts, and veins also follow the law of cross--cutting relationships
Law of Included Fragments Sediments are older than the cement and rock they are part of The pebble is older than the conglomerate rock it is found in.
However, there are exceptions to these laws…….
Exceptions to Law of Superposition Rock layers can be overturned, older layers pushed on top of younger layers THEREFORE, geologists use these clues to find the original position of rock layers…
Exceptions to Law of Superposition Graded Bedding - coarse, heavy particles - bottom layer
Exceptions to Law of Superposition Ripple-Marks - tops of ripple marks point
Exceptions to Law of Superposition Cross-Beds - curved at bottom layer, cut off at the top
Unconformity Angular unconformity Gaps or missing layers in the rock record due to erosion
Formation of an Unconformity
Picture of Unconformity
Absolute Time – finding an exact date for rocks or geologic events in YBP (years before present) Born 2004 or 1 YBP Born 1922 or 83 YBP
How do geologists measure Absolute Time? 1.Radioactive Decay 2. Biological Clocks - Tree rings, coral growth rings, glacial lake layers of sediment
Before you measure ABSOLUTE TIME you must understand the following terms…..
ISOTOPES - alternate forms of an element - some are RADIOACTIVE
Radioactive isotope - an isotope that undergoes radioactive decay - used to determine absolute age Examples: Uranium 238 and Carbon 14 Radioactivity release of high-energy particles from unstable atoms, maybe cancer-causing
Radioactive Decay radioactive isotopes give off radioactive particles until they become stable isotopes (new elements) Example: Uranium Radioactive Isotope Radioactive Decay Lead-206 Stable Isotope
Amount of time it takes 1/2 the atoms in a given sample to go through radioactive decay is called its Half - Life Lets look at the half - life of Uranium 238
Since the Half - Life of Every Isotope is …. Isotope is …. 1. Unique 2. Always the same time 3. Unaffected by: Mass, Volume, Temperature & Pressure
We can use it to find: THETHEABSOLUTEABSOLUTEAGEAGEOFOFROCKS!!!ROCKS!!!THETHEABSOLUTEABSOLUTEAGEAGEOFOFROCKS!!!ROCKS!!!
Uranium 238 Lead 206 Half-Lives of Special Isotopes 4.5 x 10 9 years (4,500,000,000 years) Carbon 14 Nitrogen x 10 3 years (5,700 years)
Unknown Radioactive Isotope half - life = 3000 years sample size = 200 g Lets complete the table below. Half LifeRemaining Unknown Radioactive Isotope (g) Number of Years ,000 15,
DECAY TIME (YEARS) MASS OF UNKOWN (G) NOW LETS GRAPH THE HALF-LIFE OF THE UNKNOWN 0 1 Half - Life 2 H - L Half - Lives
Carbon 14 half - life = 5,700 years sample size = 800 g Complete the following table. Half LifeRemaining Radioactive C 14 (g) Number of Years ,400 17,100 22,800 28, ,700
DECAY TIME (YEARS) MASS OF C 14 (G) 5,70011,40017,10022,80028,500 NOW LETS GRAPH THE HALF-LIFE OF CARBON Half - Lives 1 Half - Life
Absolute Time vs. Relative Time Most geologists use relative time because its - less expensive - easier to do
Absolute Dating & Geologic History
Fossil s -the remains of plants and animals that lived in the past - mainly formed in sedimentary rock
Fossils Form in Several Ways Form in several ways 1. Original remains - unchanged remains of a plant or animal Ex. Dinosaur bones/teeth 2. Replaced remains - soft parts of original animal replaced by minerals Ex. Petrified wood
Fossils Form in Several Ways 3. Mold - fossilized shell or bone dissolves and leaves a hollow depression in a rock Ex. Ferns, leaves, or fish 4. Cast - new mineral material fills a mold Ex. Shellfish
Fossils form in Several Ways 5. Trace fossils - fossil evidence of animal movement Ex. Trails, footprints, burrows & borings
Evolution - process of change that produces new life forms over time - fossils provide evidence of evolution
Natural Selection -theory of evolution -best adapted organisms will survive in large numbers and pass on these adaptations to their offspring
Index Fossils - special fossils that give the relative age of the rocks that contain them Index fossils must: 1. Be recognizable 2. Widespread 3. Exist for a short time
Key Bed - single rock layer that is recognizable, widespread, and exists for a short time
Correlation - matching rock layers from one area to another This can be done in several ways: 1. Walking the outcrop 2. Matching similar rock features 3. Index fossils
Correlating or matching index fossils from three outcrops.
Correlate the 3 rock sequences to make a single rock column for the geologic history of the region. (oldest on bottom)
No place on the earth has all the rocks formed through time. So geologists use correlation to make a single rock column that shows the entire history of the earth. Correlation means to match Geologists match all of the following: –rock types –rock sequences –ash layers from volcanic eruptions –index fossils
Page 8 & 9 of the Reference Tables.
Each black vertical bar represent the existence of a group of organisms. Extinction First appearance
DECAY TIME (YEARS) MASS OF UNKNOWN (G)
DECAY TIME (YEARS) MASS OF C 14 (G)
Isotopes - different forms of the same element - normal # of protons - more than normal # of neutrons Normal Hydrogen -1 proton - 0 neutrons - 1 electron Isotope of Hydrogen - 1 proton - 1 neutron - 1 electron
Radioactive Isotope of Hydrogen 1 proton 2 neutrons 1 electron Radioactive Hydrogen H-3