Geologic History Unit Notes. Relative age - general age statement like older, younger more recent

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1 Geologic History Unit Notes Relative age - general age statement like older, younger more recent Absolute age - specific age like 4,600 million years old Fundamental Principles of Relative Dating 1. Uniformitarianism The idea that the processes that shape Earth s surface today are the same processes that occurred in the geologic past. Processes don t necessarily occur at a steady, uniform rate 2. Law of Superposition Rocks at the bottom of an undisturbed exposure are usually the oldest. Exceptions caused by folding or faulting 3. Original Horizontality A rock is always older than the processes that changed it Tilted or folded sedimentary layers are assumed to have been deposited level and tilted or folded later. So the tilting or folding is a younger event than the deposition of the rock. 4. Cross Cutting - an intrusion or fault that cuts across existing layers of rock is younger ( more recent) than the layers it cuts across. 5. Principle of inclusions - The sediments or rock fragments that make up a sedimentary rock are older than the rock or rock layer itself.

2 Contact metamorphism - hot magma changes the rock it comes in contact with to a metamorphic rock. Use your ESRT to see what kind of metamorphic rock will be made. Examples: sandstone will become the metamorphic rock quartzite Limestone will become the metamorphic rock marble Igneous Intrusions - liquid magma cutting through pre-exisitng rock. Heating of interface produced contact metamorphism Layer G is older than the intrusion F. The evidence of this is that there is contact metamorphism between layer G and rock F, which means the magma came in contact with the already existing layer G. Igneous Extrusion - magma overflows onto surface. There is no contact metamorphism between the extrusion and the layer of rock above it. (layer above it was deposited after the extrusion) Extrusion E is older than layer D Unconformity - buried erosional surface. Represents a time gap or missing rock layer or layers. How an unconformity forms 1. Sedimentary rock is uplifted out of the water (emergence) 2. Weathering and erosion occurs at Earth s surface. 3. The area submerges under water again. 4. New sedimentary rock is deposited on top of the eroded rock layer.

3 Fossils are found almost exclusively in sedimentary rocks. Fossils are destroyed by heat when igneous rocks form and by heat and extreme pressure when metamorphic rocks form (on rare occasion they can be found folded or distorted in metamorphic rock) Correlation- The process of showing rocks or events that occurred in different places are the same age Correlation Techniques: A- Continuity within a layer or formation 1. A "Formation" is a layer or group of layers of rock and is the basic unit of geologic mapping. The rocks of one formation have similar features, such as rock type, mineral composition, and environment of formation. 2. Rock layers can often be matched on the basis of similarity in appearance, color, and composition. (This will only provide a tentative match) If a rock layer has a fossil in it, it means: the rock is most likely sedimentary and the area was once under water (marine environment, shallow sea, or lake etc.) * Marine fossils found high on mountains is evidence the area was once under water and then uplifted. Index Fossils - help correlate rock layers must have wide geographic distribution (found in lots of places) the species existed for a relatively short time span (one geologic time period) Volcanic Ash - is a good time marker because it is deposited rapidly over a large area all at once. Can help correlate and date rock layers. Life on Earth has shown several mass extinctions (ex. Dinosaurs - one theory is from impact of asteroid or comet causing blocking of sunlight to earth temperatures on Earth dropped) Geologic time table is divided by: fossil evidence (Humans are a new arrival) Earth s early atmosphere may have come from outgassing of volcanoes. Early atmosphere did NOT contain Oxygen. The atmosphere we have now is the secondary atmosphere (mostly Nitrogen and Oxygen)

4 Radioactive decay - the nuclei of unstable atoms give off particles and energy The original atoms decay (change) into atoms of another stable isotope. Ex. Uranium-238 (U238) Lead-206 (Pb206) Isotopes elements that have atoms with different masses, but the same number of protons Example: C12 and C14 Half-life - the TIME required for HALF of the unstable atoms in a radioactive sample to decay to a different isotope. Half-life is NOT affected by : amount of radioactive material, heat, pressure, chemical action, etc. Earth Science Reference Table (front cover) Radioactive Disintegration Isotope (decays into) Carbon-14 N 14 Nitrogen-14 Potassium-40 Ar 40 Argon -40 Uranium-238 Pb 206 Lead Rubidium-87 Sr 87 Strontium-87 Half-life (years) Exponential = standard form Shortest half life. Dates organic remains of up to 70,000 years old. Half life = 5,700 years Dates rocks. Half life = 1.3 X 10 9 years Used to date rocks about as old as the earth is. Half life = 4.5X X years RADIOMETRIC DATING Determining the age of a rock or mineral by determining the relative amounts present of the radioactive parent isotope and the decayed daughter isotope. Not used for sedimentary rocks

5 CARBON DATING Only used to date organic remains (from plants and animals) Cannot date samples older than 70,000 years old. (after that many years there would not be enough Carbon-14 left to measure) Dates how many years since the living thing died. Examples of things dated by C-14: Wood, resin, leather, bones, coal, seeds C14 can t be used to date footprints because they are not organic remains. Radioactive Decay Graphs look like this Example using Carbon-14 C-14 N-14 Radioactive Fraction Stable Decay Half-lives Years Element Element 100% 1 0 % 0 5,700 50% 1/2 50% 1 11,400 25% 1/4 75% 2 17, % 1/8 87.5% 3 22, % 1/ % 4 1/32 1/64