- Slides: 34
Relative vs. Absolute Dating
Uniformitarianism The Present is the KEY to the PAST. Laws of nature are the same now as they have always been. Past events explained and estimated by modern processes. Charles Lyell credited with this concept. Explained in his book “The Principles of Geology. ”
Other Important Words… Lithification: the consolidation of a loosely deposited sediment into a hard sedimentary rock. Strata: rock layers
Original Horizontality When sedimentary rock layers form, they are “created” in a horizontal fashion.
And, most importantly…
Relative vs. Absolute Age of Rock Relative Dating: determination of the age of a rock or geologic event by comparing with other rocks/geologic events Can you infer who is older? She is.
4 ways to determine relative age. (Also called the Principles of Geology)
Law of Superposition In undisturbed layers of sedimentary rock or lava flows… Top Layer = Youngest Bottom Layer = Oldest
Cross-Cutting Relations Any feature that cuts across a rock or layer of sediment is younger than the rock or layer it cuts Yo un ge st La ye r
Law of Included Fragments Sediments are older than the cement and rock they are part of
Fossil Correlation The same group of fossils are of the same age, and therefore, strata containing them are also of similar age.
There some speed bumps on the road to Relative Dating
Uncomformity A gap or missing layers of rock due to erosion.
Angular Unconformity Older rock under the line of erosion are tilted at an angle, while the younger strata above are horizontal.
Disconformity Strata above AND below the line of erosion are horizontal.
Nonconformity Unconformity that separates igneous or metamorphic rock from a layer of sedimentary rock.
Relative vs. Absolute Age of Rock Absolute: A form of geologic dating that gives an actual date/age in history that the item was formed or died. 6 months 82 years
Phenomenon of Radioactivity Some elements, such as uranium (U) and thorium (Th) are unstable. They decay spontaneously.
Radioactive Decay 238 U spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).
Beta Particle Emission But, 23490 Th is also unstable, and it emits a beta particle …
U – Pb Series This process is called radioactive decay, and eventually uranium (parent) decays to lead (daugher product)
U – Pb Series The rate at which this process occurs is measured in terms of a “half life”.
Half Life= Number of years for ½ of the original number of atoms to decay from U to Pb.
Radiometric Dating Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed. This is an example of uniformitarianism.
Half-Lives of Radioactive Isotopes
Examples of very old rocks found on Earth ü 3. 4 billion year old granites – South Africa ü 3. 7 billion year old granites – Greenland ü Oldest terrestrial rocks ~ 4 billion years ü Central Canada and Central Australia ü These are deep in the interiors of the continents ü Core drilling aids in excavation of these rocks.