geologic absolute age notes
- Absolute Age is the age in years, of a rock or other object.
- Geologists determine absolute ages by using properties of the atoms that make up materials.
- Radioactive Decay is the process of breaking down.
- Some isotopes are unstable and break down into other isotopes and particles.
- Sometimes a lot of energy is given off during this process.
- Half-Life of an isotope is the time it takes for half of the atoms in the isotope to decay.
- After 2 half-lives 1⁄4 of the original isotopes still remain; after 3 half-lives 1⁄8 of the original isotope still remain. (HALF LIFES ARE CONSTANT).
Q = What is meant by the term “half-life” of an isotope?
A = The half-life of an isotope is the time it takes for half of the atoms in the isotope to decay.
- Radiometric Ages are by measuring the ratio of parent isotope to daughter product in a mineral and by knowing the half-life of the parent, in many cases, you can calculate the absolute age of a rock.
- As time passes, the amount of parent isotope in a rock decreases as the amount of daughter product increases. The following ways are to date a rock: relative dating, absolute dating.
Q = What information must you have in order to conduct radiometric dating?
A = You must have an isotope with an appropriately short or long half-life. You must also know the
isotope’s half-life and be able to measure the ratio of parent isotope to a daughter product.
FORMATION OF EARTH:
- It is estimated that the earth formed along with the solar system 4.6 billion years ago (4600 mya)
- Eon = Largest segment of geologic time, Epoch = smallest segment of geologic time
- Relative time: Places events in a sequence but does not identify their actual date or occurrence (ex: a list in chronological order of what you have done today up until this time) Absolute Time: Identifies the actual dates of geologic events (ex: the exact times at which you did these things)
FAULTED AND FOLDING LAYERS:
- Finding the age of rocks with relative time: the law of superposition- in undisturbed sedimentary rocks are the oldest layers are at the BOTTOM and the youngest are at the TOP.
- Law of cross-cutting relationships - an igneous intrusion is younger than the rock it has intruded into
- (look at the whiskers on the diagram if provided)
- TOP IMAGE IS FOLDING, BOTTOM IMAGE IS FAULTING)
- Law of included fragments - If fragments of 1 type of rock are found in another rock layer, the rock fragments must be older than the rock layer in which they are found.
- Faulted and folded layers - layers of rock that have been faulted or folded must have been present before the actions of faulting or folding took place
- An Outcrop is exposed rock layers at the earth’s surface.
- A Key bed is a thin, widespread layer, usually of volcanic ash that can be used to correlate an exact point of time.
- A Fossil is any evidence of earlier life preserved in the rock. Original remains (rare).
- Replaced remains- The soft parts of the original animal have disappeared & the hard parts have been replaced by mineral material (petrified wood).
- Molds and Casts - Fossil shells or bones are dissolved completely out of the rock leaving a hollow depression in the rock. New mineral material fills the mold it forms a cast of the original fossil. Trace Fossils - Trails, footprints, tracks, burrows.
- Index Fossil - short-lived, geographically widespread occurrence.
- Measuring Absolute time - Tree Rings - Each ring represents a single year (single/fall) the width of the ring depends upon temperature & rainfall (TOP PICTURE), Varves - Glacial lake deposits, a thick light colored layer in the summer and a thin dark layer in the winter (BOTTOM PICTURE).
- Radioactive Dating: Used to date back in time, certain rocks contain radioactive isotopes.
- Radioactive Isotopes: Atoms of elements that give off radiation from their nuclei.
- Radioactive Decay: The process by which a radioactive isotope changes into a new stable element.
- Radiocarbon Dating: Uses the Radioactive Isotope Carbon-14 found in all living things. Because carbon-14 is continually absorbed by food & water it stays constant in living things. When the living thing dies the % of Of carbon-14 decreases at the rate of its half-life (can be used to date back 100, 000 years).
- Precambrian time is all of the geologic time before the Paleozoic Era, Precambrian rocks lack fossils because most of the organisms were microscopic bacteria and algae, Stromatolites are algal reefs that make up a large part of the fossils from the Precambrian, 1⁄2 Of the world’s metallic minerals are found in Precambrian rocks.
- The Paleozoic era marks the beginning of an abundant fossil record. North America was over the equator and our climate was warm with few seasonal changes.
- Cambrian Period - Trilobites & Brachiopods are the most common fossil. Trilobites are found throughout the Paleozoic era but never more abundant & widespread then the Cambrian (index fossil).
- Devonian Period - “Age of the Fishes” fishes were jawless & were covered with heavy plates.
- Ordovician - All life still in the oceans.