Historical Geology Midterm

• Number of protons = Atomic Number
• Protons + Neutrons = Atomic Mass Number
• It is the decay rate of unstable isotopes that geologists measure to determine the absolute age of rocks.
• Unstable atom has 6 protons and 8 neutrons which is also called Carbon-14.
• Radioactive decay - process whereby an unstable atomic nucleus is spontaneously transformed into an atomic nucleus of a different element.
• 3 Types of Radioactive Decay: Alpha Decay, Beta Decay, Electron Capture
• Alpha Decay - an unstable parent nucleus emits 2 protons and 2 neutrons
• Beta Decay - an electron is emitted from a neutron in the nucleus
• Electron Capture - a proton captures an electron and is thereby converted to a neutron
• Half-life - time it takes for one-half of the atoms of the original unstable parent element to decay to atoms of a new, stable daughter element
• Half-life is constant and can be precisely measured
• Half-life ranges from less than a billionth of a second to 106 billion years.
• Radiocarbon dating uses Carbon-14.
• Carbon-14 is the radioactive isotope of carbon.
• Half-life of Carbon-14 is only 5730 years.
• Carbon-14 can be used to date events as far as 70,000 years.
• C-14 is continuously produced in the upper atmosphere as a consequence of cosmic-ray bombardment.
• Cosmic rays (jhigh energy particles), shatter the nuclei of gas atoms, releasing neutrons.
• Some of the neutrons are absorbed by nitrogen atoms (atomic no. 7), causing their nuclei to emit a proton.
• As a result, the atomic number decreases by 1 (to 6), and a different element is created which is the Carbon-14.
• Isotope of carbon quickly becomes incorporated into carbon dioxide, which circulates in the atmosphere and is absorbed by living matter.
• All organisms contain a small amount of carbon-14, including you.
• While an organism is alive, the decaying radiocarbon is continually replaced, and the proportions of carbon-14 and carbon-12 remain constant.
• Carbon-12 is the stable and most common isotope of carbon.
• It is important to emphasize that carbon-14 is only useful in dating organic materials, such as wood, charcoal, bones, flesh, and even cloth made of cotton fibers.
• University of Arizona researchers used carbon-14 to determine the age of the Dead Sea Scrolls, considered among the great archaeological discoveries of the 20th century.
• Igneous rocks are the best rocks to date.
• As magma cools and begins to crystallize, radioactive parent atoms are separated from previously formed daughter atoms.
• Because they are the right size, some radioactive parent atoms are incorporated into the crystal structure of certain minerals.
• Therefore, a mineral crystallizing in a cooling magma will contain radioactive parent atoms but no stable daughter atoms. Thus, the time that is being measured is the time of crystallization of the mineral that contains the radioactive atoms and not the time of formation of the radioactive atoms.
• Sedimentary rocks cannot be dated because one would be measuring the age of a particular mineral rather than the time that it was deposited as a sedimentary particle.
• One of the few instances in which radiometric dates can be obtained on sedimentary rocks is when the mineral glauconite is present.
• GLAUCONITE: a greenish mineral containing radioactive potassium 40, which decays to argon 40.
• Glauconite forms in certain marine environments as a result of chemical reactions with clay minerals during the conversion of sediments to sedimentary rock. Thus, glauconite forms when the sedimentary rock forms, and a radiometric date indicates the time of the sedimentary rock’s origin.
• Metamorphic rocks cannot be dated because the age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock initially formed. Instead, the date could indicate any one of a number of subsequent metamorphic phases.
• Leakage may take place if the rock is heated or subjected to intense pressure as can sometimes occur during metamorphism. If this happens, some of the parent or daughter atoms may be driven from the mineral being analyzed, resulting in an inaccurate age determination.
• Every year in temperate regions, trees add a layer of new wood under the bark.
• Favorable growth conditions produce a wide ring; unfavorable ones produce a narrow ring.
• DENDROCHRONOLOGY: dating and study of annual rings in trees.
• The Geologic Time Scale is a chronological system used by geologists that relates geological strata to time, to describe the timing and relationships of events that have occurred during Earth’s History.