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historical geol 2


measured by man directly Time
heart beats Time
pendulum swings Time
atomic cyclic vibrations Time
redefined in 1967 Time
9,192,631,770 vibration cycles of a cesium atom second
rotation of the Earth day
lengthening 1 sec/year variable
1.5 Ga day 11 hours
20 hours - based on daily growth rings 400 Ma day
moon around the Earth month
Earth around the Sun year
changes in growth cycles seasons
because the Earth is inclined on its axis seasons
produces growth rings in plants seasons
bristle cone pines in the southwest U.S. example of growth rings
oldest known trees >5000 yrs.
used to calibrate 14C dates trees
produces growth rings in animals seasons
tidal control claims
fish scales growth rings
scallop growth bump growth rings
high erosion, low erosion cycles varves
back to ~15,000 years b.p. glacial
more cycles in the past indicates shorter days in the past growth rings
salt in the ocean major argument on Earth age
1899,Irish physicist estimated - Earth age ~99.4 million years old Joly
ocean thought initially salty refuted
cooling rate of the Earth major argument on Earth age
middle 1860s -a physicist Lord Kelvin
time necessary to cool from a liquid Lord Kelvin
Earth age 10-20 million Lord Kelvin
radioactive heating discovered 1896 Becquerel
sediment deposition rate since fossils appeared major argument on Earth age
total thickness versus rate of accumulation137,195 0.305m/1000 years sediment deposition rate
>450 million years old fossil bearing rocks
age estimates based on radioactive decay estimates major argument on Earth age
spontaneous breakup of the atomic nuclei of certain unstable substances radioactivity
discovered in 1896 by Becquerel - French physicist radioactivity
isolated energy producers from Uranium Curies
1898 - Curies discovered Radium, atomic number 88 Radium
radioactive disintegration decay
nuclei of helium atoms alpha particles
2 protons helium
2 neutrons helium
electrons beta particles
electromagnetic radiation (gamma) rays
- like light but with a very short wavelength gamma rays
unstable elements radioactive isotopes
spontaneous decay of individual particles is unpredictable radioactive isotopes
decay of half of the atoms is statistically predictable radioactive isotopes
(t1/2) - time it takes for half of the atoms of an isotope to disintegrate half-life
(ln 2)/l; where l = decay constant t1/2
varies from fractions of a second to billions of years t1/2
a constant relating the instant rate of radioactive decay of a radioactive species to the decay constant
naturally -in our atmosphere (14C) cosmic radiation
in the laboratory cosmic radiation
absolute dates Isotopic Dating
based on Parent/Daughter ratio when the decay is known Isotopic Dating
requires a closed system Isotopic Dating
no parent or daughter product can leave the system closed system
changes based on particle emissions/capture Isotopic Dating
parent nucleus loses an alpha particle alpha particle emission (nucleus of helium atom)
daughter atomic number is 2 lower alpha particle emission (nucleus of helium atom)
daughter atomic weight is 4 lower alpha particle emission (nucleus of helium atom)
parent nucleus loses a beta particle beta particle emission (electron)
neutron changes to a proton in the nucleus beta particle emission (electron)
daughter atomic number is 1 higher beta particle emission (electron)
daughter atomic weight is the same beta particle emission (electron)
parent nucleus gains a beta particle electron capture
proton changes to a neutron in the nucleus electron capture
daughter atomic number is 1 lower electron capture
daughter atomic weight is the same electron capture
14C dating - Carbon - Nitrogen - 1/2 life 5,730 years Carbon dating
in atmosphere 14N + e --> 14C + H Carbon dating
14C + O2 --> 14CO2 into plants and animals Carbon dating
dates 14C/12C after death Carbon dating
normal methods good to ~40,000 years Carbon dating
accelerator dates to ~100,000 years Carbon dating
tree rings used to calibrate dates Carbon dating
14C production assumed constant through time Carbon dating
14C disappears at the 1/2 life rate- 14 14 6 C --> 7 N - " decay Carbon dating
Earth age = 4.6 billion Uranium-Lead dating
1/2 life = 713 million to 4.5 billion years Uranium-Lead dating
dated oldest rocks on Earth Uranium-Lead dating
Zircon U/Pb age - 4.06 Ga oldest rocks
Northwest Territories of Canada oldest rocks
gneissic rock type (Acasta Gneiss) oldest rocks
1/2 life 48.8 b.y. Rubidium-Strontium dating
discovered 1948 Potassium-Argon dating
1/2 life = 1.3 b.y. Potassium-Argon dating
40Ar/39Ar method Potassium-Argon dating
argon loss at 50-200o C Potassium-Argon dating
metamorphism resets the atomic clock Potassium-Argon dating
usually igneous rocks dated- biotite, others Potassium-Argon dating
1/2 life 106 b.y. Samarium-Neodymium dating
1/2 life = 13.9 b.y. Thorium-Lead dating
Cenozoic; specific events in time TIME
CaCO3; water in ice cores; mineral specific MINERALOGY
ice volume and climate CAUSE
evaporation and precipitation fractionation INTERPRETATION
diagenetic problems; organism specific, ANALYTICAL
cycles - Cenozoic; event specific UTILITY
measured using mass spectrometers oxygen isotopes
rare isotopes 180
common isotope 160
lighter 16O more readily during evaporation
heavier 18O is left behind and concentrated during evaporation
ice is light-isotope enriched during glaciation
oceans become heavier during glaciation
in general, marine organisms concentrate 18O during glaciation
some organisms fractionate isotopes problems with fractionating
salinity causes local isotope anomalies problems with fractionating
temperature decreases and organisms concentrate 18O problems with fractionating
first identified by Urey in 1947 fractionation
method established by Emiliani beginning in 1954 fractionation
(1967) estimated paleotemperatures Shackleton
(1973) established $18O to 120 Ka Shackleton and Opdyke
record pushed back beyond 2 Ma Shackelton
(1984) Imbrie and others
established a well dated $18O composite to 780 Ka Imbrie and others
back to 500 k still used today as established in 1984 Imbrie and others
based on 5 marine cores Imbrie and others
showed Milankovitch periods Imbrie and others
precession at ~20 kyr orbital
obliquity at ~40 kyr orbital
at ~100 kyr eccentricity
the state of the atmosphere at a place and time as regards heat, cloudiness, dryness, sunshine wind, rain, etc., weather
the weather conditions prevailing in an area in general over a long period - classically defined as 30 yrs climate
of/or relating to the entire Earth as a planet global
as used by geologists represents the temperature, rainfall and wind over thousands of years Global Climate
Created by: lmulke1