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42 Cards in this Set

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relative dating
determination of the age of a rock or event in relation to the age of other rocks or events
absolute age
the actual age of a rock or event in years
relative dating
determination of the age of a rock or event in relation to the age of other rocks or events
absolute dating
the actual age of a rock or an event in years
principle of superposition
inference that the bottom rock layer is the oldest and it gets younger as it goes up
intrusion
when magma squeezes into preexisting rocks and crystallizes
age of intrusion
younger in realtive age than any rock it cuts through
extrusion
when laval flows on earths surface and solidifies
relative age of extrusion
younger than any rock beneath it, but older than any rocks on top of it
relative age of unclusion
older than rocks it forms in
a rock is older than ..
any fault, joint, tilting, or fold that appears in it
age of parts of sedimentary rock
the sediments are older than the rock
the cement is younger than the rock
age of igenous rocks
individual mineral crystals vary in age
why
bc they form at different temps reached by the magma as it cools or hardens over thosands or millions of years
vein age
younger than the rock around it
correlation
process that makes it pssible to show that rocks or geologic events from different places are the same or similar in age
different ways of correlation
1. exposed bedrock
2. similarities in rock
3. index fossils
4. volcanic ash and meteorite deposits
where are fossils exclusively found
in sedimentary rocks
why are fossils rarely found in igneous and metamorphic rocks
bc fossils are usually destroyed by the melting of igenous rocks and by the heat and pressure associated with the formation of metamorphic rocks
index fossils
the fossils used in correlation
2 conditions of a useful index fossil
1. widespread
2. shortlived
correlation by volcanic ash
in large eruptions, volcanic ash scattors over wide areas of eatrths surface, then settles among other sediments in many different environments-all volcanic is the same from the same volcano so it serves as specific age markers in rocks that can be far apart
divions of time in order from longest to shortest
eons, eras, periods, and epochs
why are precambrian fossils rare and difficult to identify
bc the earliest known life forms were small and had no hard parts
aloso, most rocks have either been buried by more recent rocks, eroded away, or converted to metamorphic rocks or magma
unconformities
buired eroded surfaces
what does an unconformity in the rock record of an area indicate?
that at some time in its geologic history, upliift occured, which exposed the rocks to weathering and erodsion, which removed part of the rock record. later the land sunk or the sea level rose and it waas covered in water. new seidments were then depostied on the eroded land surface, producing an unconformity
why do most unconformities show a lack of parallelism between the older rock layers and the younger rock layers
bc the older rock layers were folded or tilted during uplift
uniformity of process
the pressent is the key to the past
isotopes
variet of elemetns
radioactive decay
the nuceli of atoms of many istopes are unstable, or radioacticve, and the emit particles and electromagnetic energy, chaging into atoms of other isotopes and elements
uranium-238
one of the most important radioacctive isoptopes used in dating rocks
half-life
the time required for half of the atoms in a given mass of an isotope to decay
does anything affect a half life
no
radioactive dating
estimating the absolute age of a rock sample using the halflife of a radioactive isotope along with the ratio between the amount of orginial isotope and the amount of its decay
carbon-14 dating
can be used to date rocks and organic remains up to approx. 70,000 yrs in age
2 things that say extrusion came before top layer
1. no contact metamorphism
2. top has small or no crystals
half-life
the time required for half of the atoms in a given mass of an isotope to decay
does anything affect a half life
no
radioactive dating
estimating the absolute age of a rock sample using the halflife of a radioactive isotope along with the ratio between the amount of orginial isotope and the amount of its decay
carbon-14 dating
can be used to date rocks and organic remains up to approx. 70,000 yrs in age
2 things that say extrusion came before top layer
1. no contact metamorphism
2. top has small or no crystals
radioactive
element that is unstable and breaks down at a fixed time