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Geological Time
is measured in terms of:
• absolute age-where an actual number is assigned to describe the
age
• relative age- where it is determined which of two rock layers is
older than the other
Rock Classifications
Igneous
fire produced deep within the earth’s crust (magma). It can then be
forced out through fractures in the crust in the form of a volcanic
event (lava) to form volcanic rock or it can cool and harden without
ever reaching the surface. This later option forms plutonic rock (e.g.
granite)
Sedimentary
erosion from either water, wind, or chemical means produces fine
grains of rock matter which, in the presence of a water event, settle
out and become cemented together. (e.g. limestone and sandstone)
Fossils are commonly found in sedimentary rock.
Metamorphic
pressurized igneous and/or sedimentary rock causing changes in the
rock. Sometimes crystals grow larger, and often the rock becomes
harder, brittle and shiny. (e.g. shaleÆslate)
Geological Structures
1
fractures - cracks
faults - cracks that have shifted
2
dikes - when magma is forced into rock fractures, where it cools forming veins of
plutonic rock
folds - where layers of sedimentary rock are squeezed sideways causing a
buckling over
3
erosion surfaces - when existing rock has eroded away and then is reburied
stratum - a layer (pl. strata)
4
Finding relative ages
Law of Superposition - layers of rock on top are newer than those
beneath. (older as you go down)
Cross-cut Rule - any event that disturbs a rock layer is newer than
the rock layer itself. (e.g. fractures, dikes and erosion surfaces)
“Index Fossils” - fossils that have been arbitrarily assigned as
having lived in a certain era. (“If we find a layer that has this fossil in
it, then the rock must be older than this other layer that his this more
advanced fossil form.”)
Problem:
Standard Geological Column - an imaginary “superstack” of all
sedimentary layers and their assigned fossils arranged in order from
the oldest to the youngest. This is the “standard” (or ruler) used to
date rock strata.
Problem:
• does not exist - cannot be demonstrated anywhere on earth
• based on assumptions rather than fact
• is a result of circular reasoning
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Finding absolute age
Radiodating - based on the fact that unstable isotopes decay at a
certain rate (i.e. half-life). In decay the parent material breaks down to
decay products. The theory is that if you can measure the proportion
or ratio of parent material to decay products, then you can determine
how old it is.
uses :
14
C - decays to 14N with a half-life of 5730 years
235
U - decays to 207Pb with a half-life of 713 million years
40
K - decays to 40Ar with a half-life of 1.3 billion years
For an isotope to be useful, it must:
• be present when sample is first formed (14C in living things)
• must be sealed inside sample to keep it free from contamination
• have a half-life that allows measurement: long half-lives for old
rocks, short half-lives for young rocks.
Problem:
assumes no decay product is present at the time of rock formation.
(gives old date)
Problem:
6
Geological Time Scale
arbitrarily assigning absolute ages to each layer in the standard
geological column
Problem:
not based in fact but is a product of circular reasoning - see Standard
Geological Column
7