Transcript Document

CH. 12
GEOLOGIC TIME
Rock Records
• Rocks record Earth’s history
– Geologic events (erosion)
– Changing life (extinctions)
James Hutton
Uniformitarianism:
the forces and
processes we see
today are the same
as the processes that
have acted on the
Earth’s surface in the
past
“The present is the key to the past”
Relative Dating
• tells order rocks formed (1st,
2nd, 3rd) relative to each other
• Does not tell actual date
• Does not tell how long ago
RULES for dating rocks
law of superposition: in
undisturbed rock, the older
rocks will be at the bottom
and the youngest rocks will be
at the top
Principle of original
horizontality: layers of sediment
are deposited in a horizontal
position
law of cross-cutting
relationships: a rock cutting
through another rock is
younger than the rock it has
cut-cross
law of included fragments
or inclusions: pieces of one
rock found in another rock
must be older than the rock
in which they are found
unconformity:
Long break in the
geologic rock record
• deposition stops,
erosion occurs, then
deposition resumes
• 3 types
Contorted rock, hard to determine relative age (Copiapo, Chile)
Uncomformity
• Granite formed
• Granite exposed by erosion
• Beds 1-3 deposited
1. angular unconformity: during
long break, folding or tilting also
occurred
• boundary between
horizontal and
tilted layers of rock
Angular Unconformity
• Beds 1-6 deposited
• Beds 1-6 tilted
• Erosion
• Beds 9,10 deposited
2. disconformity: two layers
of rock separated by an
eroded surface
Disconformity
• Beds 1,2 deposited
• Erosion
• Beds 5, 6, 7 deposited
3. nonconformity: eroded
surface separates older
metamorphic/intrusive rocks
from younger sedimentary
rocks
Correlation of Rock Layers
• matching up rocks of the same age in
different places
• creates better view of geologic time
• Locally – use relative dating
• Distant – use Fossils
12.2 Fossils: Evidence of Past Life
fossil: remains of prehistoric life
• in sedimentary rocks
• tells info about past environments
• correlates rocks (time)
Types of Fossils
(1) original remains: the actual,
unchanged remains of the organism
are preserved
Ex: teeth, bones, shells
(a) frozen in ice
(b) preserved in amber (tree
sap that has hardened
(c) mummification
(2) altered remains: soft parts of the
organism have disappeared and been
replaced by minerals
Ex: petrified wood
petrification
(3) molds and casts: shells buried
and dissolve by water leaving “picture”
in rock of shape and surface
Ex: animal shell or skeleton
(4) trace fossils: indirect evidence of
prehistoric life
(a) footprints
(b) imprint
(c) tracks, trails,
burrows, borings
coprolites: fossilized dung or waste
materials
• tell about food habits
T-Rex Carnivore Coprolite (8 lbs.)
gastroliths: fossilized stomach stones
used by animals to grind food
2 conditions that favor preservation
(1) Rapid burial … protected from
environment
(2) hard parts … soft will decay
index fossil: found exclusively in rock
record of a particular geologic age
(1) must be easily recognizable
(2) must be widespread in occurrence
(3) must be limited in time
(4) must be found in large numbers
12.3 Dating with Radioactive Decay
Radioactivity: when the nuclei of atoms
break down b/c unstable
Nucleus
A
B (more stable)
B (more stable)
half-life: time it takes for half of the
atoms to decay into new stable atoms
Radiometric Dating: procedure that
uses radioactive elements & their
half-lives to date rocks
Table 1, p. 349
Uranium-238
Thorium-232
Rubidium-87
Potassium-40
Lead-206
Lead-208
Strontium-87
Argon-40
4.5 Billion
14.1 Billion
47 Billion
1.3 Billion
** Is this good for “recent events”???
Carbon dating: Carbon-14 used in
radioactive dating for recent events
C-14
C-12
• 1/2 life of carbon-14 is 5,730 years
• Dates things up to age of 75,000 yrs old
• Can only use on once living things, not rocks
Geologic time scale
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• summary of major events of the
Earth’s history
• divided by major changes in the
Earth’s surface, climate and/or
species
Eons
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The greatest expanses of time.
Divided into
Phanerozoic-youngest
Proterozoic
Archean
Hadean-oldest
45
Eras: 2nd largest segment of time
(1) Precambrian (longest)
(2) Paleozoic - “ancient life”
(3) Mesozoic - “middle life”
(4) Cenozoic - “recent life”
Eons
Eras
Divided into
Periods
Divided into
Epochs
48
12
1
11
2
10
9
3
8
4
7
5
6
How long did they last??
Precambrian –
Paleozoic –
Mesozoic –
Cenozoic –
How many hours on the clock?
How old is Earth?
How much is each hour equal to in years?
Test Questions? CH 13 pg. 363
1. Which era is the longest?
2. Which era is the shortest?
3. Which era do we live in?
4. Which era did dinosaurs live in?