How old are rocks???
PG.134-150
How landforms are made…
UNIFORMITARIANISM
CATASTROPHISM
Long time to change
Change happens suddenly
Which one is true???
Relative Dating
Determining if an object is older or younger
than other things
Like a stack of pictures (or homework
assignments)
Superposition- “younger over older”
Disturbed Rock Layers
Forces move rock… layers now out of order
Use the geologic column to find the right order
(see your book for more info)
Fault
Intrusion
Always younger than layers they cut across
Disturbed Rock Layers
Folding- rock layers bend from internal forces
Tilting- internal forces slant rocks without
folding
Unconformities- gaps in the record
Missing rock layer= time not recorded in layers of rock
Where they ever there? Or were they removed?
nondeposition
erosion
Absolute Dating
DETERMINING HOW MANY YEARS AN OBJECT HAS EXISTED
It’s radioactive!
Isotopes- atoms of the same element with equal protons, but different neutrons
◦ Atoms want equal # of protons and neutrons
most isotopes= stable
unstable isotopes= radioactive
Radioactive decay= unstable isotopes break down to become stable
Scientists use this rate to find an object’s age
Half-life
Start with original amount (parent material)
Radioactive isotopes decay into stable isotopes (daughter material)
Half the material at a time
more daughter material=older rock (more time to decay)
less daughter material= younger rock (less time to decay)
Three Types of Dating
URANIUM-LEAD METHOD
POTASSIUM-ARGON METHOD
Uranium-238 = unstable
Potassium-40 = unstable
Lead- 206 = stable
Argon, Calcium = stable
Used on really, really old
rocks
Used on really old rocks
Carbon-14 Method
Carbon 12= stable
Carbon 13, 14= not stable (combine with oxygen to be stable, CO2)
Carbon absorbed by plants, eaten by organisms
Organism dies= no more carbon
Carbon levels decrease
Fossils
NATURALLY PRESERVED EVIDENCE OF LIFE
FOSSILS IN ROCKS
MINERAL REPLACEMENT
Hard parts of dead animals buried quickly in
sediment, turned to rock
Permineralization- minerals fill pore spaces of an
organism’s tissues
Petrification- tissues are completely replaced by
minerals
FOSSILS IN TAR
FOSSILS IN AMBER
Pits of tar occur naturally and trap animals
Insects get stuck in tree sap, tree sap hardens
FROZEN FOSSILS
MUMMIFICATION
Some of the best fossils, preserved in ice
Organisms die in dry places, dry out so fast
there’s not enough time to decay
Trace Fossils
Evidence of an animal’s activity
◦ Tracks
◦ Burrows
◦ Coprolite “dung stone”
Molds and Casts
MOLD
CAST
Cavity in the ground
Cavity filled in (sediment fills the mold)
Shows the inside
Shows the outside
Fossils can tell us…
Changes in the environment
◦ Coral found in Iowa
◦ Marine fossils on mountain
Changes in life
◦ Looking at rock layers
◦ Fish, amphibians, reptiles
How old rocks are
◦ Index fossils- fossils of organisms that lived
during a fairly short timespan
◦ Ex. trilobites, tropites
◦ Found in rock layer, can tell how old rock is
FYI- Geologic Time Scale
Division of Time
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Eons
Eras
Periods
Epochs