Transcript geologic time honors

READING THE
ROCK RECORD
relative time: events are in
sequence, but no actual dates
absolute time: identifies
actual date of event
Most geologic work is
done using relative
time!
Determining Relative Age
of Rocks
law of superposition: oldest
rock layers are on the bottom
and youngest rock layers are
on top IF the layers have not
been disturbed.
law of crosscutting: any
geologic feature is younger
than anything else it cuts
across
•unconformity: layers of
rock that are missing; a gap
in the rock record
- most often caused by
erosion
Other laws of relative dating:
• Principle of Inclusions – fragments rock
contained within a body of rock must be
older than the “host” rock
– Ex. Conglomerate
• Principle of Faunal Successions –
specific fossils follow one another in a
specific order
– Ex. Dinosaur fossils in both Montana
and China
Determining Absolute Age
of Rocks
radioactive decay: over
time, radioactive elements
release a proton(s) to make
a new, lighter, more stable
element.
Major isotopes used for
Radiometric Dating:
U 238  Pb 206
U 235  Pb 207
Th 232  Pb 208
Radioactive elements decay
at CONSTANT rates
half-life: the time it takes
for ½ of the atoms of a
radioactive element to
decay
Major isotopes used for
Radiometric/Absolute Dating:
Parent
U – 238
U – 235
Th – 232
Daughter
Half-Life
Pb – 206
4.5 billion
years
Pb – 207
713 million
years
Pb – 208
14.1 billion
years
C-14 dating: used only to
date things that were once
alive
- half-life is only 5800 years;
C-14 decays into N-14
- can date more recent
remains (up to about 50,000
years)
Other dating techniques:
• Dendrochronology: counting rings
on trees
– Rings look different in times of
drought and other extreme
climate conditions
• Varve chronology – looking at
glacial sediments in lake beds
– helps us find weather patterns
for studying global warming
What is a fossil?
fossil: the remains or
traces of organisms that
lived long ago
Formation of a dinosaur fossil:
• Fossils can form in various
ways.
• Typically, the body material
is replaced by minerals.
Mold & Cast Formation
Process
1. Organism becomes encased in
sediment that is compressing
to form a rock.
2. Water gradually dissolves
organism.
3. This leaves a hole in the
rock shaped like the
organism – a mold
4. Water carries minerals
that recrystallize in the
mold making a cast
Replacement of Minerals:
1. Water partially or
completely dissolves an
organism, depositing
minerals (like quartz) in its
place.
2. Minerals are actually harder
than the original bone.
trace fossil: evidence of
life other than the remains
of plants or animals
ex. footprints, tracks,
burrows
index fossil: fossils that are
found over a wide geographic
area but lived over a narrow
range of time
- help to identify the relative
age of the rock in which they
occur
The Geologic Timetable
era: a long time segment
defined by dominant life forms
Eras are broken down into
segments called periods.
period: a subdivision of an
era
epoch: a subdivision of a
period
The Earth is 4.6 billion years old
• How do we know?
– Oldest “Earth” rocks found are
about 3.5 billion years old
– Moon rocks (no plate tectonics/no
recycling of rock) taken during the
lunar landing have been dated at
4.53 billion years old
– Meteorites (remnants of our early
solar system) have been dated at
4.6 billion years
PRECAMBRIAN TIME
•From beginning (4.6 billion
years ago) to 545 million
years ago (mya)
•Makes up 90% of Earth’s
history
•many rocks eroded
significantly
•main life form was
cyanobacteria
(photosynthetic bacteria)
•cyanobacteria added
large amounts of oxygen
(through photosynthesis)
to the atmosphere...
•made it possible for other
life (plants and animals) to
evolve
PALEOZOIC ERA
“The Age of Invertebrates”
•From 545 mya to 245 mya
(about 300 million years
ago)
Divided into 6 periods
1.
2.
3.
4.
5.
6.
Cambrian
Ordovician
Silurian
Devonian – Age of Fishes
Carboniferous – Age of Amphibians
Permian – largest mass of extinction of
recorded life
• In the US, the carboniferous
period is divided into the
– Upper Carboniferous or
Pennsylvanian
– Lower Carboniferous or
Mississippian
• warm, shallow seas
• Pennsylvania was
underwater
• North America was at the
equator
Marine life forms:
•trilobites - relative of
horseshoe crab
•brachiopods - look like
clams
•crinoids - relative of
starfish
trilobite
brachiopod
crinoid
• “Firsts”: land plants, fish,
amphibians, reptiles and
insects
• Appalachians form
• much of the limestone, coal
and schist in PA formed
during the Paleozoic Era
THE MESOZOIC ERA
(“Age of Reptiles”)
•From 245 mya to 66 mya
•“Firsts”: mammals, birds and
angiosperms (flowering
plants)
•Dinosaurs evolve and later
become extinct
•Pangaea breaks up
•Rocky Mountains form
Divided into three periods:
- Triassic  small reptiles
-Jurassic  dinosaurs flourish
- Cretaceous  dinosaurs
become extinct
The extinction of
dinosaurs marks the
end of the Mesozoic
Era and the beginning
of…
CENOZOIC ERA
“Age of Mammals”
•From 66 mya to present
•Most complete geologic
record
•Mammals and flowering
plants abundant
•Alps and Himalayas form
•Grand Canyon Forms
•Homo sapiens (humans)
evolve (100,000 yrs ago)
•Divided into 2 periods
and each period is further
divided into epochs
•We are living in the
Holocene Epoch of the
Quaternary Period of the
Cenozoic Era