Transcript History of the Earth

History of the Earth
A matter-of-time scale…
If the age of the Earth (4.6 billion years) were condensed
into one year ...
Jan. 1…
Early Feb…
Late Mar…
Mid-Nov...
Earth was born
Oldest known rocks formed
First primitive life formed
First complex life with shells or skeletons
formed
Late-Nov… First land animals
Dec 25…
Extinction of the dinosaurs
Dec 31…
Humans evolved in the evening
Dec 31…
One second before midnight, humans first set
foot on the Moon
BIG Ideas:
1.
2.
3.
Scientists use several methods to learn
about Earth’s long history.
The oceans and atmosphere formed
and life began during the three eons of
the Precambrian, which spans nearly
90% of Earth’s history.
Complex life developed and diversified
during the three eras of the
Phanerozoic as the continents moved
into their present positions.
HOW
have scientists
investigated and understood the
history of Earth and the ages of
rocks?
By
studying rocks and fossils…
Uniformitarianism:
 geologic
processes occurring today
have been occurring since Earth
formed
 the present is the key
to the past
I. Determining Relative Age
Relative Age:
 The age of a rock or an event as
compared with some other rock or
event
 A comparison (older vs. younger);
does NOT indicate an exact age
 Principle
of Original Horizontality:
most sedimentary layers of rock are
deposited in a horizontal position
A. Law of Superposition:
an undeformed sedimentary
rock layer is older than the
one above it and younger
than the layer below it.
How would you explain this?
B. Unconformities:
breaks in the geologic record
An UNCONFORMITY is…
a
GAP in the sedimentary rock
record (like missing pages from a
book).
 a BURIED
erosional
surface.

Is this an unconformity?
NO…just heavily eroded rocks
(they remain HORIZONTAL)
Law of Crosscutting
Relationships: a fault or
intrusion is always younger than
the rock layer it cuts through.
Put these rock layers in order,
oldest to youngest…
II. Determining Absolute Age
Absolute Age:
 The
actual age of the rock.
A. Varve Count
How can you tell how old this tree is?
Varves: annual layers of
sedimentary rock
- consist of a light-colored
band of coarse particles
- and a darker band of fine
particles
B. Radioactive Decay
As
elements age, they change
into new elements…
Uranium
(U) will decay into lead
(Pb) - in about 4.5 billion years
1. Half Life:
the time it takes for half the
mass of a given amount of a
radioactive element to decay
into its daughter elements.
 U-238
decays into Pb-206
 U-238 has a half-life of 4.5 billion
years
 If we were to begin with 10g of U238, after 4.5 billion years you
would have 5g of U-238 left
 After another 4.5 billion years, how
much U-238 would remain?
 How much Pb-206 would you have?
2. Carbon Dating
Organic
materials (remains of
once-living things) decay at a
measurable rate
Can
be used to estimate the ages
of samples from 0 to 70,000 years
old
Would you use
half life or
carbon dating to
date this tooth?
Why?
III. The Fossil Record
Paleontologist: Scientist that
studies fossils.
Fossils: the remains or traces
of animals or plants from a
previous geologic time.
What
type of rock are nearly
all fossils found in?
WHY?
Fossils may be preserved in a
number of ways:
1.
Molds: sediment preserves the form of
a fossil
2.
Casts: created by filling a mold with
mineral or other material
3.
Imprints (trace fossils): tracks or
footprints
4.
Original bone or shell
Trace Fossils:
Amber:
 the
fossilized remaining part of tree
resin (which is very thick and sticky
when it first flows from a tree)
 resin runs down the trunk and may trap
insects, spiders, and sometimes larger
organisms (like lizards)
 these organisms can be preserved for
millions of years with details of their
soft tissue, such as muscles and hair-like
bristles, still intact
Most
Virginia fossils are of
marine organisms.
What does this suggest about
the past environment
of our state?
Large
areas of the state have
been periodically covered by
seawater.
Where
are most fossils found in
Virginia?
Coastal
Plain
Valley and Ridge
Appalachian Plateau
How
are fossils used to
determine the ages of
rocks?
Index Fossils: fossils found
exclusively in rock layers of a
particular geologic age.
Phacops (a species of Trilobite): lived 400 million years
ago in oceans
Requirements for Index Fossils:
1.
2.
3.
4.
Must be present in rocks scattered over
a wide area of Earth’s surface
Must have unique, distinguishing
features
Organisms must have lived during a
relatively short span of geologic time
Must occur in fairly large numbers
within the rock layers

IV. The Geologic Time Scale
Divided into units:
 EON:
the LARGEST unit of geologic time
 Eons
are divided into Eras
 Eras
are divided into Periods
 Periods
are divided into Epochs
PRECAMBRIAN
 The
three earliest EONS make up
90% of geologic time
 Known together as ‘PRECAMBRIAN’
 During this time, Earth was formed
and became hospitable to modern
life.
 4.6 billion-540 million years ago
ERA: based on relative ages of life
forms found in rocks
Paleozoic (paleo = old): 540 million-248
million years ago
 Mesozoic (meso = middle): 248 million-65
million years ago
 Cenozoic (ceno = recent): 65 million years ago
to present

(zoic = of life)
During
what ERA did humans
‘appear’ on Earth?
What
ERA are we in right now?
