Transcript Earth History Notes

Earth History
How do we know how things were?
Relative Age versus Absolute Age
• 1. Compares two
• 2. Uses terms older or
younger
• 1. Gives actual number
OR information that
allows you to find the
number.
• 2. Numbers DO NOT
have to be exact.
The geologic history of the Earth is
written in layers of sedimentary rock
• The process of reading the history
following decoding rules is called
stratigraphy (layer writing).
• History is written by deposition of
sediments.
• Some records are erased by the
processes of weathering and erosion.
Law of Original Horizontality
• Sediments, that form
sedimentary rocks,
are deposited in
horizontal, parallel
layers.
• This implies that if you
find layers that are not
horizontal, some force
must have moved them.
Principle of Superposition
• Sedimentary rocks are deposited in
parallel, horizontal layers with the
oldest on bottom and the youngest
on top.
Law of Cross-Cutting
Relationships
• A rock or fault is younger than any
rock (or fault) it cuts across.
Law of Included Fragments
• Inclusions are older than the rocks
they are included in.
Guide = Index Fossils
• 1. Only lived on Earth a short period of
time.
• 2. Are easy to recognize.
• 3. Had a wide geographical distribution.
Unconformities
• If the deposition of sediments is continuous and
uninterrupted, the layers formed are said to be
comfortable.
• If deposition is interrupted by uplift, and erosion
removes layers before new ones are deposited, the
surface between two layers of very different ages
is an unconformity.
• If the older layers are tilted before new level
layers are deposited on top, an angular
unconformity results.
Point to the angular
unconformity
Half-lives
• One half-life is the amount of time it
takes for one half of something to
disappear or change into something
else.
• Radioactive elements decay (change
into other elements) by half-lives.
• Each radioisotope (radioactive
element) has its own half-life.
How half-lives workTime
(half-lives)
Start
(Time Zero)
1
Amount
Remaining
Amount
Changed
One (all)
none
1/2
1/2
2
1/4
3/4
3
1/8
7/8
Half-life example:
• If you still have $1.00 and I took $7.00
from you in the past, how much $$ did you
originally have?
• $8.00
• What fraction of your original amount do you
still have?
• 1/8th
• If I took half of your $$ each afternoon
including today, how many days ago did I
start taking your $$?
• 3 days
Marine Transgression and Regression
Practice Drawing
•
•
History in the Rocks
Fossils:
Any evidence of an organism
that lived long ago – classified
by how the fossil is formed
Types of fossils:
– CastsA mold of an organism
filled by minerals in the
surrounding rock – produces a
replica
– Trace FossilsMarkings or evidence of
animal activities – footprints,
trails, and burrows
History in the Rocks
•
Types of fossils:
– ImprintsFossils that form before
sediment hardens into rock – leaves
or feathers that fall into mud and
leave imprints
– MoldsWhen an organism is buried, it
decays leaving an empty space that
has the exact shape of the
organism
History in the Rocks
•
•
–
–
Petrified Fossils-
The hard parts of an organism are sometimes
penetrated and replaced by minerals atom for
atom, when minerals harden an exact stone copy
of the original organism is produced
Amber-Preserved & Frozen Fossils
Entire, intact organism is caught in ice or tree
sap (amber) – very rare – preserves the internal
parts of organisms important to scientific study
of extinct species