Download Geologic history - Hewlett

Earth History- Table of Contents
• RELATIVE vs. ABSOLUTE
• LAWS
• UNCONFORMITIES
• GEOLOGIC SECTIONS
Who’s got the TIME?
• RELATIVE: order/sequence
known, but not the actual date of
occurrence.
• ABSOLUTE: actual date known. If
2 dates are known, then the RATE
OF CHANGE can be known- such
as Mountain Building.
First Things First…or…
“How’d that get there?”
• In the 17th C., Nicolas Steno made an
important observation:
"Sediments are usually deposited in
horizontal layers."
He called this
“ORIGINAL HORIZONTALITY”
Finding Relative Time
The LAW of...
• SUPERPOSITION: a sedimentary sequence
will be OLDEST on BOTTOM (if undisturbed).
• CROSS-CUTTING: a body of igneous rock is
younger than rock it has intruded (cut across).
• INCLUDED FRAGMENTS: pieces of rock
found IN another rock must be OLDER (formed
first).
Geologic Dating
• Principle of Original Horizontality: Rocks
generally build up horizontally.
• Law of Superposition: any material below another
material is older.
• Principle of Crosscutting Relationships: Faults or
intrusions are younger than the rocks they cross
• Inclusions: Small pieces of rock that are not in a
rock bed are usually older than they bed they are in.
UNCOMFORMITYa buried surface of erosion separating two
rock masses. This represents a
gap in geologic time...
Superpositionyoungest
to
oldest
IGNEOUS INTRUSION:
• occurs when magma
squeezes into or between
layers of pre-existing rock.
Angular unconformity- An
unconformity in which the beds below
the unconformity dip at a different angle
than the beds above it.
STEP 1
STEP 2
STEP 3
STEPS 4-6
“SEQUENCE” of events…
1. The lower sediments were deposited as
horizontal layers in a body of water.
2. These sediments were then raised
above water level and tilted during a
tectonic event (what type of boundary?).
3. Streams & other forces of erosion carved
a nearly horizontal surface across the
tilted beds.
“SEQUENCE” of events…
4. The land surface subsided (or the water
level raised), submerging the erosion
surface.
5. A new series of sediments deposited in
horizontal layers on the erosion surface.
6. The complicated sequence of tilted and
horizontal rocks was again uplifted,
exposing them to erosion and producing
the outcrop we see today.
Disconformity
• An unconformity in which the beds above
the unconformity are parallel to the beds
below the unconformity, though layers are
“missing”.
Nonconformity
• An unconformity that separates profoundly
different rock types, such as sedimentary rocks
from metamorphic rocks.
Practice: what happened here?
Geologic link
Cross Cutting