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).
Superpositionyoungest
to
oldest
IGNEOUS INTRUSION:
• occurs when magma
squeezes into or between
layers of pre-existing rock.
Cross Cutting
Included Fragments AND…
an UNCOMFORMITY
UNCOMFORMITYa buried surface of erosion separating two
rock masses. This represents a
gap in geologic time...
….outlined below...
….a look at the Grand Canyon
and 3 types of unconformities...
Oh, and
what’s
this?
2
1
3
Angular unconformity- An
unconformity in which the beds below
the unconformity dip at a different angle
than the beds above it.
“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.
STEP 1
STEP 2
STEP 3
STEPS 4-6
“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?
S T R E S S
• Stress is a force that is capable of greatly
deforming rocks, and may result in folding or
faulting of rock
– Faults are CRACKS or FRACTURES in rocks
caused by stress…
– Folds are “bends” in rock layers
• Stress comes in three varieties:
– TENSION
– COMPRESSION
– SHEAR
TENSION
BEFORE STRESS
AFTER STRESS
Tension lengthens materials, causing them to
thin -- example= RIFTS/Mid-Ocean Ridges
…RESULTS
NORMAL faults…
HANGING WALL
(‘HANGS’ ON)
FOOT WALL
(‘STICKS’ OUT)
This example of tension results in a
structure called a GRABEN…
COMPRESSION
BEFORE STRESS
AFTER STRESS
Compression shortens materials, causing
them to thicken.
…RESULTS
REVERSE or THRUST faults…”upslope”
HANGING WALL
FOOT WALL
This type of compression results in
a structure called a HORST.
SHEAR STRESS
Shear stress is caused by side by side movement
– example= TRANSFORM BOUNDARIES!
…RESULTS