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”

ORIGINAL HORIZONTALITY

Sediments are usually deposited in horizontal layers."

Finding Relative TimeThe LAW of...

• SUPERPOSITION: a sedimentary sequence will be OLDEST on BOTTOM (if undisturbed).

• FAULTS AND FOLDS: younger than the rocks they bend or break. Overturning of rock can lead to exceptions to law of superposition

Superposition- youngest

to

oldest

The LAW of...

• CROSS-CUTTING: a body of igneous rock is younger than rock it has intruded (cut across).– Intrusion - magma flow– Extrusion – lava flow

IGNEOUS INTRUSION:

• occurs when magma squeezes into or between layers of pre-existing rock.

Cross Cutting

LAW

• INCLUDED FRAGMENTS: pieces of rock found IN another rock must be OLDER (formed first).

• UNCONFORMITY : A gap in the rock record…caused by erosion of a rock surface and a new rock being fromed on the erosional surface.

Included Fragments AND…

an UNCOMFORMITY

UNCOMFORMITY- a buried surface of erosion separating two

rock masses. This represents agap in geologic time...

….outlined below...

….a look at the Grand Canyon and 3 types of unconformities...

Oh, and what’s this?

1

2

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 2

STEP 1

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