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). Click each Law for a link to an example. INTRUDED takes you to the definition.

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.

UNCOMFORMITY- a buried surface of erosion separating two rock masses UNCOMFORMITY- a buried surface of erosion separating two rock masses. This represents a gap in geologic time... Click to continue.

Superposition- youngest to oldest Click anywhere in picture to skip back to LAWS.

IGNEOUS INTRUSION: occurs when magma squeezes into or between layers of pre-existing rock. Click back arrow to go back to LAWS.

Angular unconformity- An unconformity in which the beds below the unconformity dip at a different angle than the beds above it. Click ANGULAR to see how this is formed.

STEP 1 STEP 2 Click Step 1 to go back to sequence of events.

STEP 3 Click STEP # to go back to Sequence of Events. STEPS 4-6

“SEQUENCE” of events… The lower sediments were deposited as horizontal layers in a body of water. These sediments were then raised above water level and tilted during a tectonic event (what type of boundary?). Streams & other forces of erosion carved a nearly horizontal surface across the tilted beds. Click links to go to diagram, 1 at a time.

“SEQUENCE” of events… The land surface subsided (or the water level raised), submerging the erosion surface. A new series of sediments deposited in horizontal layers on the erosion surface. The complicated sequence of tilted and horizontal rocks was again uplifted, exposing them to erosion and producing the outcrop we see today. Click each underlined link to see example. Click arrow to go back to UNCONFORMITIES (Grand Canyon).

Disconformity An unconformity in which the beds above the unconformity are parallel to the beds below the unconformity, though layers are “missing”. Click DISCONFORMITY to go back to UNCONFORMITIES.

Nonconformity An unconformity that separates profoundly different rock types, such as sedimentary rocks from metamorphic rocks. Click UNCONFORMITY to go back to Grand Canyon.

Practice: what happened here? Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the next slide.

Sediments deposited, sea-level lowered, layers intruded, layers tilted, erosion and deposition under sea, sea-level lowered again. Click arrow to continue.

Geologic link

Cross Cutting Click to skip back to LAWS.

Speculate as to the causes of the landscape. Note igneous intrusion Speculate as to the causes of the landscape. Note igneous intrusion. What type of climate? (Arid- steep walls)

Intrusions, angular unconformities, faulting, more intrusions, etc.