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.

In the beginning- James Hutton 1700’s proposed the idea of Uniformitarianism The idea that the processes that shaped the Earth today are the same processes that occurred in the geologic past “The Present is the key to the past”.

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:States the rock is always older than the processes that changed it-ex: 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.

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.

Cross Cutting Click to skip back to LAWS.

Included Fragments AND… an UNCOMFORMITY Click to go to UNCONFORMITIES (next).

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.

….outlined below... Click to continue.

….a look at the Grand Canyon and 3 types of unconformities... Oh, and what’s this? 2 1 Click on each number in order to see the different types of unconformities. Click to arrow to continue after #3. “What’s this” = igneous intrusion. 3

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.

“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.

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 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.

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)

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 shortens materials, causing them to thicken. 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

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