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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.
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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 TimeThe 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).
IGNEOUS INTRUSION: • occurs when magma squeezes into or between layers of pre-existing rock.
UNCOMFORMITY-a buried surface of erosion separating two rock masses. This represents agap in geologic time...
….a look at the Grand Canyon and 3 types of unconformities... Oh, and what’sthis? 2 1 3
Angularunconformity- An unconformity in which the beds below the unconformity dip at a different angle than the beds above it.
“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.
STEP 1 STEP 2
STEP 3 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.
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.
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!
