Earth History - Table of Contents

1. Earth History- Table of Contents • RELATIVE vs. ABSOLUTE • LAWS • UNCONFORMITIES • GEOLOGIC SECTIONS

2. 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.

3. 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”

4. ORIGINAL HORIZONTALITY Sediments are usually deposited in horizontal layers."

5. 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

6. Superposition- youngesttooldest

7. 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

8. IGNEOUS INTRUSION: • occurs when magma squeezes into or between layers of pre-existing rock.

9. Cross Cutting

10. 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.

11. Included Fragments AND…an UNCOMFORMITY

12. UNCOMFORMITY-a buried surface of erosion separating two rock masses. This represents agap in geologic time...

13. ….outlined below...

14. ….a look at the Grand Canyon and 3 types of unconformities... Oh, and what’sthis? 2 1 3

15. Angularunconformity- An unconformity in which the beds below the unconformity dip at a different angle than the beds above it.

16. “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.

17. STEP 1 STEP 2

18. STEP 3 STEPS 4-6

19. “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.

20. Disconformity • An unconformity in which the beds above the unconformity are parallel to the beds below the unconformity, though layers are “missing”.

21. Nonconformity • An unconformity that separates profoundly different rock types, such as sedimentary rocks from metamorphic rocks.

22. Practice: what happened here?

25. 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

26. TENSION BEFORE STRESS AFTER STRESS Tension lengthens materials, causing them to thin -- example= RIFTS/Mid-Ocean Ridges

27. …RESULTS NORMAL faults…HANGING WALL (‘HANGS’ ON) FOOT WALL (‘STICKS’ OUT)

28. This example of tension results in a structure called a GRABEN…

29. COMPRESSION BEFORE STRESS AFTER STRESS Compression shortens materials, causing them to thicken.

30. …RESULTS REVERSE or THRUST faults…”upslope”HANGING WALL FOOT WALL

31. This type of compression results in a structure called a HORST.

32. SHEAR STRESS Shear stress is caused by side by side movement – example= TRANSFORM BOUNDARIES!

33. …RESULTS