Disturbed Rock Layers

1. Disturbed Rock Layers • Geologists often find features that cut across existing layers of rock. • Geologists use relationships between rock layers & features that cross them to assign relative ages to features & layers. • Features must be younger than the rock layers because rock layers had to be present before the features could cut across them.

2. Disturbed Rock Layers, continued • Events That Disturb Rock Layers Sediment is deposited to form rock layers — in horizontal layers — this has not changed over time. • If rock layers are not horizontal, something must have disturbed them after they formed. • The next slide describes four ways that rock layers may become disturbed.

3. Disturbed Rock Layers, continued • Fault - a break in Earth’s crust along which blocks of crust slide relative to one another. • Intrusion - molten rock from Earth’s interior that squeezes into existing rock & cools. • Folding - occurs when rock layers bend & buckle from Earth’s internal forces. • Tilting- occurs when internal forces in Earth slant rock layers.

4. Faults 4

5. Intrusion

6. Folding 6

7. Folding

8. Tilting 8

9. Tilting 9

10. Gaps in the Record -- Unconformities • Missing Evidence - Sometimes, layers of rock are missing, creating a gap in geologic record. Missing rock layers create breaks in rock-layer sequences called unconformities. • Unconformity- break in geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

11. Types of Unconformities • Most unconformities form by both erosion and nondeposition, but other factors may be involved. • To simplify the study of unconformities, geologists place them into three major categories: disconformities, nonconformities, and angular unconformities. 11

12. Types of Unconformities, continued • Disconformities exist where part of a sequence of parallel rock layers is missing. 12

13. Types of Unconformities, continued • Nonconformities exist where sedimentary rock layers lie on top of an eroded surface of nonlayered igneous or metamorphic rock.

14. Types of Unconformities, continued • Angular Unconformities exist between horizontal rock layers and rock layers that are tilted or folded. 14

15. Rock-Layer Puzzles • Rock-layer sequences often have been affected by more than one geological event or feature.