1. Earth’s History Introduction: The earth is about 4.6 billion years old. Much of its history is recorded in the rock. Observations of fossils, rock types, evidence of faulting, uplift and folding as well as igneous activity are all clues about the Earth’s history.

2. Geologic Principles There are four major geologic principles that must be discussed. These principles help geologists make predictions about sequences of events, and thus the Earth’s history.

3. The Principle of Uniformitarianism This geologic principle states that all the geological processes (weathering, erosion, volcanism, earthquakes, etc.) that occur today also occurred in the past in the same ways. Patterns seen today can be matched with rock layers from the past, showing the same pattern allows geologist to understand how that rock was created, and the environment of the area at the time of formation.

4. Principle of Original Horizontality States that sediments are deposited in flat lying or horizontal layers that are parallel to the surface on which they were deposited. This means that if rocks are tilted, or folded they have been deformed after deposition.

5. The Principle of Superposition States that unless a series of rock layers have been folded or overturned the oldest layer will be on the bottom. Basically, if you pile a stack of papers, what ever paper was put down first, must be the oldest.

6. The Principle of Cross Cutting Relationships States that if something (faults, cracks, veins, or intrusions-magma cooling underground, extrusions-magma cooling on the surface) cuts through rock layers, the rock layers themselves must be older then the thing cutting through them. This also implies that if chunks of another rock type are found within a layer, these rock chunks are older then the rock layer.

7. Unconformities: Gaps in time. Represent open spaces in the rock record. Caused when rock is being uplifted, weathered and eroded, or when no deposition is occurring. There are three main types of unconformities. Unconformities are represented by a wavy line in the rock column.

8. Angular Unconformities Occur when rock is first tilted by tectonic action (plate interactions or faults), then the end tilted up is weathered and eroded away. As it is eroded, it eventually becomes a low point where sediment deposits and forms a new layer. A wavy line running at an angle to the bedding surface represents angular unconformities.

9. Angular Unconformity: Easiest of the types to identify, shows older beds tilted at an angle to the new flat lying beds. The wavy line represents the unconformity.

10. Parallel Unconformities Form when parallel layers of rock of different ages are separated by an erosional surface. Comparing different locations with the same series of rock allows the extent of the erosion to be determined on some occasions. Parallel Unconformities are often called Disconformities.

11. Parallel Unconformity: Flat lying beds separated by an erosional surface. The Unconformity represents a gap in time, and is represented by the wavy line.

12. Nonconformities When sedimentary rock is deposited on top of an eroded igneous layer. In order for this to happen, some sort of subsidence must have occurred. While the subsidence was occurring, no new rock was formed, thus leaving a gap.

13. Granite Sand Stone Nonconformities: Form when a layer of igneous rock is eroded away, then a layer of sedimentary rock is deposited on top of the igneous layer. The unconformity is represented by the wavy line.

14. What do we do with these principles? These principles allow geologists to create a sequence of geologic events. The type of rock each layer is made of relates to the environment of the area at the time of formation. Examples: Limestone forms in shallow marine environments. Examples: Coal forms from the remains of plants, usually in a tropical climate similar to rainforests or swamps.

15. Relative Dating: By applying the basic geological principles discussed above geologist are able to determine the relative age of the rock layers This means that geologists can say which layers are the older than which and thus determine the sequence of events on Earth. When the age of rock is compared to the ages of other rock or events in the geological sequence. Saying “This rock is either older than or younger than something,” shows its age relative to a known.

16. Put these rock layers in correct order from oldest to youngest using the four principles. Sand Stone Slate Limestone Correct order: Limestone, Slate, Sandstone

17. Using the information from the last slide, put these rock layers and any geologic event in order from oldest to youngest using the four geologic principles. A B C D E: Tilting Correct order: D, C, B, A, E

18. 1 2 3 4 Using the information from the last slide, put these rock layers and any geologic event in order from oldest to youngest using the four geologic principles. 5: Folding Correct order: 4, 3, 2, 1, 5

19. 5 2 3 1 4 5 3 2 1 6 Using the information from the last slide, put these rock layers and any geologic event in order from oldest to youngest using the four geologic principles. D-Fault C-Intrusion B-Contact Metamorphism 6-Breccias from fault Correct Sequence: 4, 5, 3, 2, 1, D, 6, C, B