Lab 08 ONLINE LESSON

If viewing this lesson in Flash Use the control bar to navigate

Do take notes as we peruse through this lesson…

Geologic Events and Histories

Geologists spend a goodly amount of time trying to figure out the history of our planet…

To do this, geologists look at sedimentary rocks…

Sedimentary rocks are tremendously powerful records of the history of the planet…

Sediment is deposited in flat, horizontal planes called bedding planes…

Flat and horizontal…

Each of these beds represents a specific event of sedimentary deposition…

Bedding planes act as geologic markers…they start out as flat layers…

Layered sedimentary rocks provide insight into which rock layers came first and last…

This was first…

This was second…

This was last…

Knowing the order of these rocks…we can assign relative time to this rock sequence…

OLDEST

YOUNGEST

With me so far…

We can use our old friend the Geologic Time Scale…

Cretaceous rocks are younger than Jurassic rocks…

Jurassic rocks are older than Cretaceous rocks…

There are several laws that should be understood…

1. Law of Original Horizontality: sedimentary layers and lava flows were originally deposited as relatively horizontal sheets, like a layer cake.

Just as these layers are in horizontal sheets…

2. Law of Superposition: the oldest layer is at the bottom of the sequence and the youngest is at the top.

YOUNGEST OLDEST

3. Law of Cross-Cutting Relationships: Any feature that cuts across a rock or body of sediment must be younger than the rock or sediment that it cuts across such as fractures, faults or igneous intrusions

The igneous intrusion H cuts across the layers A,B,C etc…so H is younger than all the rest…

which it has been incorporated. 4. Law of Inclusions: Rocks or fossils that have become included in another rock or body of sediment must be older than the rock or sediment into which it has been incorporated.

Trilobite existed first…then the orange sediment was deposited later…trilobite is older than the orange sediment…

5. Law of Unconformities: Gaps in the geologic record that formed wherever layers were not deposited for a time or else layers were removed by erosion

3 types of unconformities exist…

Disconformity: missing layers…

Layers are missing due to erosion…

Angular unconformity …layers on top of tilted rocks…

Rocks were tilted by tectonics, eroded and new layers deposited

Nonconformity …igneous intrusions

This is a sequence of stratified rock This is a sequence of stratified rock. A is the oldest because it is on the bottom of the series of layers. Each layer is distinct. Layer D is the youngest because it is on top. This is a case of Original Horizontality and Superposition.

This is an example of an angular unconformity This is an example of an angular unconformity. Layers A, B, C and D are tilted. They were originally horizontal until an episode of tectonics forced these layers into a non horizontal position. Event E is an erosional surface. It is an example of Cross Cutting Relationships. F is the topmost layer and so it is the youngest. A is at the bottom, so it is the oldest.

Layers A, B, C, and D were originally horizontal until an episode of tectonics formed these layers into a geologic structure called an anticline. Erosional event E crosscuts the top of the anticline. Layers F and G were deposited later, then igneous intrusion H cross cut all layers, making it the youngest of all the events.

Igneous body A existed first, then came along igneous intrusion B Igneous body A existed first, then came along igneous intrusion B. C cut across later making it the youngest, while A is the oldest.

Layers A, B and C were deposited first. Fault D cut across A,B,and C Layers A, B and C were deposited first. Fault D cut across A,B,and C. Erosional surface F cut across Fault D and a layer of shale E deposited on top of F.

Color References to Lab 8

END Lab 08