Relative Dating = any method of figuring out whether an event or object is older or younger than other events or objects How is it used in geology? Why is it used in geology? What is the geologic column and how is it used in relative dating?

Superposition = a principle that states that younger rocks lie above older rocks if the layers have not been disturbed

Not all rock layers are found with the oldest on the bottom. There are forces at work that push, tilt, fold, and break rock. (see page 158) There are forces at work that disrupt rock layers so much that sometimes geologists find them upside down!

Q: How do geologists figure out the relative ages of rocks with so many forces are working against them? A: The Geologic Column (to the rescue!) Q: What is the Geologic Column? A: An arrangement of rock layers in which the oldest rocks are at the bottom. Q: How did geologists construct or produce the Geologic Column? (see page 157) A:

• Geologists combine data from all the known undisturbed rock layers around the world.

• The column is an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth.

• It is arranged from oldest to youngest.

• See Figure 2 on page 157

Q: Tell me again . . . What do geologists use the Geologic Column for?

A:

• Geologists use the column to interpret rock sequences.

• They use it to help them identify the layers in puzzling rock sequences.

The Geologic Column:

By using the Geologic Column on the left – what can we tell about the rock layers on the right?

Q: What is an unconformity? A:

• An unconformity is a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

• An unconformity represents missing time – time that was not recorded in layers of rock.

• What happened to the missing layer?

o Was it ever deposited? Why?

o Was it totally removed? How?

See Figure 4 on page 159 There are three types of unconformities.

• Disconformities • Nonconformities • Angular Unconformities

Disconformities =

• Where part of a sequence of parallel rock layer or layers is missing.

• They represent thousands to many millions of years of missing time. Nonconformities =

• Where horizontal sedimentary rock layers lie on top of an eroded surface of older intrusive igneous or metamorphic rock.

• They represent thousands to many millions of years of missing time.

Angular Unconformities =

• Are found between horizontal layers of sedimentary rock and layers of rock that have been tilted or folded.

• The tilted or folded layers were eroded before horizontal layers formed above them.

• They represent millions of years of missing time.

More good News! Rock-layer sequences are often disturbed by more than one rock-disturbing feature. Take your pick:

� Folding � Faulting � Intrusions � Uplift � Subsidence � Erosion � Disconformities � Nonconformities � Angular Unconformities � And many, many more rock-disturbing forces!

Some of the tools geologists use to unlock these rock puzzles are:

� The Geologic Column � Knowledge of fossils � Knowledge of the rock cycle � Absolute Dating