EXERCISE 17.2: Relative Ages in Cross-Cutting Situations

EXERCISE 17.2: RELATIVE AGES IN CROSS-CUTTING SITUATIONS. Refer to Figure 17.1 on p. 433 of your lab manual to answer Parts (a) and (b). Part a. Which is younger? The inclined beds in Figure 17.1 or the layer that cuts them?

The layer that cuts the inclined beds is younger FEEDBACK: The layer that cuts the inclined beds is younger

Part c.2. Using the principle stated in Part c.1, what are the relative ages of the granite and dikes in photo (a) on p. 434?

oldest: granite; intermediate age: slightly inclined dike; youngest: vertical dike FEEDBACK: The vertical dike is the youngest rock unit in photo (a) because it cuts across all the other rock units. The slightly inclined dike is the next oldest unit because it cuts across the granite. Thus, the granite is the oldest unit

Part d.2. Using the principle of original horizontality, which are the oldest and youngest (horizontal) volcanic ash layers that the fault cuts in photo (b) on p. 434?

oldest: thick white bottom layer; intermediate age: dark layer; youngest: thin top layer FEEDBACK: According to the principle of original horizontality, the oldest layers are deposited first and thus at the bottom of a rock formation

Part d.1. A fault, like in photo (b) on p. 434 of your lab manual, must be _______ the rocks it offsets.

younger than FEEDBACK: According to the principle of cross-cutting relationships, if a feature cuts across a rock unit, it must be younger than the rock unit that it cuts.

Part e.1. The process that folded the sedimentary rocks in photo (c) on p. 434 of your lab manual must be ________ the rocks.

younger than FEEDBACK: According to the principle of cross-cutting relationships, if a process affects a rock unit, it must be younger than the rock unit that it affects

This principle of cross-cutting relationships is useful for many types of geologic materials and processes, such as those shown in the photographs on p. 434 of your lab manual. Refer to these photographs in order to answer the remaining questions of this exercise. Part c.1. An intrusive igneous rock that cuts across other rock units as in photo (a) on p. 434 of your lab manual must be ______ the units it cuts across.

younger than FEEDBACK: According to the principle of cross-cutting relationships, if one rock unit cuts across another, it must be younger than the rock unit that it cuts.