![]() The most important concept in projectile motion is that when air resistance is ignored, horizontal and vertical motions are independent, meaning that they don’t influence one another. Ask students to guess what the motion of a projectile might depend on? Is the initial velocity important? Is the angle important? How will these things affect its height and the distance it covers? Introduce the concept of air resistance. This problem could be resolved, however, if we were to observe A cutting across H (i.e., the fault displacing the igneous intrusion).Review addition of vectors graphically and analytically. Given the information available, we cannot resolve whether H is older than A (or, vice versa). Oldest F, E, D, C, B, (H or A), G, I Youngest Putting this all together, we can determine the relative ages of these rock layers and geological events: I is therefore younger than both A and G. Oldest F, E, D, C, B, (H or A), G Youngestįinally, we note an erosional surface, I, at the top of the sequence (and immediately below the corn field) that cuts both A and G. ![]() Because the fault does not cut across H, we do not know if it is older or younger than that rock unit.įourth, we see that G, another igneous intrusion, cuts across A-H it is therefore younger than all of these (note that G is not displaced by A, the fault). Third, we observe that the fault A cuts across and displaces rock layers B-F. Second, we observe that rock layer H (which is an igneous intrusion) cuts into rock layers B-F. Let's work through the imaginary example above.įirst, we know from the principle of superposition that rock layer F is older than E, E is older than D, D is older than C, and C is older than B. The fault cut the curb and is thus younger than the curb itself. We know that the curb was originally straight when it was first constructed. As it turns out, the famous San Andreas fault runs below the curb at this location, which has caused the curb to be broken and displaced. You can see that the curb is offset: the bottom half does not line up with the top half. Have a look at the photographs below, which show the curb of a road in a neighborhood in Hollister, California. Because these features are the ones doing the cutting, we know that they are younger than the rocks that they cut into. When investigating rocks in the field, geologists commonly observe features such as igneous intrusions or faults that cut through other rocks. Because of cross-cutting relationships, the cut that divides the slice from the rest of the loaf is younger than the loaf itself (the loaf had to exist before it could be cut). Imagine cutting a slice of bread from a whole loaf. The principle of cross-cutting relationships states that a rock unit (or other geological feature, such as a fault) that is cut by another rock unit (or feature) must be older than the rock unit (or feature) that does the cutting. A third key principle- faunal succession-is reviewed in Section 3. Geologists employ a handful of simple principles in relative age dating two of the most important of these are are the principles of superposition and cross-cutting relationships. Relative age dating has to do with determining the temporal ordering of events in Earth's past. ![]()
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