Name:_________________________________________Date:__________Per. ______ E.N. _________

ESTABLISHING SEQUENCE Geologists can interpret events of the geological past by making careful observations of bedrock structures. The Law of Uniformitarianism tells us that the events of the geologic past are similar to the kinds of changes that we observe today. Stated another way, the present is the key to the past. These observations lead to an inferred sequence of events. Although this technique does not yield an numerical (quantitative) age in years, it does establish the order of features from youngest to oldest; that is, a relative age.

A. The three diagrams above show how younger sediments are always deposited on top of older sediments. These diagrams illustrate the Law of Superposition, which states that in any outcrop, the oldest layers of rock are generally those on the bottom.

In some outcroppings, folding or faulting has caused younger layers to lie beneath older strata. In these unusual places, we may be able to find younger fossils that are under older fossils, or we may find graded bedding that shows a gradual change from larger particles on top, toward smaller particles on the bottom. This would show that the rocks had been turned upside-down. However, if we see no evidence of overturning, we assume that the oldest layers are on the bottom.

B. The second set of diagrams shows that rocks must be deposited before they are cracked, tilted, folded, or faulted. It’s only logical that the rocks must form before they are changed by these events.

 

C. Intrusions of molten rock occur after the formation of the surrounding rock. A zone of baked rock, called contact metamorphism, (shown here by the short, black lines) forms next to the new igneous rock at the time of intrusion. Intrusions are completely within the ground (internal). Extrusions come out at the surface. They are external. A vein is a small part of the rock that has melted. Veins are also younger than the

rock in which they are found.

D. Fragments of the surrounding rock sometimes fall into molten rock before it cools to form a hard, igneous rock. These fragments (known as xenoliths) must be older than the surrounding igneous rock.  

E. Using these clues, a geologist can reconstruct the events that produced a complex rock outcrop. Sharpen your knowledge of geologic processes by naming the sequence events shown in the diagram below.

 

F. Make a list of the sequence of geological events necessary to produce the profile below.

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