# determining relative ages

## determining the relative ages of rocks

Stenos and Smiths principles are essential for determining the relative ages of rocks and rock layers. In the process of relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to try to decipher the times that an event occurred relative to the other events represented in that sequence. The relative age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, (1) Do you know which is older and which is younger? (2) Do you know how old the layers are in years? In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. Can you figure out what happened in what order in (Figure 1.1)? Write it down and then check the following paragraphs. The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. The fault cuts through all three sedimentary rock layers (A, B, and C) and also the intrusion (D). So the fault must be the youngest feature. The intrusion (D) cuts through the three sedimentary rock layers, so it must be younger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is still younger. The full sequence of events is: 1. Layer C formed. 2. Layer B formed. A geologic cross section: Sedimentary rocks (A-C), igneous intrusion (D), fault (E). 3. Layer A formed. 4. After layers A-B-C were present, intrusion D cut across all three. 5. Fault E formed, shifting rocks A through C and intrusion D. 6. Weathering and erosion created a layer of soil on top of layer A. Click image to the left or use the URL below. URL:

## instructional diagrams

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## questions

the relative age of a rock is

``````a) the age of the rock in years.

-->  b) the age of the rock relative to other rocks and geologic structures.

c) the age of the rock as determined by radiometric dating.

d) all of these.
``````

the _ rock unit lies beneath the __ rock units above it.

``````a) sedimentary; igneous

b) igneous; sedimentary

-->  c) older; younger

d) younger; older
``````

if a fault cuts a rock sequence that fault is

``````-->  a) younger than the rock sequence.

b) older than the rock sequence.

c) the same age as the rock sequence.

d) of an unknown age relative to the rock sequence.
``````

older rocks lie above the younger rocks.

``````a) true

-->  b) false
``````

in the geologic cross section in the concept, intrusion d cuts across rock layers c and b. rock layer b and intrusion d are offset by fault e. what are the relative ages of these features from older to younger?

``````a) fault e, intrusion d, rock layer b, rock layer c

b) rock layer c, rock layer b, fault e, intrusion d

c) intrusion d, fault e, rock layer b, rock layer c

-->  d) rock layer c, rock layer b, intrusion d, fault e
``````

a fault can cut through three or more sedimentary rock layers.

``````-->  a) true

b) false
``````

in the geologic cross section in the concept, the last thing to happen in the sequence was

``````a) the laying down of sedimentary rock layer c.

b) the igneous intrusion d.

c) the fault e.

-->  d) the erosion of the surface.
``````

a fault offsets three older sedimentary rock layers. this displays the principle of

``````a) horizontality.

-->  b) cross-cutting relationships.

c) lateral continuity.

d) faunal succession.
``````

a fault can shift rocks so that the layers no longer match up.

``````-->  a) true

b) false
``````

if we learn the succession of geological events in a region, it only tells us about that region and does not apply to other locations.

``````a) true

-->  b) false sources figure 1: ck-12: http://www.ck12.org/earth-science/determining-relative- ages/lesson/determining-relative-ages/
``````

## diagram questions

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