Astronomy 23/223 Homework:

Relative Ages of Rock Formations and Features

Lunar Orbiter Photograph IV-169H2

In our exploration of the moon, we carefully chose landing sites such that the samples we collected could bring useful numerical data. Since the actual sites were extremely small in comparison to the size of the moon, scientists have used relative dating to explain the ages of the geological features that were not explored on the Apollo missions.

The previous exercise examined the data manipulation that scientists can do when they have samples of extraterrestrial materials. In this exercise, we will see how dating can be accomplished without the use of physical samples. Instead, scientists are froced to extraplate information from visual images. This technique results in relative ages, rather than numerical ones.

 This directory contains two versions of Lunar Orbiter Photograph IV-169H2, both of which will probably take some time to load: (a) annotated, “clickable” image (click on any area to see a close-up) (b) un-annotated image (same as shown here; in case the labels get in your way!)

Examine these images carefully, and then prepare a chronology that includes all the formations and features listed below. The chronology can be set up as a simple list starting with the oldest formation or feature and ending with the youngest. You will quickly learn, however, that it may not be possible to define a single, unique sequence.

Once you have created your chronology, justify it by writing brief descriptions of the evidence used to place each formation or feature where it is in the sequence. Where age ambiguities occur, justify the permissible range of relative ages for the ambiguous features.

An example of how you might justify a relative age sequence would be:

“Small crater D must be younger than large crater A because it occurs within the large crater and could not have survived formation of the large crater had it been there first.” In geological parlance, we would say that crater D is superposed on crater A. This is an example of a rather obvious relationship; others are more subtle.

Here’s the list of formations and features to be ranked in time:

 large crater C brighter plains material (Pb) small crater D bright rays small crater E graben structure small crater F large crater A small crater G large crater B

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