Physical Sciences on the ACT

Physical science is the word used for the branches of natural science that focus on non-living systems, in contrast to the life sciences. It is not uncommon to see passages dealing with earth sciences on the ACT Science Test. Just like other science passages, the ACT Science Test wants to know what you can do rather than what you already know. You already know all the Science knowledge you’ll need on Test Day from your high school science class, and you may already be familiar with most of the scientific vocabulary the ACT Science Test uses. Focus on breaking down the passages analytically first. Let’s work on an example of a Physical Science passage just like you might see on Test Day!
Geologists describe the orientation of sedimentary rock layers using two angles, “dip” and “strike.”  The dip indicates how far from horizontal the rock layer is tilted, and the strike indicates the compass direction along which the rock layer has been tilted.  For example, a dip of 1 degree indicates that a rock layer is nearly horizontal – it slopes downward at an angle of only 1 degree.  A strike of 1 degree indicates that the rock layer is tilted along a line running 1 degree east of north (i.e. almost due north), a strike of 90 degrees indicates that the rock layer is tilted along a line running 90 degrees east of north (i.e. due east), and so forth.
Within a mountain range, strike values are typically similar.  For instance, a map will show that the Allegheny Mountains run from southwest to northeast, and the strikes of rock layers within this area are often approximately the same (a value of approximately 45 degrees, corresponding to northeast).  In addition, near the outer edge of a mountain range, rock layers will generally be tilted much less than near the center of the range. Table 1 shows the dips and strikes measured at several points in a square mile of the Allegheny Mountains.

Figures 1 and 2 show how the average dip and average strike vary at 25-mile intervals along a particular north-south line in the Alleghenies.




According to Figure 1, from north to south, the average dip:
A. increases only
B. decreases only
C. decreases, then increases
D. increases, then decreases
We can tell this is a Physical Sciences passage because it deals with Geology. Even if we don’t know anything about Geology, we can still use the presented data to get the question correct. The label at the bottom of the figure indicates that the northernmost point is plotted at the left edge of the graph, and the southernmost point at the right edge. Thus, the question asks how the average dip changes from left to right. It can be seen that the dip increases from slightly over 10 degrees to slightly over 40 degrees, then decreases again to slightly over 30 degrees. The answer is D.

Which of the following describes a rock layer that is tilted vertically along a line running northeast?
A. dip = 45 degrees, strike = 90 degrees
B. dip = 90 degrees, strike = 45 degrees
C. dip = 180 degrees, strike = 90 degrees
D. dip = 180 degrees, strike = 45 degrees
A dip of 1 degree indicates that the rock layer is tipped 1 degree from the horizontal. We need to know the dip that corresponds to tilting the rock layer until it is vertical. If we draw a picture of this situation, we see that the vertical line and the horizontal line make an angle of 90 degrees, so the dip is 90 degrees when the bed is tilted vertically. A strike of 1 degree indicates that the rock layer is tilted along a line running 1 degree east of due north, so a strike of 45 degrees indicates that the rock layer is tilted along a line 45 degrees east of due north, or northeast. This is confirmed by the discussion of the Allegheny Mountains, in which the typical strike of 45 degrees matches the orientation of the mountain range, which runs northeast. The answer is B.