Cross-Cutting for Kids: It’s Safer than you Think

Before taking GLG112 (Geological Disasters) two semesters ago, I had not learned about geology in a class since seventh grade. I had totally lost interest in the physical sciences. I took GLG112 because it was suggested to me by my advisor to make up for having almost no physical sciences in my transcript. So I took Lisa’s lecture class and it sparked an interest in me. For the first time in a long time, rocks and geology were actually interesting again. That’s the power of a good teacher, they ignite a curioisity in you that you did not know was there. I share the same dream as most teaching students of being the favorite cool teacher and for my last blog post I wanted to combine some of the geology I have learned with teaching.

Geology is based on many key principles, one of which is the Principle of Cross-Cutting Relationships. It sounds more complicated than it really is. In reality, it’s a simple principle. Basically, when one rock cuts into another, you know that the rock being cut into is older and the rock that’s doing the cutting is newer. That’s it! It’s all about the relative ages of the rocks. When talking about relative age, It refers to the age of the rock compared to the other rocks around it. This is different than absolute age of a rock, which is where you would date a rock using radioactive decay of isotopes to find its age. The best part of this principle is that it makes obvious sense. The old rock would have to be in place first before another rock could cut into it. One good example of cross-cutting is the formation of dikes on a volcano. A dike is formed when magma seeps through fractures in the ground (Fig. 1). Based on the Principle of Cross-Cutting Relationships, we know that the dike is newer and the outcrop is older because the dike is breaking through the surface of the other rock.

Figure 1- a picture of a dike outlined in blue, an example of cross-cutting occurring in an outcrop.
Figure 1- a picture of a dike outlined in blue, an example of cross-cutting occurring in an outcrop.

Another important tenet of geology is the Law of Superposition. This is another way of figuring out the relative ages of rocks. This law states that if you have a bed of undisturbed rocks, the bottom layer is the oldest and the newest layer is at the top. Both of these rules are great because they seem like common sense. That’s one of the principles that we have been using throughout this course when analyzing the deposits from the Minoan eruptions. The bottom layer was the first phase of the eruption and the next layer was the next phase and so on (Fig. 2).

Figure 2 - Example of the Law of Superposition, the layers at the bottom are the oldest and the layer at the top is the newest
Figure 2 – Example of the Law of Superposition at Fira Quarry, the layers at the bottom are the oldest and the layer at the top is the newest, relative to the surrounding rocks.

The trouble though, for someone like me, is how to make this interesting to kids, how do I make it important or interesting or fun for them? I think the biggest challenge for teachers is to present information to kids in a way that is meaningful. You might know that teachers use lesson plans as a way of preparing how they want to teach their students. Lesson plans can be really simple or really complex depending on the teacher writing them. I wanted to write a lesson plan while I was on this trip because I wanted to relate what I am learning here to my own major. I thought it would be fun to teach the lesson using an investigation. I want to put the students in a sort of detective scenario and have them try to figure out the story of how a bed of rocks was formed and call them the great rock detectives. It would be a fun way for public school kids from fifth grade to potentially eighth grade to learn a basic concept.

All lesson plans start with a pre-activity. The goal of the pre-activity is to introduce the topic to your students and/or gauge what your students already know about it. Pre-activities are short and sweet; a lot of teachers will do some kind of bell work or group activity as a way of warming up the class for their lesson.

As a pre-activity for a lesson on geological principles, I would want to get my students thinking about layers and chronology by asking the students to list anything that they can think of that has layers. I’d be fishing for answers like a pizza, a cake, a sandwich, their clothes, an onion, and then I’d try to prompt them to think about the order of those layers and how when you have layers you have to put them on in order. So when you have a pizza for instance, you have the crust, then the sauce, then the cheese, then the toppings or when you get dressed you put on underwear first, then pants, then a shirt, socks, etc. The goal would be that they realize that the things on bottom have to come first, and whatever the next layer is came second, and so on. With these analogies in mind, we could move on to the real lesson.

The next part of the lesson plan is the meat of it all, the activity itself, the thing you have your students do to actually learn the material and put it together with what they already know. My idea for teaching geologic principles is to set up a short investigation of sorts. I would tell all the students that they were going to be rock detectives for the day. Their assignment would be to go around and look at various “crime scene files” (pictures of rock outcrops with short written descriptions) around the room and try and figure out what happened and which order the rocks got there. I would include pictures of layered rocks like figures 1 and 2. I would make as many stations as there were groups of students, i.e. – five groups of students would have five stations to investigate. The students would have about five to eight minutes at each station to discuss the rocks with their groups and form their theories about the timelines and the stories of how the rocks got there.

Once each group had visited each station we would come back together as a group and discuss the students’ speculation. To finish up, I would introduce the relevant principle for each case. For the layered rocks, the perpetrator would be the Law of Superposition, for a photo with a dike in it, the perpetrator would be the Principle of Cross-Cutting Relationships. This lesson could expand to include the Principle of Original Horizontality and The Principle of Inclusions as well (for more information on the Principle of Inclusions, see my fellow teacher Holly’s blog, Education vs. Geology: The Magmatic Smackdown). I focused on cross-cutting and superposition here because they have been present here on Santorini but this lesson does not have to stay limited to those two.

Finally, lesson plans end with an assessment of some sort. For this specific lesson, I could use a written assessment where students look at pictures and identify the relevant principle by matching. Another idea would be to have them do another rock investigation in small groups with different pictures now that they know the principles. The students could describe/draw out examples of each principle in a science journal or worksheet. The whole point of assessment is just to see if students understand what has been taught and there are many ways to do that.

If you ask a room full of education majors why they want to be teachers, almost half will reply that they were inspired by one of their teachers. I think everyone deserves to have the experience of at least one teacher who really sparks their interest in something. For me, one of those teachers was my high school biology teacher, I loved the subject and her class so much and she solidified bio as my favorite type of science. But I had a similar experience in Lisa’s geology class too- it was the first time that the physical sciences had seemed interesting to me and I wanted to know more. That is why I chose to go on this study abroad trip. I chose this particular topic because I wanted to get back to the basics of geology and teach myself more about the physical science world and I wanted to do it in a way that related to teaching my future students using what I have gained while here.

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2 thoughts on “Cross-Cutting for Kids: It’s Safer than you Think”

  1. Hi Katie,

    I’m so glad that you chose to focus on your passion for teaching in this post. Your own personal voice really shone in this piece and your enthusiasm for teaching was expressed wonderfully. I can tell that you really put a lot of thought into your lesson plan as it was very detailed and organized.

    I thought you did a great job of not only creating the lesson plan, but also relaying the information that you’ve learned so far to us (your audience). While talking through your lesson plan about teaching young students about geology, you are at the same time teaching your audience geology! How exciting! You did a great job at explaining different concepts, such as cross-cutting, by explaining them in a clear and simple way. The information you presented was interesting and it was fun to read.

    Overall, great job on sharing with us your passion in connection with geology. This was a great last post from you!

  2. Katie – it was a pleasure to have you on this trip and get to know you better. We’ve talked about how rewarding those ‘aha’ moments are for a teacher when their students get it. I saw that many times in you these last three weeks. I’m happy to have inspired you to learn more about geology and hope that you will continue on in your education with other earth science classes. We need more teachers with a solid earth science background!

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