Often in science students see drawings not drawn to scale. The drawings are created to place emphasis on a particular section or item to bring it to the students' attention.
This need is on my mind because we just completed the Earth Layers lesson and learned that the Earth's crust is only about 2% of the Earth's layers. Yet, in many visual representations, the crust appears to be much larger -- especially when the features of the crust, such as oceans and mountains, are shown with emphasis.
We need to be explicit with our students, and show them scale images when possible so their understanding is not distorted.
Investigation Summary & Standards
Student learning includes the understanding of how changes in the Earth's surface over time. This lesson continues to develop the foundation needed to fully comprehend how the layers below the crust of the Earth impact the changes we see in the crust. (MS-ESS2-2 Construct an explanation based on evidence how geoscience processes have changed Earth's surface at varying time and spatial scales) Students are often mislead with drawings made to show emphasis on features and therefore not necessarily drawn to scale. Students will be developing their own scale models in this lesson. (SP2 Developing and Using Models) Students will also apply math skills to precisely measure their slice of the Earth as well as the proportional relationships needed to create scale. (MP4 Model with mathematics) (MP5 Use appropriate tools strategically) (MP6 Attend to precision) and (7.RP.A.2b proportional relationships).
Students in Action
In the previous lesson, Defining Earth Layers, we concluded with a video from MIT examining the Earth Layers. In the video we are told that the Earth's crust is only about 2% of the total of the Earth layers. Looking at the drawing on the student handout, we can clearly see that the Earth's crust exceeds its relative size compared to the other layers. Often we see drawings that are not drawn to scale. It is not the intent of the creator to mislead us; it is done only to emphasize a feature.
In this lesson, we create a scale drawing of the Earth's layers. A list of materials is available in the resources for this section.
The table shows the estimated average radius of the Earth's layers rounded. The distance of the radius is shown in kilometers. How can we convert the measurements from kilometers to centimeters using the scale 1 centimeter is equal to 100 kilometers? As a class we will determine how to calculate the new scale. Students calculate the rest of the scale independently then check their results with their la partner.
We note that the 70 kilometers of the crust is actually less than 2% of the the total of the Earth layers mentioned in the movie because we are looking at numbers representing the radius not the total of the Earth's layers.
Students use 2 sheets of 11 x 17 copy paper taped together end-to-end to create their slice of the Earth. To draw a line down the center, we fold the taped sheets of copy paper lengthwise or "hotdog" style and draw a line down the center of the paper. Our class size is more than 30 students so we will create slices of the Earth that are 12 degrees each. That is 13.4 centimeters across or 6.7 centimeters on the top of each side of the center line of the taped sheets of copy paper. The scale used for used for various class sizes can be found here. Students will make a small mark at the distance measured from the center. Even though we only need 30 sections and have more than 30 students, this will allow us to have extra in case of damage or absent students.
Students use a meter stick to draw a line from the bottom center of the paper to the marks made on the top of the paper creating a wedge.
Next we use the table to measure the distance of each layer and color the layers according to instructions.
Before cutting the wedge out students will round the edge using a makeshift compass. We make the compass using a string attached to a pencil. With our lab partner holding the string in place at the narrow end of the wedge we round the wide end slightly.
Students know that water makes up approximately 70% of the Earth's surface so 21 of our sections will be color blue at the crust and 30% or 9 of our sections will be green for land.
The first students finished will begin the assembly of the Earth with each student adding their section as they finish.
In this video I take a look at the completed assembly of the Earth with each of the student slices. See how this lesson creates a need-to-know for students to use math measurement skills, attend to precision and apply what they know to the real world.
This lesson was inspired by a website created by Larry Braile, Professor, Purdue University in 2000.
Throughout the Earth Science unit we will leave the scale model of the Earth's layers posted in the classroom. Students are proud of the total results. We can see the accuracy of the measurements by the relative size of each of the sections.