Igneous Rocks I

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SWBAT identify properties of igneous rocks and create a line graph demonstrating the relationship between cooling time and crystal size in igneous rocks

Big Idea

Students create a graph illustrating the relationship between cooling time and crystal size

Lesson Introduction

[Note: For additional information, including embedded checks for understanding and teacher directions, refer to this document: Whole Lesson (with comments). You can also find the same document in PDF format here: Whole Lesson [PDF].] 

This lesson is one of the main "graphing" lessons I have each unit. This graph demonstrates a critical relationship - the one between cooling time and crystal size in igneous rocks. As an igneous rocks cools, it begins to crystallize as its temperature drops below its melting point. Depending on the speed of this cooling, crystal formation is either small (so small so as to be microscopic) or large. Thus, there's a direct relationship between cooling time and crystal size - someone can theoretically look at an igneous rock and determine how long it takes to cool. This lesson mainly revolves around the creation and analysis of a graph demonstrating that fairly linear relationship. Keep in mind (and this is noted in the section below), the graph's axes are a little wonky - the y-axis is not in a traditional linear scale, but a logarithmic one. I would only attempt this graph if your students have had enough exposure to the more traditional scientific graph types. 

Do Now & Objective(s)

10 minutes

Students come in silently and complete the Do Now. After time expires (anywhere from 2-4 minutes depending on the type of Do Now and number of questions), we collectively go over the responses (usually involving a series of cold calls and/or volunteers) before asking a student to read the objective to start the lesson.

As a note, the Do Now serves a few purposes: general review of the previous day's material, re-activation of student knowledge to get them back into "student mode" and get them thinking about science, an efficient and established routine for entering the classroom, and as a strategy for reviewing material students have struggled with. 



25 minutes

In this section, students are given a data table showing a relationship between igneous rock cooling time and the size of the crystals. Their first job is to plot a graph (the axes, title, and scale have already been inserted) on the selected points. [Note: See Reflection in this section for assignment rationale]. 

When looking at this graph, ask students to pick out what they notice. They should see that the y-axis is actually in a logarithmic scale (a scale used to show data over large quantities). Be clear with students exactly how you want the graph to look and be plotted, and urge them to exercise caution when graphing (Note: You may want to plot a made up point to show them how to use the logarithmic scale). The graph itself illustrates the key and fundamental element of the lesson - there is a direct relationship between cooling time and crystal size. As exemplified in the graphical data, as the cooling time goes up, the crystal time grows larger. The logarithmic scale shows that even in the context of potentially thousands of years of cooling time, this still holds true. 

Additionally, for context, my students have had very extensive experience and practice graphing. They can graph basic line, bar, and pie graphs with aplomb - this graph is meant as an instructional curveball to challenge them conceptually on how graphical data can be represented. I give them the title and axes insofar as they already are familiar with identifying variables - the real content of this lesson comes from plotting and deriving the relationship between variables, which is in this case, a direct one. 

Graph Analysis & Reading Passage

15 minutes

After students complete the graph, there is a brief reading passage that elucidates some of the concepts they just went over in the graphing exercise, as well as introduces some new vocabulary. Following, there are some questions that give students the opportunity to describe the specific relationship between cooling time and crystal size, as well as correlating cooling time and rock texture (it may be helpful to have some actual igneous rock samples for students to see this visually - usually Obsidian (non-crystalline) and Granite (larger crystals) are two rocks that are helpful here - I use Flinn Scientific and Carolina for most of my purchase orders for science class). 

Exit Ticket & Closing

10 minutes

Students are given a few minutes to complete the Exit Ticket, after which it is graded collectively. Students pack up, and they are then posed 1-2 summary questions before exiting the room ("Can you tell me the relationship between crystal size and cooling time in igneous rocks?" and "What's the difference between extrusive and intrusive igneous rocks?")