Introduction To Relative Dating

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Objective

SWBAT differentiate between relative and absolute dating and define the laws of superposition, uniformitarianism, and original horizontality

Big Idea

Students discuss the differences between relative and absolute dating, and figure out how geologists date rock layers in this introductory lesson

Lesson Introduction

[Note: For embedded comments, checks for understanding (CFUs), and key additional information on transitions and key parts of the lesson not necessarily included in the below narrative, please go to the comments in the following document: Dating & Rock Layering (Whole Lesson w/comments). Additionally, if you would like all of the resources together in a PDF document, that can be accessed as a complete resource here: Dating & Rock Layering [Entire Lesson][PDF]. Finally, students may need their Earth Science Reference Tables [ESRT] for parts of the lesson (a document used widely in the New York State Earth Science Regents course) as well.]

This is the first, and one of the most important, lessons in the new unit. The lesson starts with a brief introduction into dating techniques, eventually flowing into a distinction between relative and absolute dating (which will be discussed again later in the unit). They then see some new vocabulary and get to practice applying it before closing out for the day. 

Do Now & Introduction

12 minutes

Students come in silently and complete the (attached) 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 I call on a student and ask them to read the objective out loud to start the lesson.

As a general note, the Do Now serves a few purposes:

  1. It serves as a general review of the previous day's material;
  2. It is a re-activation of student knowledge to get them back into "student mode" and get them thinking about science after transitioning from another content area or alternate class;
  3. as a strategy for reviewing material students have struggled with (for example, using this as a focused review for material that they have struggled with on unit assessments or recent quizzes); and,
  4. It is an efficient and established routine for entering the classroom that is repeated each day with fidelity (I never let students enter the classroom talking. While it may seem potentially severe to have students enter silently each day, this is both a school wide expectation and a key component of my classroom. In many respects, I find that students readily enjoy the focus that starting with a quiet classrooms brings each day).

 

After the Do Now, I show them a picture of the Obama family and give them a minute to discuss the question - "Can you sort all of the people in this picture from oldest to youngest?" After 30 - 60 seconds, we come back together and discuss, where I solicit a few responses. Most of them are able to do this pretty easily, after which I ask "why?" Oftentimes, students point to characteristics like size, clothing, facial structure, presence of wrinkles, etc. (there may be more - sometimes they get creative!) as evidence for their age. But in the discussion, I also mention that even though we know who is oldest, youngest, and their relative order, we don't know exactly how old they are, which is the distinction between relative and absolute dating. 

Definitions & Text

18 minutes

After the introduction, we take some time to whole group read over the brief text on relative/absolute dating. On the bottom of the first page of the Definitions & Text resource, they're given a profile of a rock layer, which I use to first talk about the idea of superposition - the concept that the bottom-most rock layers are oldest, while the youngest rock layers are found at the top (this Superposition Picture can also be used in class to illustrate the concept). We then introduce two other principles: horizontal originality and uniformitarianism.

As seen in the Definitions & Text resource, I use a quote to insinuate the definition of uniformitarianism (you can also have them look at the base word "uniform" and give that word in a sentence), which is the concept that the geological processes evident today have continued in the past, and are likely to continue into the future. The final process, horizontal originality, I use a brief demo. I take a flat-bottomed tupperware, a round-bottomed flask, and a container with some pieces of Play-Doh, and I pour some sand over the top. I ask them to concentrate on the bottom shape of the container, and the shape of the sand after it's poured in. I then pose the question to the class - again, usually focusing on the words "horizontal" and "originality" - what they think this process means. It is the concept, regardless of shape, sediments are always deposited in flat layers. That also means if the layers are not flat, something has happened to them. Students fill this out on the associated lines in the Definitions & Text as we go over these concepts. 

Practice

20 minutes

After covering the definitions and concepts of superposition, uniformitarianism, and horizontal originality, I encourage them to get started on the Practice section. 100% of these questions are pulled from former Regents examinations, so they're relevant and appropriate regarding the content and general level of rigor that students can expect to see on the assessment. Additionally, they're mostly organized to get increasingly more difficult and increase in complexity, which is why the harder questions tend to come toward the end. 

In terms of student work habits, I tend to sometimes make this decision in the moment, and as a response of what I know about the students and how they're processing the material on, but I'll either ask them to work independently, in partners, or give them the option. Usually, before starting practice, we tend to go over some steps for self-help ("What should you do if you're stuck?"), and I might reference a previously used multiple-choice or free response strategy in order to build their skills while simultaneously learning content (as an example - one popular one we always use - "If you aren't sure what the right answer is, see if you can eliminate some wrong answer choices"). I tend to circulate for compliance and then hone in on specific students while they're doing this. 

After about 10 minutes, we go over their responses. Students who finish early are encouraged to work on the exit ticket (resource below) and double-check their responses. We use a combination of strategies (active voting, cold calling, popsicle sticks, volunteers) to go over the responses, where students correct their work and ask any clarifying questions. 

Exit Ticket & Closing

10 minutes

In the last few minutes of class, I have students complete the daily Exit Ticket. For the sake of time, I have students grade them communally, with a key emphasis on particular questions and items that hit on the key ideas of the lesson (Note: This usually manifests as students self-grading, or having students do a "trade and grade" with their table partners). After students grade their exit tickets, they usually pass them in (so that I can analyze them) and track their exit ticket scores on a unit Exit Ticket Tracker.  

After students take a few seconds to track their scores, we usually wrap up in a similar way. I give students time to pack up their belongings, and I end the class at the objective, which is posted on the whiteboard, and ask students two questions:

  1. Do you feel that you mastered the objective for the day?
  2. Can you reiterate one thing you learned about ____________ (in this case, definitions of relative and absolute dating, how those processes are different, etc.)