• Observe students during class and group discussions, providing feedback in order to facilitate fair and appropriate partner work.

• Provide feedback and guide student understanding during the analysis of the animated model of continental rifting in order to help students associate common features of an engine with the more complex inner workings of Earth’s interior.

• Geologic histories and the group discussions surrounding the creation of them are relevant to the lecture in order to keep students on task.

“Today we’re going to extend your geologic histories of the Gettysburg battlefield back in time to the breakup of Pangaea.”

“What was Pangaea anyway?” A supercontinent that existed from 200–300 million years ago. “Where did Pangaea start to break apart?” In the area of what is now Gettysburg along old convergent plate boundaries of what are now Africa and Pennsylvania (North America). “How do you think this breakup of Pangaea might have affected the present day Gettysburg area?” Facilitate a class discussion.

“Let’s take a look at this animation to see what started to happen in the Gettysburg area about 200 million years ago.” Go to http://esminfo.prenhall.com/science/geoanimations/animations/35_VolcanicAct.html and click on the Continental Rift Volcanism. Focus attention on the animation to the left (formation of plateau basalts did not occur in this area) and facilitate a class discussion analyzing and summarizing the Earth processes depicted. The brown and tan layers represent the crust and upper mantle of the lithosphere. Call attention to the arrows in the tan layer that represent forces pulling in opposite directions on the Earth’s lithosphere. Move the toggle slowly through the animation. The hot material (shown in red and orange) wells up from the mantle, pushing up on the lithosphere, and producing the rifting forces that stretch the lithosphere until it breaks. The lithosphere cracks (faults) and big blocks of the lithosphere shift downward, creating rift basins (in this case the Gettysburg rift basin). Magma develops below the rift basins and moves towards the Earth’s surface, intruding into the lithosphere and extruding on the Earth’s surface. Although not pictured, some magma forces its way in between rock layers in the lithosphere, forming sills like the Gettysburg sill.

“So, what started all of this rifting anyway?” Upwelling of hot mantle material. “What causes this movement of hot mantle material?” Refer to the “Earth on a Slow Boil” article (S-8-4-2_Earth on a Slow Boil Article.doc) to discuss the Earth as a heat-engine model. Then go to http://www.classzone.com/books/earth_science/terc/content/visualizations/es0805/es0805page01.cfm?chapter_no=visualization to discuss convection currents in the mantle.

“So, let’s summarize our analysis of what happens …” Facilitate a class discussion. Heat from inside the Earth causes material within the Earth to heat up and become more buoyant (less dense). The more buoyant material rises up past more dense material. As this heated material moves up towards the Earth’s surface, it spreads apart, cools, and sinks back down. The movement of the heated material tugs in opposite directions on the lithosphere overlying the spreading center, causing it to break (rift) and great basins shift downward. Portions of the heated material melt, forming magma that intrudes into the lithosphere and flows out on the Earth’s surface as lava.

“You’re going to work today again in the same groups from Lesson 1. I’m going to return your geologic histories to you so you can create and add a rifting ‘prequel.’ Be sure to include labeled diagrams to illustrate your new chapter.”

Extension:

• For students requiring further practice with the standards, they can complete a compare and contrast the mechanisms of a combustion engine (automobile) with the convection currents present in the Earth’s interior. Students would present their information in essay form or a poster.

• Students hypothesize a scenario where Earth’s internal core has stopped producing heat. Students will create a timeline, highlighting significant events that would happen once heat production stopped. Timeline events would highlight the following:

• Climate

• Tectonic forces

• Civilization

• Living organisms