Thursday, February 23, 2017

Igneous Rocks

Our fifth Geology lesson with our co-op was all about igneous rocks.  As I taught, we put together a poster (I am kicking myself that I forgot to photograph it) so they could better visualize what I was saying.  First, we talked about the composition of the earth's crust.  Igneous rocks make up the majority.  We placed this image in our lapbooks.
Next, we talked about how igneous rocks are formed.  The word igneous comes from the Latin word ignis, which means "fire."  This was their clue.  They immediately guessed volcanoes.  I then told them that volcanoes were named after the Roman god of fire, Vulcan.  I used this little pocket volcano (bought at Michael's for $2) to demonstrate.
 Just add a little vinegar to the opening ...
 ... when it hits the baking soda within, BOOM!  We have eruption immediately.
 They were able to see that the "lava" that flowed out would cool and harden to form igneous rocks.  Note the bubbles that form holes in some of those rocks.  The "magma" that remained inside would also form different igneous rocks than those formed by the outside lava.
For a snack, I made volcano cookies.  I simply baked some dark colored cookies, then once baked, cut them into triangles.  After they cooled a bit, I used cookie icing and red decorating sugar to add lava.
 Volcano cookies!  They loved them.  I served them with ice-cold water bottles "to cool down" their mouths.  They thought this was funny.
We then talked about how igneous rocks form from volcanoes and the difference between those that are intrusive (formed slowly, below the surface) and extrusive (formed more quickly, on or above the surface).  We then looked at samples of each.  

First, we hit on a couple of intrusive rocks.  Our first was granite.  Pink granite (like the sample below) gets its pink color from the mineral presence of lepidolite.  This type of rock makes up the inner walls of the Great Pyramids.
Next, we compared the first sample to these of white granite.  You can see the large, well-formed crystals that are characteristic of intrusive igneous rocks.  Granite is the most plentiful igneous rock on dry land.  (The most plentiful worldwide is an extrusive rock which we'll get to farther down on this post.)
Our next intrusive sample was diorite.  Like granite, it has those well-developed crystals ingrained.  Diorite was found in a lot of the weapons used by the Incas and the Mayans.
After we observed each sample, we added it to our igneous rocks pocket that I printed from  These went into our lapbooks.
For those samples that I wanted them to add to their pockets that were not provided by the site's download (like diorite), I used this Dover Learning About Rocks sticker book to make slips of paper for them to add that rock's sticker to.  They would then add the slips to their pockets with the other samples (see above).  (We will use these Dover books to do the same for sedimentary and metamorphic rocks when we learn about those, too.  You can purchase these Dover books at for only $1.99 apiece.)  
Then it was time to discuss some extrusive igneous rocks.  The first we covered was basalt (seen below).  As you can see from the picture, the crystals in extrusive rocks are much smaller than those of intrusive.  This is because they form more quickly on the surface, cooling and hardening faster than those rocks below the surface.  Basalt is the most common rock in the Earth's crust.  This is because this rock erupts from deep sea volcanoes and since our planet is covered by more water than land, you can see why this one would be more common on our crust.  This rock has also been found on the moon and on Mars.
 Next, we looked at vesicular basalt.  It has holes in it that were formed by bubbles in the lava.  We had fun comparing its weight to the regular basalt sample.
 Next up?  Pumice!
 We talked about what we use pumice for ...
 ... but more importantly, how this is the rock that floats!  Of course, we tested it.  It didn't disappoint!
 Here it is, floating next to basalt, which sunk immediately.
 The last extrusive igneous rock we covered was obsidian.  It is commonly known as "volcanic glass" because of its glassy appearance.  Native Americans used it for arrowheads, tools, and knives.  It is still used in some surgical blades.
 These are considered pure samples.  The one on the right is uncleaved.
An impure obsidian is this sample of snowflake obsidian.  These white crystals you see are considered impurities, crystals of a mineral called cristobalite, which the lava picks up as it flows over the earth's surface.
After observing all of our samples, we finished filling out our igneous rocks pocket then completed this printable (from the same site we printed the pocket) to review.
 I had printed the information to go under each tab ahead of time to save them from too much writing.
 They loved putting them together and adding them to our lapbooks.
I also made this sticker for them to add ...
... and printed this image from online.  They love decorating their lapbooks with little additions like these.
For homework, I gave them this review sheet I found at  (It's a free download.)
Also, I got the idea for writing narratives in the point of view of an igneous rock at  From that idea, I made writing sheets and gave each type of igneous rock a clever name so every student got a different rock to write about.  We had Miss Polly Pumice ...
... Gregory Granite ...
... Obadiah Obsidian ...
... and Miss Basil Basalt.
They laughed as I handed out their narratives.  I can't wait to see what they return with.  Next up?  We will do a quick igneous rocks lab to identify mystery rocks (like we did with minerals), then have a field trip to a local park known for its rock structures!  Check back with us!

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