Sunday, November 22, 2009

Rock and Mineral Show

As part of our geology study we went to a local rock and mineral show. It is a great opportunity to collect minerals and to see where locally people are finding them. This is our first visit to one of theses shows; someone on the AtHomeScience Yahoo group suggested this wonderful idea.

Any child that went to the show got a grab bag of various rocks, and then my kids bought a few, too. The 5 rocks along the bottom of the picture were bought while the rest came in their grab bags.

Ds#1 noticed that one dealer was selling rocks that looked very similar to one he had recently found. We knew it was an igneous rock, granite in particular. It had no pick feldspar though it can be white. The specimen is striking for the many black crystals in it. The black crystals turn out to be biotite, a type of mica; we could tell because it was easy to flake. And since the rock also has muscovite mica, it is called binary granite.

BTW, the University of Pittsburgh and Cal Poly Pomona have informative and pictorial sites for identifying igneous, metamorphic, and sedimentary rocks.

Saturday, November 21, 2009

Saturday Science Challenge #13

Layers of the Earth

The earth has 3 basic layers--the crust, the mantle, and the core. Texts will emphasize the thinness of the crust. Making models is a wonderful way to demonstrate this, though I found my kids were particularly impressed by this demonstration.

The earth is not perfectly spherical, and the crust of the earth is irregular, so exact numbers are not really possible, but these are the ones we can use for this activity:

Radius = 6378 km
Distance to core edge = 2890 km
Distance to mantle edge = 35 km (from Wikipedia)

Take a measuring tape at least 6 m long; have someone hold it in place at the "center" of the "earth." First measure out to 2.73 m. Holding chalk and the edge of the tape together, draw a circle, or at least an arc; this first line is the edge of the core. Now bring the tape out to 6m and draw a second arc; this second line is the edge of the mantle. Ask your kids, "How thick is the crust in this model? How much longer do I extend the measuring tape to draw the crust?" Have them mark where they think the line would fall.

Answer: 3 cm

For advanced students, have them calculate the ratios. I choose to have the edge of the mantle to be 6 m, so how large is the full radius for the model? The actual distance from the center of the earth to the mantle's outer edge edge is 6378 km - 35 km = 6343 km. Now set up a ratio:

distance to outer mantle (6346) is to earth's radius (6378) as model distance to outer mantle (6 m) is to model radius (rm).

6346/6378 = 6/rm --> (6378)(6) = (6346)(rm) --> (6378)(6)/6346 = rm
rm = 6.03

You can set up a similar ratio to check the edge of the core.

Sunday, November 8, 2009

Saturday Science Challenge #12

Basic Rock Identification

Around here we rarely find rocks made of a single mineral that we can identify through color, luster, streak, hardness, and cleavage, but you can classify rocks into their three basic classes: igneous, metamorphic, and sedimentary.

All you need is one hammer and goggles for everyone participating. A geologist's pick hammer is most effective though this can be done with other hammers. And you need to find a flat, hard surface on which to do the cracking.

You can play the Rock Type Game at Geology For Kids as a warm up. You can also read the classic A First Look At Rocks by Millicent Selsam, or DK e.Guides Rocks and Minerals to get an idea what to look for when classifying them.

Cracking them open, as well as wetting them, really helps to see crystals and layering. And I didn't have to ask twice if anyone wanted to do this activity. For older kids, have them try to identify the specific types in each category. They can also map the location of their rocks, and see if different locations have different predominant types. The Moh's hardness scale is helpful for this, especially the hardness of common items you can use to test your samples. For advanced students, check out the igneous rocks, metamorphic rocks, and sedimentary rocks classification pages.

These two rocks are the same specimen. The picture doesn't show how much more lustrous the inner rock is compared to the worn outer surface.

It's granite, an igneous rock.