Let's learn a little geology
Volcanoes, named after the Roman god of fire Vulcan, are eruptions in the Earth's crust. Earth's crust is made up of rocks - sedimentary, igneous, and metamorphic ones - and is the outermost layer of our planet. When there is an opening in the crust, hot magma and volcanic ash can escape. Thus, forming a volcano.
Volcanoes are usually found where along tectonic plates boundaries. Tectonic plates are fragments of the lithosphere, aka rock sphere, the rigid outer layer of Earth (the crust and the topmost layer of the mantle). The plates fit together like a hugh jigsaw puzzle. These plates slide, converge (collide with each other) and diverge (move away from each other). Most of the world's active volcanoes occur along plate boundaries. About 4/5 of the Earth's active volcanoes are found on the Ring of Fire, a ring of volcanoes that roughly skirts the Pacific Ocean. They are mostly convergent volcanoes. On the other hand, the Mid-Atlantic Ridge, an oceanic ridge system that snakes along the bottom of the Atlantic, has volcanoes that were formed by divergence.
Everyone knows that volcanoes are cones, spewing out lava and vocanic ash, right? Wrong, actually this is only one type of volcano. Cinder cones can form in just a few weeks. Sometimes, they only reach a height of 330 feet. or or a composite cone volcano, building up layer after layer of ash, cinder, and layer over centuries. Composite cones, or stratovolcanoes, are steep sided and are some of the best known volcanoes in the world, e.g., Mt. Fuji in Japan and Mt. Hood in Oregon.
Sometimes, a volcano is simply a fissure, or opening, on the surface of the Earth's crust. These can be found anywhere, regardless of plate boundaries. Sometimes, these fissues extend for several kilometers. Several areas of Earth are covered with lava floods, that came out of these kinds of fissures. Flood lavas are common in Antartica, in South America near the Parana River, and the Columbia lava plateau in the Pacific Northwest of the U.S.
Shield volcanoes look like huge turtle shells. These types of volcanoes have low viscosity lava. Viscosity is a measure of how fast a liquid can flow - low viscosity means it takes a long time to get where it's going, think corn syrup. With the lava flowing slowly, it has time to spread out. The Hawaiian islands, Kilimanjaro in Africa and most of the volcanoes in Iceland are examples of shield volcanoes.
Lava domes are another kind of volcano. Mt. St. Helens is a great example of a lava dome. Lava can slowly push its way to the surface and cause a bulge, a rough circular mound. Because of its viscosity, the lava is sticky and will cause a dome shape. The dome can grow for several months and remains very dangerous and hot. Sometimes, there is a concomittant buildup of gases that will erupt from time to time. The other hazard associated with lava domes is pyroclastic flow, a quicky moving flow of incredibly hot rock and gas (1000°C) that pours down the side of the volcano.
Mt St Helens
Building a volcano
How to blow your top (safely)
Get the kids involved by building a simple model of a volcano using common household items. For each volcano, you'll need:
- an old flat box lid
- a bunch of old newspapers
- an empty film cannister or pill bottle
- some aluminum foil
- some ketchup
- some baking soda
- a stronger acid (I used HCl, but that may be a bit dangerous for some of you. Try lemon juice.)
- an eyedropper
- a funnel
- a spoon
What the kids do:
- Put the empty bottle in the center of the box lid. Add a few drops of your acid to the bottle.
- Using the old newspapers, build a cone shape around the bottle, making sure to leave the opening accessible.
- Cover the cone with the foil, cut a small opening so you can get to the bottle.
- Using the funnel and the spoon, put several spoonfuls of baking soda into the bottle.
- Squirt a bunch of ketchup into the bottle. Use a craft stick to stir the whole mess around. The baking soda will react with the ketchup and ooze over the edge of your volcano. I discovered that adding the HCl helped move the reaction along a little more quickly. The ketchup has great viscosity to stimulate lava flow and it shows up well against the foil background.
Let's learn a little more geology
Calderas are formed when the volcanic cone collapses on itself. There are many examples of calderas in the world, including Crater Lake in Oregon, Valle Caldera in the Jemez Mountains of New Mexico, Yellowstone Lake in Yellowstone National Park, and the bowl-shaped sea-water filled depression that once was Krakatoa in Indonesia.
The formation of a caldera begins with a set of fairly mild explosions that gradually become more and more violent. The explosions rapdily lower the level of magma in the chamber just below the dome. This leaves virtually no support for the volcanic cone. The cone then collapses into the empty magma chamber, leaving a wide, bowl-shaped caldera.
How to make a caldera
How to implode (again safely)
The kids will understand the geology of calderas once they make this easy model. You'll need to gather:
- 5-lb bag of all-purpose flour
- a balloon
- some duct tape
- some plastic tubing
- a clamp (a binder clip works in a pinch)
- an old box
What the kids do:
- Blow up the balloon, but just hold it closed, don't tie it.
- Put the clamp near one end of the plastic tubing. While holding the balloon closed, gently slide the end over the other end of the tubing. Duct tape it on tightly. This is a tricky maneuver and may need to be tried several times to get it right. Make sure the kids don't allow too much air to escape from the balloon.
- Place the balloon in the center of the box. Keep the clamped part of the tubing out of the box, so yu can get to it later.
- Gently pour the flour over the balloon. It should look like a shield volcano.
- Remove the clamp from the tubing. The balloon should deflate, causing the dome to collapse in on itself and forming a "caldera".