Objectives
- Students will learn about volcanoes.
- Students will learn about chemical reactions.
Materials
- Baking pan
- Soda bottle
- Moist soil
- Tablespoon baking soda
- Cup of vinegar
- Red food coloring
- Space where you can make a mess
Background Information
The Nature of Volcanoes
Volcanoes are built by the accumulation of their own eruptive products—lava, bombs (crusted over ash flows), and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.
Driven by buoyancy and gas pressure, the molten rock, which is lighter than the surrounding solid rock, forces its way upward and may ultimately break though zones of weaknesses in Earth's crust. If so, an eruption begins, and the molten rock may pour from the vent as nonexplosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and accumulations of fall-back fragments may move downslope as ash flows under the force of gravity. Some of the finer ejected materials may be carried by the wind and fall to the ground many miles away. The finest ash particles may be injected miles into the atmosphere and carried many times around the world by stratospheric winds before settling out.
Magma, Lava, and Pumice
Molten rock below the surface of Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Originating many tens of miles beneath the ground, the ascending magma commonly contains some crystals, fragments of surrounding (unmelted) rocks, and dissolved gases, but it is primarily a liquid composed of oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. Magmas also contain many other chemical elements in trace quantities. Upon cooling, the liquid magma may precipitate crystals of various minerals until solidification is complete to form an igneous or magmatic rock.
Lava is red-hot when it pours or blasts out of a vent but soon changes to dark red, gray, black, or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon, sodium, and potassium flows sluggishly, like thick honey, or in other cases, like pasty, blocky masses.
All magmas contain dissolved gases, and as they rise to the surface to erupt, the confining pressures are reduced and the dissolved gases are liberated either quietly or explosively. If the lava is a thin fluid (not viscous), the gases may escape easily. But if the lava is thick and pasty (highly viscous), the gases will not move freely but will build up tremendous pressure, and ultimately escape with explosive violence. Gases in lava may be compared with the gas in a bottle of a carbonated soft drink. If you put your thumb over the top of the bottle and shake it vigorously, the gas separates from the drink and forms bubbles. When you remove your thumb abruptly, there is a miniature explosion of gas and liquid. The gases in lava behave in somewhat the same way. Their sudden expansion causes the terrible explosions that throw out great masses of solid rock as well as lava, dust, and ashes.
The violent separation of gas from lava may produce rock froth called pumice. Some of this froth is so light—because of the many gas bubbles—that it floats on water. In many eruptions, the froth is shattered explosively into small fragments that are hurled high into the air in the form of volcanic cinders (red or black), volcanic ash (commonly tan or gray), and volcanic dust.
Procedure
- Start with an explanation of volcanoes using the background information.
- Place the baking pan on the ground, and set the soda bottle in the middle of the pan.
- Mound and shape the moist soil around the bottle to form a mountain. Bring the soil right up to the top of the bottle's opening, but don't get the soil inside the bottle.
- Pour one tablespoon of baking soda into the bottle.
- Color one cup of vinegar with red food coloring.
- Pour the colored vinegar into the bottle. Stand back and watch red foam spray out of the top and down the mountain like lava from a volcano.
What's happening in there?The baking soda is reacting with the vinegar to produce carbon dioxide gas. The gas builds up enough pressure to force the foaming liquid out of the top of the bottle.
Brought to you by MaryAnn Kohl, author of: Messy Art Book.
